Hydraulic Pressure Issue

[mogun]

Hi everyone,

Think about a hydraulic system with series of a pump, hose and two valve. pump is working with 3000 psi. First I closed v2 valve and right after that I closed the v1. what happen to pressure of the hydraulic fluid between v1 and v2 valve?

 

[hydtools]

Whatever pressure was ther is trapped between v1 and v2. Temperature variations will cause that pressure to vary.

Ted

 

[mogun]

hi ted, you mean to say it is 3000 psi. But we know that hydraulic fluids cannot be compressed and just transmit pressure. In the issue, we cut the pressure source, so shouldn't there be zero pressure? By the way we dont consider any variation like temp or hose expansion.

 

[3DDave]

Hydraulic fluids are almost incompressible, but still compress some, and hoses and steel lines are also elastic, so they stretch.

So the pressure remains. Or, if the temperature varies, the pressure will either decrease or increase.

 

[Compositepro]

A tiny amount of leakage through valve two will cause the the pressure to drop to zero. If valve one leaks the pressure will be 3000 psi. If both leak, they will behave as series resistors in an electrical circuit to form a voltage (or pressure) divider.

 

[FluidPowerUser]

Is this a real issue or just a theoretical question regarding fluid dynamics?

Bulk modulus, strain energy and thermal characteristics all have an effect on the pressure. Leakage over the valves also has an influence of the decay rate of the pressure.

Closing V2 creates static pressure between V2 and the pump. Closing V1 effectively traps the fluid between the valves and the strain energy in the tube/pipe is what maintains the pressure. There may also be some expansion of the fluid that contributes to the static pressure. Cooling of the fluid and leaking oil over the valves will cause the pressure to decay.

If you are seeing something different or something counter intuitive, provide the details and help can be offered.

 

[mogun]

this is just a theoritical question. We dont consider any external factor. think about there is not strain energy in the tube or thermal change etc.

 

[LittleInch]

"think about there is not strain energy in the tube"

The thing is that's just not realistic. In any event the fluid has some compression. Very little for sure but not zero.

The answers above I also agree with. A minute leak of across v2 could reduce this to zero pressure, but assuming valves are tight then pressure will be whatever it is when you shut it in.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

You can't arbitrarily not consider the modulus of elasticity of the pipe (and everything else that's exposed to the pressure) and the compressibility of the fluid.

If you insist on that position ... What is 0 divided by 0? Is it 5? Is it 42?

 

[FluidPowerUser]

OP has provided no details of the issue.

Fluid?
Application?
Rate of decay?
Type of valve?
Type of pump?

The external factors that the OP has dismissed are part of the issue and should not be neglected.

We need facts about the issue, then we might be able offer an explanation.

 

[Compositepro]

In a recent thread I criticized some answers for considering too many relatively insignificant factors to a theoretical question. However, in this case it is only these "small" factors that completely determine the answer. There are no other "much more" significant factors to consider.

The finite bulk modulus of oil and Young's modulus of pipe and hoses in hydraulic systems is what determines the stiffness of the systems. The effects are often very real and noticeable in real life. And it is best to not learn this the hard way, through experience.

 

[hydtools]

Valve characteristics will matter, also. When closing do they displace fluid or not? An infinitely thin gate valve? A ball valve? A plug valve?

Ted

 

[FluidPowerUser]

I’ve run the setup in a simulation and with ideal fluid, lines and neglecting all other characteristics, the pressure stays at 3000 PSI when both valves are closed.

As expected in the simulation, but not a real representation of what will happen when considering the real effects of thermals and materials etc

 

[3DDave]

This is apparently more of a "mass-less spherical cows, in a vacuum, on a frictionless surface, with zero thermal energy" question. It's a question that exists outside of engineering, or even science and physics.

 

[FluidPowerUser]

That is why I asked if this is a thought experiment or real system characteristic.

You can’t neglect real system boundary conditions and expect to get real results.