NFPA 20 A-11.2.6 Section (d) Clarification Please!

[DTR]

Gentlemen,

Your assistance would be appreciated please, the above clause in NFPA 20 states that the Jockey Pump stop point should equal the churn pressure of the Pump plus the minimum static supply pressure, I have a 10m High Fire Water Storage Tank, I have an 8" Outlet on the Tank and 8" Inlet on the Pump Suction Flange so there is no difference in heght between the two connections, can I therefore assume that the minimum static pressure available will be zero?? Or do I have to make an allowance for static gain...if so...what would it be and on what basis?

Many thanks.

 

[SprinklerDesigner2]

For the most part we're Americans. Meters are complicated and for foreigners. We'll take the kings foot and divide by 12 and 16 anyday.

I assume this is a ground storage tank?

32.75*.433=14.1.

Since the tank will/should always be nearly full you are going to have a 14 psi static pressure but if the tank is empty, nearly empty anyway, you should figure zero static as far as sizing the jockey pump.

 

[FFP1]

NFPA requires the suction tank to be full at all times and overflowed or filled on a weekly basis; therefore, the static pressure which needs to be added to the fire pump churn pressure is the vertical distance in feet from the suction of the fire pump to the top of the water level when the suction tank is full multiplied by 0.433 to obtain psi. You will probably need to add 13-14 psi to the fire pump churn pressure for your scenario. Of course, this approach only applies to pump and tank arrangements when water from the suction tank is strictly gravity fed.

The pressure settings for the fire pump and jockey pump are very important. These settings must be correct to prevent false water flow alarms and water hammer issues.

 

[DTR]

SD2,FP1,

Thank you for taking time to reply...SD2...I am from the UK and of a certain age that I too value the foot or yard as much as the centimetre or metre!The water storage tank is a simple NFPA 22 above ground tank giving positive suction conditions.

Unfortunately for me, you have both given me valid answers to add the Static Gain and to not add the Static Gain to the Pump Churn pressure...I need a casting vote!

I understand all the requirements for the Tank in line with NFPA...and I certainly understand the importance of Fire Pump Settings.....and usually I add the static gain to the churn pressure every time because it is never an issue, however on this particular project if I add the static gain to the churn pressure it takes me above 175 PSI for the Jockey Pump stop pressure and that is not good! I was hoping that I could deviate from what I usually do and not add the Static Gain and " hang my hat" on the above clause for NFPA 20 Code Compliance and keep the Jockey Pump stop pressure below 175psi.

Any further comments or a casting vote would be appreciated!

The sizing of these particular Fire Pumps was not the easiest due to mitigating circumstances (customer insisting on end suction when I need about 2100 usgpm!!and also limiting my motor sizing due to his switchgear capacity!!)so please don't beat me up over poor pump selection because I already know!

Thanks again guys.

DTR

 

[FFP1]

You cannot use anything other than the static pressure when the suction tank is full........based on your comments above, the system pressure would spike well above 175 psi each time the fire pump is activated (weekly fire pump tests!!). This would trap an unacceptable pressure in all of the sprinkler systems.

Provide an adequately sized pressure relief valve arranged to discharge into the suction tank and set it to flow a small volume of water at 170-175 psi. The best location for the pressure relief valve is between the discharge side of the fire pump and the check valve. Then set the jockey pump stop pressure at 170 psi. Jockey pump should be set to activate at 155 psi and fire pump should activate at 150 psi. This approach allows the properly sized fire pump, prevents water hammer issues and prevents system pressures above 175 psi (as long as there are no significant air pockets in the sprinkler systems). I am sure you are already aware that maintaining the system pressure above 175 psi is not allowed.

I hope this helps.