Relief valves

The relief valves are safety devices for discharging a certain amount of gas when the pressure at the control point exceedes a given level (bubble-bursting pres-sure) due to any temporary occurance such as the momentary shutting of the on/off valve and/or the overheating of the gas with no flow requirement. Discharge of the gas can, for example, avoid any intervention by blocking devices in the case of transient causes not regarding damage to the regulators. Clearly, the amount of discharged gas depends on the entity of the excess pressure in relation of the pressure setting .These safety devices operate on the principle of comparison between the thrust on the diaphragm  or on the piston , deriving from the pressure of the gas to be controlled, and the thrust deriving from the setting spring. Of course the weight of the mobile unit and the static and dynamic residual thrust on the cocked plug also influence the comparison. When the thrust deriving from the gas pressure exceeds that of the spring, the plug is raised and a certain amount of gas is discharged; otherwise the plug is released and it closes the valve seat due to the sole effect of the pressure of the gas being controlled, not because of the setting spring. This means that the cocked plug cannot be damaged by any wrong handling of the setting-adjustment nut.