PIPE SYSTEM HYDRAULICS - Environmentally Conscious Materials Handling - page 247

1. The valve should not produce excessive pressure drop when full open.
2. The valve should control over about 50 percent of its movement (i.e.,
closing the valve half way should reduce the flow rate by 10%).
3. The cavitation intensity should be limited to the appropriate level.
4. The valve should be operated so that the resulting transient pressures do
not exceed the safe limits of the system.
5. The valve operator and shaft for quarter-turn valves should be compatible
with the valve maximum torque requirement.
6. Restrictions in the range of valve-opening operation should be identi-
fied. Some valves should not be operated near the closed or fully open
positions.
5.2 Check Valves
Check valves are used to prevent reverse flows resulting from pump shutdown,
for example, or pipe rupture. Check valves are designed to pass flow efficiently
in the forward-flow direction and to close quickly when the flow reverses. Most
check valves are swing-type, spring-actuated, or a combination of both. When
selecting a check valve, the characteristics of the valve should be compatible
with the characteristics or requirements of the system. Selecting the wrong type
or size of check valve can result in poor performance, severe transients, and
frequent repairs (Kalsi and Tullis 1993). For a description of the characteristics
of common types of check valves, see Kalsi and Tullis (1993) and Tullis and
Tullis (2005). Five characteristics of check valves should be considered in the
selection process:
1. Closure speed of check valves relative to the rate of flow reversal of the
system
2. Stability of the disk and its sensitivity to local turbulence effects
3. The flow rate required to fully open and firmly back seat the disc
4. The head loss at the maximum and typical flow rates
5. Sealing effectiveness and ease of maintenance
It is a mistake to oversize a swing check valve located just downstream from
a disturbance such as a pump, elbow, or control valve. The disk will not firmly
back seat, and it will be subjected to severe motion and accelerate wear. All check
valves have components that experience wear. If the valves are not inspected and
maintained periodically, these valve components may eventually fail, rendering
the check valve ineffective. Operating check valves with the disk firmly seated
in the full-open position during normal operation will reduce the amount and rate
of component wear and will increase the useful life of the check valve.
It is also important to consider the protection against harmful transient pressure
wave generation by check valve operation. Transient pressures are maximized