NFPA 20 Question

[jaycarper]

I just started at a new facility and after reviewing their annual fire pump data I noticed the pressure settings for the fire pump were off. Also this facility has had fire pipe ruptures in the past which I think was due to the water hammer effect based on these pressure settings: Currently the fire pump starts at 100psi and the jockey pump starts at 120psi. The system with no flow at max churn operates at 145psi. As a result I want to change the jockey pump start to 135psi and the fire pump start to 130psi.

Is it correct to assume that having the jockey and fire pump start and stop so far apart from the operating pressure that it would cause a waterhammer and changing it to the NFPA recommendations would help prevent this?

Also when I went to the fire system contractor to make these changes he told me that this was a bad idea and that the NFPA was going to be changed and if I followed the NFPA and tighting the on/off pressure spread it was cause more damage to the system and pumps. Is there any basis in his concern and has anyone heard that the NFPA 20 is being challenged on their pressure recommendations?

 

[FFP1]

Water hammer is a very common problem and the fire pump activation settings are one of the most common causes.

You are correct. You should adjust the activation settings for the fire pump and jockey pump. I would make sure the jockey pump stop pressure is 145-150 psi (you might have dry systems or other fire protection system components which are set for the maximum expected system pressure which sounds like 145 psi => fire pump churn pressure). The adjustments you have proposed will definitely reduce the likelihood of water hammer. The "fire system contractor" you referenced is simply wrong.

 

[ContractorDave]

I agree with FFP1.

NFPA 20 2007 on-line edition section A.14.2.7 explains how the pressures should be set, and appears to be what you are recommending. On what authority is the contractor claiming the settings should be set at something else? Has an engineer set it this way? Has the manufacturer determined this? On what authority does he claim that NFPA will be changing this?

As was explained to me in a past post, it's better to have the system properly set (at the pressures it will run at) and discover the problems BEFORE an emergency requires the system to operate without breaking.

Regards
Dave

 

[LCREP]

Been on vacation just got back so I am late responding.

Depending on what type of pressure switch you are dealing with a 5 psi difference between the stop and start pressures may be difficult to get. My experience is if you have a mercoid switch, the kind with mercury in a clear tube, 5 psi will not be possible, it will be more like 10 psi. Based on fire pump 145 max churn I would set it up as follows:

Jockey

On 135
Off 145

Fire Pump

On 125

****************************************
Fire Sprinklers Save Firefighters’ Lives Too!

 

[jaycarper]

Thanks for all of your help.