Knocking pipes - resolved but mystified

[Kezzaa]

Hi all , I have had an issue for months - water supply pipes knocking randomly at all hours of day and night - I tried a couple of fixes to no avail - water arrestor , cistern flushing valve.
Recently I fully opened the cold mains stopcock and voila - no more knocking.

So relieved to get a good nights sleep without being disturbed but curious why pipes would knock when stopcock partially open and not knock when fully open !

Can any experts shed any light ?

Kezzaa

 

[Aire]

It could be caused by your water pressure. If you've got high pressure and a partially closed stop tap, this could cause resistance which in turn causes vibration which travels down your pipes.

 

[Ric2013]

Bit late for me to understand anything much, but no doubt something to do with the jumper on the stopcock being able to travel onto and off the valve seat acting a bit like a one-way valve.

Why was the stopcock partially closed? While I always back them off 1/4 turn from fully open just to make them easier to unseize if they do seize, I can see several disadvantages in having them any less open than this.

 

[Kezzaa]

I hadn’t realised stopcock wasn’t fully open . Does that suggest stopcock faulty ?

 

[Ric2013]

There were other replies that obviously haven't made it onto the new forum. Basic answer was that sometime people shut down stopcocks in a misguided attempt to reduce water pressure.

The actual result is reduced FLOW (in technical terms the standing pressure remains the same, but the working pressure falls) and then when you open two taps at once they don't supply much water.

If your stopcock turns on and turns off, then I'd suggest not faulty. They are usually quite simple designs that show their faults openly, so if it seems to work, then it probably does work!