Design of the Valve Actuator - The design engineer has to consider the operating conditions such as;
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The atmosphere and potential corrosion. If the Actuator is being utilised on an Offshore Platform then particular care must be taken in selecting the actuator body materials and internal mechanism. Particular emphasis on this should be taken on the tubing associated with pneumatic actuators or any that ‘breathe’ on the return stroke, potentially sucking in salt or corrosive air into the internal mechanism. Under this scenario a technique called closed loop breathing is used (an excellent schematic of a typical system can be found here). Selection of any accessories such as quick exhaust, solenoid valves and limit switches etc must also consider the conditions. In the Offshore Oil and Gas Industry 316SS Actuators are sometimes selected.
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Torque requirements must be carefully considered as too little power can mean that any stiction in the valve means that the valve may stick in the cycle. Too much power may actually cause the valve mechanism to shear.
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Pneumatic Valve actuator 0 – 100% cycle may be various pressures, control valve actuators are generally 3 – 15 psi (20 – 100kpa). However they may sometimes be set differently to this.
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On – Off Valve Actuators may be set to other pressures, commonly 0 to 100 psi, this is to keep the size of actuator as compact as possible.
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Hydraulic Valve Actuators utilise much higher pressures, especially on very large ball valves, this design is used to obtain the higher torques required and to keep the actuator and valve footprint as compact as possible.
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Smart Positioners may be used both on Control and On – Off Actuator Valve combinations, these are a superb maintenance tool in that the Valve/Actuator signature at new conditions can be taken. Any changes outside parameters then mean that any maintenance is condition based.