Page 103 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.53

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 oper-
ating 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 adequate safety factor should be
considered to ensure 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. Failsafe operation. With the automatic failsafe operation, the energy necessary 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 tem-
perature 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 hour 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 equiva-
lent 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 spe-
cial consideration. The sudden physical shock associated with fast operating speed
combined with fast cycling rates can damage valve and actuator parts. Pneumatic actu-
ators 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 suitabil-
ity 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 but not bubbletight.
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.

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