Reference Manual

2−8
Table 2-2. Recommended Seat Load Per Leak Class for
Control Valves
Class I As required by customer
specification, no factory leak test
required
Class II 20 pounds per lineal inch of port
circumference
Class III 40 pounds per lineal inch of port
circumference
Class IV Standard (Lower) Seat only—40
pounds per lineal inch of port
circumference (up through a
4–3/8 inch diameter port)
Standard (Lower) Seat only—80
pounds per lineal inch of port
circumference (larger than 4–3/8
inch diameter port)
Class V Metal Seat—determine pounds
per lineal inch of port
circumference from figure 2-9
C. Packing Friction
Packing friction is determined by stem size,
packing type, and the amount of compressive load
placed on the packing by the process or the
bolting. Packing friction is not 100% repeatable in
its friction characteristics. Newer live loaded
packing designs can have significant friction forces
especially if graphite packing is used. Table 2-3
lists typical packing friction values.
D. Additional Forces
Additional forces to consider may include bellows
stiffness, unusual frictional forces resulting from
seals or special seating forces for soft metal seals.
The manufacturer should either supply this
information or take it into account when sizing an
actuator.
Actuator Force Calculations
Pneumatic spring-and-diaphragm actuators
provide a net force with the additional air pressure
after compressing the spring in air-to-close, or with
the net pre-compression of the spring in
air-to-open. This may be calculated in pounds per
square inch of pressure differential.
For example, suppose 275 pound-force (lbf) is
required to close the valve as calculated per the
process described earlier. An air-to-open actuator
with 100 square inches of diaphragm area and a
bench set of 6 to 15 psig is one available option.
The expected operating range is 3 to 15 psig. The
Figure 2-8. Recommended Seat Load
A2222−4/IL
pre-compression can be calculated as the
difference between the lower end of the bench set
(6 psig) and the beginning of the operating range
(3 psig). This 3 psig is used to overcome the
pre-compression so the net pre-compression force
must be:
3 psig X 100 sq. in. = 300 lbf.
This exceeds the force required and is an
adequate selection.
Piston actuators with springs are sized in the same
manner. The thrust from piston actuators without
springs can be calculated as:
Piston area X minimum supply pressure =
minimum available thrust
(maintain compatibility of units)
In some circumstances an actuator could supply
too much force and cause the stem to buckle, to
bend sufficiently to cause a leak, or to damage
valve internals.
The manufacturer normally takes responsibility for
actuator sizing and should have methods
documented to check for maximum stem loads.
Manufacturers also publish data on actuator
thrusts, effective diaphragm areas, and spring
data.