POWER June 2022 - 35
OPERATIONS & MAINTENANCE
blown out through the valve without
problems. However, when the higher
temperature water passes through the
valve to the low-pressure side, it will
" flash " into steam and the flow will
" choke. " When flow chokes, the mass
flux (mass flowrate) through the valve
is limited. The abrupt change in mass
flux from sub-cooled water to hot twophase
flow creates a large velocity
change that generates a water hammer
shock wave and turbulence at the
valve (Figure 5).
At any point where the water pressure
drops below the vapor pressure, the water
will flash to steam. Flashing water
fills the pipe with vapor bubbles, which
can accumulate into a vapor pocket, leading
to a water hammer when the vapor
suddenly condenses.
Heater drains to the condenser are
susceptible to this CIWH Mechanism.
Leaky desuperheater spray valves and
steam trap failures are typical causes of
hot condensate flashing to steam.
To prevent Mechanism 4 transients:
■ Install a small bypass around shutoff
valves that create cold stagnant pipe
legs to get the water moving and prevent
a sudden temperature change
when the shutoff valve opens.
■ Slow the change in the water velocity
by slowly opening flow control valves.
■ Steam condensate drains piped to a
low-pressure condensate collection
header should enter through the top
of the header. This allows the flash
steam to disperse in the steam space
and not condense under the liquid surface
where vapor bubbles could form,
collapse, and create a water hammer.
In addition to condensate and steam
interaction events, there are other types
and causes of water hammer transients,
which are not discussed here. Water slug-
induced water hammer occurs when condensate
puddles in the pipeline become
slugs when steam isolation valves open
and the water becomes entrained in the
steam. Rapid pump or valve operation will
create transients. Water column separation
creates a void, which will also result
in a water hammer when conditions
cause the water columns to rejoin.
These transients can be modeled and
analyzed using conventional commer5.
Sub-cooled water followed by hot saturated
water into a low-pressure zone. The change
in mass flowrate as the condensate exits the
valve creates a water hammer. Courtesy: Michael
F. Czyszczewski, PE
cial software for incompressible and
compressible flow modeling using the
" method of characteristics. " Most water
hammer software is not capable of
modeling two-phase flow CIWH events.
See the EPRI reference mentioned earlier
for details. ■
-Michael F. Czyszczewski, PE
(mczyszczewski@asme-member.org) is a
mechanical engineering consultant with
45 years of design experience
in the power industry.
October 3-6, 2022
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Denver, Colorado
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POWER June 2022
Table of Contents for the Digital Edition of POWER June 2022
POWER June 2022 - Intro
POWER June 2022 - Cover1
POWER June 2022 - Cover2
POWER June 2022 - 1
POWER June 2022 - 2
POWER June 2022 - 3
POWER June 2022 - 4
POWER June 2022 - 5
POWER June 2022 - 6
POWER June 2022 - 7
POWER June 2022 - 8
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POWER June 2022 - 40
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POWER June 2022 - 43
POWER June 2022 - 44
POWER June 2022 - Cover3
POWER June 2022 - Cover4
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