About two days ago, my colleague asked me about inconsistency in our design basis of fire fighting system of our on-going project. In our design basis, we stated that the capacity of jockey pump is 50 gpm (the capacity of main fire pump is 500 gpm for electric motor fire fighting and diesel engine fire fighting, respectively). But vendor who supply fire fighting package stated that they will supply jockey pump with the capacity of 25 gpm. It would be a problem if the Construction Management (the one who monitors and review the product of the project) notices this. The question that will be come up is are you sure the capacity of jockey pump (which is 25 gpm) is enough in case there is a fire? The question rises in my mind is
How to calculate jockey pump capacity in fire fighting system?
Before we answer the quetion, it’d better to understand what jockey pump is and what the function in fire fighting sytem.
A jockey pump is an important component of fire fighting system. Jockey pump or pressure maintenance pump is a small apparatus that works in conjuction with a fire pump as part of a fire-protection sprinkler system. It is designed to keep the pressure elevated so that the main fire pump is prevented from running unless absolutely necessary. From time to time, unwanted pressure drop, small water leaks, or even temperature changes may “fool” the fire pump into starting when it is not necessary.
A jockey pump is electric motor driven. It is controlled with a pressure switch in the main trunk. It will switch the pump on and off periodically to maintain trunk main pressure at high level.
How to Size Capacity of a Jockey Pump
I have browsed the answer to this question and I found several answer. My colleague who asked me the question also has his own answer. He thought the capacity of a jockey pump should be about 5-10% of main fire pump. I also found the exactly similar answer. In this source, jockey pump with the capacity of 5-10 gpm is usually acceptable. And at different source, for example in this source, the jockey pump capacity is 1% of main pump capacity. So, which one is true?
I think it will be better to refer NFPA (National Fire Protection Association). Based on NFPA 20 year of 1999 Standard for the Installation of Stationary Pumps for Fire Protection , pressure maintenance pumps (jockey pumps) shall have rated capacities not less than any normal leakage rate. The pump shall have discharge pressure sufficient to maintain the desired fire protection system pressure. So, the first thing to do to size the capacity of a jockey pump is to look at the system and to calculate normal leakage rate.
Based on NFPA 24 year of 1995 Standard for the installation of private fire service mains and their appurtenances, maximum leakage per 100 joints is 2 quatz per hour or 0.5 gallon per hour. The standard requires the water and the pressure to be replenished within ten minutes. We also need to take into account :
- Net positive suction pressure available
- Effect of rise to shutoff the system to prevent over pressure (see here)