Pressure setpoints for a set of pumps (Affininity Laws)

• AbdullahS
In summary, the conversation discusses the calculation of an appropriate setpoint for pumps supplying cold water to a building in order to achieve electricity and cost savings. The current setpoint is 750 kPa, but the local standards require a minimum pressure of 50 kPa at the farthest outlet. Calculations are needed to determine the necessary pressure at the pump in order to achieve the desired pressure at the end of the pipe. The nameplate of the pump does not provide enough information, and the affinity laws must be used to properly apply the pump. It is suggested that pressure be controlled at the end of the pipe instead of at the pump, as the system flow rate can vary greatly.
AbdullahS
TL;DR Summary
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,

AbdullahS said:
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.

AbdullahS and berkeman

1. What are the Affinity Laws?

The Affinity Laws are a set of mathematical relationships that describe how changes in speed, diameter, and flow rate affect the performance of a pump. These laws are commonly used in pump design and operation to determine the appropriate setpoints for optimal performance.

2. How do the Affinity Laws relate to pressure setpoints?

The Affinity Laws provide a way to calculate the changes in pressure setpoints for a set of pumps when there are changes in speed, diameter, or flow rate. By using these laws, engineers can accurately determine the appropriate pressure setpoints for a specific pump system.

3. What is the significance of pressure setpoints for pumps?

Pressure setpoints are crucial for ensuring the efficient and safe operation of pumps. These setpoints determine the amount of pressure that the pump must generate to move the desired amount of fluid through the system. If the pressure setpoints are not set correctly, it can result in pump failure, decreased efficiency, or even damage to the system.

4. How can I calculate the pressure setpoints for a set of pumps using the Affinity Laws?

To calculate the pressure setpoints using the Affinity Laws, you will need to know the original speed, diameter, and flow rate of the pump, as well as the desired changes in these parameters. The following formula can be used: (P2/P1) = (N2/N1)^2 x (D2/D1)^2 x (Q2/Q1), where P is pressure, N is speed, D is diameter, and Q is flow rate.

5. Are there any limitations to using the Affinity Laws for calculating pressure setpoints?

While the Affinity Laws provide a useful tool for estimating pressure setpoints, they do have some limitations. These laws assume that the pump is operating at a constant speed and that the fluid being pumped is Newtonian (having a constant viscosity). In real-world situations, there may be variations in speed and fluid properties, which can affect the accuracy of the calculations.

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