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parallel pump problems

parallel pump problems

parallel pump problems

(OP)
Dear fellow members,

I'd like to ask your expert opinion on my current situation.  I'm currently handling left-over projects that have 5 booster pumps in paralell delivering liquids from storage tank (produced water) to a pig launcher.  3 of the booster pumps are old existing pumps and another 2 are fairly recently bought with matching H/Q characteristic with the existing (not really identical).  These pumps fail frequently and they took turns (imagine that!).  so operation guys came to my department (project) to look for solutions and hopefully create something to solve their predicament.  

the first thing when I look up when I step in is the calculation by FEED contractor.  The old pump itself basically reused from another project, rated at 600 gpm @ 735 ft, water temp 180 F.  the newer 2 pumps also have the same rating, 600 gpm @ 735 ft to suit the old one.  So before they actually bought the 2 new pumps, FEED contractor have studied the system and made recommendation for 5 pumps in parallel.  So the combined pump curve only intersect the system curve at 4 & 5 pumps operating.  At single, double and triple pump operation the system is well below the pump curve (way to the right).  So I wonder does this possible?

by right, operating 1,2 or 3 pumps will have cavitation problem, since it will push operation way to the right curve (it doesnt even intersect the system curve!)and while the pumps do have problems and regulary in-out service for maintenance, there were no report of cavitation damage on impellers.  there are erosion damage on the casing, however (it turns out there were sand sediment in the fluid that was not considered). there are even report on casing leak and damage.  I still wonder if this is because of the sand or other things.

So back to the my question, can parallel pumps designed this way? that the system curve only intersect the combined curve of 4 & 5 pumps?
 

RE: parallel pump problems

(OP)
Here is the pump-system curve



correction, only 1 & 2 pumps operating in parallel dont intersect with the system. The 5 booster pumps operates 4x25%, with 1 pump standby. And here is the P&ID



Any comments?

RE: parallel pump problems

Yes, you can design this way and in rality, two pumps will probably work, but clealry out of their operating zone. Three looks ideal.

The issue I can see is that no one has thought too much about how to start this system or operate with less than three running. Given you appear to have a flow meter, there should be a control valve with set point based on flow driven by how many pumps are operational, hence set at 600 for one, 1200 for 2 and then fully open / 1900 once the third starts.

Multiple paralell pumps are common, but you need a good control system / operator to make them work effectively AND a means on controlling flow on start up and when you only have one or two available. My guess is that when one of the old ones went off line or was reqaired, they kept working with two and hence operated them at >>100% of rated flow and power and contributed to their condition.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

Congratulations to the OP for supplying good data and a clear description of the problem, it's a shame more don't follow this example.
What else would help is the individual pump curves with NPSHr and the NPSHa for the system - this will allow for discussions as to how far the pumps can run out on their curves before you need to apply artificial head into the system so that the pump curves / system curves can intersect.
However, I would say that operating 1 or 2 pumps is a big ask and unlikely to be a reasonable possibility, far too much head to dissipate to get them onto their curves.

As pointed out by LittleInch, 3 pumps appears reasonable -- but NPSHr for the pumps would be worth having for checking.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: parallel pump problems

(OP)
Thanks for the reply guys.

@littleinch, when I first learned about the calculation I straightaway thought that this was really recipe for disaster, having a curve like that, and nobody notice that unfortunately. But since u said it's possible, and after hearing report that there were no cavitation damage on impeller perhaps I have to reconsider my stance. the system did work though with a lot of problems. Yes there is a control valve at the thick line, and there is also restriction orifice after control valve, just to drop the pressure upstream pig launcher. the pig max inlet is about 200 psig. the pump is well oversized for the system (it's a left over, just for the sake of using it so they dont need to buy new pump long time ago), reality on the field so we made adjustment on the system to make the pump works. Control valve is dictated by level switch on the water tank upstream of the pump, not by flowmeter (no flowmeter).

