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Flow stall after pipe drops down
8

Flow stall after pipe drops down

Flow stall after pipe drops down

(OP)
Hello,

we have the following problem: A heat exchanger (15 m) is fed by a pump (0 m). It is a pipe network where other heat exchangers on parallel pipes are located much lower. The pressure gauges read low to negative pressures right before and behind the heat exchanger. Throttling behind the heat exchanger seems to solve the problem. It is suspected that the flow stalls after the heighest point of that branch of the piping system. Nonetheless the flow rate through that branch is higher than through the branch that contains the other heat exchangers, located lower.



1) Can anyone direct me to literature or suitable key words for google, to get more information on the problem of the flow stalling? We suspected because the flow experiences a free fall and accelerates the vacuum in the heat exchanger is created.

2) Another mitigation which was thought of: install a throttling valve behind the pump before the pipes branches in order to increase the pressure. The increased pressure should make sure that the 15 m heat exchanger is supplied with medium. My questions here are:
2.a) The pump head matches the pressure losses of the pipe network. If I install a throttling valve the pressure will rise, but only before the valve. The increase in pressure should match the pressure drop across the valve. So in my opinion it is not possible to control the pressure in the pipe network with this method.
2.b) I am correct in assessing that the pressure at branching point does need to be greather than 15 m + pressure losses across the pipe + pressure loss across the heat exchanger. I suspect this does not pose a problem as long as the pressure loss across the other path is high enough. This would lead me to think we can ensure proper operation by throttling in the branch where the 15 m heat exchanger is NOT located.

Basically I'm trying to determine at which points we can try to control the flow in a manner, where we have no negative presssure at 15 m.

I'd appreciate your input on the situation,

have a nice weekend!

RE: Flow stall after pipe drops down

This is an simple problem and a solution can be found if you would supply a complete flow diagram. Add the flows, pipe diameters, and valves onto your sketch. Is this a recirculating loop?

RE: Flow stall after pipe drops down

A picture is worth 1000 words. Maybe 1,000,000 words if you talk about pipe networks!

Please make an accurate network diagram. Show all equipment in proper place, known flowrates and pressures at the points in the diagram. Show all pipe lengths, diameters and elevations.

15m could be length or head. Please make that clear, or show pressures in bars or psig.

You usually don't want to throttle the pump unless there is too much pressure or flow.
Valves in front of the heaters ... maybe.

Give us a nice diagram, then we can talk intelligently. Otherwise .. there is no otherwise.
Ball is in your court.

RE: Flow stall after pipe drops down

markboc,

No one can really figure this out as your description is confusing.

What exactly is yur problem? No flow through the highest HX? Is that waht you mean by the flow "stalling?"

PLease use descriptions such as up stream or downstream. "Behind" is too vague.

Assuming you mean that the valve is downstream of the junction point from the other HX's, you might be getting more flow if the pressure losses for the same flow rate are lower for the higher HX than the lower ones, i.e. it is short circuiting the flow.

In any closed system you normally pressurise the loop such that the highest point is above atmospheric pressure. Then all you need to do with your pump once you've removed all the air is overcome frictional losses.

Your questions
1) Try closed circualtion loop
2) Makes no sense to put the valve U/S the junction point. You are correct - won't work
2b) Yes you are correct.

As said by others - Is this a closed system or open?. If a closed system then you normally need a pressurisation input to keep the pressure constant. If an "open" system then you might need pressure control D/S the high heater to stop it pulling a vacuum in the HX or causing the water to boil at sub atmospheric pressure.

But see 15-03s post and see if you can give us the required information. It's not a hard system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

You say "we have the following problem". What problem? I did not see one. Negative pressures? Why is that a problem? Flow stalling? What is that?

It sounds like you may be suffering from vapor lock where air is accumulating in the higher heat exchanger, but who knows?

RE: Flow stall after pipe drops down

Please give this person time to provide appropriate information.

RE: Flow stall after pipe drops down

(OP)
Thank you all for your input! I also realized prior to looking back at this thread that my post may have been confusing and I wasn't wrong. Let's start again.



This is a recirculating loop, the tank is vented if the symbol I drew there isn't clear.

This is the current situation and I supplied all information I have. Pipe diameter is DN500, exception: the two branches containing hx1, hx2, hx3 are DN200. The overall length is a couple of hundred metres. I don't have an isometry at hand. The medium is water.

1) The pressure indicators PI 1 and PI 2 indicate negative pressures and hx1 is often damaged. We suspect due to the vacuum occuring there. The idea here is that due to the free fall D/S of hx1, the fluid accelerates and the flow stalls. I'm an ESL speaker, so I don't really know how to describe this better. If 500 m^3 / h go in and the fluid velocity increases, then more than 500 m^3 / h would need to flow D/S of hx1, unless the wetted crosssection in the pipe decreases. This should reflect my original question 1), I'd like some further information on this phenomenon or at least a better description so I can find out myself.

2) Throttling V1 seems to solve the problem or so have I been told. If the acceleration / stalling theory from 1) is correct I can understand this.

3) The current idea is to use V2 to control the pressure U/S of hx1. This is where I have the most trouble in understanding / I'm opposed to the idea. I think every increase in pressure due to throttling V2, let's say the 6 bar rise to 8 bar, wouldn't change the fact that we still had 3.5 bar U/S of the junction point. All "extra pressure" gets dropped across V2.

4) In order to control the pressure U/S of hx1 I'd personally install an additonal valve here:

While throttling V1 seems to work, it also decreases the flow rate and I think in order to make sure hx1 AND hx2, hx3 are supplied with the proper flow, you would need to be able to increase the pressure losses in that branch.
Basically this question is: How to pressure control U/S of hx1?
From what I already got as input, pressurizing the tank may also be an option? I'm not convinced V2 has anything to do with "controlling" the pressure. From what I think I know, the only way to increase the pressure at a certain point, is to make sure D/S of that point there is some increased. D/S of joining the two branches would also be a viable location for a throttling valve. In order to maintain / gain flow control capabilities for the two branches I'd prefer installation in either of the branches.

Thanks for taking the time to read and answer this!

RE: Flow stall after pipe drops down

Thank you for the nice diagram! Usually everyone asks so many questions and only get a long series of incomplete answers and it takes forever to figure out what is going on, if ever. This is great. I'll give you a star for that diagram. It is clear and now you will get some good and timely comments. I will start with mine.

1) you need more pressure at P1 and P2. Open V2 fully. Throttling V1 will tend to raise pressures at all upstream points, but will tend to reduce pressures at all downstream points. If pressure after V1 is reduced too much, you may reduce flow in the downcomer to less than full cross sectional flow, I.e. partial flow. If pressure is reduced lower than water's vapor pressure, water vapor, cavitation might occur there.

2) Throttling V2 will increase pressure at the pump discharge, but reduce pressure and flow everywhere else. That is not likely to be helpful, unless you want to reduce flow everywhere. The 3.5 bar will reduce, tending to cause more vacuum at all heaters, especially at hx1. By throttling V2, you might be reducing flow so much that pipe friction lessens, which tends to increase downstream pressures everywhere somewhat. That might increase pressures at P1 and P2 some, but presumably you do not want to reduce flow going to the heaters. This possible effect is determined by how your pump and responds to the lower flow (usually increases output pressure) and by how much pressure drop you get across V2 at the lower flow (less). You have to look at your pump curve and valve pressure-flow-position characteristics to know haw that will work, or won't work to your advantage at the heaters.

4) Adding the red valve and throttling there increases upstream and hx1 pressures, but tends to reduce overall flow in the system. It will also tend to increase that slightly reduced system flow going to hx1 and hx1's pressure, but it will also reduce flow and pressures at the other heaters. It could be helpful, if not enough flow is going to hx1, or if pressure at hx1 is too low. You don't ever usually want to increase pressure loss anywhere, especially with P1 near vacuum, so let's just say we want to increase pressure everywhere in the hx1 leg.

Comment:

It can be tricky to balance flows going to multiple heaters, unless you can achieve exact hydraulic equilibrium. As we know, throttling the new red valve, increases a reduced system flow to hx1 and increases pressures in that leg, but reduces flow to HXn's and the pressure there in their leg. You may be able to find the proper throttling point and still have acceptable flows going into each leg, but pressure in the HXn leg may suffer. If you have excess pressure in that leg now, you could possibly afford to throttle the red valve and reach a solution for both legs. If pressure in that HXn leg is minimal already now, then throttling the red valve will only reduce their pressures more. If that happens, you will probably be forced to consider changing the pump for one that can produce a higher discharge pressure at your 800m3/h flow rate. Adding a valve into each heater leg would not solve the problem, as I think you know. Each valve would fight the other. It can be tempting to try that, but in the end, it is not a solution.

Increasing tank pressure will only raise the pressure at every point in the system equally. That would increase the P1 and P2 pressures and all pressures everywhere, but accomplish nothing else. If low pressure at HX1 and/or cavitation in that leg is your only problem, it might solve your problem, but increasing all pressures is not ideal.

Adding a valve downstream of both legs would raise pressure everywhere upstream, but tend to reduce system flow. It could solve your low P1,2 pressure, but it would also lower pressure at the pump inlet, potentially causing NPSH trouble. If you would not get NPSH trouble, you might think about that, but you would not be able to benefit by balancing flows going to each heater leg, if you needed to do that.

I would recommend that you add the red valve, if possible. If pressures and flow going to the HX heaters drop too much, then think about a new pump, or trying to increase its RPM, if that is possible.

RE: Flow stall after pipe drops down

(OP)
Thank you very much 1503-44 for your indepth answer!

Quote (1503-44)

1) you need more pressure at P1 and P2. Open V2 fully. Throttling V1 will tend to raise pressures at all upstream points, but will tend to reduce pressures at all downstream points. If pressure after V1 is reduced too much, you may reduce flow in the downcomer to less than full cross sectional flow, I.e. partial flow. If pressure is reduced lower than water's vapor pressure, water vapor, cavitation might occur there.
So throttling V1 may actually help with the problem. Partial flow was the term I was looking for with stalling. What I took from your post is that it may be possible to find the sweet spot in throttling such that upstream pressure is increased enough without dealing with partial flow. Currently with V1 open we think we already hit partial flow and hence have the vacuum problem.

