Water Hammer Case Study
Water hammer is the term used for the shock resulting from the sudden change in velocity, starting or stopping of the motion of a column of fluid. The term is also applied to events that result in similar pressure transients, such as a water slug being forced through a system by entering steam, rapid expansion of a cool fluid when suddenly exposed to heat or fluid of a higher temperature, or the sudden and rapid collapse of a steam bubble or pocket of hot fluid when suddenly exposed to cooler fluid or rapid cooling action. The latter type event should not be confused with cavitation. The consequences of all these types of events vary in terms of possible damage risk to components and equipment, and in terms of the risks to personnel.
The following assumptions are made regarding the transmission of a pressure wave due to
water hammer:
1. The fluid in the pipe is elastic, of homogeneous density, and in the liquid state.
2. The pipe wall material is homogeneous, isotropic and elastic.
3. The velocities and pressures in the pipe are uniformly distributed over a transverse cross section.
4. Flow velocity is much less than the speed of sound.
5. The pipe is full of water (there is no open surface wave velocity to consider) when the pressure wave is traveling through it.
6. The velocity head is negligible relative to pressure changes.
7. Friction in the pipe may be ignored.
8. Water levels at the ends of the pipe or inlet and outlet reservoirs do not change faster
than the wave propagation time from one end to the other.
The following properties are commonly used to calculate the parameters necessary to understand water hammer:
Density
Density is the mass per unit volume.
𝜌 = m/V (lbm/ft³)
where
m = mass (lbm or slugs)
V = volume ( ft³ )
Travel Time of Pressure or Sound Wave
The time that a wave of a particular pressure will take to make a round trip in a closed section of pipe may be calculated from the celerity equation:
t = 2L/C (sec)
where
L = length of pipe (ft )
for rigid pipe
C= (EB/𝜌)¹/² ft/sec
EB = bulk modulus of fluid ( lb/ft²)
Newton’s Second Law
Newton’s Second Law may be applied to find the theoretical pressure changes and forces due to water hammers
F = m a (lbf)
a = acceleration of fluid column ( ft/sec² )
m = mass (lbm or slugs)
m = mass (lbm or slugs)
The pressure spike in a pipeline caused by a closing or opening a valve can be estimated as
dp = 0.070 dv L / t
where
dp = increase in pressure - pressure spike (psi)
dv = change in flow velocity (ft/s)
t = valve closing time (s)
L= upstream pipe length (ft)
The resulting force from unbalanced pressure or from a water slug colliding with a closed
valve or other transverse obstruction may also be calculated:
valve or other transverse obstruction may also be calculated:
Force from unbalanced pressure
where
A = crosss ectional area ( ft² )
Force from a water slug:
Example 1
Calculate the celerity of a pressure wave in water at 200°F in a rigid pipe. The bulk modulus
of the water is 45x106 lbf/ft² and the density is 1.868 slugs/ft³ or (lbf-sec²/ft⁴)
C= (EB/𝜌)¹/² =4908 ft/sec
For the wave above, calculate the round trip travel time through 20 ft of pipe.
t = 2L/C =0.00815 sec
Types of Water Hammer Events
Water hammer occurrences may be normal and anticipated as part of the design and operation of the plant. These may occur due to pump starts or stops, control or isolation valve operation, check valve closure, safety or relief valve operations, turbine trips and the filling of normally empty systems. Usually, designs and procedures minimize the effects of these events and no damage occurs from them. There are other abnormal and unanticipated types of water hammer events which are severe,
and which may cause damage or profoundly affect plant operation. These events can be very complex and difficult to analyze, and often occur in the presence of two phase flow. The pressure transient amplitudes in two phase flow are not necessarily easy to analyze in complex systems where secondary or reflected waves may actually exceed the first transient in amplitude, depending on the configuration of the pipe. The resulting forces are equivalently complex.
The pressure wave is affected by the interferences and boundaries encountered. As a result,
wave transmission factors and reflection factors may be calculated specifically for the type of
obstacle encountered. This type of calculation should be done as part of the analysis of
piping system design if water hammers are expected and detailed analysis is desired, but are
outside the scope of this module.
