Basic Concepts of Water Pressure for Plumbers

Question

I am learning how to install water pipes and I was told that to check if a pipe is leaking water I need to check the water pressure. But I don't understanding how water pressure works inside the pipe.

For example, a water pump is connected to a valve and to a tap like this:

pump ----> valve ---->tap

the water pressure is caused by the pump. Now if I close the valve and blocking the water from the pump to the tap, what will be the water pressue in the segment of pipe between the valve and the tap? i.e.

pump ----> X ----> tap

My understanding is that the pressure should drop to zero as this segment is disconnected from any "source of water pressure" (the pump). But If I connect a meter to the tap before closing the valve:

pump ----> X ----> meter

The reading on the meter doesn't drop to zero. Instead, if there is a water pressure drop, it indicates a leakage. But what causes the pressure when the valve is closed?

I know air pressure inside a sealed system is caused by the Brownian motion of the air molecule does the same thing applies to water?

Answer 1

There's always a bit of elasticity in any system, even copper pipes.

If you attached the pump to a balloon then closed the valve you wouldn't be surprised to see the pressure maintained in the ballon because obviously the rubber skin of the balloon has been stretched and it's exerting a pressure on the air.

Your pipework is much, much, less elastic than a ballon but nevertheless when you pressurise the water in the pipes there is a bit of give, and that keeps the pressure up even after you've closed the valve. However because the amount of stretching is so small losing even a small amount of water will bring the pressure back down to zero. That's why it's so easy to spot leaks.