Quote (1503-44)

That is not likely to be helpful, unless you want to reduce flow everywhere.
We actually have a multitude of problems / issues we want to adress in this project. Since V2 is already installed and currently used to control the flow rates we want to upgrade it to an automatic valve in order to reduce flow rate overall. Right now it is highly unlikely that we need the full flow rate ever again (the heat exchangers are actually cooling too much right now). However I am pretty confident that V2 is the right position to introduce temperature control and it is not disputed in my team. Hence I didn't focus the question in that direction. I still want to add it now, since you pointed it out. Main focus is pressure control for hx1 / avoiding the vacuum or possible cavitation there.

Quote (1503-44)

you might be reducing flow so much that pipe friction lessens, which tends to increase downstream pressures everywhere somewhat.
That does not sound like the ideal solution to me. Also see my next point regarding that.

Quote (1503-44)

This possible effect is determined by how your pump and responds to the lower flow (usually increases output pressure) and by how much pressure drop you get across V2 at the lower flow (less). You have to look at your pump curve and valve pressure-flow-position characteristics to know haw that will work, or won't work to your advantage at the heaters.
It's a centrifugal pump, pressure increases with lower flow. I'm under the impression that almost all increased pressure will be lost at V2, hence no meaningful gain due to that.

Quote (1503-44)

4) Adding the red valve and throttling there increases upstream and hx1 pressures, but tends to reduce overall flow in the system. It will also tend to increase that slightly reduced system flow going to hx1 and hx1's pressure, but it will also reduce flow and pressures at the other heaters. It could be helpful, if not enough flow is going to hx1, or if pressure at hx1 is too low. You don't ever usually want to increase pressure loss anywhere, especially with P1 near vacuum, so let's just say we want to increase pressure everywhere in the hx1 leg.
Throttling V1 together with the red valve would be a good solution then? Both would increase upstream pressure to hx1 and with V1 I can ensure the other hx get enough flow.
Additionally throttling V2 then helps to reduce the cooling power of all hx if not needed.

Quote (1503-44)

Adding a valve into each heater leg would not solve the problem, as I think you know. Each valve would fight the other. It can be tempting to try that, but in the end, it is not a solution.
We already have control valves upstream to every single heater, which from what I know I currently set to manual. Ideally as you mentioned I would like to avoid having multiple control valves in automatic mode as I fear the same result as you: they will fight each other.

Quote (1503-44)

but it would also lower pressure at the pump inlet, potentially causing NPSH trouble.
Doesn't the tank decouple the downstream pressure of the pump and the pump inlet? It's a rather big tank sitting above the pump. Given all parameters I highly doubt cavitation at the pump inlet will be a problem though.

Quote (1503-44)

I would recommend that you add the red valve, if possible. If pressures and flow going to the HX heaters drop too much, then think about a new pump, or trying to increase its RPM, if that is possible.
Frequency inverters to control the pump rpm were thought of but the invest was too high (including all changes to the current infrastructure that comes with it).

My take away from our exchange is:

- We probably have partial flow d/s of hx1
- Throttling V1 may mitigate the issues until it doesn't (throttling too much)
- Red Valve is a meaningful solution
- Red Valve + V1 may solve all problems, and may be used as pressure control
- V2 can still be used as flow control
- we do not want to use the valves we currently have at the Hxns

Most people involved in the project think that V2 can be used as a pressure control solution for Hx1 and right now I'm almost alone with my opinion that it can't be used that way.

Thank you all again so far!

RE: Flow stall after pipe drops down

markboc,

My friend mr 44 has said it all quite well, but what I can't understand is what you're actually trying to achieve?

Is is less total flow than 800 m3/hr (how are you measuring this flow?)
What is the design flow through each HX?
Are you trying to measure or control temperature somewhere?
If this is the cold side of the HX, where is the heat going? - Is that the first HX in the common inlet?
What is the design flow of the pump?

Assuming someone actually designed this system as opposed to just throwing it together, this data should be available.

My guess is that your system is flowing too much water and the pump has been over designed in terms of head/pressure and is operating at the right hand side of the curve. If, as you say the HX are cooling too much this indicates excess flow. It's quite common for margins to be added to margins and worst case design inputs and you end up with a pump which is too big in reality.

So the real solution, In My Opinion (IMO), is to introduce a flow control valve to force the pump back into the correct location with a valve located down stream of all the HXs. This will cause the pressure upstream to increase. Then you can play with all the control valves on the HXs to get your required split in flow between your different HXs and ensure that P1 & P2 are both > 0 barg.

Anyone suggesting closing V2 would help things is talking rubbish.

I suspect closing V1 will help by reducing the total flow and hence raising pressure. It would force more flow through HX2 & 3 and hence raise the differential pressure across those two HXs sufficient to stop it vapourising in HX1.

All the red valve would do is increase flow through HX1. Now that might be enough to raise the pressure to prevent vacuum conditions appearing, but if you already have too much flow through hx1 that doesn't solve anything.

Pressurising the tank would also help as this would increase pressures throughout the system and would make it more like a pressurised central heating system or closed circulation system in a block of flats which has the same issue. Basically the key to all this is making sure that the pressure at all times in the highest part of the system is >0 barg. DO that and then you can play with other controls to control flow to your hearts content.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

How about adding a vacuum breaker at a high spot downstream of HX1? That will stop the cavitation/damage.

Good Luck,
Latexman

RE: Flow stall after pipe drops down

In this case I don't think a vacuum breaker would do any good. It would just introduce air into the system and not really change the pressure in the HX.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

It’ll raise the pressure at the HX above atmospheric. With VB that line will run mostly water full and air, versus mostly water full and water vapor.

If the downcomer is running full and not self-venting, most of the air will just sit there and not flow to the tank, except due to solubility.

Good Luck,
Latexman

RE: Flow stall after pipe drops down

(OP)
Thank you too!

Quote (LittleInch)

Is is less total flow than 800 m3/hr (how are you measuring this flow?)
Yes, with the exisiting V2 we want to be able to supply less water in order to reduce the cooling duty of the heat exchangers. Flow is measured with inductive flow meters. My diagram is one single state of which I got the most information. Occasionally up to 1000 m^3 / h are used.
The main problem here is, that someone has to actually go down and adjust V2 in case the product gets too hot. To mitigate that the people running the plant usually just don't throttle V2 at all / very little -> product doesn't get too hot. Unfortunately that also prevents any meaningful usage of the cooling water.

Quote (LittleInch)

What is the design flow through each HX?
I don't know right now and I'm not sure if the appropriate documentation still exists / can be found easily.

Quote (LittleInch)

Are you trying to measure or control temperature somewhere?
Ideally the product is run as hot as possible since we have no upper limit on the water temperature. One could control the water temperature to be at least X, however then it also needs to be checked that the product does not exceed Y.

Quote (LittleInch)

If this is the cold side of the HX, where is the heat going? - Is that the first HX in the common inlet?
Good catch, as I didn't have all the files at hand (it's the weekend and engineers also take time off ;) ) I completely forgot that. Yes somewhere d/s of the union point is another heat exchanger to actually take away the heat.

Quote (LittleInch)

What is the design flow of the pump?
Again, no idea. It's curve is 80 m head at 0 m^3 / h and something like 50-55 m at 1000 m^3 / h if I remember correctly.

Quote (LittleInch)

Assuming someone actually designed this system as opposed to just throwing it together, this data should be available.
The plant itself is multiple decades old and grew organically. Basically the whole system was not really designed as it is used right now. Parts of the plant went and some came. Probably I could find the orders for the heat exchangers and ask the vendor for which flow rates they are designed (I need to do that anyway before we start implementing the new control) it is not given that the design flow rates actually match the current requirements. Probably the design flow rates exceed what is necessary.

Quote (LittleInch)

Anyone suggesting closing V2 would help things is talking rubbish.
It only achieves flow control but not pressure control which is what people want to prevent damages at Hx1. This really boosts my confidence in bringing that point across again.

Quote (LittleInch)

Basically the key to all this is making sure that the pressure at all times in the highest part of the system is >0 barg. DO that and then you can play with other controls to control flow to your hearts content.
This is what I want.

I'll check next week if positioning a valve d/s of all Hx is possible and try to sell that one.

Quote (LittleInch)

All the red valve would do is increase flow through HX1. Now that might be enough to raise the pressure to prevent vacuum conditions appearing, but if you already have too much flow through hx1 that doesn't solve anything.
Unless I also reduce the flow with V2?

Otherwise V2 for flow control in combination with the existing control valve(s) should be my only viable option?

@latexman, you mean a relief valve?
It has been done on other heat exchangers as far as I know but was considered for Hx1 and was discarded. I'd need to investigate as to why exactly.

RE: Flow stall after pipe drops down

No, a vacuum breaker, or a spring loaded check valve that allows air to open the check valve and flow into the water pipe.

Good Luck,
Latexman

RE: Flow stall after pipe drops down

I would use V2 to control flow through the pump, which will of course define system flow too. In fact I thought that is the "flow control" valve. If there is any other flow control valve, then you should try to use that one. Two flow control valves might fight each other, or work together, but it usually makes control more complicated. You should only control with one. Keep the other full open, if you can. Anyway, your pump will respond by adjusting discharge pressure to whatever new flow results. V2, or the other flow control valve, should be sized to allow you to flow at that rate that while not dropping the pressure too much (when the flow rate is 800m3/h-1000m3h). Thus, if pump and V2/PCV are sized and matched correctly, pressure out of the valve will be sufficient to move that 800m3h-1000m3h flow through the remainder of the system, preferably without cavitation at any point. Especially at the heaters.

As the system is right now, throtteling V1 increases pressure and reduces flow at HX1. That will tend to increase flow and pressure in the other HX leg. But it may reduce the pressure downstream of V1 too much, partially-full flow causing cavitation, cascade flow and vapor formation and flow instability, including vibration and noise, if severe. Installing an air valve there may not work well, because releasing anything, water, air or vapor there will only try to reduce the pressure there more. It will not increase pressure there and that is at least part of that problem.
An air inlet (vacuum breaker) valve there would try to increase the pressure to up to 0 barg, but then you will get air in the lines. Controlling 2 phase flow is a lot more difficult and full of potential instabilities. I would not do that, unless other solutions are not acceptable. Plus it might not increase pressure at HX1, only at the breaker valve itself and downstream. The addition of air changes the continuity of mass balance in that leg, so the hydraulics may be difficult to predict. And if pressure at HX1 does not increase, those damages might continue.