The scenarios that lead to severe water hammer events must be prevented if at all possible due
to the potential consequences listed below. The severity of the effects may be worsened if the
acoustic resonances of the piping, or the structural resonances of the configuration are excited
by the event(s).
The consequences of a water hammer event may be:
1. Pressure boundary failures, including pipe breaks, which are often considered the most severe consequences, but are generally rare.
2. Damage to components, which may be costly and time consuming to repair, and sometimes go undiscovered.
3. Flange or instrumentation leakage.
4. Support damage to restraints and snubbers.
5. Observation of water hammer event without damage.
There are seven well known mechanisms or operating scenarios which can lead to severe water hammer The first four listed are classified as condensation events. Number 3&7 may be controlled by the head generated by a pump rather than by bubble collapse.
1. Sub cooled water with condensing steam in a vertical pipe (water cannon).
2. Steam and water counterflow in a horizontal pipe (steam/water counterflow).
3. Pressurized water entering a vertical steam filled pipe (steam pocket collapse).
4. Hot water entering a low pressure line (low pressure discharge).
5. Steam propelled water slug (water slug).
6. Rapid valve actuation (valve slam).
7. Filling of a voided line (column rejoining).
There are sometimes events similar to water hammers associated with severe cavitation, but
those events are not considered as part of this module
1-Water Cannon
The flow of steam into a pool of subcooled water is stopped or reduced significantly, trapping a pocket of steam above the liquid surface. Rapid condensation of the steam draws water rapidly into the exhaust line. The water impacts on the fully or partially closed valve, causing a pressure transient in the waterfilled pipe. This mechanism is somewhat unique to BWRs. See Figure 1. This type of hammer typically results in moderate damage to components such as rupture disks and check valves
Plant systems that are susceptible to this type of water hammer are BWR:
“High Pressure Coolant Injection” (HPCI) system
or
“Reactor Core Isolation Cooling” (RCIC) system
2-Steam/Water Counterflow
This usually occurs when a small flow of subcooled water is being injected into a large horizontal pipe leading to a reservoir of high pressure steam. Rapid condensation across the stratified twophase
interface results in rapid steam flow counter to the direction of water flow. Contact with the subcooled liquid causes the steam to condense. The high velocity steam flow creates waves on the surface of the liquid. If these waves make contact with the top of the pipe a low pressure “pocket” is created and a water slug is formed. As the steam in the pocket condenses, the differential pressure across the slug increases which causes the slug to accelerate. As the steam pocket disappears, the slug is stopped quickly, and large amplitude pressure waves are generated to propagate through
the system. See Figure 2.
Plant systems that are susceptible to this type of water hammer are:
BWR PWR
HPCI Feed Water
Main Steam Steam Generator
Condensate Residual Heat Removal
Aux Feed Water
3-Steam Pocket Collapse
A steam filled pipe may exist at any elevation and is usually caused by the leaking of steam or hot water, which flashes from a higher pressure region. The pipe orientation, the fill rate and inertia of the liquid, and the pressure from a pump or other filling device control this mechanism. Bottom filling results in steam bubble collapse and a resultant pressure transient. Top filling will not usually result in a hammer, so long as the fill rate does not exceed the bubble rise rate. If top fill is too rapid, a slug may form, collapsing the steam bubble and causing a pressure transient. See Figure 3.
A steam pocket collapse event will occur (verses a steam/water counterflow event) when the piping is greater than 3o above horizontal. This is because there is insufficient contact area between the steam flow and the surface of the water to generate waves which give rise to water slugs.
