Surge Control Valves must be capable to operate the compressor below the surge control line.
Most information required for the sizing of the surge control valves is available on the compressor map.
As the compressor suction pressure may vary, various calculations need to be made. 
Most compressor users or surge control system suppliers have their own standard for the safety margin included in the valve capacity.
This figure is based on the system dynamics, valve response and characteristic.
Commonly used margins are that the valve must be capable of passing 100% flow-rate at 50% percent opening. 
As substantial amounts of energy are dissipated in the Surge Control Valve, the noise from the control valve needs to be calculated.
Generally, limits range from 85 dBa to 100 dBa.
Apart from noise the fluid velocity in the valve outlet shall be kept within limits to avoid pipe vibrations.
The anticipated noise level, before external attenuation, shall never exceed 110 dBA with fluid velocities below 0.3 Mach.
Above those levels structural damage may occur to piping components. 
Mokveld offers a series of valve trims to reduce noise.
The axial flow principle of the Mokveld valve results in inherently less noise compared to conventional globe or ball valves.

MOKVELD AXIAL FLOW CONTROL VALVE
Mokveld control valves have been developed over three decades of research and field applications.
They combine high quality engineering with a thorough understanding of the oil and gas production and process industries.