High Mach numbers at valve exit 6

[JohnPritchard]

Can someone explain why most valve manufacturers seem to place an upper limit of Mach number of about 0.3 at the valve exit? All the literature I have seen tends to give excessive noise as the major reason for this limitation. Is noise the only issue, or is wear and tear (because of vibration) on the plug, stem etc. something to worry about? In addition, if the Mach number approaches 0.3 at the valve exit, I would presume that there are higher mach numbers inside the valve (e.g. at the vena contracta). Is the Mach 0.3 simply a proxy for a higher number elsewhere in the valve?

 

[hacksaw]

Valve outlet noise.

Almost impossble to correct without major piping re-work.

 

[davefitz]

Valve outlet noise. Also, there can be some issues related to aerodynamic forces inside the valve trim.

There can be developed "oblique shock waves" at mach numbers above 0.4 if the valve geometry has sharp turns or reducers. These shock waves cause noise.

Also, the vena contracta area can be as small as 33" of the valve outlet nominal area, so acoustic choking might occur, but usually for normal Xt's > 0.2 the choking is likely due to oblique shock waves at the transition from vena contracta to outlet

 

[mtmurthy]

Valve outlet velo 0.3 Mach is generally used.
But nowadays industry accepts velocity till 0.7 Mach.There is a possiblity to have noise less than 85 dBA with mach less than 0.7 Mach. However, with noise atteunation trims, manufacturers restrict the allowable outlet velo up to 0.3 Mach or at the maximum 0.5 Mach. More outlet velo ( more than 0.7 Mach) lead to pipe vibration, malfunctioning of positioners(if the valves are fitted with) and ofcourse trim wear. Valve outlet velocity can be reduced by using larger valves ( more outlet area) but we should note that valve size cannot be more than pipe line size.

 

[4carats]

One cannot look at velocity (Mach Number) without look at density of the exit fluid. A better way is to look at the trim exit fluid Kinetic Energy. See "Practical Guides for Measurement and Control, Control Valves" by Borden, ISA, 1998. In most application, a trim exit fluid Kinetic Energy of 70 psi is good enough. In gas venting, this trim exit fluid Kinetic Energy will be less than 1 psi almost in every case (because of noise). Once the magnitude of the trim exit fluid Kinetic Energy is defined, the difference in static pressure across the valve divided by the trim exit fluid Kinetic Energy will be the loss coefficient (i.e. K-factor) of the trim required. Many valves failed because they do not have enough K-factor in them.

 

[spud1]

Davefitz's reply gives the reasons behind the limitations for valve outlet velocity.

If you get a copy of the European Standard "Control Valve Aerodynamic noise prediction method" (IEC-534-8-3:1995) the scope and limitations section limits the standard noise prediction to 0.3 Mach - hence the industry standard limit. Noise prediction for control valves is mostly based on this and ISA standards.

The latest European Standard (IEC-60534-8-3:2000)gives a method of noise prediction up to 0.8 Mach.

The latest version of Shell’s DEP (Design & Engineering Practice- which tend to be copied by all and sundry for new petrochemical plants) I have for control valves (DEP 32.36.01.17-Gen 1994) references IEC-534 i.e. the Mach limit at 0.3 mach.

Depressurising systems and vent valves are excluded from this DEP and in these cases a velocity of 0.7 Mach is allowable with some restrictions which limit the velocity to 0.3 Mach is the valve is to be operated more than 10 times per year.

In principle the velocity head calculations for trim exit, which 4carats refers to, can allow higher valve outlet velocities however these must always be viewed along with the velocities in the seating area to ensure the valve does not choke. In some cases the velocity head can be maintained below the recommendations of the ISA while the valve outlet velocity reaches the sonic limit.

Where higher outlet velocities are used great care should be takes with the outlet piping configuration so that the noise reduction achieved at the valve is not sacrificed to increases in the noise of the outlet piping.

 

[BHMech]

John,
the main issue is noise and vibration limits. special valve trims can be designed to cope with the noise and vibration inside the valve but not in the down stream pipe system. You might consider raising the downstream pressure somehow thereby reducing velocity. I would have to look at your application more closely.

 

[zdas04]

The velocity vs. induced density curve has a significant inflection that statrs at 0.55 and ends at 0.7. On top of what the others have said, in this "transition region" throttling characteristics are quite unpredictable so a valve that had good linear flow characteristcs can become non-linear. Above 0.7 Mach (really above about 0.6) the flow-induced density has turned the stream into an incompressible flow whose density is very high and has throttling characteristics like a liquid instead of a gas. If the valve was designed to throttle gas this is not a good thing.

David

 

[4carats]

Some valves are deliberately designed to choke. Fuel flare applications are a typical example, so as to keep a constant flow of fuel downstream. So long as the energy can be managed, all problems such as noise, velocity, downstream pressures, vibrations, Mach No., etc. are managed. Shock waves occur when fluid Mach No. is 1. Downstream of the shock waves, the fluid became subsonic. But the energy released by shock waves can be so huge that the piping and/or the valve stem and trim can shatter. In design of control valve trim, the loss coefficient (i.e. K-factor) is to be increased so as to avoid the excessive trim exit fluid Kinetic Energy. How excessive is excessive depends on your requirements. In general, ISA recommends 70 psi trim exit fluid Kinetic Energy for most applications.

 

[clarenceworley]

Hello, I would like to know if it is possible for outlet valve pressure to drop down pressure at vena contracta.If not, how is it possible not to have pressure recovery at the outlet, for high flow regimes, as I read in aa article.

Thank you

 

[HMILD]

The responses have made interesting reading and have added complexities to the criterion of Mach < 0.3 that were never expected.

This criterion of Mach less than 0.3 is a historical rule of thumb used in piping design to assure there would not be excessive noise generated at the valve exit. Nothing more or nothing less than this.

I have seen references to this criterion that go back to 1972 and I am sure a search of older literature will show that it was an "accepted design practice" for many years before this. A control valve noise prediction method did not exist before 1989 (ISA S75-17) and the valve exit noise prediction was not published until 1995 (IEC 534-8-3). So a design practice developed early on to avoid excessive piping noise.

There is no physical reason today to apply such a limitation. With the current physics based noise prediction methods available in the IEC and ISA standards the affect of an exit velocity on noise is quite predictable. These standards account for the noise created by both the valve trim and the valve exit. They account for all of the fluid conditions such as the temperature, density and speed of sound. They account for differences in valve type also. These standards are continually upgraded as more knowledge is gained however they do a very commendable job now.

Depending upon the maximum noise level one wishes it is quite likely that a Mach Number will be less than 0.3 however the calculation may also result in a much higher number. Most piping systems will also accommodate the higher velocities without a problem although checks for pressure forces and coincident frequencies in the piping system is always prudent; regardless of the Mach Number.

 

[hacksaw]

0.3 mach is used for a number of reasons. if you are designing for low valve noise, a high exit mach no. may circumvent your improvement by increasing pipe noise. that is much more difficult to treat.

to say that "piping will accomodate" higher piping velocities, > 100 m/s say, flies in the face of the consequences of high fluid velocities.