Vent and blow down noise is a function of upstream pressure and temperature, type of gas being vented, the valve size and type, plus the effect of down-stream piping. Each vent silencer is designed to attenuate the noise level to the required sound pressure level criteria at a given distance from the silencer.
The size of the vent silencer is specified by diameter and the total length. The diameter of a silencer is a function of flow rate of the gas it can safely pass and the silencer length is a function of the degree of noise reduction to be achieved. High priority in design is placed on the inlet diffuser for back pressure and distribution of flow.