NFPA 20 Check Valves in Sensing Line 1

[GeorgeMech]

Guys,

Any reason why NFPA requires 2 check valve in the pressure sensing line and why is it require to be separated by 1.50meters.

Thanks!

 

[skdesigner]

It is a safety measure for the controller's pressure transducer. The check valves are arranged to allow water to flow from the controller to the system. When the pressure drops enough, the controller signals the driver to start. If the checks weren't there, the pressure surge would hammer the controller like you wouldn't believe.

The purpose for (2) checks is to dampen the pressure surge through the sensing line. The pressure surge will hit the first orifice, slow down significantly (we are talking a 3/32" hole), lose pressure from friction through the 5' of pipe, hit the second orifice, reduce again and be at a manageable pressure when it gets to the transducer. The 5' between the checks is likely not an exact science, but deemed acceptable enough to be an across-the-board requirement for all NFPA 20 fire pump installations.

Also, check valves aren't the only way to do it. NFPA 20-2010 4.30.4.2 states "Where the water is clean, ground-face unions with non corrosive diaphragms drilled with a nominal 0.09375" orifice shall be permitted in place of the check valves.

 

[3cio]

Yes,