Bray 31U - Manual (Page 3)

Resilient Seated Butterfly Valves – Torques
All information herein is proprietary and confidential and may not be copied or reproduced without the expressed written consent of BRAY INTERNATIONAL, Inc.
The technical data herein is for general information only. Product suitability should be based solely upon customer’s detailed knowledge and experience with their application.
Introduction : 3
Torques
INTRODUCTION
There are a number of torques which butterfly valves may
experience such as:
T
su
- Seating and Unseating Torque
T
d
- Dynamic Torque Resulting from fluid flow
T
bf
– Bearing Friction Torque
T
ss
– Stem Seal Friction Torque
T
e
– Eccentricity Torque resulting from
disc offset from centerline of stem (either single,
double or triple offset)
T
h
– Hydrostatic Torque
Factors which influence the butterfly valve torque values
shown above are:
Type of Seat and Seat Material
Interference of Seat I.D. and Disc O.D.
Shaft Diameter
Valve Diameter
Bearing Coefficient of Friction
Angle of Opening
Shut-off Pressure
Fluid Velocity
Disc Shape and Configuration
Piping System and Location/Orientation of Valve in Pipe Line
System Head Characteristics
Physical Size of Disc/Shaft Obstructing Flow
Disc Edge Finish
With respect to Butterfly Valves, the two major conditions for
determining total valve operating torque (T
T
) exists as follows:
CASE I
(Angle = 0° , Disc in Closed Position)
T
T
= T
h
+ T
bf
+ T
ss
+ T
su
Analyzed
Total Torque for Case I using a symmetrical disc butterfly valve is
the sum of hydrostatic torque, bearing friction torque, stem seal,
friction torque, and seating/unseating torque.
A. Hydrostatic Torque (T
h
)
We will ignore discussion of the hydrostatic torque values as they
are generally insignificant compared to the seating/unseating,
bearing friction and stem seal torque values (the safety factor
applied to seating/unseating, stem seal friction and bearing
friction torque values more than compensates for the hydrostatic
torque which is usually less than 2% of these total torques).
B. Bearing Friction Torque (T
bf
)
Bearing friction torque occurs because pressure forces against
the disc are transmitted to the stem. As the stem is forced against
the bearing supports, bearing friction torque is created between
the stem material and the support material as the stem is turned.
Bearing friction torques are normally included in the seating/
unseating torque values.
Bearing friction torques can be determined by using
the following equation:
T
bf
= .785 C
f
D
v
2 (d/2) ∆P
Where:
T
bf
= Bearing Friction Torque
C
f
= Coefficient of Friction (approximately .25 for non-
corroded stem to cast iron body) (dimensionless).
D
v
= Valve Diameter (Inches)
d = Diameter of Shaft (Inches)
∆P = Pressure Differential (psi)
C. Stem Seal Friction Torque (T
ss
)
For all practical purposes stem seal friction torque values are
insignificant when compared to seating/unseating and bearing
friction torques. Stem seal friction torques are normally included
in the seating/unseating torque values.