You will need to do an accurate hydraulic analysis to determine how to stop cascade flow, partially-full flow and cavitation downstream of V1, as there isn't much pressure there, so all little tiny pressure losses are important and must be accounted for in the analysis. If that is the problem. It may not be severe, but the flow is relatively high, so noise and vibration might be worrisome. If not, try to throttle V1 a bit and see how things respond. Some cascade flow may be acceptable and it will raise pressure at HX1 while reducing flow there, but it will force more flow to the HXn leg.










RE: Flow stall after pipe drops down

The first question, it seems to me, is to know explicitly, how much flow rate each hx requires to do their job. Then, given that, does the pump provide sufficient flow to achieve the head needed for your problematic hx.

TTFN (ta ta for now)
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RE: Flow stall after pipe drops down

Absolutely correct IR. Generally that pump head would be enough to preclude cavitation anywhere, if the flow has not been increased above that used for the original design. Trying to increase flow rates too high above what the original design condition specified is the primary cause of these kinds of problems.

RE: Flow stall after pipe drops down

Big Inch, you're contradicting yourself.

Item 1 in your first reply I think is spot on "you need more pressure at P1 and P2. Open V2 fully. " and "Throttling V2 will increase pressure at the pump discharge, but reduce pressure and flow everywhere else. That is not likely to be helpful,"

But now you're saying use V2 to control the flow....

It's pretty clear now that this is not a system which has been designed properly so pressures and flows are all wrong and nothing matches or makes sense.

The ONLY WAY to make life better is, as you rightly say, more pressure at P1 AND P2, to maintain these above atmospheric pressure.

After you do that then just control each HX or series of HX on temperature.

You won't do that if you do anything with V2 other than open it up fully and control flow downstream of the HXs to raise the pressure upstream of this currently non existent control valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

No contradiction. I am thinking about the appearance of that possible "other flow control valve".
Marcboc, some more info about where that other control valve is would help.
Is it FCV or PCV?

If you want flow control, the best place for that is near V2, or somewhere in that main line, the closer to the pump the better.
And where are the flow meters?

Lowering the system flow will have a tendency to increase pump head and reduce all friction pressure drops in the entire system, so average system pressure would increase with that and it is possible that P1 and P2 will rise with that effect. In fact I think that is what is going on now. Marc says that it helps some when you throttle with V1.

RE: Flow stall after pipe drops down

(OP)
I'll adress the other questions raised in depth tomorrow.

Just to clariy: There is no other flow control valve than V2. Currently V2 is a manual valve (flap?) and will be upgraded to an automatic rotary plug valve. Maybe this upgrade started the confusion that there maybe another flow control valve.

All other valves (existent d/s the Hxn right now and the maybe to be installed red one) should then control the flow ratio and foremost ensure that there is enough pressure at Hx1.

Thank you all for taking your time with me!

RE: Flow stall after pipe drops down

I disagree.

If you use V2 to control flow all that will happen is the flow will reduce and the pressure at P1 and P2 will fall further than they do at the moment. V2 is shown upstream of the HX's.

If you throttle V1 then what you're doing is increasing the pressure upstream by reducing the overall flow rate and forcing more flow through HX 2 & 3. This will raise the system pressure upstream of V1 and hence remove the boiling / partial vacuum at P1 & P2.

Only by raising the pressure at P1 and P2 to > 0 barg will this system have a chance of working properly. The easiest way to do that, IMHO, is to install a control valve anywhere on the common return leg downstream of all heat exchangers before it gets back to the tank. You probably want to control on flow at that valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

Quote (LittleInch)

If you use V2 to control flow all that will happen is the flow will reduce and the pressure at P1 and P2 will fall further than they do at the moment. V2 is shown upstream of the HX's.

What you say I belive is universally true only if the pump is held to a constant discharge pressure. Otherwise we do not have enough info to tell. We need pump and valve Cv info to be sure how the system will respond. When the flow to a pump is reduced, as you know, head generally increases, that amount depending on the pump curves dH/dQ. So that adds some amount of head into the system.

With the drop in flowrate, all friction losses decrease. That also "adds head", or rather subtracts less head from the system.

So, now, look at what happened when you throttled V2. Yes, you "added" some pressure loss to the system, but the question is how much loss did you add, or rather subtract from the system by doing that. If the throttling of V2 at a now reduced flow created more pressure loss than was added by the increasing pump head and by the lesser friction losses in pipes, yes, then PI1 and PI2 will decrease, but if you do not get a pressure drop across V2 greater than the absolute value of the sum of the pressure gain from pump head increasing and the effect of lesser friction losses, then PI1 and PI2 will increase. BUT, as I implied, the overall response is dictated by pump-pipe head "gains" vs the loss at V2. So, without knowing pump curve and valve Cv, we don't know which way the PIs will go. It could be either way. It could increase pressure at the PIs, if the pump curve is steep, but might not if the pump curve is flat. Since operating points for these relatively large flows are usually within the steep regions of pump curves and pipe friction is high, I choose to give the narrative of an increasing net system head, pump head increase and reduced pipe friction equaling a net gain, ie. greater than valve loss, with a small valve position change, but correct, I can't be sure until we look at curves. Obviously, if V2 is closed completely, PIs go to near 0 barA, but the pump also goes to deadhead max.

A valve downstream of all HX will have the same effect as upstream. It makes no difference where you put that additional pressure drop (as long as it is not on the branches). The sum of all dPs would remain the same, given the same pump head and same flow across the valves. My preference is to have pump control valves near the pump. Downstream location is a good place to control backpressure on the HXs and cascade flow. It would also have similar but possibly reduced effect at the pump, as long as cascading stopped. If cascading continued even in a small amount, no effect would be seen at the pump until cascading stopped and the line "tightened up".

Don't you agree? I'm (pretty) sure I can put it into my Stoner program and demonstrate, but I'd rather not have to do that. But I will, if you say so.

This may also be why the system appears difficult to control. Mark mentioned that throttling V1 seems to help at times, but too much, when the V1 pressure drop exceeds all other gains, then PIs drop fast and that starts damaging heaters with low pressures and cascade flows. Maybe..

RE: Flow stall after pipe drops down

(OP)
What do you mean by cascade flow? I couldn't really find an explanation.

Quote (1503-44)

A valve downstream of all HX will have the same effect as upstream. It makes no difference where you put that additional pressure drop (as long as it is not on the branches).
This would mean V2 could be used to control the pressure in the system, I read from all the posts that this is not the case?
I get that you can still get increased pressure depending on the pump curve and cv. Did your statement refer to that case? It wouldn't work if the pressure loss across V2 is greater than the gain in pump head though. Otherwise I would think that a control valve downstream of the union would have a greater effect, since the pressure upstream is increased which would certainly help mitigate the issue of too low pressure at P1 / P2.
If we replace V2 we can choose the new valve . The one we currently looked at has cvs of 1500 m^3 / h.
This is an interesting point actually as it got over my head that you actually can get a higher pressure downstream by using a control valve directly after the pump, even if not in all cases.
Placement of a possible control valve to increase pressure at p1/p2 is my main concern. From what I gathered I thought we agreed that the position it currently has, V2, is not viable.

Quote (1503-44)

It would also have similar but possibly reduced effect at the pump, as long as cascading stopped. If cascading continued even in a small amount, no effect would be seen at the pump until cascading stopped and the line "tightened up".
May be the language barrier, but I did not get that.

Quote (LittleInch)

So the real solution, In My Opinion (IMO), is to introduce a flow control valve to force the pump back into the correct location with a valve located down stream of all the HXs. This will cause the pressure upstream to increase. Then you can play with all the control valves on the HXs to get your required split in flow between your different HXs and ensure that P1 & P2 are both > 0 barg.
This would strike me as the best solution, too.

Thank you really for your help, this helps me a lot in understanding my problem!

I already added gathering information on the design specs of all Hx for next week.

RE: Flow stall after pipe drops down

Cascade flow occurs when pressure drops below the vapor pressure of the water corresponding to its temperature at any given point. Pressure is no longer sufficient to drive enough flow in the pipe to keep it full, so the volume of liquid flowing at that point drops, the pipe is no longer flowing full and water vapor pressure, now being higher than the pressure imposed on the surface of the liquid water itself, is liberated to fill the empty space created by the reduced liquid volume. When that happens on a downward sloping pipe, it is said to "cascade" down the slope, similar to the motion of a river waterfall or river with air above it. It is not like usual gravity driven flow in a river, or like flow of water through a half-full pipe in a sewer system, because the pressure on the water's surface in those situations is always atmospheric pressure of one barA, greater than the water's vapor pressure, unless the water is boiling. In this instance, pressure exerted on the water surface is less than its vapor pressure, even at low temperatures, so the water effectively boils at low temperature and water vapor is created; I think of it as "low temperature steam".

Yes, actually any valve closing in the system will tend to raise pump pressure, as long as the pump is running freely, i.e. is not flow or pressure controlled in some manner. We often don't initially think like that, because most of the time we usually want to have a good control of pump discharge pressure in long pipelines to protect downstream pipe and equipment, often owned by others in a completely foreign network, from a pumps very high deadhead (no flow head) pressure, but in some more limited systems like yours (actually a loop back to itself), it is OK to let pump pressure float, as long as pipe and heaters are designed for that max pressure.

The control valve can technically have the same effect on the pump if it is placed anywhere that is not on the heater branches themselves, although it is more intuitive for us to see how PI1 pressure is directly affected by the increasing backpressure that would result from a valve placed immediately downstream of the last junction. Actually closing a valve placed on either heater branch will tend to raise upstream pressure too, but that effect will be mitigated, as that increased pressure will be immediately dissipated somewhat by forcing more flow down the other branch.

