Environmental Engineering Reference
In-Depth Information
expected flow conditions in order to identify the maximum flow torque condition.
There are four primary sources of torques for quarter-turn valves:
1. Seating torque . Torque is created by the disk moving into or out of the
seat (sealing surface).
2. Bearing friction torque . Torque is required to overcome the frictional
resistance that forms between the valve shaft and its bearing surfaces in
the valve body.
3. Packing friction torque . Torque is required to overcome the friction resis-
tance that forms between the valve shaft and the packing material used
to prevent leaks between the shaft and valve body.
4. Hydrodynamic (flow) torque . Torque is created by the flow momentum
and differential pressure across the disk.
These torque values are usually determined experimentally and should be
available from the valve manufacturer. All four torques should be evaluated to
determine the maximum torque for a given valve. Operating torque is normally
greater when the valve is being opened because the hydrodynamic, bearing,
and packing torques all oppose the direction of valve motion. During valve
closure, the bearing and packing torques act in the direction opposite to that of
the hydrodynamic torque. The seating or unseating torque can also be significant.
[I think this works]. It is the responsibility of the valve manufacturer to provide
the flow and torque characteristics and limits for their valve. It is the responsibility
of the system engineers and operators to see that the valves are operated within
these limits.
5.1.4 Restricted Operating Range of Valve Openings
Many conventional control valves warrant a restricted operating range of valve
openings, as they cannot safely and/or accurately regulate flow near the closed
and full-open positions. Near the closed position, two potential problems include
seat (sealing surface) damage due to high velocities and inability to accurately
set the valve opening when the valve operator experiences hysteretic effects in
its valve positioning. Near full open, some valves lose control, meaning there
is no change in flow rate with valve position change. For globe-style valves,
this occurs when the stroke is too long. The full-open characteristics of butterfly
valves are influenced by the shape of the disc and changes in the flow pattern
around the disc at large openings. For some disc shapes, the flow can actually
decrease as the valve disc position moves between 90 and 100 percent open.
This problem is magnified when the valve is installed in a long system where
the valve loss is small compared to the system friction loss. Some quarter-turn
valves experience a torque-reversal near the full-open position, which can cause
the disk to flutter and which increases the potential for accelerated wear.
In summary, when analyzing a flow control valve, six criteria should be con-
sidered:
 
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