Sprinkler system question

[GPM1230]

Hi All. I'm studying the IFSTA Fire inspection and code enforcement 7th edition. On page 375 it covers Controllers for fire pumps. they boost flow to the sprinkler system in areas where the municipal ( or other) water system is not enough.

My understanding is that the system is under pressure and when a sprinkler head opens, the pressure drops and the fire pump cuts in. The book explains this and goes on to say " The pressure at which the pressure switch is set to start the fire pump must be higher than the pressure in the system." What the??? Is this a typeo, or am I missing something?

Thanks

[InsuranceLCRep]

GPM,


Most fire protection systems with automatic fire pumps have two pumps, a jockey pump to maintain system pressure and the fire pump to supply water and pressure. The jockey maintains pressure in the system and prevents the fire pump from coming on for small leaks such as a leaking main drain.

The jockey and fire pump pressures are set in the controllers as follows:

The fire pump is churned, meaning it is started with no flow, i.e. hit the start switch. The pressure on the discharge side of the pump is noted, say for example it is 150 psi.

The jockey pump would be set to come on at about 140 psi and go off at 150 psi.

The fire pump would be set to come on at 130-135 psi and go off at 150 psi.

The above assumes the fire protection engineer who designed the fire protection system choose the correct size and pressure of the pump based on the available water supply.


Hope this helps

[GPM1230]

Thanks, I follow what you are saying and I agree. I used to work on domestic water pumps and am familiar with pressure switches. In your case the system pressure is kept at 150 psi by the jockey pump. The book says that the cut in pressure of the fire pump must be higher than the system pressure. That does not make sense. The only thing I figure is that when they refer to system pressure it is the pressure on the suction side coming from the towns system. Let’s say that pressure is 100 PSI and the cut in pressure is set at 80 PSI,( lower than system pressure " from the town") then the pump would not cut in when a sprinkler opened, until the flow reduced the pressure to 80 psi. That’s the only thing that makes sense to that original statement found in the book. Do you agree?

[HEYVERN]

Your exactly right. The cut in setting of the fire pump pressure must be set higher than the static water pressure of the city supply. Another common mis conception is that a fire pump is installed on a sprinkler system to boost pressure not flow. If more flow is required than the city can supply, we have to install a storage tank.

[GPM1230]

Thank you, I also e-mailed the editor and confirmed it. He noted my concern on the wording in the text.
I can't say they will change it, but hey, I did my part.