The cascading, or cavitation would continue until system pressure increased above vapor pressure and the pipe starts flowing full once again. That is because anywhere the pipe is not flowing full, the pressure at all of those points will always be equal to vapor pressure. In that regard, partially closing a downstream valve, but not enough to get pressure back above vapor pressure at the cascading point, will not stop water vapor from filling the still partially filled pipe. Pressure there still remains equal to vapor pressure ... and all pressures upstream of that place never "feel any need" to increase. They just see the same old constant downstream pressure, the vapor pressure, so they are all happy to just keep on doing whatever they were doing before tou tightened up on that valve. Once the cascading segment starts flowing full again, the pressure goes higher than vapor pressure and the pressure is free to climb higher. Then it starts transmitting that increasing backpressure all the way upstream again, because the pressure "discontinuity", caused by the never changing vapor pressure, has disappeared. Now increasing backpressures are free to travel all the way back upstream to the pump.

Placing the control valve downstream of all heaters is good, because it is easy to see how increasing backpressure there will cause all upstream pressures to increase and additionally it will reduce flow from the pump. That is what you want to do. However you must make sure that the heaters and all upstream pipe and equipment, instruments, everything, is designed for the pump's deadhead pressure; the maximum pressure that the pump can produce. If you control with V2, then you could keep the pump's deadhead pressure from reaching the heaters with a maximum pressure set for V2's outlet at a pressure lower than deadhead, if you needed to do that, if for some reason like the heaters have a lesser design pressure than pump's max. Either one could work, as long as max design pressures "harmonize". If a downstream control valve closes and pump does not stop instantly, you will get max pump discharge pressure at all points upstream of that valve.

Don't overlook finding the pump curve, or at least a known max possible discharge pressure, and the Cv coefficients of the valves. If you can find the Cv of the heaters, that would also help. If not some idea of a known pressure drop for their design flow rates.

By the way. If EN is your second language, you must be damn good at your first. If you don't mind my asking, what language would that be? If you prefer to keep that secret, no worries. I'm just a curious sort of guy.

RE: Flow stall after pipe drops down

Possible Control Option

It also occurs to me this morning, thinking about control options, that you could install that downstream control valve (PCV3) with its input pressure from PI2. Configure it to maintain PI2 at some set pressure > vapor pressure. That would stop cascade flow. You could also control pump flow by using V1 to a great extent. As long as PI2 is >= PCV3's set pressure, V1 would actively control the system, as PCV3 would remain fully open. If you tried to increase system flow by opening V2 more, the pump's output pressure would go lower and friction losses everywhere increase, all downstream pressures would tend to reduce. If PI2 tried to drop below PCV3's set pressure, then PCV3 would begin closing to keep PI2's pressure above its set pressure and thereby not allow cascade flow to start. PCV3 would then effectively take active control of system and opening V2 further would not produce any additional flow. I have used that control strategy on many pipelines that had steep slopes coming down from mountains just before reaching an export terminal at sea level. Without a PCV3 type control of backpressure, the flow coming down the slope would have tried to increase indefinitely, causing partially full pipe, vacuum/vapor pressures and cascade flow.

More About Cascade Flow

Flow down a sloping pipe, or down a vertical pipe will occur by upstream pipe pressure and by gravity. Flow down a hill can continue without pump pressure. Gravity takes over. For any given pipe slope and pipe diameter, there is a critical flow rate. It is the flow rate that has a frictional head loss exactly equal to the head gained by the fluid moving downhill. If flow is below critical flow rate for that slope, friction is lower than gravitational energy gained, therefore the liquid's velocity can increase. When velocity increases, Bernoilli says pressure reduces (at least until vapor pressure is reached). You can increase flow to the critical flow rate without adding pump pressure.

Conversely, if actual flow is greater than the critical flow rate, then friction loss is greater than that gained by gravity elevation drop and velocity must decrease. Bernoulli says when velocity decreases, then pressure increase, so flow tries to decrease and no further flow rate can be attained on that slope without adding pump pressure.

RE: Flow stall after pipe drops down

Another possible means of control.

Instead of installing additional valves, it may be possible to get control of low pressure in HX1, by using V1 as a control valve. Configure V1's pressure controller to act like PCV3 above. That would allow you to keep HX1 pressures higher than vapor pressure, but it would not control cascade flow downstream in that leg. If PI2 decreased, V1 would begin to close, raising pressure. Backpressure would build towards upstream, lowering flow in the HX1 branch. Higher pressure reaching the junction would divert the flow previously going to HX1 to go into the HXn leg and some amount of backpressure would eventually reach the pump, thereby tending to reduce system flow. A new flow pattern would result, a lesser flow to HX1 and probably a slightly lesser flow to the HXns. But knowing exactly how much the flows redistributed themselves, depending on the pump curve and how much pressure drop each leg has after the new flows stabilise. I think that may have been what the original designers intended by locating V1 where it is. It allows some control of system flow rate as well as some proportioning of flow between heater legs. A good analysis can tell how well it could work over various V1 positions. But this method will not control cascading, so that must not reach severe levels.

The same strategy could also be used in combination with a new PCV3 to try to achieve better control of flow proportioning going into one HX leg or the other while giving you control of the cascade effect.

At this point, you now probably have far too much to think about.

RE: Flow stall after pipe drops down

This is a common situation and Latexman is correct. The only way to fix it is to break the vacuum downstream from V1. In fact, I would not even go as "high tech" as a vacuum breaker. I have built several similar plants over the years and they all included a simple vent downstream of V1.

You currently have sub-atmospheric pressure at V1 and there is still enough pressure to get the water back to the tank. So raising the pressure to 0 gauge will only improve the situation.

You can be fairly certain that the damage to hx1 is due to cavitation.

It is important to prevent air being sucked down the vent and causing 2-phase problems further downstream (as already mentioned by others). The downleg from point A to point B must be made self-venting, which would be 600 mm for a flow of 500 m3/h. [Edit]Only the section from point A down to the 8m level needs to be this larger diameter.[/Edit]

Once these changes have been made the flows through the two branches can be balanced by setting V1 and the red valve. Many years of experience have shown me that this is the simplest, most reliable and easiest to operate solution.



Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

I feel that flow balancing can easily be done if required by introducing throttling valves/orifices in different HX branches.

Important factor is to prevent possibility of vacuum condition in pipeline. This may disturb the flow by cavitation as well as release of air at low pressure.

The loop is similar to power plant auxiliary cooling circuit where a number of auxiliary coolers are cooled by closed circuit water, the same water is cooled by CCCW coolers. A number of pumps supply water in closed loop similar to here.

The difference here is that we provide an expansion tank connected to pump suction at about 15-20 m elevation (needs calculation) . This tank in addition to providing pressure all over the circuit to prevent any vacuum conditions and also provide for thermal expansion volume for water.

The 8 m level in suction tank may be too low. It will be worthwhile to think of a small expansion tank at a higher level. This is possible if there is a margin available in design pressure.

Caution: If such a tank, the operating pressures of piping and equipment will increase. The design pressure of the system is to be checked.

RE: Flow stall after pipe drops down

Katmar,

Quote (www.valmatic.com/products/air-valves/vacuum-)

breakerhttps://www.valmatic.com/products/air-valves/vacuu...
A Vacuum Breaker is mounted at critical pipeline high points, penstocks, or tanks and allows for rapid inflow of atmospheric air to reduce vacuum conditions in piping systems.

Quote (katmar)

It is important to prevent air being sucked down the vent and causing 2-phase problems further downstream (as already mentioned by others). The downleg from point A to point B must be made self-venting, which would be 600 mm for a flow of 500 m3/h. [Edit]Only the section from point A down to the 8m level needs to be this larger diameter.

If it is important to prevent air being sucked down the vent, why have a vent and thus give air an opportunity to flow into the vent? Furthermore, correct me if I am wrong, if a reduced flow to HX1 is asked for, pressure could rise at the vent above atmospheric and start ejecting water from the pipe through the vent to atmosphere. Is it not so?

Personally I would only consider adding a vacuum breaker to any pipe only if the pipe has such thin walls that it could collapse under vacuum conditions, when atmospheric pressure is greater than pipe's internal pressure. Cascade flow can be easily tolerated at low levels, so I have yet to see the need for vacuum breaker valves or vents.



RE: Flow stall after pipe drops down

Goutam, introduction of multiple valves in each leg usually only serves to increase pressure in the system. Only one valve will be actively controlling anything. Closing any other valve slightly more only tends to increase pressure everywhere upstream and reduce it downstream, just as would closing the original valve. Any time you opened one valve more, you would have to start closing the other to maintain a constant system flow. The secondary valve closing would only divert flow from it to the other leg, which can usually be done simply by opening the original valve a little more anyway. At least if the valve is sized properly to do so. Multiple valves in this situations add probable unnecessary complexity and most likely will not give any real additional control over flow to the heater legs than what one valve is providing already. Multiple valves are often a sign of trying to over-control the system, unless there are 3 or more branches going to parallel heaters. IMO, you can usually do with 0 valves with no branches, 1 valve with 2 branches, 2 valves with 3 branches, etc. N-1 valves usually can be made to work, because tou can control the branch with no valve using the pump curve. IMO.

Pressures below vapor pressure in pipe is not always such a great problem that it needs to always be avoided.

RE: Flow stall after pipe drops down

Mark, please try to get pipe diameters, lengths and elevations. I think someone will want me to prove what I say. Actually I would be OK with that, but I'll need good info to do it.

RE: Flow stall after pipe drops down

This is a simple problem. You either should have a return leg from each heat exchanger back to the storage tank or have a separate pump on each heat exchanger loop.

RE: Flow stall after pipe drops down

@1503-44 The fact that the OP has observed pressures below atmospheric confirms that there is a vacuum in the downleg (marked A-B in the sketch) and this means that somewhere in the downleg there is a liquid-vapor interface. Having a vacuum, and even worse a variable vacuum, plays havoc with the plant - both in terms of operability and physical damage. Having a vent (or a vacuum breaker) allows air into the pipe and stops the pressure fluctuations in the vapor space.

