Hi Rise system pressure requirements 2

[ContractorDave]

Here's a quicky: I have an 18 story building with a 100 psi fire pump and 70 psi suction residual available. There are no plans available to review design requirements but at 170 psi in the parkade to the 18th floor let's say we lose maybe 82 psi, there's still 88ish in the mech penthouse. My problem is this building is 25 years old and it has had 175 psi static (maintained by excess pressure pumps) on it from day one and it's starting to take it's toll here and there. My question is, since the system will pressure up to it's 170 psi almost immediately, is it necessary to maintain such a high static pressure?

Regards
Dave

 

[LCREP]

Dave,

Yes, NFPA 20 would require the jockey pump to maintain the system between 160-170 psi with the fire pump starting at about 150-155 psi. The reason is to prevent water hammer.

Tom

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[ContractorDave]

Hi Tom

Thanks for the reply but do you mind if I query you further about this? I do not see where NFPA 20 would require that you have upstream pressure for 'any' reason (this would be an operational issue specific to a given system), let alone one that would prevent water hammer. As I draw it out and look at it, it appears to be a good solution. Since the standpipe is full of water to begin with, hydraulics suggest that if the system is at set at a more benign 120psi, when the fire pump kicks in it will quickly rise to 170psi. At the top of the riser given the above assumed 82psi loss for elevation, you would see a rise from 38 to 88; a 50 psi increase. On the face of it, this would not appear all that dramatic. But an engineer might tell me I'm not taking into account velocity of pressure increase (or some such thing) and this pressure increase would indeed be detrimental.

I had thought that because this system was designed to produce 88 psi at the top of the riser, there might be a specific code that states I shall maintain said 88psi at all times... but then I think of dry systems where no pressure is maintained.

I have another solution in where a separate riser can be run to the floors where the residual pressures are sufficient but of course this would have to be calc'd out.

Regards
Dave

 

[FFP1]

Dave,

You already indicated (or suggested) the system piping has developed a few leaks. One of the worst things you could do would be to lower the activation pressures for the jockey pump and fire pump. During a real life fire incident, the system pressure will drop and the fire pump will create a water surge equal to or greater than (due to water hammer) the given churn pressure of the fire pump. The resulting water hammer could potentially cause damage to the system piping when the system is needed most. The fact that the sprinkler piping is older increases the likelihood of damaged piping due to water hammer; let's not increase the pressure differential which increases the probability even more.

Properly resolve the system leaks and make sure there is little or no trapped air pockets which can lead to excess system pressure during significant temperature fluctuations!

 

[ContractorDave]

Thx gentlemen.

 

[LCREP]

lightecho

See NFPA 20, 2007 edition section 14.2.7 and A 14.2.7, it has detailed info on how to set the jockey and fire pump pressure settings.

Tom

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Fire Sprinklers Save Firefighters’ Lives Too!

 

[ContractorDave]

Now that's interesting Tom.
I was thinking of bringing up some examples where I am aware of new installs that don't conform to A.14.2.7. Thank you for pointing this out

Regards
Dave

 

[LCREP]

Light,

This is common mistake I see with new and existing installs. NFPA 25 requires the settings be checked but many times it is overlooked.

Tom

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Fire Sprinklers Save Firefighters’ Lives Too!