The severity of a steam pocket collapse event is dependent the rate of fill (from a pump or any driving head) as well as the condensation rate of the steam pocket. Plant systems that are susceptible to this type of water hammer are:
BWR PWR
Core Spray Safety Injection
RCIC Coolant Charging
Reactor Water Clean Up Steam Generator Blow Down
Residual Heat Removal Residual Heat Removal
Feed Water Feed Water
Moisture Separator ReHeater
Feed Water Heaters
HPCI Aux Feed Water
4-Low Pressure Discharge
This is caused by hot water entering a lower pressure line. Subcooled water from a lower
temperature, often stagnant, leg is admitted to a hotter leg. As water quickly passes
through the valve, downstream flashing may lead to formation and propulsion of a water
slug. When the water flashes to steam on the down stream side of the valve, the pressure
around the steam pocket that is created increases dramatically. The flowing water
upstream of the valve is suddenly reduced or stopped. This produces a water hammer
event similar to the Valve Slam type. As the steam pockets are swept away and new ones
are formed, the water hammer upstream of the valve will continue. Additional water
hammer events are created as the steam pockets downstream of the valve condense and
collapse. See Figure 4.
Plant systems that are susceptible to this type of water hammer are:
BWR PWR
Moisture Separator ReHeater
Moisture Separator ReHeater
Condensate Condensate
Feed Water Feed Water
Residual Heat Removal Residual Heat Removal
Reactor Coolant System
Steam Generator BlowDownPiping
5-Water Slug
Steam propelled water slugs may occur in piping which collects condensate upstream of a
closed valve or in low point of normally empty steam lines downstream of closed valve.
This type water hammer often occurs in poorly designed or sagging piping. The opening
of a valve allows steam flow to accelerate the water slug, sweeping it up even if the water
does not fill the pipe initially. Relatively small steam pressure may result in significant
slug velocity, and large forces can result when the slug impacts at an elbow or hits a
restriction in the pipe. See Figure 5.
Plant systems susceptible to this type of water hammer event are:
BWR PWR
Main Steam Main Steam
Aux Steam Aux Steam
HPCI Feed Water
RCIC S/G Blow Down
Reactor Coolant (PZR)
6-Valve Slam
Rapid closure of a check or other type valve or abnormal valve opening or closing events,
such as those caused by actuator failure, result in pressure transients. In addition to the
severe transients from the closure event, column separation and rejoining may occur on
the low pressure side of the rapidly closing valve. This type of water hammer event is not
limited to any particular plant design or system
1-Instantaneous Closure
2-Rapid Closure
3-Slow Closure
7-Column Rejoining
If check valve or other leakage occurs in a system with an elevation or head change (of
over 34 feet), a vacuum may be formed after a pump shutdown. This will occur if a keepfull
system is not operating or is inadequate to prevent it. The void at the high point
collapses after pump restart, causing a substantial pressure transient in the water filled
piping.
Plant systems that are susceptible to this type of water hammer are :
BWR PWR
HPCI Safety Injection
RCIC Feed Water
Core Spray Service Water
Service Water Residual Heat Removal
Residual Heat Removal Circulating Water
Circulating Water
8- Transients in NonFlowing Systems
Valve opening or a sudden release from a pipe can also generate a water hammer. In this
case, the fluid flow is away from the standing column or reservoir toward the opening,
i.e., toward the low pressure end. This rapid change can result in a pressure wave in the
standing water which has not yet started to move. This is a variation of the column
separation type event.
There is also the possibility of water hammer in stagnant or “dead” legs where no flow
exists. This is due to a water hammer pressure wave from the main pipe dividing and
traveling down the standing fluid column, or from excitation of an acoustic resonance in
the stagnant leg. The wave can simply rebound in the dead leg, and eventually die out, or
it could excite an acoustic or structural resonance of the pipe causing an intense transient
response.
The steps needed to determine the source of the water hammer are:
1. Examine marks and impact deformation to identify the direction, sequence and
extent of pipe movement.
2. Study local deformations of the pipe that do not have associated external impact to
determine if a water slug occurred.
3. Study the pattern of line distortion and any system damage to determine if a
pressure wave event occurred.
4. Decide if the event was due to a pressure wave or a water slug.
5. By combining the evidence of direction of pipe motion and the type of water
hammer, identify the source or originating point of the water hammer.
6. Investigate the possible water hammer generation mechanism and scenarios at the
source of the event.
