Blow Off Silencers with/without expansion orifice

During free venting or blowing off of pressurized 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 the 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 a 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 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 the case (hole diameter, the distance of stages, etc). A 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) 

Note: 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 manufacturer allows 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.) the silencer could be designed at optimal cost

NOTE: Blow Out silencer:

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. The differences to a blow off silencers are as follows: 

1.expansion orifice is designed to a maximum velocity of steam inside the blow out pipe
2.removable expansion orifice 

Data sheet: EN 

Example for a rating of blow out silencer: 

If the boiler is designed to 40 bar / 450 °C, the blow out pipe should be cleaned acc. to VGB directive with approx. 20 - 25 bar / 400°C.

If the valve (slide valve) is opened during the short time of e.g. 10 seconds a steam mass flow is generated total independent from the performance of the HRSG. The resulting mass flow is depending on the pressure drop of the outlet system and the Cv value of the valve.

By experience, calculations and measurements at several plants a mass flow, in this case, will be about 100 to 129 tons per hour for a duration time of 10min. (Base: pipe size 8”) 

Silencer for rental:

For 1-time blow out cases, you can rent a silencer from us. The Rentalsilencer can be connected with an NPS 12“ nozzle. The rental fee contains the overhaul in our shop after usage and if necessary the repair work – incl. ultrasonic or X-Ray testing of the critical welding lines.