Pump Outlet Pressure Setting

[Ray Burrell]

Quick question:

I have emergency fire pumps (diesel and electric) that are rated at 136 psig maximum and the controls are set to maintain the discharge header at 149 to 159 psig using the jockey pumps, the electric fire pump is set come on at 144 psig and the diesel at 134 psig. Our construction EPC has told us that the controls are set this to address the head in the water tank that supplies water to the fire system.

Essentially, they are telling us the set points are higher than the pump curve to account for the extra head in the tank. Does it make sense for the pump to be set to come on at a pressure higher than its rating?

Thanks,

Ray

 

[TugboatEng]

Yes, the pump curve is additive to the inlet conditions for centrifugal pumps. With that said, many manufactures add the velocity head to the discharge pressure so the pump will never perform to the curve when observed with a pressure gauge.

 

[TiCl4]

Yes. The pumps head measurement is a differential head - you gotta add the suction head to determine total outlet pressure.

You might want to clarify something - I think you mean the pumps are rated at 136 psig dP at deadhead conditions, but you didn’t give a flow rate at the given pressure. Pump curves would help tremendously.

 

[mk3223]

The pressure of the piping system may be help to have a hydraulic diagram for a better understanding as shown:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1602957777/tips/Document1_qvirt2.pdf[/url]

 

[SB1976]

Yes, the pressure settings for the FW Pumping units are in conformity to usual industry practice.

This is how it operates :
1. Under normal scenario (i.e. no fire scenario), jockey pumps keep the FW header pressurized at 149-159 psig.

2. In the event, fire breaks out and one or more hydrant is/are opened for mitigation, the header pressure starts to fall.

3. When it falls to 144 psig, the Electric Motor driven FW Pump will kick-in to develop the required header pressure.

4. When the header pressure falls further (to 134 psig) due to more hydrants opening implying higher demand for FW, the diesel driven FW pumps kick-in additionally to make up for the loss in system pressure.


Pump rating is 136 psig implies that it is the pressure withstanding capability of the pumps, which is not related to start-up sequence of each individual pumps, since start-up is initiated based on system pressure. It could have been based on flow also, but in your case, it is pressure based.

 

[Artisi]

Following on from S.Barman,if the electric units fails to start or goes out during operation, the diesel unit must start up immediately.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1503-44]

Maximum Pressure rating and max differential pressure shown on pump curve are different numbers entirely. Surely you are not operating the pump at a higher pressure than its max pressure rating. I hope.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.