Transient Vibration (Water Hammer???)

[jfaucher]

I have a transient vibration problem on a test cell using a hydraulic dynamometer. An upstream pump is supplying 80 PSI of water pressure to a 4" I.D. pipe, which is controlled by a valve. Downstream the valve, 18 feet of (about) staight piping lead to an absorber dyno. When the valve is slowly opened, a transient penomenum occurs, during which severes vibrations shake the pipe. Once the valve is fully opened (few seconds later), the water flows normally without vibration, feeding the dyno.

It appears that changing the speed the valve is opened doesn't change that much the issue.

Someone can help me to understand the phenomenum?

Thanks!



Jean-Pierre Faucher, ing.

http://www.cel-aerospace.ca

 

[itsmoked]

You are getting a resonant oscillation due to some sort of variable-space and the pipe run topography. The variable space allows for energy storage to support the oscillation.

It's rather like a capacitor in a tuned electrical circuit.

It can be remedied by changing something - anything - that shifts the system resonance point to somewhere that fails to resonant.

Things to consider:

The valve's gate may be loose in it's channel. A gate valve can commonly suffer from this. The gate then rattles providing the initial source for the oscillation.

The pump can run at a speed or have an impeller pitch that drive the problem.

There could be air trapped in the system that provides the energy storage.

There could be a flexible section, like a hose, that can expand and contract to store the energy.

There could be an accumulator or anti-water hammer unit that stops water hammer but causes this problem.

A long stretch of straight pipe can exacerbate the issue.

What you need to do is 'change things' in the hope of moving the system's resonance point somewhere the system has no excitation frequencies at or remove the driving source,(like a loose valve gate).

Examples:
Put a valve on the other end of the piping and partially close it.
Add inline direction changing fittings.
Change to a different type of valve.
Change pump speed.
Change pump type.
Remove anti hammer accumulator(s).
Remove flexible sections.
Add flexible sections.
Increase pipe diameter.
Reduce pipe diameter.
Add a nozzle after the valve.
Change pipe diameter in a section.

You get the point. Scan my list and do what you think is easiest first and work you way thru the system until it stops. Any of several of the above suggestions or any of ones you can think of will likely solve your problem.

Do let us know what worked as this is a common problem and we love to hear the solutions.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[racookpe1978]

First, try to simply support (rigidly tie down) the straight length of pipe - before AND after the valve, up and downstream of the straight length - with supports that will positively restrict the motion up, down, left, and right. In other words, make all whole spools in that section be rigid, then relax small sections a little bit until the vibration shows up again. Decide how much movement is acceptable for the short periods of time that flow is changing.

If that helps, then try simple rubber mounts (wrap rubber insulators for example) around the support points to allow a little bit of movement from temperature changes and minor movement or adjustments. Full sized vibration dampers and spring restraints probably won't be needed: This really is is a little pipe in a short run.

 

[MJCronin]

Jean-Pierre

How many and what type of pipe supports do you have on this piping system ?

What is the maximum vertically unsupported length of 4" pipe downstream of the valve ?

-MJC

 

[racookpe1978]

Why limit the "maximum supported length" question to vertical runs of the 4" pipe?

 

[cvg]

sounds like cavitation. increasing back pressure on the valve may eliminate it.