Page 142 - Valve Selection Handbook

Manual Valves 129

Figure 3-87. Opening Torque
Characteristics of Butterfly Valves in
the Actual Pumping Installation.
(Reprinted from Schiffand Hafen, 40
Courtesy of VAG-Armaturen GmbH.)
Curve a: Symmetrical Disk, Flow
From Either Direction
Curve b: Off-Set Disk, Flow Towards
Shaft
Curve c: Off-Set Disk, Flow Towards
D138isk

pumping installation. The decreasing pressure drop, in turn, decreases the
corresponding operating torque as the valve opens. The maximum operat-
ing torque shifts thereby from the region of the fully open valve to the
region of the partially open valve, as shown in Figure 3-87. This torque is
lowest if the disc is of the offset type and the direction of flow is toward
the disc. However, this advantage is obtained at the penalty of a somewhat
higher pressure drop. If the valve is large and the flow velocity high, this
torque characteristic can be exploited to lower the cost of the operator.

Standards Pertaining to Butterfly Valves

Appendix C provides a list of U.S. and British standards pertaining to
butterfly valves.

Applications

Duty:
Stopping and starting flow
Controlling flow
Service:
Gases
Liquids
Slurries
Powder
Granules
Sanitary handling of Pharmaceuticals and food stuffs
Vacuum