However, there will be times when the flow rate changes and the liquid-vapor interface level rises. This would mean that either you accept the air being compressed or you vent it. I much prefer the vent option and maintaining a constant pressure downstream of V1. I know that you can get vacuum breakers that also vent over-pressures, but these devices need maintenance and sitting at the top of the plant where nobody sees them can mean that they get neglected. A simple vent with a 180 degree bend is cheaper and better.

The first time I saw an installation like this I also expected water to spray out at some point. In the 25 years the plant ran it never happened. If the vent is about 2m high and the piping hydraulics are reasonable it just won't happen.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

@bimr That might make it easier to balance the flows through the branches, but it will not eliminate the vacuum problem. The vent I have described above is the usual way to do it - and it works so well that people are not even aware that there might have been a problem.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

@1503-44
It is usually done in power plant aux cooling circuit. We provide globe valve/orifice at each branch to balance flows. You may be right in saying that one has to manipulate all the valves in all the branches to get the right balance of flows.

Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Flow stall after pipe drops down

Thanks for answering my questions guys,

Goutam I agree. At Enron we did put valves on all branch lines. After 5, or 6 branches, one more valve didn't add much additional cost and sometimes having a little extra redundancy does not hurt.

Katmar, since I never do that with thick pipe walls, I'll take your word that it worked, but I think I would still not do that as long as my pipe wall was thick enough to handle vacuum and I would carefully study flow reduction and pressure rise situations, as during system start ups and shut downs. I have used a vacuum breaker on one large diameter, thin walled water supply pipeline, but only once. Otherwise I specified pipe walls that were able to tolerate vacuum.

RE: Flow stall after pipe drops down

When water in closed pipe is drained, the flow is almost always chaotic. Why? The pressure goes from - to + to - to +, glug, glug, glug, right? The dynamics are terrible. Vibrations and cavitation can result. That is why we always open a high vent, to introduce air for displacement and to get a constant (atmospheric) pressure, then the dynamics stop and flow is smooth.

The HX1 pipe is somewhat similar, right? The water falling down the 15 m is pulling the flow along, like a drain. OP said, "The pressure indicators PI 1 and PI 2 indicate negative pressures and hx1 is often damaged." Emphasis on often is mine. So, open a vent. The dynamics will stop. OP will have to learn anew how to balance all the HXs, but they will no longer be fighting the dynamics.

Good Luck,
Latexman

RE: Flow stall after pipe drops down

The problem is that the pressure is simply too low at HX1. Why is it operating at such low pressures. IMO, the system lacks sufficient pressure. The pump should have been supplied with more head to avoid that problem entirely. Why even take a chance on doing all the damage that is actually happening with that kind of design. If I had a choice, I would simply keep it always running well above vapor pressure... with increased and a controlled backpressure at all flow rates guaranteed. If that cannot be guaranteed at all flow rates from 0 to the max required with the present configuration, personally I would prefer to install a backpressure control valve that could be guaranteed to do exactly that, but no harm in looking at other potential solutions.

Way Forward

This system should first be analyzed to see if it can be properly operated with its present configuration and at the latest flow rate requirements.
Determine what any limitations to flow rate may be necessary to operate within other limitations.
Decide if any flow rate limitations are acceptable to meet current needs.
If not, then attempt to meet current flow rate requirements by making the minimum number of changes to the current configuration and without introducing any unnecessary complexities.
Determine what all the various options are and what the limitations may be, if any, for each option.
Include analysis of start up, ramping, operating ranges and shut down cases.
Select from the options that allow sufficient operating range, flexibility, ease of control and equipment safety and protection from cavitation, if that is indeed necessary.
Evaluate the cost effectiveness of each technically acceptable alternative.
Make final selection.

RE: Flow stall after pipe drops down

38 posts and a number of options.

The thing for me looking back at this is that there is something which just doesn't look right.

Working backwards from the tank, there is an inlet into the tank apprently at 8m, so this is a fixed back pressure.
There is then about 2-300m of pipe PLUS another HX not shown which is actually the chiller HX - " I completely forgot that. Yes somewhere d/s of the union point is another heat exchanger to actually take away the heat." So we have maybe another 8-10m frictional losses and losses accross the chiller HX.

So that gets us to approx 15m+ head at PI2. There must be losses across HX1 so PI1 should be > 15m and hence above atmospheric pressure.

The data given on the sketch shows 3.5 barg, so 35m D/S of the first HX. Even allowing the additional height of 15m, 20m head loss in pipework between that point and PI1?? Doesn't make sense.

I get what Katmar is saying and it will work if the pressure D/S Hx1 doesn't rise above 15m head.

SO I don't think we have the complete picture and it may take some time for anyone to figure this system out, but as I see it at the moment there are some discrepancies in what information we have.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

Right. And not too surprising that we still have missing info.

I hesitate to suggest adding a vent even still, because there is apparently not one there now (Markboc?), so the original designers didn't see the need for it.
I'm not sure that the pressure won't ever go higher. Unknown pump curve and other things.
And if it does take in air, is there no danger of upsetting NPSHA or air trapped at high points, if all the entrained air does not find its way out of the system at the tank.
It just seems unnecessary, at least so far, and otherwise just an additional complication.

RE: Flow stall after pipe drops down

(OP)
First of all thank you all for your valuable input!

I didn't have access to my system over the weekend so I couldn't get you all the information earlier. I updated the diagram (please excuse that I mistakenly thought the Hxn were in series when they in fact were in parallel, but they are still on a separate branch from hx1, so in principle the situation shouldn't have changed).

I corrected the volume flow from 800 m^3 / h to 1000 m^3 / h, for that flow rate I have the single flows available (rounded as they were taken from a low resolution screenshot from the PCS). I also have the pressure of 4 bar available. All other pressures ( 6 bar and 3.5 bar ) are not on the PCS, hence I can't know them during the time the flow rate data was aquired. So take them with a grain of salt. Honestly I thought that this question would be more of a qualitative than quantitative nature. The possibility to get PCS info and then get the missing readings in the plant is an option and if needed I can get to that.
Especially regarding LittleInchs last remark, this would be the only option to make sure that all values correspond to each other and that a state of operation is shown that can indeed be analyzed like he tried to.



I printed out the thread and marked all questions directed at me as well as possible solutions and other remarks, so I'm breaking with the tradition of quoting every user.

Your questions

Pump curve


CODE -->

0	72
72	72
139	73
217	73
294	73
362	73
421	73
511	72
607	71
710	69
793	66
852	64
934	62
1056	57
1117	54
1189	51
1222	50 

Unfortunately I couldn't find much information on the heat exchangers for now. I did find out that hx1 is designed to work at 250 m^3 / h.

The control valve we were looking at should be a FCV (again, completely unsuited to achieve any pressure control other than maintaining the 4 bar). cv value is 1500 m^3 / h and DN250.

Pipe diameter is DN500 everywhere except the branches are DN200. There should be a little part after hx3-hx5 merge that is DN350 before it goes back to DN500 again. 5 m d/s of the pump is currently DN300 and it is thought of changing it to DN250 if the new V2 valve should be aquired.

The length of the common return leg is at approx. 150 m (this is the distance from the estimated union point and the pump, measured directly from an aerial image + some buffer). So I'd estimate maybe 200 m for the discharge side of the pump until union. It does not help that the return leg is underground and I don't have an isometry. Due to the age of the plant it's not unlikely that I won't get an isometry either. I talked to some people who were also looking for an isometry at some point in the past years and didn't find any, too.

I hope that answered all questions that wre asked since my last post. To answer 1503-44'S questions, I'm a german native speaker ;)

Solutions

1) Valve in common return leg
2) Vent
3) "the red valve"
4) Placing a control valve anywhere (this is what I got from one of 1503-44's posts, that the location really doesn't matter.

Currently I would prefer option 1) (and control the flow ratio with the existent valves) but I haven't had time to evaluate each solution in its entirety. Placing the red valve in combination with using the existing valves would probably achieve the same.

My questions / remarks

1)

Quote (The downleg from point A to point B must be made self-venting, which would be 600 mm for a flow of 500 m3/h.)

Do you mean installing a larger pipe for the download in addition to the vent?

2) With the pump curve and cv value, how would one proceed to calculate if there was any pressure increase downstream the valve? So far I've only seen that you input the pressure loss and cv and get the flow, or input the flow and pressure loss and get cv. For FCV/PCV (if I understood correctly) where either flow or pressure loss is fixed, this makes sense. But for example V2 right now is just a flap. Pressure loss and flow are not independent from one another. Would I take some flow, get the pressure loss, adjust the point on my pump curve, get the new flow and iterate until no change occurs?

3) Cascade flow cannot be tolerated as the damages at the heat exchanger cannot be tolerated or at least they don't want to aquire new heat exchangers as often as they have to now. All other Hx last longer.

4) @goutam_freelance changing or repositioning the tank is not possible.

5) @LittleInch "The data given on the sketch shows 3.5 barg, so 35m D/S of the first HX. Even allowing the additional height of 15m, 20m head loss in pipework between that point and PI1?? Doesn't make sense." give one or two days I'll try to get a current picture of the situation. Unfortunately I would also love to have more information than I currently have. Again, I was convinced the problem is of quantitative nature. As you see in the updated sketch, there should be now other chiller d/s of hx1. Hx 2.1 / Hx 2.2 seem to take away the heat. But I need to verify that tomorrow with someone and cross check the PIDs.
But now that you state it, it is strange where the pressure loss comes from. Certainly not from the 150 m of DN500 pipe.

6) "The pump should have been supplied with more head to avoid that problem entirely." you mean by throttling somewhere in the system? A larger pump with a higher head wouldn't do anything (or I am completely mistaken) because the plant dictates where I end up on the pump curve, probably at much higher flow rates then.


If I forgot to answer someones questions, please feel free to remind me again I simply must have overlooked it then.

Thanks again!

RE: Flow stall after pipe drops down

Now that we see that there is no heat exchanger in the common return line I am even more convinced that a simple vent at the high point is the answer. As I said before, the fact that the pressure gauge between Hx1 and V1 indicated a vacuum tells me that if this pressure is increased to 0 gauge the situation can only improve.

In case anyone thinks I am making all this stuff up, have a look at page 205 in the article by Larry L Simpson in the June 17, 1968 edition of Chemical Engineering. I didn't invent this technique - it was well known a long time ago and I just implemented it a couple of times.

