During free blowing off / venting of pressurised steam or gas, kinetic energy in terms of waves with high pressure amplitudes is released. At minor amounts of venting media (500 – 1000 m³/h) and relatively low expansion pressures (1 – 2 bar), this effect causes 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).
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 an attenuating absorbing splitter system analogous to 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.). Using 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 a silencer, following information is needed:
At blow off silencers for safety valves the allowable back pressure caused by the expansion orifice depends on the construction of the valve. Most safety valve manufacturers allow a maximum backpressure of 15% of the set pressure. This allowable backpressure should be utilised because of the following reasons: 1.) the differential pressure over the valve is lower, 2.) piping behind the valve could be designed at smaller size, 3.) silencer can be designed at an optimal cost
To clean pressure steam pipes for HRSGs or turbines during commissioning of a plant, high pressure steam is used for blowing out the pipe with preferably a high velocity of steam.
To reduce the noise at the opening of the pipe blow our silencers are used.