Water Pressure in an Isolated System

In summary, the expert describes a scenario involving a metal pipe filled with water and a pump supplying 150PSI. They then discuss the possibility of pressure in the pipe and the role of suction head pressure. They also suggest that a sketch of the setup would be helpful in understanding the situation.
  • #1
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.

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

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.

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

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

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?
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  • #2
Scott Pratz said:
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.
  • #3
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.

1. What is water pressure in an isolated system?

Water pressure in an isolated system refers to the force exerted by water on the walls of a container or system when it is completely sealed off from the outside environment. This pressure is caused by the weight of the water and is measured in units of force per area, such as pounds per square inch (psi) or Newtons per square meter (N/m2).

2. How is water pressure affected by depth in an isolated system?

In an isolated system, water pressure increases with depth. This is because the weight of the water above exerts a greater force on the water below, resulting in higher pressure. This relationship is known as Pascal's law, which states that pressure in a fluid is equal in all directions and increases with depth.

3. What factors affect water pressure in an isolated system?

The main factors that affect water pressure in an isolated system are depth, density, and gravity. As mentioned before, pressure increases with depth. Density also plays a role, as denser liquids will exert more pressure than less dense liquids at the same depth. Gravity also contributes to water pressure, as it is the force that pulls the water towards the bottom of the container or system.

4. How does temperature affect water pressure in an isolated system?

In an isolated system, temperature does not directly affect water pressure. However, changes in temperature can affect the density of water, which in turn can impact water pressure. As water is heated, it expands and becomes less dense, resulting in lower water pressure. Conversely, when cooled, water contracts and becomes more dense, leading to higher water pressure.

5. How can water pressure in an isolated system be measured?

Water pressure in an isolated system can be measured using a pressure gauge, which is a device that detects and displays the amount of pressure being exerted. The gauge can be attached to the walls of the container or system and will show the pressure in units of force per area. Another way to measure water pressure is by using a manometer, which measures the difference in pressure between two points and can be used to calculate the pressure at a specific depth in an isolated system.

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