With regard to your latest Question #1 - the downleg from the point A to the 8m level should be made self venting. Below the 8m level it will remain flooded with liquid and does not need to be self venting. The vent pipe can be much smaller. Probably 100 mm NB is plenty big enough.

There needs to be a valve in each heat exchanger branch for flow balancing purposes - and it appears that this is already the case. Setting the valves manually is a trial and error process because adjusting any one of the valves influences the flow in all the other branches as well. Automatic flow control through each of the branches would be great, but expensive.

The drop from Hx3 down to the common point will remain flooded because the inlet to the tank is at 8m. There will not be any vacuum created in the outlet from Hx3.

In response to the most recent post by 1503-44, there is no danger of air from the vent being trapped at high points in the return piping if the downleg from point A is designed to be self venting. The vent is not just an additional complication. It is the central piece in the solution. Without it there will always be pulsations and/or vibrations.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down


"V2 right now is just a flap" I'm not sure what that means. Is it a one-way flow, "check" valve?

2) Yes, hydraulically a valve is just another "flow element", exactly as is a pipe and anything else, but instead of a fixed pressure drop for a given flow, we can vary the pressure drop, provided that we can adjust its open-close position. You are right. It is just another element in the iterative solution of the "assume a flow, sum the pressure gains and losses and see if they equal the boundary conditions. If the system is a closed loop, the sum =0. If it is not closed then you must know the difference between inlet and outlet pressure and the sum should equal that difference. If not, adjust the flow.

When you model valves, first you usually assume they are fully open, then you close them, or partially close them and see how that affects your last iteration. Actually valves have a position in the range of fully open to fully closed. So if you want to look at what happens when not fully open, then we need another curve showing O/C% position vs Cv. That is important for control valves, but not too interesting for valves that are used for on/off purposes only. Those are either Cv=full, or Cv=0. Flapper check valves can often be treated the same way, as the flap moves quickly from 0 to full and the software knows that, but technically they also have a Cv vs position curve and sometimes you need to enter that data to know their specific effect of position in detail.

Pumps are analyzed in much the same manner as pipe. They are just another flow element like a pipe, except they add head for a given flow rate. Assume your flow rate and see how much head to add.

3) I do think cascade flow is best avoided, unless you have no practical alternative.
4) resolved.
5) I'm digesting that part. LittleInch?
6) A pump with a higher head potentially just gives you more pressure to work with and you don't have to deal with trying to operate equipment in or too near the "cavitation zone".

Katmar, I agree with you, 0 > -n, but I just don't see the need for it. At least not yet.

RE: Flow stall after pipe drops down

Quote (katmar (Chemical))

@bimr That might make it easier to balance the flows through the branches, but it will not eliminate the vacuum problem. The vent I have described above is the usual way to do it - and it works so well that people are not even aware that there might have been a problem.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

Installing a vent is more or less the same as a return leg from each heat exchanger back to the storage tank.

This installation appears to be too expensive in capital and operating costs not to conduct a complete evaluation.

markboc (Bioengineer)(OP), do you have a complete piping and instrumentation diagram (P&ID) which shows the piping and process equipment together with the instrumentation and control devices.

RE: Flow stall after pipe drops down

Quote (bimr)

Installing a vent is more or less the same as a return leg from each heat exchanger back to the storage tank.

In my opinion the two options are significantly different from each other. Here are 2 important differences:

1. A 2m long vent made of 100 mm NB pipe will be vastly cheaper than 3 separate 200 mm NB return lines, each around 150 m long.
2. The vent will ensure that the pressure immediately downstream of V1 will be constant, and will be at atmospheric pressure. With a separate return line the pressure at this point will be variable and is likely to fluctuate between a positive and a negative (i.e. vacuum) gauge pressure.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

OK, my take.

markboc, As I think you're realised, the key to doing anything is data and establishing that your flow diagram is actually correct. Especially if you have an old plant, drawings are just a guide as to what is actually happening out there. Where someone says there is a pressure guage is different to where it is on paper etc.

Some points from your data.

1) 1000m3/hr looks to be quite high for the pump - my guess it is is best at about 800m3/hr, but at least you're not at end of curve.
2) Running HX1 designed for 250 m3/hr at 600m3/hr is a bad idea and may be the cause of some of your issues. This will give you quite a high differential pressure but may also damage the tubes or shell in the HX itself. Needs to be checked out
3) Katmars vent pipe is interesting and after a bit of reflection I think it could be good. However it would need a clean run all the way from the union point back to the tank with no valves anyone can close. If anyone closes a valve downstream the vent then the system will pressure up with the pump running and squirt water out of your vent. but it is easy to do.

4) your point 5) is my point at the start above. Gathering data and confirming it on an existing old system which has been re configured many times and had lots of people "fiddle" with it to get it to work is 90% of the effort, but without out it nothing makes sense on the numbers you have provided. There is no substitute to literally following all the pipework on site and then drawing it up or checking against your existing drawings. Check all valves which are supposed to be fully open are fully open, any filters are clean, trace any strange junctions or connections and then you might find where the missing 2 bar / 20m head has gone.... Also check all the gauges are working properly and actually connected.

Keep us informed of progress...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

Quote (Markboc)

4) @goutam_freelance changing or repositioning the tank is not possible.

I think you misunderstood my post. I was not suggesting relocation of tank. I assume it will be quite sizeable. Instead a small expansion tank typically 2 m^3(needs calculation) can be located at an elevated level, if possible. The existing tank may be blanked off.

Below mark-up was my suggestion. We follow this type of system religiously for power plants. I hope you will not have any contradicting esoteric process requirements in your plant



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RE: Flow stall after pipe drops down

What purpose does the original tank then serve?

You can't make water flow up hill.

All you're doing is turning an open system into a closed system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

Quote (LittleInch)

You can't make water flow up hill.
Pump differential head will ensure flow to the top point as is being done now.
Closed system is better. You can avoid losses in the tank.

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RE: Flow stall after pipe drops down

Quote (katmar (Chemical))

In my opinion the two options are significantly different from each other. Here are 2 important differences:

1. A 2m long vent made of 100 mm NB pipe will be vastly cheaper than 3 separate 200 mm NB return lines, each around 150 m long.
2. The vent will ensure that the pressure immediately downstream of V1 will be constant, and will be at atmospheric pressure. With a separate return line the pressure at this point will be variable and is likely to fluctuate between a positive and a negative (i.e. vacuum) gauge pressure.

Don't think you understand. Your concept creates a (15 + 2) meter long vertical gravity standpipe with a gravity drain back to the tank. The reason that it will work is that you have created separate return legs. Agree that it will work but the standpipe has to be high enough so that it doesn't overflow.

Since it is now an open to the atmosphere cooling system, the owner would also have to institute a chemical treatment program to control organics and corrosion.

RE: Flow stall after pipe drops down

Folks, the system is already open at the tank. If they do not have a chemical treatment program already, they should have.

Good Luck,
Latexman

RE: Flow stall after pipe drops down

Right on both counts.

RE: Flow stall after pipe drops down

(OP)
Thanks again for all the good input!

Quote (katmar)

With regard to your latest Question #1 - the downleg from the point A to the 8m level should be made self venting. Below the 8m level it will remain flooded with liquid and does not need to be self venting. The vent pipe can be much smaller. Probably 100 mm NB is plenty big enough.
Earlier you mentioned

Quote (katmar)

The downleg from point A to point B must be made self-venting, which would be 600 mm for a flow of 500 m3/h. [Edit]Only the section from point A down to the 8m level needs to be this larger diameter.[/Edit]
Bear with me here please, but I don't the woods from the trees.
We should install a vent like in your sketch, got that. This will raise pressure to 0 gauge and prevent the vacuum.
Then you talk about changing the leg A - B to 600 mm, and then you say 100 mm? What exactly do you mean by self venting? Getting a smaller pipe in addition to the vent? And the vent pipe can be DN100 whilst the leg gets a larger diameter of approximatly DN500 (from DN200 which it is now)?
Wouldn't adding a long/high enought small vent be sufficient if we also lower the flow rate to 250 m^3 / h as per design spec?

Quote (1503-44)

"V2 right now is just a flap" I'm not sure what that means. Is it a one-way flow, "check" valve?
There is also a check valve in place (omitted in the sketch) but I mean that V2 is currently of this type:

This is what I meant by flap.

Quote (bimr)

Installing a vent is more or less the same as a return leg from each heat exchanger back to the storage tank.
Adding individual legs to the tank is completely out of budget. The loop I have shown you runs through the whole plant. The storage tank is in an building. With costs of scaffolding, hundreds of metres of pipe, breaking through walls etc. this is not an option.

Quote (bimr)

do you have a complete piping and instrumentation diagram (P&ID) which shows the piping and process equipment together with the instrumentation and control devices.
Yes, but unfortunately I cannot share these.

Quote (LittleInch)

1000m3/hr looks to be quite high for the pump - my guess it is is best at about 800m3/hr, but at least you're not at end of curve.
Two separate flow measurements as well as the sum of the individual measurements verified operation of 1000 m^3 / h. See updated diagram at the end.

Quote (LittleInch)

2) Running HX1 designed for 250 m3/hr at 600m3/hr is a bad idea and may be the cause of some of your issues. This will give you quite a high differential pressure but may also damage the tubes or shell in the HX itself. Needs to be checked out
At the end of this project I hope we will turn the valves at each Hx back to automatic and lower the flow.

Quote (LittleInch)

If anyone closes a valve downstream the vent then the system will pressure up with the pump running and squirt water out of your vent. but it is easy to do.
This would add the cost of at least one DN200 pipe. I will raise this issues but I'm rather confident that a simple chain at the valves downstream will do. Usually people do not run around open / close valves as they like. We have a pretty strict work permit procedure that is to be followed. Good point nonetheless. Which brings to recognizing all the good post's here. I'll add the stars when I get to it. The response is almost too much to keep up with. But I'm grateful for that.

Regarding point 4), I went around and still missed some on site gauges (like Hx3). I cannot verify each and every point you suggested. Many places of the piping system are not easily accessed, too. But I see where you're going at.

