Fire Pump - why the bronze check valve with 3/32 orifice

[msm15]

All the plans I look at for a fire pump system associated with a sprinkler/standpipe system show a "Bronze check valve w/ 3/32" orifice in caliper" in line with the take off coming coming off the discharge line of the pump and going to the fie pump controller.

What is the reason for such a check valve, and what does " 3/32" orifice in caliper" mean ?



Thanks

 

[msm15]

See attached for basic schematic

 

[chicopee]

I think that if you are testing the fire pump static pressure when the OS&Y valve on the pump discharge side is shutt off, the 3/32 orifice will allow a small amount of water to flow thru the pump thereby preventing the pump from overheating.

 

[bimr]

There are actually two (2) check valves, one is not correctly labeled on your drawing. See the drawing in the link.

The check valves are installed in the pressure sensing line at least 5 ft. (1.5 m) apart with a 3/32 inch (2.4 mm) hole drilled in the clapper to serve as a dampening affect on the pressure sensing.

Regarding "3/32" orifice in caliper", it actually reads clapper. The "clapper" is the swinging disc mechanism inside of the check valve. See the last page of the link below:

http://www.mh-valve.com/component/k2/item/download/23

You probably should review your detailed equipment drawings so you don't miss something.

 

[Pumpsonly]

The check valves are installed with flow direction from controller to the pump discharge. When the fire main is being pressurized by the pump the small hole on the clapper of the CV allow a small flow rate to the controller preventing high pressure surge damage to the pressure sensor. It serves the same function as a restriction orifice.
When the fire main pressure dropped due to demand, the CV clapper swing open to allow quick drop in pressure in the pressure sensing line thereby start the fire pump.

 

[bimr]

As stated in NFPA 20, the reason for the check valves (with orifice) is to serve as dampeners (to prevent water hammer) in the sensing line when the fire pumps are started. The reason has nothing to do with check valves opening quickly. It is acceptable to substitute orifice plates for the check valves if you are operating with clean water.

"There shall be two check valves installed in the pressure sensing line at least 5 ft. (1.5 m) apart with a 3/32 inch (2.4 mm) hole
drilled in the clapper to serve as dampening."

"Exception No. 1: If water is clean, ground-face unions with noncorrosive diaphragms drilled with 3/32-in. (2.4 mm) orifices
shall be permitted in place of the check valves."