The relief valve is for relieving brief surge pressure in control systems thus preventing the safety shut-off valve from being activated unintentionally. The VDG reduces too high a pressure increase due to gas creeps which are the result of gas pressure regulators that are not tight closing. It’s normally install together with the safety shut-off valve.
Specifications:
Gas type: NG, LPG, town gas and non-aggressive gases
Inlet pressure: 1 bar
Response Pressure: 100 to 250mbar or 180 to 330mbar