Pipe hammer arrestor

[F]

I've got a pipe hammer arrestor installed but the hammer is still there. It's got what looks like a bicycle tyre valve on top of it: do I need to pressurise it?

[Tim Watts]

On Saturday 14 December 2013 22:15 F wrote in uk.d-i-y:

Hammer arrestors are usually smaller affairs:

What have you got?

[Tim+]

Maybe, maybe not. It all depends on resonance in your system. Have a good look over your system for "dead legs". We had a problem until I discovered and eliminated an old dead leg.

Tim

[F]

One like this:

[newshound]

Any instructions with it? I would be inclined to check the pressure with an ordinary car tyre pressure gauge, I bet it is zero and that you need to pump it up to something like 50 psi.

I *imagine* there is a rubber diaphragm in it rather like a mini boiler expansion tank. With the air side pressurised it will be roughly half full of water, half full of air. So when a positive pressure pulse comes down the system water is forced into the chamber, providing "damping" as it goes into the arrestor and (hopefully) reducing the hammer. But as another poster said, if you have a significant "dead leg" somewhere filled with air, or perhaps an inverted U which does not flush clear, you may still have problems.

[F]

Absolutely no instructions.

I'll get the bike pump out...

[sm_jamieson]

o pressurise it?

I had a problem that went away when a small dead leg was created by closing some disused pipework. As you say, its down to resonance, and sometimes a shock wave hitting a sharp bend in the pipework. Connecting things with fle xible connectors can help - presumably they absorb the energy. Simon.

[John Rumm]

De-pressurise the water side first though...

If you know what your water pressure is, stick an equal pressure of air in there. Failing that, go for about 3 bar.