Mains Pressure

[Rod]

The other day I replaced an old and leaking stopcock under the kitchen sink - with a proper full bore valve.

However, once I had done that, I found that there was a horrific bang every time I turned the tap off other than *very* slowly and gently. So, at least as an interim solution, I closed the new valve down a bit - and reduced the bang considerably. (That is, it would only happen when really trying.)

Then I realised that the other water in the house that is fed directly from the mains (one WC cistern and the cold water tank and CH header/expansion) were also very prone to banging - which was new.

On to the next idea - when I had turned off the mains to do the job, had I inadvertently turned it up higher when restoring water? Well - no. I checked before turning it off - it was fully open - and back half a turn.

So I have now turned the under sink full bore valve full on and turned the mains stopcock down. (Currently a bit too far - at about half-to-one turn from fully closed.) Even at this setting the flow is pretty good.

As I had been working on the kitchen sink, there had been a couple of vans and around half a dozen people on the main road nearby. They had been staring at some water utility 'holes' - the covers of some were open. The vans were marked with "PRV Maintenance" and a company name (one of the outsource the work outfits that do most of the work for the utilities - but I have forgotten which one). Looking up PRV I found only one related meaning - Pressure Reducing Valve. Could they have done something like turn up the pressure?

Make any sense to anyone?

[Bob Mannix]

They may have been reducing the pressure as they have realised that this is the cheapest way to meet government leak reduction targets (less pressure, less water lost). They have a statutory minimum to maintain so pressure and leak management programmes concentrate on tuning the supply systems down to the statutory minimum. It is, of course, possible that subsequent to such a tuning operation (or coincidentally), someone has complained that they are not getting their statutory minimum or, just as likely, a new development tacked on the end of the supply pipe is under pressure and the pressure has been increased to compensate further back, leading to an overpressure in your road.

As to your banging, this is generally caused by loose pipework whacking something as it flexes under pressure step changes such as shutting off a fast flow. One approach would be to get someone else to help and to try and isolate where exactly the physical bang is coming from and the direct cause. Whatever the incoming pressure (within normal limits) a domestic installation should be possible without the banging!

[Doctor Drivel]

Fit in a full-bore mains stop-tap. Then near the kitchen sink tap fit a small shock arrestor which cost around £10. Sorted.

[Dave Liquorice]

The banging needs to sorted out properly. Just fixing the moving bit of pipe might deal with the audible bang but the hydralic shock will still be there. Over time that has been known to force apart compression joints...

Cures vary from fiddling with valve to reduce flow rates, pressure reducers or shock absorbers.

[Rod]

Will that cure the cisterns?

(Cheapest I could find was nearly £13.)

[Rod]

Thanks.

I am certain the pressure has not reduced - at full lick we have a veritable Niagara coming out the tap! Seems even faster than before. Can't think of any likely building changes in the area.

Well - I took your pipe looseness comment seriously and, despite being fairly sure all was well, checked thoroughly. And found one definitely wobbly pipe and a questionable section. Fixing those solidly has improved things a bit - but still getting some so I will have to search for any other possibilities. (The difficulty is that some of the pipework is seriously inaccessible.)

[TheScullster]

"Rod" wrote

In that case it would appear that the branch line was acting as a shock arrestor, rather like a damper, with the reduced bore of the stopcock providing the damping effect. As others have said, it looks like it is time to install a proprietory shock arrestor somewhere to see if this resolves the issues.

Phil

[Doctor Drivel]

£3 is going to kill you? The jumper on the stoptap look like it is locking. A full-bore tap will take shocks back into the mains, the jumper may be acts as a check valve. But to be sure a shock arrestor near the most used and offending tap - the kitchen. It should sort it. A 15mm x 1/2" fbsp x 15mm tee in the pipe under the sink, near the kitchen tap

A full-bore tap may mean you can suffer pressure reductions as it improves "flow". If they reduce the mains pressure you flow suffers. Remove restrictions (a stoptap) and you flow improves at a reduced pressure.

[Doctor Drivel]

Replace this with a full-bore valve.

[Tim S]

Doctor Drivel coughed up some electrons that declared:

Just an NB: I don't think the "upside down copper bulb" cheap shock arrestors are suitable for potable water. There are fancy ones with diaphragms which are, but these cost more.

Certainly worth checking.

Cheers

Tim

[Doctor Drivel]

[Rod]

Drivel,

I put the price in parentheses as a matter of information but not desperately important. Some were very considerably more expensive.

I asked the question because we are now getting bangs from the sink, the washing machine, the WC cistern and the cold water tank. So I was questioning whether putting the arrestor by the sink would be effective for the others. The thought crossed my mind that the best location might actually be near the incoming main stopcock.

If I were to fit a full-bore valve (instead of the main stopcock), would I need to add a check valve? (AIUI, some form of check valve is mandatory. Or, as they say in Edinburgh, a meringue.)

[And will someone somewhere please decide whether it is 'arrestor' or 'arrester'. Is this another one the trade tends to spell one way and the rest of the world the other?]

Tim,

The ones I have seen specify 'potable' - anything that didn't, I have ignored. But worth pointing out.

[Doctor Drivel]

Not mandatory. The shock goes into the mains when full bore. On high pressure this can still cause thunks when taps are turned off, so a larger whole house shock arrestor after the maintap is needed.

Change to a full-bore maintap and see. If OK leave. If reoccurs fit a small arrestor on the most offending tap. If all of them then one large arrestor at the main tap, or kitchen tap, which is usually first on the line anyway, and easily hidden under the sink and easy to get at.

Yes use both spelling if searching.