Water Hammer

[pcnerdman]

Hi...I have a 3hp Munro Centrifugal pump.   I use a Square D  30/50 switch to keep the system pressurized.  I have 2 pressure tanks.  A 35 gallon and 30 gallon.     This provides about 2 minutes between pump cycles during normal usage.  There are 22 users connected to the system and it is impossible to monitor their usage.   I have burned up 3 pressure switches this year.   After reading your pages, I moved the pressure switch close to the tank and this has helped a lot.

But now,  the water hammer, that causes the switches to short cycle and spark, is now completely apparent in the 2" brass check valve  (In early spring of this year I blasted a PVC spring loaded valve apart.   ( I thought it may have been a manufacture defect)  So I switched to a flapper check valve.

This year, the irrigation lateral that brings the water to me was repaired and it now has 12psi of head pressure, (last year it was more like 3 -4 PSI of head).    Anyway, when the switch cuts out (or, if I trip the breaker near the cutout pressure) the check valve slams closed and creates enough water hammer the water bounces through the plumbing and bangs the switch.  I believe this has been my problem all along, but things happen so fast at cutout pressure, I could not tell what the problem was.  I thought it was switch location.  One by one I eliminated what it was not.  Now i know, it is the check valve slamming shut that smashes the whole system.

Any suggestions would be welcome.   The system is 7 years old.   I had a 2hp pump for several years ( with no water hammer) but it burned up last year so this year I installed a 3hp pump.  Now the 3 hp pump keeps up with demand but the water hammer is killing switches and is also hard on the pump and pipes.   I did upgrade the switch to a Squared D 30amp pressure switch (for the bigger contacts) and that's how I finally found the problem with the check valve. 

Now it is clearly in the check valve.  The pump has 2" PVC line on the suction side.  And 1.5" on the discharge.
In an attempt to solve the water hammer I switched to a brass swing check valve.   Now when the pump shut off, the brass valve "Slams" shut (50 psi on the pressure side and 12 PSI on the suction side).   The check valve is located 8" from the pump on the suction side to hold prime.   But this year, with increased head pressure, I might be able to move it to the pressure side.  Any Thoughts?

[Cary Austin]

Swing checks are terrible for water hammer.  Any check valve with a spring-loaded poppet is better.  The spring pulls the valve closed as soon as the flow stops.  With a swing check, the flow has to reverse to close the check.  Stopping the flow after is reverses causes water hammer big time.

However, even with a good spring-loaded check valve you can still have water hammer when the pump starts or stops.  With that size pump you could be producing 50 or 80 GPM when the pump shuts down.  Any check valve will be wide open when the pump stops, and will slam shut from the full open position.

A Cycle Stop Valve (CSV) can help with water hammer in a couple of ways.  The first way the CSV will limit, almost eliminate cycling.  The CSV will turn the constant rate pump into a variable rate pump.  The CSV will hold a constant pressure on the system and the pump will continue to run with reduced amperage and flow, instead of cycling on and off at full amperage and full flow.  As long as more than 3 GPM is being used, the pump will run continuously.  If the pump doesn’t shut off and on, it doesn’t cause water hammer.

The CSV won’t cause the pump to run all the time, only when more than 3 GPM is being used.  Then when the pump does shut off, the flow rate is only 3 GPM.  At 3 GPM the check valve is only open the thickness of a piece of paper when the pump shuts off, and water hammer is eliminated.  The CSV also makes the pump start at 3 GPM, which also eliminates water hammer on pump start.

Not only will a CSV solve your water hammer problem but, the tenants will love the steady, constant pressure compared to the on/off, on/off you have now.