what causes a noisy valve?

[notnats]

I am using a modified globe valve to regulate flow from a filtering plant. A relief valve is fitted upstream from the globe valve. This valve cracks at 2.5 bar and at full flow of about 5000l/h has a back pressure of 8 bar. The medium is light oil about 15 cSt.

To get good flow control we have replaced the normal poppet in the 25mm globe valve with a tapered plug topped with a 45 deg face to match the valve seat when fully closed.

The system works well except that as the valve is closed, at about 2000l/h and about 5 bar the valve starts to vibrate and howl.

As the valve is re opened, at the point where the howling stops the pressure suddenly rises by about 0.5 bar.

I suspect that I am going from laminar flow to turbulent and the howling vibration could be cured by machining slots or grooves to create more turbulence.

Has anybody any suggestions?

Jeff Stanton

 

[Burdy]

Notnats, I would imagine that you have a little knowledge of flow or you wouldn't have modified the plug!! Noise in a valve is caused by many things,you mentioned turbulent flow, Cavitation is another. However before anyone can attempt to address your problem it may be worth while going to your control valve supplier who will be able to take your particular problem and tell you exactly what you should do(because they know the valve CV and characteristics, plus will have a sizing program). In trying to solve this you could make things worse.
The data you have supplied is really inadequate to make any assumption about your problem. Although I expect your pressure drop is to high and you are actually getting choked flow which dissappears when the pressure drop decreases.
B