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TDH - Complex Rising Main
2

TDH - Complex Rising Main

TDH - Complex Rising Main

(OP)
Profile attached.

I want to check the pump total head.

Should I average the
minimum head = 143.4-140.5 + total pipe flow friction losses
and
maximum head = ((154.4-144.4)+(149.0-147.0)+(149.0-148.5) + total pipe flow friction losses)?

Or simply TDH = 154.4 - 143.4 + pipe flow friction losses from this line.

Thanks!

RE: TDH - Complex Rising Main

(OP)
(I feel like taking the maximum head btw).

RE: TDH - Complex Rising Main

The thing that will make the difference is what is on the end of the line and what your total pipe friction loss is. Given that this is a pumped system abut 1 km long by the look of it, my guess is that your friction losses are a lot more than 10m. Your exit point is lower than your inlet (I asusme you're flowing left to right?), therefore your first equation is the one to use. Providing that your pump develops more than 10m you will make it over the first hill. Once you have more than 10m , forget about the highest point in respect of pump head and just look at the end point elevation.

The pipe friction loss on the other hand should be such that it allows the pressure in the pipe to be above atmospheric pressure at all times. Otherwise the fluid will pull a vacuum on the outlet leg. Also if you have no valve on the outlet and stop pumping, the pipe will self drain to a certian extent.

Draw your results on a graph ( see attached) and if you have or need a back pressure on the end point include that in your calcualtion and just add it to the line friction to obtain your required pump head above the start point.

Remember pump head as shown on apump curve / data sheet is actually differential head so if you have positive pressure or head on the suction of the pump this needs to be subtracted from the required pump discharge head to get the "pump differential head". Sorry if this is obvious.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
Flowing left to right (and wastewater) - it will end in a manhole with a discharge 2 m below the end point (140.5).
With an air valve on the top of the hill (154.4).
Static head calculated with the stop water level.
Adding losses from the aspiration line to the discharge line.

I'm not sure I get things straight.
Geometric Head = 11 meters (154.4-143.4)?
Friction losses = Total length of the pipe









RE: TDH - Complex Rising Main

I probably confused things, but geometric head is now 143.4 - 140.5 - 2. Friction loss is friction loss. Looks like you have atmosperic pressure at the end point?

Providing your friction losses are more than about 12m for the entire line, then forget about the first high point.

However if all you are doing is pumping the water up to the top of the first hill (i.e total friction losses less than 10m), the all your pump needs to do is 10m plus friction losses in the first 193m.

I ask again - what are your calculated friction losses (m/100m) or total and then I can draw the head loss onto your profile and tell you a bit more.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
Geometric Head is 143.4 - 140.5, no? (as 140.5 is the pipe outlet then it falls straight (2 m) in a manhole with a discharge at 138.5).

For a peak flow of 35 L/s, I get 18.8 meters of losses (150 mm CHW = 150 length = 883 m + EK = 3.0 for bends and valve).
Losses = 1.8 m in the aspiration line.

I was thinking that for 35 L/s, the TDH would be 34.4 m (1.8 + 18.8 + 13.9)
You seem to tell me that Geometric head is not 11 meter (water level to the hill) but rather... 3.1 m.

RE: TDH - Complex Rising Main

(OP)
So is it ?
13.9 m (sum of all the hills)
11 m
3.1 m
or even 1.1 m

RE: TDH - Complex Rising Main

(OP)
(3.1 but 2.9)
(1.1 but 0.9)

RE: TDH - Complex Rising Main

geometric head is 143.5 minus 140.5 if the pipe ends in an open manhole. What happens after that is irrelevant to the pump.

See attached graph.

Your problem is that because you have an open end, there isn't sufficient back pressure in your pipe to keep the pipe full of liquid past the 149m high point at kp 723m. This point now effectively become the end of your pipe for hydraulic purposes as the flow will be gravity from that point onwards and try anf low faster than the incoming flow (called slack flow). Thus moving back from that high point with your calcualted losses, your pump discharge head to do your flow is 168m-144.4m = 24m assuming that your pump is at the 144.4 m level - the yellow line.

Forget all about the other head losses and concentrate on friction only, plus the high point at the end of the pipeline which actually defines your hydraulic end point, hence static head is actually 149 - 144.4 plus friction of say 18 allowing for the reduced length = 23m - essentially the same as the graphical solution ( I added 1m for graphical purposes).

What you seem to neglect is that for a pipe which is filled with liquid and kept above 0 barg at all times, whilst there is effort to move the liquid up the hill, this is balanced by gravity helping it down the hill on the far side. Think of this as continuous train of trucks for the entire length all joined together. Once you've got trucks from one end to the other then the ones going down the hill will help to pull those going up the hill. The problem you have is that at the 149m mark, the trucks become detached and run down the hill under their own volition, faster than the trucks being pushed along. Thus gaps start to appear between the trucks.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
So TDH 24 m. I'll get pressure below 15 PSI (atmospheric) in the system
It'll be able to get over the hillpoint (154.4 m).
(?)

RE: TDH - Complex Rising Main

Yes (24m).

You will get pressure below 15 psia in the section beyond the last high point, so it might not be smooth flow going into the manhole at the end point.

You will be able to get over all the hills.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
You know what? this is an actual pumping system (real).
It has a 34 L/s pump with a TDH of .. 17 m.