Quote (goutam_freelance)

I think you misunderstood my post. I was not suggesting relocation of tank. I assume it will be quite sizeable. Instead a small expansion tank typically 2 m^3(needs calculation) can be located at an elevated level, if possible. The existing tank may be blanked off.
Thank you for the clarification. I doubt that we can blank off the existing tank and go with a smaller one. I need to check but I doubt they put at tank of that size into the building without having the need for it. I take away that you want the / or a tank on the suction side of the pump in an elevated position, so you could still add it in addition to the existing tank. But since the system is inside a building this is not easily (or cheaply) done.

Quote (bimr)

Your concept creates a (15 + 2) meter long vertical gravity standpipe with a gravity drain back to the tank.
I didn't take away that this concept introduces a separate line back to the tank. Did I misunderstand?

Quote (Latexman)

Folks, the system is already open at the tank. If they do not have a chemical treatment program already, they should have.
The water used is DM water (VE water in german, hopefully correctly translated). If additional measures are taken I don't know. But I would suspect that such a system is in place or they would have figured that out decades ago.

And as promised I went on site today and got readings that correspond to each other. Unfortunately I missed the PI for Hx3 but I can get that tomorrow if the flow rate stayed the same.



When we get to a solution here and you are interested, subscribe to this thread so I can post an update (probably end of the year) how the solution worked out. The least I can do is to reward you with real world feedback!

edit:

Quote (1503-44)

This possible effect is determined by how your pump and responds to the lower flow (usually increases output pressure) and by how much pressure drop you get across V2 at the lower flow (less). You have to look at your pump curve and valve pressure-flow-position characteristics to know haw that will work, or won't work to your advantage at the heaters.
To conclude this, with the FCV valve, that keeps the pressure constant we can't achieve anything. But it _may_ be possible if we had a PCV valve, that we net gain pressure if our pump curve is appropriate. But compared to the other suggestions I don't think this would be a good option, as it depends too much on the characteristics and may not work if the pump curve and valve don't play nice together, right?

Kind regards and thanks again!

RE: Flow stall after pipe drops down

Last question.

What is the water level in the tank?

Could you raise it to the max to give you a slightly higher back pressure d/s HX1? You would only need 2m ABOVE the return pipe.

Your "flap" valve is normally called a butterfly valve. Flap could be misunderstood as a non return valve.

Following pipe in an old plant is sometimes very difficult alright especially when they go through walls and underground...

Only other option possible is to install a fixed orifice plate in any flange in the pipe from the union point back to the tank including the flange back into the tank. To get a relatively small pressure drop to increase the back pressure should be quite simple and a simple plate should be able to be inserted between any RF flange. And cheap...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

Quote (markboc (Bioengineer)(OP))

Quote (bimr)
Your concept creates a (15 + 2) meter long vertical gravity standpipe with a gravity drain back to the tank.
I didn't take away that this concept introduces a separate line back to the tank. Did I misunderstand?

The existing return is a pressurized pipe:

1. If the air is removed through venting;
2. If operating with enough flow velocity to push the air out and back to the atmospheric tank.

Adding the vent at the top changes the operating scheme to a pressurized pipe to the upper heat exchanger and then it is a gravity flow return pipe downward to the atmospheric tank.

The scheme suggested by katmar has more or less created a flow scheme with separate legs (into the gravity return standpipe).

You will need some type of chemical treatment. Otherwise, the heat exchangers may foul and you will also have MIC.

RE: Flow stall after pipe drops down

How is the heat energy being removed from this system.

RE: Flow stall after pipe drops down

One of those inline HXs I believe

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flow stall after pipe drops down

It seems that the pump can get some flow to V1, but at the outlet pressure is less than 1 barA.
Without the vent, assume partial flow in the pipe at V1's outlet and say you only have vapor pressure, or something less than atmospheric. That pressure is driving the downstream leg flow back to the tank. Now drill a hole in the pipe and pressure increases to 1 barA and then you get more flow through the downstream leg. Or in other words, that vacuum pressure is no longer trying to suck liquid from the tank back to V1. Since you now have 1 barA at V1 and 1barA at the tank, gravity balances the downstream segment. Add a meter height of water into that segment from V1 and a meter at the other end is forced out into the tank. There is an additional friction loss there, if you do it fast

RE: Flow stall after pipe drops down

Thanks, I'm not sure I agree with that; I had thought that the flow through the other exchangers created a Venturi effect at the T, where the flow is sucking air and water from the downstream side of hx1, slightly reducing the head on the upstream side of hx1, allowing it to work sometimes.

TTFN (ta ta for now)
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RE: Flow stall after pipe drops down

I'm pretty sure they are just trying to run over original capacity, so they are winding up with low low pressure at the highest HX. Happens all the time in old plants. I might try do a proper model of it. Maybe enough data already to start. I can probably bracket it in for now.

RE: Flow stall after pipe drops down

With the latest information I am very confident that the vent system will work well. If there is a flow of 1000 m3/h in the return line and it has an equivalent length of 200 m of 500 NB pipe the friction pressure drop from the commoning point back to the tank will be less than 0.1 bar. The measured pressure at the commoning point is 0.84 bar and the tank return height is approx 8 m. This all ties together very well and indicates that the water will not back up into the vent.

If the pressure at the base of the downleg from Hx1 is 0.84 bar then the downleg cannot be liquid filled to the 15 m level as that would result in a 1.5 bar pressure at the base. The downleg will be liquid filled to approximately the 8.4 m level and above that will be filled with vapor. At present there is no way for vapor to enter that space so it is created by the boiling of the liquid. This boiling is partly what causes the vibration and resultant physical damage. Putting a vent at the top of the downleg allows air to enter and fill the space that is required to be vapor filled. As the flow rate through the heat exchangers varies the pressure required to return the water to the tank will also vary, and so the level in the downleg will vary and the vapor space can breathe air in or out through the vent to compensate.

The air flow through the vent is very low. Describing it as breathing is quite accurate. A 100 mm NB pipe will be large enough, although it could be even smaller if it does not have to be self supporting.

The upper 6 or 7 m of the downleg will be air filled and there will be water from Hx1/V1 falling through it. The water will entrain air and if steps are not taken to allow the air to be de-entrained it will be carried into the return piping and cause problems. This de-entrainment of the air is called self-venting and it is achieved by keeping the Froude Number below 0.3. There are plenty of discussions on Eng-Tips on the Froude Number and the design of self-venting pipe and I won't go into it here. But what it means is that where there can be air and water together the pipe diameter must be larger. About 1 m of depth is required below the liquid level in the downleg to achieve the separation of the air. This means that the section of the downleg above 7 m should be self venting and the lower section can be designed purely on frictional pressure drop.

This is shown in the sketch below


The flap valve for which you included a picture is called a butterfly valve in English (Absperrklappe in German).

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

(OP)
Sorry for delayed response! I was preoccupied with other projects.

Quote (LittleInch)

What is the water level in the tank?
Could you raise it to the max to give you a slightly higher back pressure d/s HX1? You would only need 2m ABOVE the return pipe.
Above what return pipe? I could see if we have a LI for the tank (probably).


Quote (LittleInch)

Only other option possible is to install a fixed orifice plate in any flange in the pipe from the union point back to the tank including the flange back into the
This was also one of the original ideas but to be flexible with changing operating conditions and the fact that the time frames in which can work on the pipe are scarce, we would prefer valves.

Quote (bimr)


How is the heat energy being removed from this system.
LittleInch is correct, I mentioned it earlier in a post. hx2,1 and hx2,2 do remove the heat.

Quote (1503-44)


so they are winding up with low low pressure at the highest HX. Happens all the time in old plants. I might try do a proper model of it. Maybe enough data already to start. I can probably bracket it in for now.
Happens all time in old plants: I'm pretty confident we run into the same caveats as other older plants.
If doing a model is not too much work for you this would be greatly appreciated! I'm already grateful for the time you and the others spent on this thread / problem.


Thank you katmar for the clear explanation!

I will keep you posted on the progress.

RE: Flow stall after pipe drops down

Sure Mark. I started setting up a basic system. I could use some approximate pipe lengths between all devices and temperatures of water at the pump and hopefully going into and out of the HXs. Are the pipes insulated? I stopped, because you disappeared for awhile. Now that you are apparently alive and well, and interested in continuing, I'll start filling in the remaining bytes.

RE: Flow stall after pipe drops down

(OP)
Thank you very much!

We're currently in the construction phase of other projects and that consumes quite a bit of my days currently. So bear with me if my response times are a little bit slower than usual.

Orange = Distance in m
Red = Temperature in °C



The lengths are rough estimates, the temperatures correspond to the flow rates and pressures of my latest update.

RE: Flow stall after pipe drops down

OK. Me too.

I'll start with that and let you know here as soon as I have something to tell you.

RE: Flow stall after pipe drops down

Would not be a good idea to add another pump for the upper heat exchanger line only for the required pressure and flow after the tee connection since the pressure loss and head are major problem for this line?

RE: Flow stall after pipe drops down

mark,

I went back through all the postings above scraping data and made this diagram of what I think the system looks like.
Is the bottom of tank at 4m elevation?

It would help if you could check my diagram for proper configuration and data values.

Thanks

RE: Flow stall after pipe drops down

Mark, Do you have the pump's rating parameters at BEP and driver's max power?
BEP RPM
BEP HEAD
BEP FLOW
BEP POWER
DRIVER POWER RATING

How old is this system?

RE: Flow stall after pipe drops down

Mark, Built a model that seems to correspond reasonably well with the info above.
Checking water @70°C, vapor press is 31kPa_A and all pressures are higher, so no indication of cavitation. Are exchanger damages erosive? That will occur with high velocities.



