Reference Manual

2−9
Table 2-3. Typical Packing Friction Values (Lb)
Stem Size
(Inches)
ANSI
Class
PTFE Packing
Graphite
Ribbon/
Filament
Single Double
5/16 All 20 30 – – –
3/8 125
150
250
300
38 56 – – –
125
– – –
190
600
900
1500
250
320
380
1/2 125
150
250
300
50 75 – – –
180
– – –
230
600
900
1500
2500
320
410
500
590
5/8 125
150
250
300
600
63 95 – – –
218
– – –
290
400
3/4 125
150
250
300
75 112.5 – – –
350
– – –
440
600
900
1500
2500
660
880
1100
1320
1 300
600
900
1500
2500
100 150 610
850
1060
1300
1540
1–1/4 300
600
900
1500
2500
120 180 800
1100
1400
1700
2040
2 300
600
900
1500
2500
200 300 1225
1725
2250
2750
3245
Values shown are frictional forces typically encountered when using standard
packing flange bolt-torquing procedures.
Actuator Sizing for Rotary Valves
In selecting the most economical actuator for a
rotary valve, the determining factors are the torque
required to open and close the valve and the
torque output of the actuator.
This method assumes the valve has been properly
sized for the application and the application does
not exceed pressure limitations for the valve.
Torque Equations
Rotary valve torque equals the sum of a number of
torque components. To avoid confusion, a number
of these have been combined, and a number of
calculations have been performed in advance.
Thus, the torque required for each valve type can
be represented with two simple and practical
equations.
Breakout Torque
T
B
=A(DP
shutoff
)+B
Dynamic Torque
T
D
=C(DP
eff
)
Specific A, B, and C factors, for example, rotary
valve designs are included in tables 2-4 and 2-5.
Maximum Rotation
Maximum rotation is defined as the angle of valve
disk or ball in the fully open position.
Normally, maximum rotation is 90 degrees. The
ball or disk rotates 90 degrees from the closed
position to the wide-open position.
Some of the pneumatic spring-return piston and
pneumatic spring-and-diaphragm actuators are
limited to 60 or 75 degrees rotation.
For pneumatic spring-and-diaphragm actuators,
limiting maximum rotation allows for higher initial
spring compression, resulting in more actuator
breakout torque. Additionally, the effective length
of each actuator lever changes with valve rotation.
Published torque values, particularly for pneumatic
piston actuators, reflect this changing lever length.
The Selection Process
In choosing an actuator type, the fundamental
requirement is to know your application. Control
signal, operating mode, power source available,
thrust/torque required, and fail-safe position can
make many decisions for you. Keep in mind
simplicity, maintainability and lifetime costs.
Safety is another consideration that must never be
overlooked. Enclosed linkages and controlled
compression springs available in some designs
are important for safety reasons. Table 2-6 lists
the pros and cons of the various actuator styles.