I was tempted to put a pump with a TDH of 31.4 m to avoid getting pressure below atmospheric in the system.
Could be more $.

Thanks for your time.

RE: TDH - Complex Rising Main

(OP)
I'll check operation time and real flow measurement.

My conclusion is that it doens't pass 34 L/s now with a 17 meter pump.

RE: TDH - Complex Rising Main

The only way as i see it to avoid sub atmospheric pressure is to instal a back pressure control valve on your exit point. Is it worth it? Up to you. Most pipeline engineers would say yes, but in reality this is a low pressure water system and often works at this sort of pressure.

There are fine margins so one or two metres either way will make a big difference, e.g. is the high point measurement the pipe or ground level?. The head at the high point could easily be-2 to -3 m and hence drag down the requited head at the inlet. Friction losses are notoriously conservative so you could easily get close to your flow with a 17m pump.

It's just not the best way to do it.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
I Think that the 17 m pump can't pump 34 L/s that's all.

RE: TDH - Complex Rising Main

Quite possibly not, but I doubt it's that far away, prob somewhere between 25 to 30 l/sec. Will be interesting to see.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
I'm gonna do a drop test.
No flow recorded for that small pump station.

It's probably not dramatic though as you say (pumping more often).

RE: TDH - Complex Rising Main

(OP)
LittleInch,

Is it important to maintain pressure above 0 PSI in the system? I wouldn't take the risk to rely on the siphon effect...

I'm thinking about maintaining it above 14.7 PSI.

RE: TDH - Complex Rising Main

Yes I think it is important to avoid creating vapour / vacuum which collapses later on and leads to unstable flow. Above 14.7 psig is fine. The pipeline won't stop working if you go below this, but it is generally recognised as good practise to operate at all times > 0 psig

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

I would not be surprised to see a comb air/vac valve at the high point. this would maintain atmospheric pressure on the downhill leg(s), allow air into the line and promote gravity flow. In fact, there may be one at each high point. regardless of how this system operates, you will need a pump that can fill the first leg of the pipeline all the way to the top. You might find quite a few relevant discussions of sewer force mains hydraulic performance in the Waste disposal & treatment Forum


http://www.eng-tips.com/search.cfm?pid=161&act...

RE: TDH - Complex Rising Main

Once you allow air into it then all bets are of though I do understand why water / sewage systems work this way, I still think it's better to keep it all at positive pressure.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
There is an air valve on top on the 1st leg but it's closed/stuck and not operating anymore.
I wanted to change that design to allow the pressure to be above atmospheric all the way. To do so, I have to increase the TDH (pump) though.
I'm gonna do a drop test next week to see what this pump can discharge (with the air valve closed/on).

RE: TDH - Complex Rising Main

(OP)
Back on Geometric head: If I want P > atm all the way : is it (154.4 - 143.4) + (149 - 147) + (149 - 148.5) = 13,5 m or...

RE: TDH - Complex Rising Main

I thought we'd sorted that out earlier. Your calculation method is not correct, see my post earlier with the graph attached. For a full pipe design you need to concentrate on friction losses plus any end point head losses/ gains and forget about the intermediate hills. You need 23m at the start to keep the pipe full up to the last hill when it starts to run away from you. Unless you put on some sort of restrictor at the end point or greatly increase flow rate, this is just physics.

LI

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

Yes, that's a very interesting book, I'll keep that for another day, but those pages only apply when you have air or gas in it. You're trying to operate this like a "proper" pipeline with pressure greater than atmospheric at all points. When you do that, that calculation isn't valid and the book says that.

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

this is effluent, it naturally contains large quantities of air and in addition, the BOD and COD will also create other gases. So it is critical that you maintain sufficient pressure and velocity throughout the pipeline in order to avoid pockets of gas filling the high points. That will create a "vapor lock". Also, be aware that if you increase the pressure and eliminate additional oxygen intake into the pipeline, it will almost certainly maintain anerobic conditions all the way. Since most lift stations do not pump either constant flow rates or constant flow, this can be challenging. And if your pumps cycle on and off, then you will definitely need to maintain back pressure or you will allow your pipeline to fill with air.

RE: TDH - Complex Rising Main

(OP)
cvg : I'll go read the Waste disposal & treatment Forum.
See that there's alot of posts about similar issue.

RE: TDH - Complex Rising Main

(OP)
Did the drop test.

15.1 L/s.

Putting it in my calculation sheet : TDH = 15 m.
The pump was supposed to be 36 L/s with TDH of 17 m.

RE: TDH - Complex Rising Main

The pump might be abe to pump that, but if the pipe resistance is greater than it can pump then the flow reduces. There maybe some reduction in diameter / silting up going on here but also that the system curve is not accurate. I don't thin we've ever seen a system curve or diam / type of pipe to be able to judge your calcualtion sheet.

I'm a bit confused about this now...

My motto: Learn something new every day

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

RE: TDH - Complex Rising Main

(OP)
The discharge line might be clogged (silted) if air vent is closed and vacuum effect is not there to help it flow by gravity after the 1st leg.
I'm not really surprised as I need at least a 24 m pump to pump 34 L/s.
Simulating it to keep the discharge line above atm. pressure, I need more than 28 m at the pump to pump 34 L/s.

What I don't know is what will happen if I put a 34 L/s - TDH 28 m in that wet well.

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