Fire pump start and stop

[TAGE]

Dear All

I'm curious about NFPA20 regarding installation of pumps for fire. I have two questions as follows

1. In my (basic)point of view and apart from the jockey pump, I understand that A/C motor-driven fire and/or Diesel engine-driven fire pump will be set to automatically start at certain set pressure and then be stopped manually by personnel in order that it can operate the best without any unexpected interruptions ; however I found some wording in NFPA20, Appendix A-11-2.6 talking about "Fire pump start-stop" set pressure recommendation. So i'm eager to which one is correct and practical.

2. In case we have two main fire pumps; A/C motor pump as the primary and diesel fire pump as the backup on. What the appropriated set pressure should be if "churn" pressure of each pump is different.

Please give me some guidelines to answer my problems.

Many thanks in advance for your kind help

 

[LCREP]

What is the churn pressure of each pump?

 

[LCREP]

NFPA 20 says 5 psi between each setting, I find with most pumps controllers it is difficult to get 5 psi, most times I use 10 psi. So if I have 2 fire pumps and both churn at 150 psi I would set it up like this.

Jockey on at 140 psi off at 150 psi
Pump 1 electric on at 130 psi
Pump 2 diesel on at 120 psi

 

[TAGE]

Thanks so much indeed LCREP
Answer your 1st question: The churn pressure of the primary and secondary are 135 Psi and 175 psi respectively. So what should we do next?

And your recommendation for 10 psi different setting sounds practice, I agree accordingly. : )

How about my first question regarding pump start-stop method? Looking for some ideas

 

[LCREP]

I would make the 175 psi pump primary and the 135 psi pump secondary. I would set it up like this:

Jockey on 165, off 175

Primary on 155

Secondary on at 135


The reason for keeping the jockey pump close to the pump pressures is to reduce water hammer on start up. I really do not like the jockey pump and system pressure so high 175 psi. BUT I would hate to have the 135 psi pump on 1st and have the 175 psi pump start at 125 psi, giving us a 50 psi hammer. You do not say how many gpm each pump is, but if they are close in gpm the 2nd fire pump should not come on at least during the operation of the sprinklers in the design area. If so the open sprinklers will relieve any potential water hammer.

 

[FFP1]

I agree with LCREP, but there is one problem with his suggestion.......you cannot use 135 as the start pressure for the secondary fire pump if the controller uses a mercoid switch. The mercoid switch will never reset. After automatic start, the pump will not shut off even when pushing the stop button because the system pressure is too low. The activation pressure must be lower to allow the mercoid switch to reset.

If you really need the 175 pump to be the secondary pump consider a PRV for the 175 pump IF THIS APPROACH allows adequate flow and pressure for the fire protection systems. Then follow NFPA 20 for activation settings.