My undestanding is that generally "flashing fluids" will produce high noise either at high or low velocity, worse at high velocity. While non-flahsing fluids will produce high noise during high velocity only (high pressure drop). Is this right?
Your question is too general to be answered without a specific fluid given, and also a specific valve including a full description of the pipeline before and after.
Trying to be systematic:
a) Water: probably not, roughly checking out a couple of common control valves from max given pressure to athmospheric.(eg. worst case)
Reason, as you indicate: noise is created by obtaining cavitation and /or flashing conditions. Control valves are in addition built to give best possible flow and regulation, eg. lowest cavitation possibillity.
Even using 'non control valves' (worst construction type valves for control, for instance butterfly) you could probably provoke cavitation to be heard, but not a full cavitation at highest noise level.
b) Other fluids, including gases. Rephrasing of question: Is there a fluid that under a given pressure and temperature, could reach flashing or cavitating conditions by a pressure drop of 30 PSI through a given control valve?
My feeling is 'unlikely' for this one, but I am not able to 'prove' it.
Others out there better to comment on this? - We are anyway moving into 'theorethical engineering'.
If you cross the vapor pressure to the low side, you will flash. If the valve recovery coefficient is high enough to recross to a pressure higher than VP, you will revert to a liquid. If the +/- 30 psi drop crosses below then back above the VP, you've got it.
The way to answer the question is to ask how can you make noise with low pressure. There are many ways, but most are avoided by good design. But one example woult be loose parts in the valve. I've seen a whole house shake from a loose washer in a faucet valve. It was high freaquency water hammer.
Whether a control valve can produce noise depends on:
1. state of fluid: gas or liquid?
2. if liquid, how far is its pressure from vapor pressure?
a) if liquid and much higher than vapor pressure, no noise;
b) if liquid and very close to vapor pressure, could be cavitation or flashing depending on the outlet pressure. Some noise, but the real threat is actually erosion to damage valve trim;
c) if gas, probably at least some noise.
We can calculate the expected noise. This needs data such as P1, P2, T, Q, (if not water: molecular weight, specific heat, viscosity, .........). Need data to be specific.
One additional possibility: at certain flows does the frequency of the noise move into an inaudible part of the frequency spectrum? This would give the subjective appearance of it being 'loud' or 'quiet' at certain flows.
Noise is generated by mass flow rate and velocity.
Certain valves have greater acoustical efficiency (the ability to turn energy into noise) than others.
That being said, a control ball valve will make a lot more noise than a globe control valve.
Liquid flows: If the inlet pressure is 200 psi and there is a 20 or 30 psi drop, it is unlikely to make much noise, but if the inlet pressure is 30 psi and the valve discharges to atmospheric pressure, the flow is almost guaranteed to be cavitating. Cavitation can be very noisy, even from relatively low inlet pressures.
Someone mentioned flashing in an earlier post: In my experience flashing is generates a low rumble but is not opressively loud. Cavitation generates a sharp crackling noise and can be deafening.
Gas flows: General rule of thumb is if the (absolute) outlet pressure is less than about half the (absolute) inlet pressure, the flow will be shocking sonic in the vena contracta and the valve will be noisy. This is the noise of a jet engine on afterburner and the decibel numbers can go into three digits easily. Compressible noise control valve trims split the flow up into small streams and run the flow through many stages so the pressure ratio in each stage remains less than the critical drop. What this REALLY does is to control velocity so the flow never goes sonic. A muffler works very much the same way.
Back to the original question: a 30 psi drop through a ball or butterfly valve discharging to atmosphere can be really, really noisy.
-Gas valves
Some refinery unit column overhead vapor lines (18"+) usually have a butterfly valve for pressure control before the condenser.
Some of these, with 7psig inlet pressure and 1 psid can make over 110dBA noise due to the high flowrate. Recent increases in average unit size have made this problem worse. When the valves were 10 or 12" it wasn't such an issue.
