Stopcock question

tony3619

Hello, 
Does opening the main cold water stopcock more increase water pressure? I'm worried about altering the position to carry out some work and rupturing a pipe when turning it back on. 

greatcrested

Depends how far open it was to start with. If it was partially open, thus restricting the flow, then opening it fully will increase the pressure.

What makes you think you might rupture a pipe though?

If concerned, then when you turn it off to do the work, count the number of turns needed to close it, and then count back when you re-open.

tony3619

It's a new house, I've seen loads of little problems and I dread to think what the pipe work is like. ..lol. 

It's probably nothing it's just I heard having it fully open shouldn't increase pressure enough to cause a problem and didn't know if this was true.

Jeepers_Creepers

Opening the stopcock won't affect the static pressure.

Ie, if you don't have any taps running inside the house - no water flowing - then the pressure on both sides of that mains stopcock is the same whether it's cracked open just a quarter-turn or opened fully. If your incoming mains pressure is, say, 4 bar, then the pressure in your domestic pipe will also be 4 bar. 

Until you open a tap.

The difference then is that a fully open stopcock will be able to supply more flow to your domestic system so will be able to maintain the household pressure a bit better - it might drop to only, say, 2 bar dynamic (flowing).  However, if your stopcock is cracked open just to a trickle, then as soon as you open a tap in the house the static domestic pressure of 4 bar will collapse completely as the tap will be allowing water out at a greater rate than the stopcock can supply it.

'Pressure' and 'flow' are connected, but are often confused/mixed up.

So, if all you are concerned with is the 'pressure' in your domestic pipes, then it doesn't matter how open or closed the stopcock is when you are not drawing water.

What makes you think your mains pressure is riskily high? Have you had it tested? I would suggest that if it's found to be much above, say, 4 bar, then you should consider fitting a pressure reducing valve to bring it down to around 3 bar. Turning taps on and off - especially with the fast on/off action of quarter-turn ceramic types - can give your pipework quite a dynamic shock at 4 bar+, and is - I think - best avoided. 

J_B

Excellent explanation - well done!!  😀

grumbler

Jeepers_Creepers said:

 Turning taps on and off - especially with the fast on/off action of quarter-turn ceramic types - can give your pipework quite a dynamic shock at 4 bar+, and is - I think - best avoided. 

Are you sure this depends on the pressure? Essentially, it's just the amount of water in the pipes flowing at given speed that gets stopped very fast. I don't see how pressure can affect this - only through the higher flow rate available.

Jeepers_Creepers

Flow rate isn't a problem in itself - it doesn't provide any 'shock'. You could have a 100 gallon tank with a hole in the bottom, and you'd stop the flow with your little finger. Like a bath tap supplied from a tank just a few feet above in the loft - it flows like a flowy thing, but I wouldn't worry at all about dynamic shock from shutting it off; the pressure behind that flow will be like 0.1 or so bar - now't. 

But, yes, absolutely, a combination of flow and pressure being shut off quickly provides that unwanted percussive shock. So, as you say, if the stopcock were to be turned almost right off to a trickle, then the 'shock' would disappear.

But, the static pressure would remain. 

If the mains pressure was being delivered at 20 bar (wouldn't happen of course - it's just to make a point) and the homeowner shut off their stopcock until only a drop per day passed through, his pipes and joints would still blow (after quite a few days!). Ie - the pressure on the 'house' side would also ultimately reach 20 bar - kaboom. However, if the homeowner opened and closed his taps rapidly, it wouldn't have any 'shock' as the pressure would collapse instantly the tap was opened, and would still be near-zero when re-shut.  

It's fascinating; the parallels with leccy - volts and amps - are pretty much exact. 

grumbler

Jeepers_Creepers said:

Flow rate isn't a problem in itself - it doesn't provide any 'shock'. You could have a 100 gallon tank with a hole in the bottom, and you'd stop the flow with your little finger.

Because it's a tank, not a pipe. The flow/speed is close to zero except of the small amount near the hole. Put the same volume of water moving at the same speed in a pipe - and the effect will be different. And regardless, with a finger it's different to do this fast enough, especially when the pressure rises in the process.

"Hydraulic shock (colloquial: water hammer) is a pressure surge or wave caused when a fluid,  in motion is forced to stop or change direction suddenly"

"When a pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving, thereby building up high pressure"

https://en.wikipedia.org/wiki/Water_hammer

Jeepers_Creepers

Good point - well made :-)

tony3619

It looks like I have a pressure reducing valve fitted.

What exactly does this do? Keep the same flow / static pressure no matter how open the stop !!!!!! is?

grumbler

It limits the output pressure.

It can't keep the same flow - and you really don't want it to do this.