σoperating is greater than σid. However,
size and pressure scaling effects must be
considered.
To apply the lab results to an actual
application, scale effects for the actual
valve size and pressure conditions need
to be applied to σmr. Methods of scaling
the sigma index for such variables have
been established in ISA-RP75.23.01.
The following equation for the scaled
valve coefficient, σv, is used to apply the
scaling effects for size (SSE) and pressure (PSE) to a reference or recommended coefficient represented by σR. σv= [σR (SSE) - 1] PSE + 1
Where: σR = The reference Sigma,
or in this case σmr
SSE = Size Scale Effect
(determined by the
manufacturer)
PSE = Pressure Scale Effect
(determined by the
manufacturer)
Guidelines for determining and
applying SSE and PSE scale effects can
be found in ISA-RP75.23.01. Using
data from the manufacturer, SSE is calculated to be 1. 29 and the PSE is calculated to be 1. 19 for this example. Substituting these numbers into the
equation above we have: σv = [ 2.59 ( 1. 29) - 1]
1. 19 + 1 = 3.69
This means that the σv value of 3.69
is the true coefficient of σmr to which
the σoperating should be compared.
Because the σoperating value is less than σv, the valve is likely to be damaged at
these conditions unless cavitation control trim or harder materials are used.
The type of valve used can make a
difference in the level of resistance to
cavitation achievable for a given process
(see Figure 6, which lists example sigma
values of various valve types and trims).
The σmr for a given valve configuration
is determined by the manufacturer
based on testing and experience. Meth-
ods other than vibration testing may be
necessary for some specialty multistage
valves to determine σ values for incipient cavitation or choked flow. Experience shows that a high degree of correlation between predicted sigma values
and actual performance values can be
developed.
ANTI-CAVITATION/
CONTROL MECHANISMS
Profiles were presented earlier (See Figures 1, 2 and 5) to illustrate the energy
transfer between velocity and pressure. A
variety of mechanisms that trigger this
energy conversion can be employed to
control or eliminate cavitation. Many
types of anti-cavitation trim solutions will
employ multiple mechanisms to achieve
the most effective results.
Cavitation Bubble Isolation – The simplest method of controlling mild cavitation is by isolating the cavitation
bubbles away from metal surfaces. The
most common method for doing this is
to use cages with opposing drilled
