Water Pressure in an Isolated System

[Scott Pratz]

So I'm more into electricity, so fluids is not really a forte at all. But I have been thinking about this, and there must be some fundamental issue in the way that I view fluid pressure.

Conditions:
1. I have a metal pipe, incompressible
2. There is water in the pipe, (incompressible? ... I thought)
3. There is a pump on one end of the pipe, supplying 150PSI
4. There are discharge and suction pressure gauges attached to the respective pump locations
5. There is a deadhead on the other end of the pipe

Actions:
1. while the pump is running, I close a perfect valve (whatever type) that does not leak by on the discharge side of the pump.
2. I stop the pump.

Questions:
1. Is there 150PSI in the pipe?
2. Where does the pressure come from? What has compressed to allow this pressurization?

Actions:
1. I have a discharge pipe (on the same level as the pump (i.e. no head pressure)
2. I have a suction pipe (on the same level as the pump (i.e. no head pressure)
3. The pipes are filled with water before the pump starts (assume pipes match pump gpm)
4. Start the pump

Questions:
1. How does the gauge read actual suction pressure when there is no attempt for anything to expand on either side of the pump
2. same for discharge, if there is nothing restricting flow, how does the pipe gauge read any pressure in the pipe?

 

[mfb]

2. There is water in the pipe, (incompressible? ... I thought)

To a good approximation, but nothing is perfectly incompressible.
If you have a closed pipe system, completely filled with water, it can maintain its pressure. As soon as you open a valve towards lower pressure, a tiny amount of water can come out, quickly reducing the pressure.

A sketch of the setups would help if that does not answer the questions.

 

[montoyas7940]

The metal pipe is elastic and will expand some very small amount as pressure is increased. It is this springiness that maintains the pressure if the pipe is closed -mostly. And as MFB said -nothing is perfectly incompressible. So the water will yield some as well.

For the second action, gauges are sometime calibrated in an absolute scale, psia for example.
It is common for pump suction head requirements to be specified in absolute because cavitation is affected by saturation temperature and absolute pressure. So you may be seeing absolute pressure represented on the gauges.

Also, there must be some suction head pressure or there would couldn't be any fluid at the suction of the pump. Simply atmospheric pressure is enough in many cases.