series pumping 2

[jrwchem]

COnsider two pumps in series. Pump A is feeding Pump B.

A couple questions I have been stumbling over:

Somebody has told me that it is possible for Pump B to flow at a higer rate than pump A and therefore draw a vacum on the suction side of Pump P.

Is this actually possible? Or will as many text books say that the flow is the same through both pumps (mass conserved in steady state operation??

Many thanks.
Jon

 

[JJPellin]

I assume we are talking about centrifugal pumps. Over the long term, the pumps must pump the same amount. The system tends to be somewhat self-stabilizing. If the flow from the second pump were to increase and draw down the pressure in the piping between the two pumps, the differential pressure across the second pump would go up causing it to drop in flow. At the same time, the differential pressure in the first pump would go down, causing it to increase in flow. They will always come back to the same flow. There are some potential problems with series pumping. The higher suction pressure to the second pump can cause problems designing a mechanical seal and support system. Both pumps have to be matched well. Just because they are running at the same flow rate does not mean that both are running well. The two pumps need to be able to run at that rate reliably. Neither of them can be running below a suitable minimum flow. Neither of them can be running "off the end of the curve" at an excessively high flow.

Johnny Pellin

 

[dcasto]

Only if the the head on the first pump and its flow allow it AND the density decreases on the fluid such that the ACFM matchs. HUH? There can't be a system like that. The thing is define a higher rate. in acfm terms its true, in mass terms no-way.

 

[Artisi]

Just to add a note to Johnny Pellin's excellent reply, the pumps should also have a continually rising H/Q curve especially important if the system friction head component is flat.

 

[jrwchem]

Thank you all.

This is what i thought: That in steady state the flows should be the same.

I have seen a configuration where both pumps have discharge control valves. It was explained to me that the control valve after the second pump was there to prevent the second pump from flowing more than the first (and thus draw a vacuum on the pipe between the two pumps). The original design controlled the suction pressure of the second pump with the control valve downstream of the second pump. It never worked right.

ANy comments about the above configuration. It seems like overkill to me, and that the system could be controlled equally well with a single control valve.

 

[BigInch]

Sounds like a good way to find out if the pumps have an internal relief valve.

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[BRIS]

I assume from your question that pump A and pump B are located in the same station ?

In which case I cannot see how you will get a vacuum (inadequate NPSHA) on pump B. I cannot see any benefit to a control valve between Pump A and B. The only reason i see for two control valves would be to prevent an overpressure on pump B. this would not be a very safe solution. Normally you would provide one control valve after Pump B (if at all).

However, if pump A and B are separated by a long length of pipeline then the NPSHA at pump B may be inadequate. I am presently looking at increasing the capacity of a gravity pipeline by the addition of two pump stations in series 150 km apart. Because of limits on the pressure rating of he existing pipeline the pumps will increase the hydraulic gradient by drawing down the pressure and we have the situation that you describe - i.e the critical design condition is to achieve adequate NPSH on pump station B. We have a control valve downstream of pump station B which is primarily to actual friction losses in 250 km of pipeline being less than long term aged design losses.

In answer to your original question:

1) The flow is the same through both pumps (it cannot be anything else.
2) It is possible to get a "vacuum" (inadaquate NPSH) on pump B, but only if there is a very high head losses upstream of pump A and between pump A and B
3) A control valve downstream of pump b would allow control of the system and would not be uncommon.
3) A control valve between pump A and B would increase the vacuum on B and would more likely be provided to either A) prevent pump B being over pressurised (not an advisable solution) or B to maintain adequate NPSH on pump A (also not a good solution if pumps a and B are in the same station).

 

[EdStainless]

I like the comment by Artisi.
I have seen matched pumps, closely spaced (<20') in series. The curves were not very flat, there was no control valve after the second pump, and the pumps were slightly oversized. The pumps ran fine with some back pressure on them, but when a valve was suddenly opened the pumps cavitated to pieces. As the flow increased the pumps took turn having too low of inlet pressure.
Hey, it wasn't my idea.
We ended up using a fixed restriction on the outlet of the second pump.

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[BigInch]

Well it all depends...

on the slope of the individual pump curves, the total of all pump's curves and the system's curve intersections, and, in the case of unstable (or transient) condtions, how long the system's or pump's flow takes to respond to a change in either head or flow and return to a steady state. If the pump has a controlled rpm, head, or flow then add how fast the controls and the pump can react to or against the piping system. If it is possible to reach a steady state, of course a steady flow and head will be obtained. If its not possible to reach a steady state, there are 3 possible outcomes, 1.) if the system is semi-stable, a "steady state oscillation" of one form or another will be reached, or 2.) if the system is totally unstable, either a runaway or a shutdown condition is reached. Determination of the operating characteristics of multiple pumps, their controls and the interaction with the system is one of the main advantages of transient hydraulic simulations being done before turning the key.

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