Beside the casing damage, pumps also experienced severe vibration problems (which i suspect that would happen if only 1 or 2 pumps running). but with such system, u have to run 1 pump first then 2nd come online, then the 3rd etc, right? when 1 and 2 running, can it sustained well runout condition? or u can (or have to) actually make simultaneous 3 pumps online with this kind of system? My guests so far is that operation actually throttled the gate valve upstream & downstream control valve to boost system resistance, but this I havent got any feedback from them.

I will try to post the casing pics showing the suspected sand erosion damage. It's quite perplexing to hear that casing damaged to the point it got holes (from maintenance team), while impeller is fine.

RE: parallel pump problems

One thing we forgot was that when you get multiple pumps like this, and as you said there is no such thing as identical - you can get some inequality and the more the pumps the bigger it could be, hence at 1800 gpm, one pump could be doing 800 with the others 500 each if the head is only a few percent off. You need to check the motor amps of each of them to see, but a difference of anything less than 10% is probably the best you can do. Four pumps will even flow out a lot more and should be your default mode based on your info.

I can see that there is no flow control - there is no control valve shown on the PO & ID inless I'm much mistaken (looks like an on off isolation valve to me)- my point is that there should be but it is often overlooked and will result in your pumps not starting properly. Is that a re-cycle line on this system? Is that supposed to be used at start up to allow three pumps to come on line before you open the main valve??

If you start one pump with no more resistance than the mian line I can't see it doing anything other than tripping on excess amps if not vibration..

congrats on a decent post with most of the info - doesn't happen too often....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

What do you mean that one and two pumps do not intersect the system curve. Extend the curves! One pump will run at about 925 gpm. Two pumps will run at 1550 gpm

The system curve does not appear to be plotted correctly. System curves are plotted in absolute head, but you appear to have plotted it on the pump curve, ie. pump curves are plotted using differential head, or there is no decrease in suction pressure as flow increases. Pump curves must be superimposed on system head diagrams, but it seems that you show no suction head decrease as additional pumps are turned on, as all pumps have the same absolute +/-925 discharge head. Are you holding suction pressure constant in some manner even though you are running from 1 to 4 pumps? Normally suction pressure decreases when flows increase by 4X.

Independent events are seldomly independent.

RE: parallel pump problems

(OP)
BigInch,

I don't really get it. System curve plotted using absolute head? Normally we only use absolute head in calculating NPSHa. And I thought suction pressure should increase while flow increase? suction pressure increase, discharge decrease ---> flow increase? please do correct me if i'm wrong.

LittleInch,

Yes, pump is not identical, I've replot the pump curve and superimpose it below:

Below is enhanced section of previous snap-shot on the discharge line. It has control valve (pressure), and an orifice. And there is a recycle line back to the main suction tank. I was told that operation normally closed this line (on normal operation)

RE: parallel pump problems

(OP)
Sorry I forgot to explain on the pump curve, the red dot line is the old existing pump and the solid black line is the new pump. and btw here are the foto of the current casing condition with suspected sand erosion.



let say when the casing di deteriorated as above, does that affect the overall pump performance, perhaps created a "new" performance curve?

RE: parallel pump problems

~As ever new information makes a lot of difference.

First though I would like to take issue with a couple of BI comments - I have always looked on these graphs as being needed to be plotted from the same elevation point - normally pump discharge. Thus any static head is the start point of the system curve at 0 flow, with the pump curve based on pump differential plus whatever it's inlet head is. From your graph and description (pumping from storage tanks) there is probably only a low inlet head, but in other situations the inlet head will normally fall as more flow goes through the pumps hence greater friction on the inlet line leading to lower inlet head, and therefore lower outlet head as you add more identical pumps and increase flow..

The only common thing your pumps will see is the outlet header pressure. The difference between the pumps is quite a bit and the real issue is if the back pressure increases above 850 m differential head such that one of your new pumps will have very little or no flow and the older ones end up with all of it.

Still can't work out how your sytem is supposed to work or start, but I think you really need to start two pumps at the same time as a minimum as one pump, whilst it could end doing 900+ GPM is clearly well off the end of the curve and should not be operated like that. Given that you have a control valve you should be able to create a start mode where you fix the position of the valve that equates to 600gpm for one pump before starting another one and then release the control back to the pressure control.