Water Hammer Prevention, Elimination and Mitigation
Prevention or elimination of water hammers is the preferred solutions. Mitigation should be
pursued if accommodation to the effects of water hammers must be done instead of
elimination. The mitigation would reduce the effects on the piping system and other
components so that they might be able to withstand the forces resulting from the pressure
transients. Along with mitigation, some redesign or reinforcement of the piping and support
system may still be necessary
Some design and procedural techniques and changes to consider are:
· Analysis, design and construction, including modifications, to reduce the effects of
anticipated water hammers and to mitigate the effects of unanticipated ones.
· Installation of vacuum breakers to replace steam void and prevent vacuum formation.
· Installation of designs that prevent inadvertent drainage and creation of high point
voids.
· Designs which eliminate dead legs and stagnant sections without adequate vents and
drains.
· Ensure that asbuilt
pipe configurations and slopes are correct in order to prevent the
formation of condensate traps or pooling from inadequate draining.
· Finding ways to keep systems water solid or ensuring that techniques already in place
do the job adequately.
· Use of maintenance and operating techniques that prevent the accumulation of steam
in normally waterfilled
systems.
· Use of void detection devices.
· Improved venting and draining procedures.
· Use of appropriate warming techniques and procedures.
· Fill locations and fill rates should be tightly controlled and monitored.
· Eliminate valve slams through the use of controlled closure devices, and ensure that
those devices work properly.
· Select valve actuators which are designed to minimize opening or closure problems
that might result in water hammers.
· Ensure that valve actuators function properly without sticking and jumping, and that
level controls are set to avoid unnecessary repetitive and/or rapid cycling.
· Provide pressure equalization paths and devices that reduce the risks from having
widely variant pressures across a valve or pump prior to operation.
· Provide appropriate and adequate pressure relief valves or schemes.
· Install surge tanks, or surge suppression devices in pump systems where appropriate.
· Prevent reverse rotation of pumps after power failures.
· Prevent automatic or quick restarts of pumps while there is still reverse flow in the
attached pipe.
F = dp A (lbf)
where
A = crosss ectional area ( ft² )
Force from a water slug:
F = 𝜌 A v-²
Example 1
Calculate the celerity of a pressure wave in water at 200°F in a rigid pipe. The bulk modulus
of the water is 45x106 lbf/ft² and the density is 1.868 slugs/ft³ or (lbf-sec²/ft⁴)
C= (EB/𝜌)¹/² =4908 ft/sec
For the wave above, calculate the round trip travel time through 20 ft of pipe.
t = 2L/C =0.00815 sec
Types of Water Hammer Events
Water hammer occurrences may be normal and anticipated as part of the design and operation of the plant. These may occur due to pump starts or stops, control or isolation valve operation, check valve closure, safety or relief valve operations, turbine trips and the filling of normally empty systems. Usually, designs and procedures minimize the effects of these events and no damage occurs from them. There are other abnormal and unanticipated types of water hammer events which are severe,
and which may cause damage or profoundly affect plant operation. These events can be very complex and difficult to analyze, and often occur in the presence of two phase flow. The pressure transient amplitudes in two phase flow are not necessarily easy to analyze in complex systems where secondary or reflected waves may actually exceed the first transient in amplitude, depending on the configuration of the pipe. The resulting forces are equivalently complex.
The pressure wave is affected by the interferences and boundaries encountered. As a result,
wave transmission factors and reflection factors may be calculated specifically for the type of
obstacle encountered. This type of calculation should be done as part of the analysis of
piping system design if water hammers are expected and detailed analysis is desired, but are
outside the scope of this module.
The scenarios that lead to severe water hammer events must be prevented if at all possible due
to the potential consequences listed below. The severity of the effects may be worsened if the
acoustic resonances of the piping, or the structural resonances of the configuration are excited
by the event(s).
The consequences of a water hammer event may be:
1. Pressure boundary failures, including pipe breaks, which are often considered the most severe consequences, but are generally rare.
2. Damage to components, which may be costly and time consuming to repair, and sometimes go undiscovered.
3. Flange or instrumentation leakage.
4. Support damage to restraints and snubbers.
5. Observation of water hammer event without damage.
There are seven well known mechanisms or operating scenarios which can lead to severe water hammer The first four listed are classified as condensation events. Number 3&7 may be controlled by the head generated by a pump rather than by bubble collapse.
