Page 54 - Valve Selection Handbook

Fundamentals 41

4000 ft/s), and the static pressure increases by 13.5 bar for each 1 m/s, or
2
about 60 lb/in for each 1 ft/s instantaneous velocity change.
If the valve does not close instantaneously but within the time of a
pressure-wave round trip of 2L/a, where L is the length of the pipeline,
the first returning pressure wave cannot cancel the last outgoing pressure
wave, and the pressure rise is the same as if the valve were closed instan-
taneously. This speed of closure is said to be rapid.
If the valve takes longer to close than 2L/a, the returning pressure
waves cancel a portion of the outgoing waves so that the maximum pres-
sure rise is reduced. This speed of closure is said to be slow.
To minimize the formation of unduly high surge pressures from open-
ing or closing valves, stop valves should be operated slowly and in a
manner that produces a uniform rate of change of the flow velocity.
Check valves, however, are operated by the flowing fluid, and their speed
of closure is a function of the valve design and the deceleration charac-
teristic of the retarding fluid column.
If the surge pressure is due to a pump stopping, the calculation of the
surge pressure must take into, account the pump characteristic and the
rate of change of the pump speed after the power supply has been cut off.
If the distance between the check valve and the point of pressure wave
reflection is long, and the elevation and the pressure at this point are low,
the system tolerates a slow-closing check valve. On the other hand, if the
distance between the check valve and the point of reflection is short, and
the pressure at this point is high, the flow reverses almost instantaneously
and the check valve must be able to close extremely fast. Such nearly
instantaneous reverse flow occurs, for example, in multipump installa-
tions in which one pump fails suddenly. Guidelines on the selection of
check valves for speed of closure are given in Chapter 4.
Calculation of the fluid pressure and velocity as a function of time and
location along a pipe can be accomplished in several ways. For simple
cases, graphical and algebraic methods can be used. However, the ready
availability of digital computers has made the use of numerical methods
convenient and allows solutions to any desired accuracy to be obtained.
See Reference 25 for a description of this calculating method.
In some cases it may be impossible or impractical to reduce the effects
of waterhammer by adjusting the valve characteristic. Consideration
should then be given to changing the characteristic of the piping system.
One of the most common ways of achieving this is to incorporate one or
more surge protection devices at strategic locations in the piping system.
Such devices may consist of a standpipe containing gas in direct contact