Two Pumps

Jim_16

I Walked into a boiler room today on a backflow test. I noticed the relief valve on one of two boilers leaking a pencil lead stream. Pressure at 30 psig The expansion tank was not the problem. Two boilers two pumps in paralell, only one pump running when I got there. After some investigation I found the new system was designed for 600 gpm. Each pump is rated 300 gpm. Below 30 deg. both pumps run. Yes the boiler short cycles even though it is staged. The fill valve is piped to the top of the air seperator and is set correctly. The suction side of the pumps drop to near 4 psig when both pumps run. Allowing for more makeup water to enter. When Dual pump cycle is ended relief valve trips. A half hour backflow job turns into 4hrs. And some questions . Any Ideas?

Jerry Boulanger_2

If the pressure at the suction

of the pumps drops that low when they run there is a high pressure drop between the fill connection and the pump suction, such as a plugged strainer. It would not affect the fill valve unless the fill connection is not connected at the same place as the expansion tank, since running the pumps cannot change the pressure at the expansion tank connection.

The relief valve popping when one pump shuts off is likely from water hammer - the check valves on the pumps should be non-slam.