Pump Sizing -

[Stewartf87]

Hello, I am looking at a pumping station with 2 inlets of differing suction pressures, flowing to dedicated pump sets (to utilise the larger suction pressures and reduce pump size) and converging to flow through a common rising main. I am looking to confirm the system curve for when both pump sets are running and essentially the head requirements for the pumps. The flow will differ between pump sets also. Will the higher suction pressure pumps being the main driving force determine the system curve to be used for pump sizing? Thanks.

 

[LittleInch]

From what I understand of your question I don't think the pumps have anything to do with the system curve.

The system curve in this case is the set of pressure versus flow for the system downstream of your pumps.

You then match the pumps to the system curve to see what flow you get or what pressure you need for a certain flow. At the unction of the discharges from both pumps the pressure will be the same.

With two different inlet pressures you have two different calculations to perform, but this is quite simple. As you say you are doing this so that you can minimize the pump sizes.

Maybe a graph or schematic would help.

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

 

[Stewartf87]

Hi LittleInch,

thanks for the response. Yes I agree, the pumps do not affect the system curve. But with the varying inlet pressures to each of the pumps, and essentially the starting point of the system curve, will this not change when both pumps are operating? I agree its fairly simple to calculate on each pump individually, it is when they are both running at the same time, what affect this will have on the system curve and to essentially determine what pressure each pump set will operate at.

 

[EdStainless]

No, the starting point is the discharge flow and pressure, and then you work back from that to size the pumps.

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P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Your description states you have two separate systems which are joined together at the discharge of both pumps. This point is the start point for your system curve.

Your system curve if there is no variable static element will be a fixed curve which is not affected by what is happening upstream of the start point.

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

 

[Stewartf87]

Hi LittleInch,

thanks again and yes you are right, two separate systems joined together at the discharge of the pumps. and yes, there is no variable static element as the pumps and discharge points are fixed. Does the varying suction pressures not change the operating conditions for the pumps downstream i.e. higher suction pressure, lower required head for the pumps? And this is where my confusion has been in determining this required head for each pump set as the suction pressure will vary and essentially the system curve. But if this does not affect the system curve I will have another look from that point of view.

Thanks for your explanation.

 

[LittleInch]

I suspect you will need to come up with a set of possible conditions and then you can see what gives you the largest power and head / flow and what gives you the least.

Lets use some figures to explain based on your description.
Case 1

Pump 1 Normal flow
100 m3/hr discharging at 10 bar with a 4 bar inlet - DP of 6 bar @ 100 m3/hr

Pump 2 Normal flow
50 m3/hr discharging at 10 bar ( same as above) with a 6 bar inlet - DP of 4 bar @ 50 m3/hr

System curve - 150 m3/hr at 10 bar
200m3/hr at 15 bar
100 m3/hr at 7 bar

Case 2

Pump 1 high inlet
150 m3/hr @ 15 bar discharge with 10 bar inlet - DP of 5 bar @ 150m3/hr
Pump 2 50 m3/hr @ 15 bar discharge with 11 bar inlet

Etc Etc
So yes varying inlet pressure will affect flow because it's combined discharge pressure from both pumps which will determine flow.

So if one pump has more inlet pressure, but the other lower, that may just mean you still run at the same total flow, but one pump is now flowing more than it did before, but the other is flowing less

Only you can figure out the combination and what represents the low flow / low discharge pressure and which represents the high flow/high pressure cases



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