Water pressure vs water flow

[davembk]

I’m after some help with understanding how water pressure is less once it’s past the stop valve.

Last night I had to urgently replace our leaking mains stop valve with a gate valve which was all I had.

It’s working ok but I know how it’s made differently, so that eventually it won‘t shut off too well.

But apparently there’s another reason why you should not use a gate valve as a stop valve:

I’m reading that a gate valve is not supposed to be directly on the mains inlet because it is only meant for low pressures. That seems to infer that once past the stop valve the water has less pressure.

I’m not a plumber but either way, I don’t really understand this.

Surely regardless of how much or how little the initial stop valve restricts flow, the actual pressure is still the same anywhere else in the house (except gravity feeds) because the stop valve has only restricted the flow, and not the pressure, which is something different.

http://www.ehow.com/about_5533714_water-flow-vs-pressure.html


So to me, regardless of the flow, any gate valve in the house, unless it's on a gravity feed for example, must still be getting the same pressure as the stop valve.

Thanks, Dave

 

[seco services]

But apparently there’s another reason why you should not use a gate valve as a stop valve:

A stopcock acts as a non return valve a gate valve don't.

 

[twgas]

Pipework and fittings connected to the main are high pressure, tank fed is low pressure as once the water is discharged from the ball valve the only pressure is provided by the head of water.
There are other systems involving pumps and other bits and pieces but mains and tank (gravity) are the normally encountered systems.

 

[ianmcd]

Both of the above are absolutely correct, if you rely on a gate valve to isolate mains pressure then good luck and make sure you have waders on

 

[Charnwood]

I’m reading that a gate valve is not supposed to be directly on the mains inlet because it is only meant for low pressures. That seems to infer that once past the stop valve the water has less pressure.

It does not infer anything of the sort. By "directly on the mains" means it should not be fitted onto a pipe carrying mains water. this is due to the pressure of mains water and, as mentioned, they act as a non-return valve, the jumper being designed to drop down when there's reverse flow.

A gate valve does not have a seal as such. It is a metal plate that slides down a metal slot. The parts should be precission engineered so that water should not pass, but at mains pressure, Chinese precission engineering ain't to be trusted.

 

[hazetimesfive]

i wonder if anyone will answer his real question

 

[plumbarius]

Hello davembk Gate Valves are not allowed on mains pressure I repeat NOT? If for any reason they turn the mains off you can draw dirty water back in to the mains and poison everybody I Would change it as soon as you can. 1ft of water will give you .434 psi of pressure stood, Pressure and flow are related the more the pressure the more the flow
it's resistance that reduces both, If you connect a very long long hose pipe to your stop cock eventually you could have it long enough so nothing came out the end (Resistance). The Pressure at Your Stop Cock will be more than at say your Bath tap but would need good gauges to measure it. Charnwood gave a good answer about a stop cock take note and change the gate valve This is Serious stuff. Alex