ESD (Emergency Shut Down)Valves have torque safety factors that can vary from 1.3 up to 2, depending on project specifications, and can also vary from Break to Open, running & End to close positions. Speed of emergency stroke operarion can also vary from 1 second total, to 1 second per inch of diameter, to 3 seconds per inch daimeter. The Safety factor has an effect on (some) valve shaft diameter, or material, due to much higher requisites (due to increase in resistance torque) as requested torques exceed valve MAST. Most of ESDV are low pressure pneumatic operated (unless you are in the desert on a gas pipeline, where line pressure= to actuator sizing pressure), by low pressure anything between 3 & 5 Barg is considered...a Air Filter Regulator should be included in instrumentation to ensure that supply pressure to actuator does not increase over sizing pressure...increase of pressure to a piston = increase to torque output of actuator. The speed of operation can also cause problems, that are mostly solved by intervention on actuator & instrumentation; QEV (Quick Exhaust Valve) are fitted on discharge to allow faster stroke. This may cause problems to valve, when slamming shut (damaging seal/seat & possibly connections between valve & actuator...lots will argue that it's enough to set mechanical stoppers in actuator, however these are not designed to take this sort of shock...it would not be stopping only actuator stroke (fast) but would also be taking high loads caused by inertia (valve disk can be quite a heavy mass traveling at high speeds). The actuator manufacturer needs to add a hydraulic damper to come into action during the last few degrees of closure (slowing the speed of stroke to a "rasonable" speed before it makes contact with travel stops, but still respecting stroke time requirements).

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