Actually, giving this a bit more thought, this looks like a back presusre valve to me. Hence your system curve may be incorrect and in fact is a horizontal line at whatever the pressure set point is until it meets your system curve when it will then be fully open. This might already then allow you to start one pump if the pressure set point is above the equivalent head of about 625 feet. Find out what that set point is and convert it to feet head then draw a horizontal line on your system curve and it will also tell you a lot about how your system works

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

Take it off the test bench. Is discharge pressure held constant. If discharge pressure is held constant, then yes, the discharge head is constant in differential and absolute terms, if not, then no. Is suction pressure held constant? If not, then the same issue applies. Remember Bernoulli. Z + v2/2g + P/γ. If neither suction pressure is held constant, nor discharge pressure, then it's a free running pump and absolute discharge head varies, so you don't know where the static head and HGL are in relation to the system. You must first know how the system will operate before you superimpose pump and system curves. If not, you assume Z is 0 for all points in the system.

Where a pump should be operated is in relation to BEP and Power Rating. A pump sized for 600 could be operated off the end of the curve, and it will overdraw power in overspeed conditions, if not controlled to do otherwise, it will operate at the system intersection point, wherever it is, including if it is off the end of the curve. The pump doesn't know anything about end of curve limits.

Independent events are seldomly independent.

RE: parallel pump problems

(OP)
Well, I've just got news from operation that they just got mechanical seal failure on one of the pumps. Come to think of it, is it possible that the pumps experience discharge cavitation? As I quote from one website:

"Discharge cavitation occurs when the pump discharge pressure is extremely high due to output restriction. A restricted discharge that reduces the pump efficiency to a low value (10 – 20%) forces the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller the increased fluid velocity and heat buildup within the pump housing causes a vacuum to develop at the housing wall that turns the liquid into a vapor. A pump that experiences discharge cavitation for a period of time shows premature wear of the impeller vane tips and the pump housing and can result in the pump shaft failure."

With the picture of the casing that i supplied, what do you think? I don't have impeller failure (at least not reported) but pump casing failure has already happened. If this is the case, is it possible that the failed pump goes into deadhead operation? Would that make sense? what do you think?

RE: parallel pump problems

Oh boy. "discharge cavitation with a vacuum turning liquid into a vapor", is it?? This is recirculation and failure to maintain minimum flow-through, due to pump not being to overcome static head (including valves) at minimum flow rates, heating of liquid to above the water's vapor pressure, causing boiling off of the water into water vapor. No liquid cooling causes overheating, your seals dry out and burn up, if not blow out due to higher and higher pressures as the vapor continues to heat into steam. Your already high suction temperature isn't helping. Do you know what the vapor pressure is of water at 180F? I doubt you meet NPSHR.

Independent events are seldomly independent.

RE: parallel pump problems

Well I was a bit worried when you said that the re-circulaiton line, which is designed to prevent v low or no flow was
"I was told that operation normally closed this line (on normal operation)". Operators often have little or no comprehension about what pumps can and can't do or how to operate them efficiently and just develop certain ways of working which they find works, but can lead to problems in operation. These are quite decent sized units and if you dead head them, all that energy has to go somewhere - water can heat up very quickly and cause all sorts of damage.

As I said earlier, the difference between your pump curves becomes crucial as the pressure climbs / flow decreases as the "new" pumps can be severly affected even if the old ones are still flowing.

Big bro' is right though , you won't get a vacuum at the pump discharge, but all sorts of bad things start to go on when you dead head pumps.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

As stated way back, individual pump curves showing full pump peformance would help in answering the initial question " At single, double and triple pump operation the system is well below the pump curve (way to the right). So I wonder does this possible? " "So back to the my question, can parallel pumps designed this way? that the system curve only intersect the combined curve of 4 & 5 pumps?"


