Page 140 - Valve Selection Handbook

Manual Valves 127

T b = torque to overcome bearing friction
T s = torque required to seat or unseat the disc

The hydrodynamic torque varies with the valve opening position and
the pressure drop across the valve. In the case of symmetrical discs, this
torque is identical for either direction of flow, and its direction of action
is against the opening motion throughout. If the disc is offset, as in the
disc shown in Figure 3-84, the hydrodynamic torque differs for each
direction of flow, and the lowest torque develops when the flow is toward
the disc. With flow toward the shaft, the torque acts against the opening
motion throughout. However, with flow toward the disc, the torque acts
only initially against the opening motion and then, with further valve
opening, changes its directions.

Figure 3-84. Offset Disc Configuration in Butterfly Valves.

The bearing, seating, and unseating torques, on the other hand, always
act against the operating motion. The magnitude of the bearing torque
corresponds to the resultant hydrodynamic force on the disc, while the
magnitude of the seating and unseating torques is independent of flow.
In the case of interference-seated butterfly valves of sizes up to DN
400 (NFS 16), the seating and unseating torques normally dominate, pro-
vided the flow velocity is not too high. It is sufficient in this case to size
the operating gear for these torques. However, the magnitude of the seat-
ing and unseating torques is influenced by the type of fluid handled and
the operating frequency. For example, if the fluid has good lubricity and
the valve is operated frequently, the seating and unseating torques are