POWER March 2013 - 21

The choked pressure drop, for valves installed without inlet/
= FL x (P1
2
- FFPv), where FL
); PC
factor provided by valve manufacturer; P1
/PC
= liquid pressure recovery
= upstream pressure
= critical pressure of liquid,
outlet fittings, can be predicted by the following equation:
∆Pchoked
at valve inlet, psia or kPa; FF = liquid critical pressure ratio
factor = 0.96 − 0.28SQRT(PV
psia or kPa; and PV = vapor pressure of the liquid at flowing
temperature, psia or kPa.
The choked pressure drop corresponds to choked flow in the
valve created by the formation of gas bubbles when the fluid
pressure drops below the vapor pressure at the valve vena contracta.
The formation of gas bubbles at the valve vena contracta
depends on the downstream pressure (P2
could be in cavitating service or flashing service.
), meaning the valve
Consider Effects of Cavitation
Cavitation occurs in the valve only in single-phase liquid service
across the valve. As the liquid flows through the control
valve, the pressure falls from the inlet pressure until a point
is reached when the local fluid pressure falls below the vapor
pressure. At this point, vapor bubbles are formed. The potential
for cavitation damage occurs when the downstream pressure
(P2
) again rises above the vapor pressure and the vapor
bubbles collapse.
As mentioned previously, if the actual pressure drop is
higher than the choked pressure drop, the choked pressure
drop is the allowable pressure drop for control valve sizing.
However, at these conditions, fully developed cavitation can
be expected with its high potential for damage to valve internals
and downstream piping.
If the actual pressure is less than the choked pressure, the
actual pressure is the allowable pressure for control valve
sizing. But, to establish the damage potential from cavitation,
this value of actual pressure drop must be compared
to the ∆P associated with the cavitation index provided by
the valve manufacturer. Instead of the cavitation index, some
valve manufacturers use the cavitation coefficient, calculated
as KC
= ∆P x (P1
- Pv). The cavitation coefficient KC
assumes
that a valve may function without damaging cavitation at
any pressure less than the pressure drop calculated using the
coefficient.
Another commonly used cavitation index () is defined by
the Instrument Society of America (ISA) in publication ISARP75.23-1995,
where  = (P1
- Pv)/(P1
- P2
).
The valve manufacturer can provide the minimum recommended
value for sigma at various conditions, including
incipient cavitation, onset of damaging cavitation, or manufacturer's
recommended value. These values may need to be
adjusted for pressure scale effect (PSE), size scale effect
(SSE), and pipe reducer effect (defined in ISA-RP75.23-1995)
in case the reference conditions used for establishing  differ
from the service conditions. The adjusted value of  under
service conditions may be higher than the manufacturer's
recommended value (after adjustments for PSE, SSE, and pipe
reducer effect).
Based on the above considerations, cavitation in control
valves can be mitigated by two methods:
■ Modify system operating conditions so that either valve outlet
pressure remains below the vapor pressure, thus creating only
flashing conditions but no cavitation, or minimize valve pressure
drop so that  exceeds the valve manufacturer's minimum
recommendation.
March 2013 | POWER
www.powermag.com
■ Use multistage trims or anticavitation trim in the control
valve. This type of trim divides the overall pressure drop into
several stages, thus preventing the pressure at the vena contracta
of any individual stage from falling below the vapor
pressure. Some flashing service damage can be minimized by
use of hardened trim material or upgraded metallurgy for valve
body and use of target flanges in the downstream piping.
Finally, if the downstream pressure is lower than the vapor
pressure at the flowing temperature of the fluid, the fluid will
flash, resulting in a vapor-liquid mixture. This mixture moving
at high velocities often causes erosion in the valve internals and
downstream piping. Some flashing damage can be minimized by
use of hardened trim material or upgraded valve body metallurgy
and use of target flanges in the downstream piping
Additional Design Considerations
Designing heater drain piping and the associated control valve is
complex and requires careful evaluation to ensure that the heater
drains function properly and are capable of passing the required
flow over the intended range of operation. To aid the engineer
responsible for checking the adequacy of the design, use the
checklist provided in the sidebar " Design Review Checklist. " By
following this procedure you can be assured of producing a robust
design that will operate under all expected plant operating
conditions for many years. ■
-S. Zaheer Akhtar, PE is the assistant chief & technical advisor
to the PGESCo manager of engineering, on assignment from
Bechtel Power Corp. to PGESCo, Cairo, Egypt.
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21
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