The photo of the pump casing, appears to be typical corrosion erosion, the result of pumping and maybe re-circulating sand round and round within the casing. The main consideration would be casing wear ring / impleller wear ring clearance to maintain performance..


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: parallel pump problems

(OP)
Artisi

what are the chances that the pump experience discharge recirculation? can it have the same erosion damage on the casing even if there were no sand content in the liquid? plus we just got seal failure too..

RE: parallel pump problems

if the pumps are throttled and operating left of BEP, then there is recirculation taking place and the further left you go the worse it gets, you will also very poor inlet conditions as well, inlet flow will not be entering the impeller eye correctly and not flowing onto the blades as designed. Both will result in noise, vibration, shaft deflection, and probably unbalanced axial loads etc etc.
I notice in your photo the inlet has a vortex breaker built in, if some pumps don't have this you could also be suffering pre-rotation of the inlet flow which will only worsen the problems.
As these are high head pumps they should be running at BEP +- maybe 10 - 15%, 20% should be the max.
Yes, if suffering from discharge recirc.you will / could have erosion damage.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: parallel pump problems

(OP)
Let say I have a minimum flow problem, how does a recirculation line back to the suction tank would help? because the suction tank itself have continuous incoming stream of fluid and adding back small portion from recirculation line should not make any different anyway, unless there are no more flow into the tank?

RE: parallel pump problems

A dedicated miniflow line on each pump leg ensures that, if one of the pumps is being dominated by the other(s) with a stronger pump curve, the weak pump maintains its minimum flow. The diagram you posted is too blurry for me to see a miniflow line.

I've only quickly browsed this thread but I haven't seen any pressure readings posted, only the system curve at beginning. If the actual discharge pressure (does it vary?) is a little higher than what is shown on the curve, then the new pumps may need an auto recirc valve in 4 or 5 pump operation (e.g. http://www.agivalves.com/prod01.htm). Alternatively, you could put a restriction at each pump discharge to reduce strong/weak pump problems (for non-permanent situation try partially closing discharge valves).

RE: parallel pump problems

hitchhiker,

As others have commented, you are to be complimented for providing substantial information from the outset!!!

The only thing certain about any given number of identical pumps is that no two will every be truly identical. With a mix of older and newer pumps with probably not well documented actual operating history (and likely unintentional operating abuse), their individual actual operating characteristics will likely vary much more than one may casually presume.

Presuming that your diagram fairly well represents the suction piping system configuration, the suction (OK, inlet, if one cares to be fussy) conditions at the several pumps will not all be equally influenced by variations in the total flow rate. This may or may not be an issue of concern for some of the pumps.

Having dealt with some astoundly troublesome systems of mis-matched pumps operating in parallel, the most useful characteristics that I found worth primary attention are the suction and discharge temperatures at each pump. The discharge temperature of a pump will be greater when the pump is doing proportionately more spinning than pumping thereby serving to become more of a mechanical water heater than a pump. If the suction temperature is greater at a pump, that is a good indication of troublesome recirculation. The greater the several temperatures vary from their counterparts at other pumps, the more attention their associated pump will deserve in your studies.

Because the temperature rise across a good-running pump is normally very small, the calibration of the instruments is important. Given the apparent problems of the pumps in your system, it would be no surprise to find some of the temperature problems to be quite obvious. Where I am trying to monitor good-running pumps, I prefer using thermocouples connected to indicate the temperature differential across the pump since that avoids the problem of calibration differentials between two separate instrument systems measuring the two temperatures.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

RE: parallel pump problems

(OP)
clay87,

there is only one common recirculation line from the discharge header as you can see in my previous post. each individual pump doesn't have dedicated miniflow line. let say the old pump (stronger) dominates the new (weaker), and with recirculation line the flow from the new pump goes back to the suction tank. How does this line ensure that it will make the new (weak pump) will pump above its minimum flow? since the suction tank itself have continuous incoming stream as well?

ccfowler,

unfortunately i don't have temperature reading data yet. I will have to push the o/m guys to provide it. I agree with you, that looking at this condition it'd be beneficial to have temperature reading. I only have pressure data and I will post it on monday.

