Blow Off Silencers with / without expansion orifice
During free venting or blowing off of pressurised steam or gas kinetical energy in terms of waves with high pressure amplitudes is released. This effect causes already at an minor amounts of venting media (500 – 1000 m³/h) and relatively low expansion pressures (1 – 2 bar) a quite high sound pressure level – depending on application – of 115 to 125 dB(A) in 1m distance from blow off opening (e.g. piping, etc.). At bigger blow off flow and higher expansion, e.g. blowing of safety valves, the sound pressure level SPL can increase up to 150 dB(A). This radiation of noise can be reduced effectively with blow off silencers, preferably with expansion stages (orifice).
[ further pictures refer to References ]
The effectiveness of noise reduction of such blow off silencers is caused by the expansion orifice right after the nozzle inlet (multi hole expansion stage), followed by a attenuating absorbing splitter system analogous the baffle type silencer. At the expansion orifice the media is streaming through several cylindrical stages. The multi hole cylinders are designed in dependence with the individual application of the blow off case (hole diameter, distance of stages, etc). At special design of the multi hole cylinders and the absorbing splitters a noise reduction (sound transmission loss) of up to 60 dB(A) can be achieved.
For the design of the silencer the following information is needed:
• Gas- or steam flow
• Temperature and pressure upstream (before) valve
• permissible sound pressure level at immission point (in defined distance), e.g. 85 dB(A) in 1m distance
• diameter of inlet nozzle (if defined)