Pressure setpoints for a set of pumps (Affininity Laws)

[AbdullahS]

TL;DR

Calculating the pressure set point required for a set of pumps serving a residential building.

Hi Guys,

The problem I am facing at the moment is to calculate the appropriate setpoint for a set of pumps supplying cold water to a building so that electricity and cost savings can be achieved.

Here is the situation:
There are three pumps connected in parallel. They are located on the ground floor of a building which is roughly 21m high. All three pumps are connected to VSDs. The pressure at the water mains is 450 kPa. The current set point is 750 kPa which results in a 3kW pump to operate at 95% speed.

The local standards mention that that minimum pressure at the farthest outlet cannot be less than 50 kPA. This means that in order to sustain the water column and provide 50 kPa of pressure, ignoring pipe loses, the pressure at the outlets of the pumps need to be 256.01 (=50 + 1000*9.81*21/1000) kPa. Surely, that does not mean that we do not need any pumps, right? I cannot get my head around how it works.

Also the nameplate of the pump indicates that its a 3kW pump with pressure head of 33.7m and flowrate of 17 cubic meters per hour. If I do reduce the pressure setpoint let's just say 650 kPa and use use affinity laws in conjunction with the the data from the name plate, I get shaft power as 1.3kW and a flow rate of 13 cubic feet per hour. Is that the correct way to go about it?

Any help on this will be greatly appreciated.

Regards,

 

[jrmichler]

The local standards mention that that minimum pressure at the farthest outlet cannot be less than 50 kPA. This means that in order to sustain the water column and provide 50 kPa of pressure, ignoring pipe loses, the pressure at the outlets of the pumps need to be 256.01 (=50 + 1000*9.81*21/1000) kPa.

Your numbers look correct. You need to calculate and add the line losses to find what pressure is needed at the pump to get the desired pressure at the end of the pipe. As a side note, 50 kPa is very low pressure in a residential building.

The nameplate does not give enough information to properly apply a pump. You need the pump curve (search the term). You then use the affinity laws to scale that curve to different pump speeds. You find your operating point by sketching the system curve on the pump curve and finding the point of intersection.

Keep in mind that the system flow rate varies over a wide range, and the flow can change quickly. Instead of controlling pressure at the pump, you may be better off to control pressure at the end of the pipe. Then the pump would shut off completely during periods of low flow. Keep in mind that the building owner may want the pressure at the end of the pipe to be higher than that specified by the local standards in order to satisfy the residents.