Page 51 - Valve Selection Handbook

38 Valve Selection Handbook

The impinging of the opposing liquid particles of the collapsing vapor
bubble produces locally high but short-lived pressures. If the implosions
occur at or near the boundaries of the valve body or the pipe wall, the
pressure intensities can match the tensile strength of these parts. The
rapid stress reversals on the surface and the pressure shocks in the pores
of the boundary surface lead finally to local fatigue failures that cause the
boundary surface to roughen until, eventually, quite large cavities form.
The cavitation performance of a valve is typical for a particular valve
type, and it is customarily defined by a cavitation index, which indicates
the degree of cavitation or the tendency of the valve to cavitate. This para-
meter is presented in the literature in various forms. The following is a
21 22
convenient index used by the United States Bureau of Reclamation. -

where
C = cavitation index
P v = vapor pressure relative to atmospheric pressure (negative)
P d = pressure in pipe 12 pipe diameters downstream of the valve seat
P u = pressure in pipe 3 pipe diameters upstream of the valve seat

Figure 2-11 displays the incipient cavitation characteristics of butter-
fly, gate, globe, and ball valves, based on water as the flow medium. The
characteristics have been compiled by the Sydney Metropolitan Water
Sewerage and Drainage Board, and are based on laboratory observations
and published data. 23 Because temperature entrained air, impurities,
model tolerances, and the observer's judgment influence the test results,
the graphs can serve only as a guide.
The development of cavitation can be minimized by letting the pres-
sure drop occur in stages. The injection of compressed air immediately
downstream of the valve minimizes the formation of vapor bubbles by
raising the ambient pressure. On the debit side, the entrained air will
interfere with the reading of any downstream instrumentation.
A sudden enlargement of the flow passage just downstream of the
valve seat can protect the boundaries of valve body and pipe from cavita-
tion damage. A chamber with the diameter of 1.5 times the pipe diameter
and a length of 8 times the pipe diameter including the exit taper has
24
proved satisfactory for needle valves used in waterworks.