Page 43 - Valve Selection Handbook

30 Valve Selection Handbook

Figure 2-6. Approximate Effect of Partial Opening of Diaphragm Valves on
Resistance Coefficient. (Courtesy of Engineering Sciences Data Unit. Reproduced from
Item No. 69022, Figure 75.)

Figure 2-7. Approximate Effect of Partial Opening of Butterfly Valves on Resistance
Coefficient. (Courtesy of Engineering Sciences Data Unit. Reproduced from Item No.
69022, Figure 12.)

value and scatter of data, as obtained from both published and unpublished
reports and from results obtained from various manufacturers.
In the case of partially open valves and valves with reduced seat area, as
in valves with a converging/diverging flow passage, the energy of the flow
stream at the vena contracta converts partially back into static energy.
Figure 2-8 shows the influence of the pressure recovery on the resis-
tance coefficient of fully open venturi-type gate valves in which the gap
between the seats is bridged by an eyepiece.