Report: HEADERS
Device kPag-in kPag-out Qm3/h
------ -- -- --
S1H 39.000 76.529 1075.595
DH1 596.671 594.847 1075.595
HX21 405.553 370.845 1075.595
HX22 407.820 373.204 1075.595

HX1 225.041 -17.827 540.705
HX3 344.746 45.444 236.907

HHX31 45.444 91.000 236.907
H4 400.705 398.842 534.890
HX4 398.796 91.473 120.032
HHX41 91.473 91.000 120.032
HX5 395.052 92.039 177.951
HHX51 92.039 91.000 177.951
HR1 91.000 90.934 1075.594
HR2 90.934 90.602 1075.594
HR3 85.986 10.000 1075.595

Report: TRANSFER_LINES
Device kPag-in kPag-out Qm3/h vel m/s VaporPress kPa_A
------ -- -- -- -- ---
T1 405.553 475.119 1075.595 1.639 30.524
T2 407.820 370.845 1075.595 1.634 25.549
T3 400.705 373.204 1075.595 1.633 24.434
THX10 225.041 400.705 540.705 5.462 24.434
THX11 91.000 -18.031 540.705 5.470 26.720
THX30 344.836 398.842 236.907 2.393 24.434
THX50 395.119 398.842 177.951 1.798 24.436
TR1 85.986 90.602 1075.595 1.639 30.456

RE: Flow stall after pipe drops down

Using V1 as a control valve,

Lowering Flowrate settings at V1.
you can lower the flow through HX1 from 600m3/h down to 210m3/hr.
Unstable flow begins at 210m3/h and reaches severe levels at 200.
Flow at 200 will not stabilise. V1 discharge pressure is within 20 kPa of vapor pressure.
Probable downstream vaporization with vapor pocket collapse generating transient pressure waves that affect the valve and HX1 exchanger.
Decreasing flow into the 190 to 140m3/h range takes time to reach stability, but eventually those flow rates will stabilise. Best not to operate in that range.
130 m3/h is more stable, but takes a short time. Pump flow is 800m3/h.
80 m3/h and lower to 0 are more stable. Pump flow is 767m3/h to 709m3/h.

Raising Flowrate Settings at V1.
Raising flow through HX1 shows signs of unstable flow at 140 to 150m3/h
Other flow rate settings appear to be stable.

RE: Flow stall after pipe drops down

What does your model say if V1 is moved from immediately downstream of HX1 and rather placed at the bottom of the downleg near the combining point. I don't think it would be as stable as using a vent at the 15 m level, but it should give better stability than where it is now.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

Patience please?
It should be very stable with another 15m of head on it.

RE: Flow stall after pipe drops down

I improved the stability of the previous runs with V1 as a control valve by reducing the PID gain and slowing down the actuator a lot, but there are still indications of instabilities at 200 and 500m3/h through HX1. You could rig V1 as a control valve, if you have to.

A control valve placed just before the intersection at "BRANCH3" works very well.
There is some instability when set to 400m3/h, but it soon settles out.
The control valve's PID is getting its input signal by reading flow at HX1.
If you can't put a FIT there, I think it can be set up as a PCV, but I didn't look into that.

I ran the model a number of times dropping the control valve's flow set point by 100 m3/h each time with the following results.
The valve controls the flow to HX1 according to its set point and all remaining flow from the pump is diverted to the HX3,4 & 5 group. You can see the approximate flow rates entering each HXn branch. If you do not need other flow distributions to those HX3,4 & 5 heaters, then you're done. If a finer control on flow to the heaters in that group is needed, I would suggest adding 1 control valve to each of two heaters, set those to the required flow rates and let the third heater carry the remaining flow. The total flow distributed to the HX3,4 & 5 heater group will be whatever is not going to HX1. You could redistribute that to this group as you choose with the two additional control valves. A third valve could be used, but it will not give you additional control as you might think. If you have 2 valves partially closed and partially close a third valve (without backing off one of the first two), all that will happen is the "equivalent pressure drop" in this group will rise higher than needed and that will actually start diverting some flow from this group back over to HX1. Then the flow controller sees the flow trying to increase, so it begins to close some more, which just results in a pressure back up on the pump curve. That tends to increase pressure at a lesser flow rate. If you do put valves on all 3, just keep one of them fully open.

RE: Flow stall after pipe drops down

(OP)

Quote (1503-44)


Is the bottom of tank at 4m elevation?
Yes.

Quote (1503-44)


How old is this system?
From what I gathered from the documents, the pump is from 1980.

Quote (1503-44)


Mark, Do you have the pump's rating parameters at BEP and driver's max power?
RPM 1480, not sure if it's BEP but it's 85 % efficiency: head 60 m, Q 1050 m^3/h, power 195 kW (rated 260 kW)

Quote (1503-44)


Mark, Built a model that seems to correspond reasonably well with the info above.
Thank you very much, again, greatly appreciated!

Quote (1503-44)


Are exchanger damages erosive? That will occur with high velocities.
I'd need to check that, at first glance even at 550 m^3 / h I wouldn't think of that. Unfortunately I don't have access to the documentation remotely right now.

Quote (1503-44)


Checking water @70°C, vapor press is 31kPa_A and all pressures are higher [...] HX1 225.041 -17.827 540.705
Is this actually the output of your model? I'm surprised because the -17.827 do seem to fit the real world reading perfectly. I'm just confused because you came to the conclusion that all pressures are sufficiently high.

and now in response to your calculations / findings in general:

With instabilities you mean the flow does not stabilize in the sense of a constant flow rate but varies rather. You are not talking about partial flow but about the instability in the sense of control systems?
I'm totally with you that it's best to not use the last control valve rather than diverting all other flow through the hx without the valve.
Am I correct in assuming that if an overall lower heat exchange (during winter or different demands in the plant in general) is needed, additionally regulating the overall flow rate with V2 (or a FCV there) would be possible?
It's really awesome to be able to pin numbers to the different scenarios! From your data alone I would even say that V1 is enough at this point and using two additional FCVs for hx3 and hx4 for example would just be the icing on the cake. The flow rates do seem to be well within the specifications of the heat exchangers.

Which software did you use to make these calculations?

Did you also consider the case where V2 (or an additional valve) is placed in the common return leg?

@katmar since a vent is tied to a lower invest we are still investigating that route as well.

Have a nice weekend!


RE: Flow stall after pipe drops down

I entered water's vapor pressure curve into the model. I set pressure drops through the HX's to match what you told me at your flow rates and the program calculates proportional drops to match velocities of all other flow rates I wanted to run. I entered your pump curve too. I just guessed and set the wall thickness of the pipes at 5mm.

Yes the instabilities were the result of the control configuration for V1. At first I had a fast acting valve with a high gain used in the PID controller. It caused the valve to react too quickly on slight flow deviations, closing too much and allowing pressure to drop below vapor pressure and then rise above. When the vapor pocket collapses, pressure waves were created that affected the flow rate and started the controller cycling again. Slowing the valve down solved almost all of that. Moving V1 to 0m elevation helped a lot. 400m3/h cycles some, but it stabilises quickly now.

I did not try any scenarios using other valves, or even activating V2, as there seems to be a lot of flexibility of making just about any flow rate you want already with V1 alone. Adjusting V2 to a new flow rate will change all the flows above proportionally, so there is also considerable redundancy present. If you need to proportion the flow going into the HX 3, 4 & 5 group differently, then right, just add some more valves in there.

The simulation shows that the system as is can be operated in a manner that there is enough pressure to stay above the vapor pressure. That does not mean it is being operated that way. And if flow deviations are not being responded to quickly and being properly controlled, the instabilities I observed might persist long enough to some damage. I have no info about that. It is apparent that a careful hand is required to keep V1 properly adjusted in the system with its present configuration. What could seem like minor uncontrolled changes in flow rates may have a significant negative effect on HX1.

I believe that moving V1 to near 0m elevation will make control easier and result in a more stable operation at all flow rates within the design operating range of the equipment. Of course if those ranges are exceeded, all bets are off. Flow rates higher than the maximum case above will lower the pressure further, which will eventually get near or lower than vapor pressure at HX1. Keeping flow low enough so that the pump is operating at higher points on its curve and giving sufficient pressure to stay above vapor pressures at HX1 is important. Holding pressure there to 0 kPag would be the equivalent of Katmar's drilling a hole there solution, albeit more costly ... I suppose. But maybe not if the pipe diameter has to be increased???

Rather than have flow rate control for V1, a pressure control might provide a more secure means to avoid low pressure at HX1. V1 could be controlled to adjust its open/close position to keep HX1 pressure always sufficiently above vapor pressure, ie. set to -15 to zero kPa. The flow rate would automatically reduce to correspond to that set pressure. More info about how production operations uses HX1 and how they would like to control the flow rate into it is needed.

The program I used is a precursor to this one currently offered in DNV's lineup. Actually it was the first program written for dynamic simulation of piped fluids. I've been using versions of the same program since when I had to send and receive telex files to/from a mainframe in Pennsylvania in 1986. I actually did some client partnership work with the original program owners and wrote the code for their ActiveX based graphic displays to model tanks. Before that they could not graphically display liquids in a tank! That's often quite important for oil pipeline operation simulations.
https://www.dnvgl.fr/news/dnv-gl-releases-new-vers...

RE: Flow stall after pipe drops down

All participants have certainly given their keyboards a hard workout? Considering Hx1 is 15m above the pump (and other Hx's) I dont see any means of venting the line running up to Hx1. Essentially it is the high point in the system and will trap and air/vapour, thereby reducing the pipe diameter, increasing velocity, creating turbulence, and rendering the Hx less efficient.

RE: Flow stall after pipe drops down

I'm not going to re-read the whole thread, but I do not recall anyone suggesting installing a vent before HX1. There is no reason for air to be in the system upstream of HX1 and no venting is required there. The vent is required at the top of the downleg after HX1.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Flow stall after pipe drops down

I'm not suggesting a permanent vent. A 1/2 inch ball valve at the highest point. When putting the system into operation vent any air that accumulates at the highest point, close it off and forget about it. Considering it's a closed loop system, once it has been vented on start up there should be no further accumulation of air.

RE: Flow stall after pipe drops down

The purpose of the vent is not to release accumulated air. The vent is there to break the vacuum that forms at the top of the downleg by controlling the pressure there to 0 barg. This is a continuous duty and cannot be performed by occasionally opening a manual valve.

The real problem that the OP was experiencing was mechanical damage to HX1. This is most probably caused by boiling of the water (because of the vacuum) in the downleg resulting in fluctuating pressures and cavitation.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

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