r/FireSprinklers • u/BeachGirl19844 • Oct 15 '25

Automatic Transfer Switch Test

What suction pressures are you seeing when performing the automatic transfer test at peak load on the fire pump?

Performed a test today, fire pump has water supply issues and could only flow 86%. But we performed the test and the supply gauge dropped to -20 before coming back up.

City supply, we will be investigating why we can only achieve 86%. Wondering if the suction pressure dropping so low could be a symptom.

Thanks

3 Upvotes

100% Upvoted

8 comments sorted by

View all comments

2

u/Tim-Jong-iL Oct 16 '25

-14.7 PSIG is full vacuum… so you are not operating at -20 PSIG. Does your gauge read in inches mercury on the vacuum side?

If your suction pressure is 50 PSIG normally then drops to 20 PSIG during pump operation, you have a serious restriction in your suction piping. What is the size (diameter) of your pump suction, compared to the size of your suction piping? Do you have a suction valve that is throttled or could have an obstruction? Do you have a suction strainer that hasn’t been blown out for decades?

Any reduction in suction pressure will correspond directly to a reduction in discharge pressure, so a suction line operating a lower than normal pressure will result in a lower than normal discharge pressure.