The actuator output force ensures that it matches the force required to drive the valve.
Selection of valve actuators – angular stroke
The actuator output torque ensures that it matches the torque required to drive the valve.
Suitable for selection of pneumatic, electric or electro-hydraulic actuators.
The forces required to operate the globe valve include:
A = the force required to overcome the static unbalanced force of the spool
B = force required to provide seat closing force
C = force required to overcome packing friction
D = additional force required for some specific application or structure
Total force required = A + B + C + D
Unbalanced force (A)
The unbalanced force is the force caused by the fluid pressure when the valve is closed, and can be expressed as:
Unbalanced force = net differential pressure × net unbalanced area
It is common practice to use the maximum upstream gauge pressure as the net differential pressure unless the process design always ensures that there is a back pressure at the maximum inlet pressure. Net unbalanced area is the port area of an unbalanced single-seat valve with upward flow. Depending on the form of the stem, the unbalanced area may need to account for the area of the stem. For balanced valves, there is still a small unbalanced area. This data can be obtained from the valve manufacturer.
Force required for valve seat to close (B)
Seat closing force, usually expressed in pounds of force per linear inch of port circumference, is determined by the closing rating requirement. Use the following guidelines to determine the seat closing force required to meet factory quality testing for ANSI/FCI 70-2 and IEC 60534-4 leakage classes II to VI. Because the severity of working conditions is different, do not take these leakage levels and corresponding leakage amounts as indicators of field performance. To prolong seat life and closing ability, a higher than recommended seat closing force can be used. If tight shut-off is not a primary consideration, a lower leak rating can be used.
Packing friction (C)
The size of the packing friction is determined by the size of the valve stem, the form of the packing, and the compressive load acting on the packing by the medium or bolt. In terms of friction characteristics, packing friction is not 100% repeatable. Live loaded packing styles have high friction, especially when graphite packing is used.
Additional force (D)
Additional forces may be required to actuate the valve, such as bellows stiffness, abnormal frictional forces due to seals, or special sealing forces required by soft metal seals. Valve manufacturers should provide this information or consider these factors when selecting actuators
Valve actuator selection principle - Zorgon (Zhejiang) Automation Technology Co.,Ltd. (cnzorgon.com)
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