I have had many discussions concerning the prediction of valve noise using the current IEC 534-8-3 standard. My discussions usually involve how the actual configuration of the valve installation affects the theoretical Sound Pressure Level(SPL)produced by the equations in the IEC standard. The IEC equations were verified in laboratory conditions and the SPL predicted is at one meter downstream of the valve and one meter from the downstream pipe surface. I am not sure exactly how the pipe downstream is configured in the lab but I would assume it is a nice straight clean run.

In real-world applications I try to balance the theoretical with the practical. This involves not specifying expensive anti-noise trim if it is not required in order to insure operators that the holy grail of sound, 85 dBA, is not exceeded. One area I try to consider is the piping system that contains the valve. In many cases we have block valves (usually ball valves) immediately up and down stream of the control valve. Sometimes there are bypass lines, also with ball valves installed. If one adds the usual pipe supports, tees, elbows, etc. encountered in a real-world installation, what affect does this have on the theoretical SPL numbers predicted by the IEC. My gut feeling is that, while the IEC standard gives us something to consider, in reality they are bogus.

I would like to hear others take on this. I am under constant pressure to specify large, expensive, specially equipped globe valves in low pressure, low pressure drop applications just because the IEC equations predict an SPL over 85 dBA.

In the real world the valve(needle valves) is adjacent to as many as six reflective surfaces and the sound can bounce around for quite a while.  The actual measured sound pressure level in a real installation can easily be 6dB higher than calculated due to reflection/reinforcement/acoustic persistence. Then again: if the pipeline pops up out of the ground in a grassy field, connects to the valve, and drops back underground, the measured noise can be less than calculated.  

Noise from isolation valves is a minimal contributor because if their noise is more than 3dB less than the control valve's noise it doesn't contribute-due to the exponential nature of adding decibels.  There may be turbulence in an isolation valve, but not throttling, so the noise generated high velocity seen in the control valve does not occur.

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