The following factors require consideration in the selection of an actuator:
1. Valve type and size. The valve-operating torque results from the inherent size and characteristics of the valve itself and the type of seat. The amount of torque necessary to overcome static imbalance must be obtained from the manufacturer.
2. Pressure drop. The operating torque increases with an increase in pressure drop across the valve. A valve operating at full-rated pressure will require significantly more operating torque than one operating at a low-pressure drop. Depending on the source of pressure, it is probable that the pressure differential will vary throughout the valve’s entire stroke. This condition is important if the actuator torque output must be carefully matched with that of the valve.
3. Service-operating conditions. Will the valve be required to be only opened or closed or will it also be required for throttling flow? Actuators for ON / OFF service will be selected only on breakaway torque. For quarter-turn valves requiring throttling, calculating the torque is more complicated because additional torque is required to counterbalance the momentum of the flowing fluid. Unbalanced forces generate ‘‘hydrodynamic torque.’’ The actuator torque output must be well above the operating torque to achieve smooth operation.
4. Seat material. Most valves have a metal closure member sealing on a soft seat made of elastomers. Metal-seated valves may require as much as 50 percent more seat material as needed for soft seat valves.
5. Fluid being transported. Since air and gas do not provide any lubrication, their operating torque requirements add to the frictional forces. Water and other media may provide excellent lubrication. Liquids carrying solids clog clearances between stem and bearings. The fluid may also corrode internal parts, so that in time the torque valve may rise considerably, up to twice that when new. An ade- quate safety factor should be considered to assure reliable and continued operation.
6. Bidirectional seating. If operating conditions require the reversal of flow, additional torque may be required for seating.
7. Fire safety. The valve may require secondary metal-to-metal seating if the primary seat is destroyed by fire. This will require more operating torque.
8. Fail-safe operation. With the automatic fail-safe operation, the energy nec- essary to close or open the valve requires a larger size actuator than one without a failsafe requirement.
9. Temperature of fluid. Torque requirements are lowest at room temperature. High temperature and cryogenic bearings require higher operating torque. Fluid temperatures above 300 F may require a special operating and mounting assembly, often a stem extension. Ambient temperatures must also be considered, for example, actuators located outdoors require special consideration.
10. Cycling rate. Pneumatic and hydraulic actuators cycling in excess of 30 cycles per h are considered to have high operating rates. The same is true for electric actuators cycling in excess of 10 percent of their duty cycle (operating for 1 cycle and resting for a time equivalent equal to 9 cycles). An extended duty motor should be obtained for this condition.
11. Cycle speed. Fast cycle speeds of less than one-half standard cycle times require special consideration. The sudden physical shock associated with fast op- erating speed combined with fast cycling rates can damage valve and actuator parts. Pneumatic actuators may need quick exhaust valves, special solenoids, and larger actuators. Higher speeds are accomplished using different gearing devices, which may increase torque output, or an electronic speed control, which will not affect torque output.
12. Stem orientation. Orientation of the valve stem in a position other than vertical will require mounting in a manner that may cause stem seal leakage or galling due to side thrusts induced by an overhung load on the actuator. The use of heavy-duty couplings and mounting brackets will minimize these problems.
FIRESAFE VALVES
By nature of their service, some valves require a firesafe designation. There is no single generally recognized definition of firesafe or a code that can be used to determine suitability or acceptance. A simplified definition is that a valve must not melt in a fire or leak after a fire and that the seat must close adequately.
The standard used most often for the CPI is the API 607 rating. For water fire- service lines, FM is the most conservative, although a listing with UL may be acceptable depending on the specific insurance carrier used. Specific companies often have ratings that must be used when projects are designed for them.
Firesafe valves require testing to meet minimum recommended performance standards when operating in a firesafe environment. These recommendations are:
1. Valve type and size. The valve-operating torque results from the inherent size and characteristics of the valve itself and the type of seat. The amount of torque necessary to overcome static imbalance must be obtained from the manufacturer.
2. Pressure drop. The operating torque increases with an increase in pressure drop across the valve. A valve operating at full-rated pressure will require significantly more operating torque than one operating at a low-pressure drop. Depending on the source of pressure, it is probable that the pressure differential will vary throughout the valve’s entire stroke. This condition is important if the actuator torque output must be carefully matched with that of the valve.
3. Service-operating conditions. Will the valve be required to be only opened or closed or will it also be required for throttling flow? Actuators for ON / OFF service will be selected only on breakaway torque. For quarter-turn valves requiring throttling, calculating the torque is more complicated because additional torque is required to counterbalance the momentum of the flowing fluid. Unbalanced forces generate ‘‘hydrodynamic torque.’’ The actuator torque output must be well above the operating torque to achieve smooth operation.