RE: parallel pump problems

So long as the reciculation line goes back into the same tank that the inlet line comes from, then there is no NET effect, i.e. flow out of the tank into the pump equals the flow through the recycle line. It's only when the two are from different locations that you run into problems.

If you can, measure temp, pressure and running amps from each pump and motor when it's working which will tell you a lot about how equal the pumps really are. Whilst it is tempting to use the isolation valves as control valves to equal up the flow, this is a short term measure only and cannot be recommended for any long term operation. When you've worked out which ones are stronger than the other, fitting RO's may be the thing to do.

As ccfowler points out, if the actual piping layout follows that of the diagram then you have inbuilt problems as overall resistance for each pump should be the same for inlet and outlet piping length, no of elbows etc from the common inlet point to the common outlet point. Sometime didn't think about this when they designed it, but unfortunately this isn't uncommon.

You have provided much useful info, but I await what the set pressure is of your outlet back pressure control valve before being able to understand how the system works.

Keep it up.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

Recirc off the header will allow control of group flow, but not allow control of individual units.

Independent events are seldomly independent.

RE: parallel pump problems

An option might be to fit an orifice plate to the discharge of each unit sized to force the pump to run at a point on its curve close to the required flow rate (seems this might be 600gpm) which hopefully is somewhere near to BEP.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: parallel pump problems

Artisi, that is an excellent recommendation. At least it would tend to force some semblance of balance in the mix of problems present without introducing any great costs. Brilliantly elegant and inexpensive!

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

RE: parallel pump problems

(OP)
Guys, thanks for all the very insightful replies. I'm currently working with operation to obtain more data. Will let you know the reading once I get it.

RE: parallel pump problems

(OP)
Guys, while waiting for the data, I got another question. Is there any guide or recommended practice on number of pumps in parallel? Like for instance in my current case I have 5 pumps. Personally to me, 5 is too much because the flow addition normally quite marginal, but yet I've seen 8-10 pumps or more designed in parallel in my current company. any comment? have you ever design such system as well?

RE: parallel pump problems

The number of pumps in parallel is unlimited if the system is correctly designed.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: parallel pump problems

Multiple parallel pumps work best when the system curve changes a lot depending on how many units are flowing, e.g. a/c units or multiple outlets. For a static system curve they don't work very well as you note as additional pumps make very little additional flow, just a reduced flow through each pump.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

(OP)
@artisi,
true, but is there any "rule of thumb"? we can have many, but too many?

@littleinch,
even at dynamic dominated system would make the addition more insignificant, with steeper system curve, am i right?

Those I see here with more than 5 pumps in parallel usually on water injection system (produced water) to wells in multiple location. It is largely determined by static since each well would have certain minimum pressure. the pipeline to the wells would give friction head but considerably small compared to the min well pressure. The feed to the pumps depends on how much water produced (normally by surge/water tank level, as you've seen in above system). Given the incoming feed fluctuate througout the day, it is logical to use parallel pumps. But as we've discussed,in static dominated system,up to certain number of pumps it doesn't give benefit because of marginal addition.

I'm fairly new in this company, looking at this practice triggers my curiosity.

RE: parallel pump problems

Little Inch, I don't understand your assertion. Take, for example, flood water pumps around New Orleans. Every pump is in parallel and the system curve for each pump is identical and static. There is no limit to the number of pumps in parallel.

RE: parallel pump problems

Parallel pumping means that pumps have a common discharge header and one pipe - See the diagram earlier on in this post. What I believe you refer to is multiple pumps with their own discharge pipe. That is not parallel pumping, but pumps mounted together. Not the same thing.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

In my example the pumps are pumping from the same place and to the same place but with effectively VERY large inlet and outlet pipes. The point is that one can design a system to work or not to work. I still do not understand what you were trying to say about system curves. Perhaps you could elaborate.

