Environmental Engineering Reference
In-Depth Information
light, crackling noise. High levels of cavitation can generate tremendous levels
of noise and vibration, and can cause damage to the valve, fitting, or pipe wall
through material removal. An example of cavitation damage in a globe valve is
shown in Figure 9.4.
Different valves have different cavitation characteristics. Different applications
also warrant different levels of cavitation restrictions. A control valve that experi-
ences continual use should be operated below the cavitation damage threshold. A
pressure-relief valve, whose purpose is to keep line pressures below a maximum
level for process or safety reasons, is typically required to operate at the max-
imum discharge possible. For this type of application, pressure relief typically
supersedes valve cavitation damage concerns, and/or the valve operation events
are so infrequent that no appreciable damage will occur over the life of the valve.
Additional information on valve cavitation is presented by Tullis (1989, 1993).
5.1.2 Hydraulic Transients
Most pipe analyses correspond to steady-state flow conditions. When a valve
opening position is changed, a pump is started or shut down, or a pipe rupture
occurs, an unsteady pipe flow condition is generated in the pipe system. Sudden
flow decelerations or accelerations can produce traveling transient pressure waves
with magnitudes significantly higher or lower then the steady-state line pressures.
The duration of transient pressure waves are typically short-lived. The resulting
damage, however, can be severe if the pipe is not designed for the extreme
positive and negative pressures associated with transient events. As an example,
if the flow velocity in a steel pipe were decreased instantly (or over a very
short period of time) by 1 m/s, transient pressure head increase of approximately
100m would result. Every pipe system should have at least a cursory transient
analysis performed to identify the possibility of serious transients and decide
whether a detailed analysis is necessary. If an analysis indicates that transients
are a problem, there are at least five methods of controlling transients:
1.
Increase the closing time of the control valve.
2.
Use a smaller valve to provide better control.
3.
Design special facilities for filling, flushing, and removing air from
pipelines.
4.
Increase the pressure class of the pipeline.
5.
Use pressure relief valves, surge tanks, air chambers, and so on.
For more details regarding hydraulic transients, see Tullis (1989, 1993) and
Wylie and Streeter (1993).
5.1.3 Torque
Selection of the correct operator and shaft size for a specific quarter-turn valve
and application requires knowing the maximum torque that the valve will experi-
ence during operation. This requires analyzing the system for the entire range of
Search WWH ::
Custom Search