4. Seat material. Most valves have a metal closure member sealing on a soft seat made of elastomers. Metal-seated valves may require as much as 50 percent more seat material as needed for soft seat valves.
5. Fluid being transported. Since air and gas do not provide any lubrication, their operating torque requirements add to the frictional forces. Water and other media may provide excellent lubrication. Liquids carrying solids clog clearances between stem and bearings. The fluid may also corrode internal parts, so that in time the torque valve may rise considerably, up to twice that when new. An ade- quate safety factor should be considered to assure reliable and continued operation.
6. Bidirectional seating. If operating conditions require the reversal of flow, additional torque may be required for seating.
7. Fire safety. The valve may require secondary metal-to-metal seating if the primary seat is destroyed by fire. This will require more operating torque.
8. Fail-safe operation. With the automatic fail-safe operation, the energy nec- essary to close or open the valve requires a larger size actuator than one without a failsafe requirement.
9. Temperature of fluid. Torque requirements are lowest at room temperature. High temperature and cryogenic bearings require higher operating torque. Fluid temperatures above 300 F may require a special operating and mounting assembly, often a stem extension. Ambient temperatures must also be considered, for example, actuators located outdoors require special consideration.
10. Cycling rate. Pneumatic and hydraulic actuators cycling in excess of 30 cycles per h are considered to have high operating rates. The same is true for electric actuators cycling in excess of 10 percent of their duty cycle (operating for 1 cycle and resting for a time equivalent equal to 9 cycles). An extended duty motor should be obtained for this condition.
11. Cycle speed. Fast cycle speeds of less than one-half standard cycle times require special consideration. The sudden physical shock associated with fast op- erating speed combined with fast cycling rates can damage valve and actuator parts. Pneumatic actuators may need quick exhaust valves, special solenoids, and larger actuators. Higher speeds are accomplished using different gearing devices, which may increase torque output, or an electronic speed control, which will not affect torque output.
12. Stem orientation. Orientation of the valve stem in a position other than vertical will require mounting in a manner that may cause stem seal leakage or galling due to side thrusts induced by an overhung load on the actuator. The use of heavy-duty couplings and mounting brackets will minimize these problems.
FIRESAFE VALVES
By nature of their service, some valves require a firesafe designation. There is no single generally recognized definition of firesafe or a code that can be used to determine suitability or acceptance. A simplified definition is that a valve must not melt in a fire or leak after a fire and that the seat must close adequately.
The standard used most often for the CPI is the API 607 rating. For water fire- service lines, FM is the most conservative, although a listing with UL may be acceptable depending on the specific insurance carrier used. Specific companies often have ratings that must be used when projects are designed for them.
Firesafe valves require testing to meet minimum recommended performance standards when operating in a firesafe environment. These recommendations are:
1. Minimum internal leakage. A valve must offer acceptable seating prior to and after exposure to high temperatures without depending on supplementary pressure from spring-loaded or other devices and without depending on a critical seal.
2. Minimal external leakage. The valve body design should minimize external leakage by using fire-resistant stem seals and avoiding large gasketed body joints.
3. Continued operability. A valve must be operable despite fire damage. The body and actuator must resist warpage and damage from high temperatures.
VALVE RATINGS
There are a number of designations used to indicate the pressure ratings of valves. Valves are pressure rated by their ability to withstand pressure within a range of temperatures. Standard pressure ratings have been established to match ANSI ratings of flanges and fittings and are designated by class, conforming to ANSI B 16.34 ratings. Two types of designation are WSP and WOG. WSP, or working steam pressure, rates the ability to handle steam at the specified working pressure. WOG, or water, oil, and gas, rates the ability to handle cold water, oil, and gases at the assigned working pressure. When the two ratings are given, WSP is called the primary rating. When only one rating is given, the valve is not generally used for the service not mentioned. The rating 150 lb refers to the working pressure in psig for which the valve is rated. If a valve is primarily used for water service, a common designation is WWP, or water working pressure. This designation rates the ability to handle cold water. The valve class designates the working pressure of a valve. A class 300 rating indicates a valve with a working pressure of 300 psig.
Cold temperatures mean ambient temperatures from 32 to 90 F For high temperatures, the valve pressure shall be derated. For high pressures, the temperature rating shall be aerated. The temperature limitation on most metallic valves is generally based on the capabilities of the seat and interior trim materials.