POWER June 2022 - 32
OPERATIONS & MAINTENANCE
How to Prevent CondensationInduced
Water Hammer
Condensation-induced water hammer (CIWH) is the term commonly used when
describing a variety of transients that occur when water vapor is rapidly condensed
by cold water. This article describes the various types and causes of CIWH, explains
how to estimate the velocities and pressures involved, and provides readers with
solutions that can be used to prevent it.
Michael F. Czyszczewski, PE
S
team and liquid water in a pipeline
do not always play well together.
Situations where they are unexpectedly
brought into contact can be
volatile. A hydraulic transient is a shortterm
event initiated by a rapid change
in pipe flow velocity or pressure. The
event generates pressure pulses that
travel like waves downstream and upstream
from their point of origin. The
pulses impact and reflect off of obstructions
as they travel through the
pipe. This produces a hammering noise
and kicking-like pipe movements. The
term " water hammer " is generically
used when referring
to these transients
even when other liquids and
gasses are involved.
Vapor Pocket Formation
Water vapor is wet steam. If you add
heat to a quantity of water at the boiling
point, it will begin to change (vaporize)
from a liquid into a vapor. If you continue
to add heat, the liquid and vapor will
remain at the same temperature until
all the liquid is converted to vapor. This
is known as being at saturated conditions.
A pipeline that conveys gas and
liquid water at the same time is said to
contain two-phase flow.
If hot saturated water in a pipe experiences
a decrease in pressure, it will
begin to vaporize. Small vapor bubbles
will form. These bubbles are buoyant so
they tend to rise and collect into pockets
at high points. Pockets can also be
created if the pipe geometry isolates a
quantity of steam during a system refill
or shutdown. For example, steam
can get trapped in a vertical U-bend if
both risers become blocked with water
when filling.
Vapor Pocket Collapse
A liquid is said to be sub-cooled if it is at
a temperature below the saturation tem32
perature
for a given pressure. If a vapor
pocket comes into contact with a subcooled
liquid, it will begin to condense
at the vapor/liquid boundary. The condensation
rate increases as the temperature
difference increases. When the temperature
difference is greater than about 35F,
all of the vapor will suddenly condense.
Vapor occupies much more space than a
liquid; therefore, a low-pressure void is
created. The water surrounding the pocket
will accelerate into the void. This implosion
occurs in a fraction of a second.
The velocity (VI
, ft/sec) of the advancing
water just before impact is reduced to
zero at impact, and can be calculated as
shown in Equation 1:
VI = ∆V = √((( 288 gc
( 1 - α )))
( PU
- PD
)) / ρ ) ( α /
where α is the void fraction (conservatively
about 0.5); gc
is the gravitational
constant (32.2 ft-lbm/sec2-lbf); and ρ is
the fluid density (lbm/ft3). The pressure
in a vapor pocket (PD
) is the vapor pressure
corresponding to the temperature
of the surrounding water. Because the
distance traveled is very small, friction
has a minimal effect and is neglected.
The abrupt velocity change generates
a water hammer pressure pulse. The velocity
of the pulse as it travels through
the pipe is the acoustic (sonic) velocity
(c, ft/sec), and can be calculated using
Equation 2:
c = √(( 144 gc
G / ρ ) / ( 1 + ( G / E ) φ ))
where G is the bulk modulus of compressibility
of the liquid (psi); E is the
modulus of elasticity of the pipe (psi); φ
is a pipe boundary condition parameter,
which for a thin wall tube fixed at both
ends is D/t; where D is the pipe inside
diameter (in.); and t is the pipe thickness
(in.). Air entrainment would reduce
the velocity.
www.powermag.com
The theoretical maximum pressure
pulse (∆P, psi) can be found by using
the familiar " Joukowski " water hammer
equation, shown here as Equation 3:
∆P = k (( ρ c ∆V ) / ( 144 gc
))
where the variable k is 1.0, if the vapor
pocket collapse occurs next to a hard surface
such as a pipe dead end or a closed
valve, otherwise, k = 0.5. Note that ∆P is
not the total pressure, but is the increase
or decrease from the steady state pressure
that existed before the transient.
Water Slug Formation
When there is stratified two-phase flow
in a horizontal pipe, the water level in
the pipe will increase due to condensation
at the steam/water interface. This
decreases the pipe cross-sectional area
occupied by the steam, which increases
its velocity. When the difference in
velocity is great enough, there will be
a transition from stratified flow to slug
flow. Surface waves will form and move
at a higher velocity than the liquid. The
waves become bigger as the flow becomes
more turbulent. If a large surface
wave blocks the pipes cross-sectional
area, steam entering the pipeline will
exert pressure on the wave's upstream
side, which will propel it down the pipeline
as a water slug.
The Froude number (Fr) is a parameter
typically used to assess the formation
of gravity waves in open-channel
flow. Studies have found that there is a
minimum velocity (VMIN
) above which a
transition from stratified two-phase flow
to slug flow will not occur. This flowrate
corresponds to a Froude number of 0.5.
Industry practice is to conservatively use
a Froude number of 1.0 when pipe sizing,
to provide a design margin. VMIN
can be
calculated using Equation 4:
VMIN
= Fr √( gc
D / 12 )
POWER | June 2022
http://www.powermag.com
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
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