5 replies to this topic

[ChemEng11]

Hi All, 

 

I am designing a fire water pumping system which includes two fire water tanks, one main electric fire pump, one diesel fire pump and 2 jockey pumps (to maintain the pressure in the system). I would like some clarification regarding the fire water start and stop settings to be defined based on NFPA guidelines. 

 

As per NFPA 20, A.14.2.6(4) - 

"Fire Pump Settings. The fire pump system, when started by pressure drop, should be arranged as follows:

(a) The jockey pump stop point should equal the pump churn pressure plus the minimum static supply pressure.

( The jockey pump start point should be at least 10 psi (0.68 bar) less than the jockey pump stop point.

© The fire pump start point should be 5 psi (0.34 bar) less than the jockey pump start point. Use 10 psi (0.68 bar) increments for each additional pump."

 

As per hydraulics, the current fire water system is designed as :

Main Fire Water Pumps:

Rated Flowrate: 454 m3/h

Differential Head: 85.6m head

Discharge Pressure: 8.5 barg (required based on hydraulics)

 

Jockey Pumps:

Rated Flowrate: 14 m3/h

Differential Head: 92.3m head

Discharge Pressure: 9.2 barg (based on rule of thumb - 10 psi higher than FW pump)

 

Minimum static supply head: 0.7m (0.07 bar)

Churn pressure: ??

 

As we are still in design stage, I do not have any final pump curves from vendor to determine the churn pressure (pressure of pump at zero flow). Is it safe to assume an initial guess for the churn pressure based on assumed shutoff pressure calculations as below: 

Churn pressure = Differential pressure * 1.25 = 8.6 * 1.25 = 10.75 ~ 10.8 barg 

 

Please confirm the following two points based on my understanding:

1. Based on the above churn pressure assumption, the pump start stop I have calculated are as per below: 

Jockey Pump Stop Point = 10.8 + 0.07 = 10.87 barg

Jockey Pump Start Point = 10.87 - 0.68 = 10.19 barg

Main Fire Pump Stop Point = 10.8 + 0.07 = 10.87 barg

Main Fire Pump (electric) Start Point = 10.19 - 0.34 = 9.85 barg

Main Fire Pump (diesel) Start Point = 9.85 - 0.68 = 9.17 barg

Is this understanding correct?

 

2. If the above fire pump settings are correct, then how does it correlate with the required fire pump discharge pressure of 8.5 barg calculated based on hydraulics?

 

Your help is highly appreciated! Thanks in advance. 

Edited by ChemEng11, 22 March 2023 - 09:37 AM.

[Pilesar]

See worked example here: https://img1.wsimg.c...ings - 2019.doc

[breizh]

Hi ,

To me the diesel pump is the back up of electrical pump and should start when the fire pump is down for any reason. Start and stop point should be very similar.

I get use to see a set pressure close to 12 bars, not less.

Don't forget that you need UFM controller for your fire Pumps.

My 2 cents

Breizh 

[ChemEng11]

@breizh

 

Thank you for the input. In this case, we have the entire site backed up by emergency diesel generator, hence it has been agreed to provide only electric pumps with standby (and no diesel pump).

 

Yes, the start and stop point should be as per NFPA but I am not sure my understanding above is correct. 

[breizh]

Hi,

Not sure you are right about having only an electrical pump, you need to revert to your insurance broker in charge of your facility and/or the fire brigade in your area.

About pumps start and stop, the first to start and stop is the Jockey pump then the main pump if the pressure in the system is below the set point.

Jockey should be starting quite often to keep the pressure within the system, very seldom for the main pumps (test runs) 

Good luck

Breizh

[amiirasri]

1. I think for now it is reasonable to use the estimate of churn pressure = 1.25 * rated pressure. You can update this later based on vendor data.

 

Overall I think your set points look reasonable. However, I disagree on providing any stop point for main fire pumps. My experience has been that the main fire pumps are stopped manually only.

 

2. Hydraulic calculation helps you with pump selection not for selecting start and stop points.