RE: parallel pump problems

Compositepro; fully agree with your comment/s, exactly what I was hoping to infer with my earlier post re "correctly designed".

hitchhiker: No rule of thumb just common sense, if increasing flow only increases total head by a small amount then you can keep adding pumps until the total of the system exceeds the useful operating head / conditions of the individual pumps - however, if increasing flow results in a large increase of total head then it stands to reason that the number of pumps could very quickly become limited by the total head being imposed on each pump unit.
Again, the system, the pumps,and the control system must be engineered for each application.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: parallel pump problems

Composite pro,

A system curve exists for a set run of pipe plus any fixed items which create head loss such as filters or heat exchangers. However the shape of the system curve can change if you add or subtract those fixed units also in parallel. You see this in ac systems where additional pumps in parallel work well when combined with additional chillers.

For systems like you describe, the system curve is fairly flat, but has a high static element compared to the pump delivery head. For instance if the pump duty head is say 30m, with a static lift of say 20m, but with a very large pipe so the system curve takes a long time to get to 30m, equivalent to several pumps, then adding extra pumps increases flow more or less in proportion to the number of pumps. On the other hand if you look at the curves the OP posted in his second post, then you can see the additional flow for the fourth pump is very small compared to it's nominal capacity and a fifth would add virtually no extra flow because the system head at that flow rate has equaled the pumps delivery head.

I think we all agree that if you design it right it will work, but once you reach an optimum number of centrifugal pumps, adding more has little effect unless you change the system curve at the same time by e.g. opening another valve or flow path creating a lower head loss for the same flow rate.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: parallel pump problems

As mentioned, there is no rule of thumb for 'how many' parallel pumps that I know of. Parallel pumps (with a common discharge header) are usually specified to minimize pump size and/or provide some sort of redundancy. For reasons the OP has observed, using additional pumps to handle changing 'demand' has its limitations.

RE: parallel pump problems

3 or 4 pumps will pretty much make any vfd system redundant. All reasonable flowrates can be reached with pumps running at very good overall efficiencies, using only control valves, and with nearly full discharge head available if necessary.

Independent events are seldomly independent.

RE: parallel pump problems

(OP)
another thing, would you recommend individual control valve at each pump discharge or 1 common control valve at the discharge header like the design above? Which one will work better?

RE: parallel pump problems

Eitehr CV configuration produces pretty much the same effect when looking at what the system will output overall, however 1 CV requires all pumps to be outputting the same discharge pressure at all times, whereas multiple CVs would allow for individual control of recirculation (internal or external) for each pump and would be more effective at paralleling pumps with different characteristic curves into a common header. If you have the same curves for each pump, there would be no real difference if you wanted to put 1 CV for all pumps, or 1 CV for each pump. One possible exception would be that a problem with the 1 common CV for all pumps would shut the whole station down. In that case, I'd probably consider using at least 2 CVs between header and downstream pipeline, in parallel, 1 operating and the other blocked off when on standby.

Independent events are seldomly independent.

RE: parallel pump problems

(OP)
Thanks. What about the recirculation line? Ideally it's better to control from individual discharge goes to common recirculation header (separate recirculation line from each pump) or take common recirculation line from the discharge header as shown on P&ID?

RE: parallel pump problems

Separate recircs. This allows testing, startup, and pump swaps while minimizing system disturbance.

RE: parallel pump problems

Agreed. With a large number of pumps, what you can do with recirc off the header could be done by turning on/off individual pumps to a large extent. I also think that, in general, recirculation lines are better on individual diesel driven pumps, where the pumps must run for awhile at low power as the diesel warms up. On electric units, they start fast so power is available nearly immediately and long recirculation times where products heat up are avoided. You would only need recirculation for flow control, which you can do with a large number of pumps just by the on/off button, with, or in some cases without, the 2 CVs to control downstream pressure to the pipelines. A recirc off the header could still be installed, if it turned out to be necessary at all. That depends on how closely you can operate these pumps to the duty point you want without the CVs, or if you will have enough flow variations that operating at within efficient range of a basically one-head duty point, or close to it, isn't possible.

Independent events are seldomly independent.

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