Can you provide additional info on the conditions? Is the supply city-fed or a water tank. Is the basis of the question from a design or NFPA 25 inspection?
NFPA 20 permits up to -3 psi at the pump suction flange when pulling water from a storage tank that has its outlet at or above the pump suction elevation. Other than that, I believe 0 psi or greater is required at the suction flange when pulling from a water supply.
Are you getting the -1.5 psi at the system demand or at 150% of pump capacity?
Simple answer - Do not use a split case fire pump in that application. NFPA may permit it - but the mfg or the water supplier may not.
It depends on many conditions and if the AHJ allows you to do so.
If you have -1.5 psi at 150% because of friction losses in the suction pipe, this is related to NPSH and probably a too long suction piping that is not desirable.
If it is because you are taking suction under lift with an horizontal split case pump (your water level is below your pump level), it is normally not approved by NFPA as you should use a vertical pump in that case (NFPA allows the use of horizontal split case pump under head).
In Europe, we often have horizontal fire pumps under lift. And it was possible in older versions of NFPA 20 (prior to 1974).
In that case, we use a priming tank.
But remember this is not anymore in line with NFPA... NFPA 20 does not allow horizontal fire pumps taking suction under lift since its 1974 Edition, after several foot valve failures that caused the loss of the priming water destroyed the pumps running them dry. ...
But the following arrangement (if approved by AHJ) may prevent you to have problems with your current installation...
The maximum depth from which the pump would suck should not exceed 10 ft 6 in.
In practice, the pump should be installed at the nearest position to the suction location. When sucking from deeper points, separate suction lines should be arranged for each pump instead of a single collector in multi-pump systems.
The suction pipe should be immersed in water with an elbow, and a filter flap (or foot valve) should be installed to the suction pipe end side to keep water between the water level in the tank/pond/river and the pump casing.
There is no need to install a shut-off valve onto the suction line. The filter at the suction flap end should be occasionally cleaned up and prevented to be clogged with dirt and foreign substances.
A priming tank should be installed to fill the volume between the backflow preventer and the foot valve (upstream and downstream).
• The bottom of the priming tank should be at least 5 ft above the top of the pump
• Automatically filled tank only are accepted: volume at least 130 gal. or 3 times the volume of the suction pipe + pump casing, whichever is the greater
• The priming tank should be connected to the pump discharge upstream of the check-valve, with minimum 2 in. diameter pipe
• The priming connection should be fitted with a gate valve and a check-valve, as close as possible to the pump
• The priming tank should be fitted with a low level gauge, set at 2/3 of tank volume, to automatically start the pump on a low level condition, and allowing tank refill through a calibrated orifice
If you are taking the suction from storage tank, then it's allowed as per NFPA 20. Section 188.8.131.52 otherwise not less than o psi. This means NPSHa can be lower when using tank.
NFPA 20, A.184.108.40.206. It is permitted that the suction pressure drop to –3 psi (–0.2 bar) for a centrifugal pump that is taking suction from a grade level storage tank where the pump suction elevation is at or below the water level in the water storage tank at the end of the required water flow duration. This negative suction pressure is to allow for the friction loss in the suction piping when the pump is operating at 150 percent capacity.
NFPA20 permits 0,2bar negative pressure at %150 flow when pump room floor and atmospheric water storage tank bottom is at same level.
This means that any NFPA20 fire pump must have NPSH required value at %150 flow lesser than 0,8 bar approximately.
Pump manufacturers should achieve this performance to be listed.
if your pump require less NPSH (Net positive suction head) than it can still supply enough pressure at %150 flow according to its curve even your suction line is not applicable according to NFPA20.
Keep in mind that generally low rpm pumps require less NPSHr
"This means that any NFPA20 fire pump must have NPSH required value at %150 flow lesser than 0,8 bar approximately."
Sir, Do you have any reference for this sentence?
If yes, would you please provide this reference number?
I need to know this.
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