POWER June 2022 - 34

OPERATIONS & MAINTENANCE
desuperheater spray systems are especially
susceptible.
To prevent Mechanism 2 transients:
■ Long horizontal pipe lengths will allow
the water slug to achieve higher velocity
and momentum. Shorten horizontal
pipe runs to increase the rate at which
they fill, thus minimizing the condensation
rate. Pipe runs shorter than 24
pipe diameters are recommended.
3. Top filling a vertical steam-filled pipe with
pressurized water. Slow filling does not experience
water hammer while fast filling does.
Courtesy: Michael F. Czyszczewski, PE
■ Prevent stratified flow by not adding
water to a steam-filled system. Alternatively,
ensure that pipes run full of
water at all times.
■ Transients typically occur when horizontal
pipe sections have insufficient
slope for drainage. A minimum incline
of 0.5 in/ft is recommended to insure
the pipe runs full.
■ Pipes inclined more than 3 degrees
from the horizontal will act like a vertical
pipe, and will, therefore, not be
vulnerable to Mechanism 2 CIWH.
■ Maintain and repair steam isolation
valves. Stratified two-phase flow can
occur if valves permit steam to leak
into and condense in a voided pipe.
Similarly, maintain and repair condensate
isolation valves. Hot condensate
will flash to steam if it leaks through a
valve into a lower-pressure pipe.
4. Bottom filling a vertical steam-filled pipe
with pressurized water. The sequence of
events is shown from left to right. The water
column strikes the valve creating a pressure
pulse. Courtesy: Michael F. Czyszczewski, PE
water can be found using the criterion
shown in Equation 10, which is based on
a Froude number of 0.5:
m ≥ 2.227 ρ D2.5
Also, flow model tests have shown that
stratified counter-flow conditions will not
form at low flowrates, where the mass
flowrate during a water fill meets the criteria
in Equation 11:
m ≤ C D2 e-0.005 (LH/D)
where LH
is the horizontal pipe length (ft);
and C is a coefficient that is a function of
pipe diameter (1.5 to 36 inches) given by
Equation 12:
C = -0.00001745 D4 + 0.0001736 D3 -
0.06152 D2 + 0.9425 D + 4.280
Counter-flow CIWH has the potential to
produce very damaging transient forces.
Problems most often occur during startup.
Condensate return lines and steam
34
■ Maintain and repair steam traps.
■ Keep pipe insulation intact. Heat loss
due to missing or insufficient insulation
will increase the condensation
rate and the severity of the transient.
Mechanism 3-Pressurized Water
Entering a Vertical Steam Filled Pipe.
This CIWH Mechanism occurs in vertical
steam-filled pipes or horizontal pipes
sloped more than 3 degrees (which act
like a vertical pipe). When taking a system
out of service, steam can become
left behind in pockets. Saturated water
can also leak past shut-off valves where
it could flash into steam on the low-pressure
side and form vapor pockets.
Although condensation plays a role,
the primary contributor to the severity
of a Mechanism 3 transient is the inertia
and pressure of the water filling the pipe,
and whether the water enters the vertical
pipe from the bottom or from the top.
Top Filling. As sub-cooled water
comes into contact with the steam pocket,
vapor bubbles will form in the water.
If the pipe is filled slowly, the water will
primarily flow down the vertical pipe walls
(annular-film flow) giving the bubbles a
chance to rise in the center of the pipe
and escape. However, if the water fill rate
www.powermag.com
is faster than the bubble rise velocity, the
vapor bubbles will be trapped, and they
will condense and collapse under the
pressure exerted by the entering water.
Collapsing vapor bubbles are much smaller
than a collapsing vapor pocket, so the
resulting water hammer pressure pulses
are considered moderate, and not severe
(Figure 3).
The maximum safe filling velocity (VF
,
ft/sec) for annular flow in a vertical pipe
can be calculated using Equation 13:
VF = 0.67 √((( ρf
Where, ρf
- ρg ) / ρf
and ρg
) (( gc
D ) / 12 ))
are the densities at saturated
conditions for the liquid and gas
phases, respectively. Note that when
pressures are low, the gas density is near
zero, so the density terms can be neglected.
Also note, if the pipe is inclined
more than 15 degrees from the vertical,
the annular flow pattern becomes modified.
Vapor bubbles will gather along the
upper part of the pipe cross section and
their rise velocity will increase, therefore,
the safe fill velocity should be increased
by a factor of the √2.
Bottom Filling. When filling the system
from the bottom, the water column
front will become turbulent as it compresses
and condenses the vapor pocket.
The pocket will implode, creating a lowpressure
zone, which accelerates the
water column upward. A large pressure
pulse is generated when the water column
front strikes the closed steam valve
(Figure 4).
To prevent Mechanism 3 transients:
■ In a top filling situation, fill the system
slowly to promote an annular flow
pattern and give the vapor bubbles a
chance to escape.
■ In a bottom filling situation, install a
small bypass around isolation valves,
which could be used to slowly pressurize
areas where vapor pockets
could form.
■ Keep the system full to prevent the
creation of steam pockets.
■ Vent areas where vapor pockets are
expected to
form prior
to system
start-up. The air will cushion the blow
of a water slug.
Mechanism 4-Hot Water Entering
a Lower-Pressure Pipe. Figure 5 shows
a pressurized water vessel with a
closed discharge pipe. The discharge
piping contains stagnant water that
will cool over time. When the valve reopens,
the sub-cooled water is quickly
POWER | June 2022
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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
POWER June 2022 - 9
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POWER June 2022 - Cover3
POWER June 2022 - Cover4
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