1. Sub cooled water with condensing steam in a vertical pipe (water cannon).
2. Steam and water counterflow in a horizontal pipe (steam/water counterflow).
3. Pressurized water entering a vertical steam filled pipe (steam pocket collapse).
4. Hot water entering a low pressure line (low pressure discharge).
5. Steam propelled water slug (water slug).
6. Rapid valve actuation (valve slam).
7. Filling of a voided line (column rejoining).
There are sometimes events similar to water hammers associated with severe cavitation, but
those events are not considered as part of this module
1-Water Cannon
The flow of steam into a pool of subcooled water is stopped or reduced significantly, trapping a pocket of steam above the liquid surface. Rapid condensation of the steam draws water rapidly into the exhaust line. The water impacts on the fully or partially closed valve, causing a pressure transient in the waterfilled pipe. This mechanism is somewhat unique to BWRs. See Figure 1. This type of hammer typically results in moderate damage to components such as rupture disks and check valves
Plant systems that are susceptible to this type of water hammer are BWR:
“High Pressure Coolant Injection” (HPCI) system
or
“Reactor Core Isolation Cooling” (RCIC) system
2-Steam/Water Counterflow
This usually occurs when a small flow of subcooled water is being injected into a large horizontal pipe leading to a reservoir of high pressure steam. Rapid condensation across the stratified twophase
interface results in rapid steam flow counter to the direction of water flow. Contact with the subcooled liquid causes the steam to condense. The high velocity steam flow creates waves on the surface of the liquid. If these waves make contact with the top of the pipe a low pressure “pocket” is created and a water slug is formed. As the steam in the pocket condenses, the differential pressure across the slug increases which causes the slug to accelerate. As the steam pocket disappears, the slug is stopped quickly, and large amplitude pressure waves are generated to propagate through
the system. See Figure 2.
Plant systems that are susceptible to this type of water hammer are:
BWR PWR
HPCI Feed Water
Main Steam Steam Generator
Condensate Residual Heat Removal
Aux Feed Water
3-Steam Pocket Collapse
A steam filled pipe may exist at any elevation and is usually caused by the leaking of steam or hot water, which flashes from a higher pressure region. The pipe orientation, the fill rate and inertia of the liquid, and the pressure from a pump or other filling device control this mechanism. Bottom filling results in steam bubble collapse and a resultant pressure transient. Top filling will not usually result in a hammer, so long as the fill rate does not exceed the bubble rise rate. If top fill is too rapid, a slug may form, collapsing the steam bubble and causing a pressure transient. See Figure 3.
A steam pocket collapse event will occur (verses a steam/water counterflow event) when the piping is greater than 3o above horizontal. This is because there is insufficient contact area between the steam flow and the surface of the water to generate waves which give rise to water slugs.
The severity of a steam pocket collapse event is dependent the rate of fill (from a pump or any driving head) as well as the condensation rate of the steam pocket. Plant systems that are susceptible to this type of water hammer are:
BWR PWR
Core Spray Safety Injection
RCIC Coolant Charging
Reactor Water Clean Up Steam Generator Blow Down
Residual Heat Removal Residual Heat Removal
Feed Water Feed Water
Moisture Separator ReHeater
Feed Water Heaters
HPCI Aux Feed Water
4-Low Pressure Discharge
This is caused by hot water entering a lower pressure line. Subcooled water from a lower
temperature, often stagnant, leg is admitted to a hotter leg. As water quickly passes
through the valve, downstream flashing may lead to formation and propulsion of a water
slug. When the water flashes to steam on the down stream side of the valve, the pressure
around the steam pocket that is created increases dramatically. The flowing water
upstream of the valve is suddenly reduced or stopped. This produces a water hammer
event similar to the Valve Slam type. As the steam pockets are swept away and new ones
are formed, the water hammer upstream of the valve will continue. Additional water
hammer events are created as the steam pockets downstream of the valve condense and
collapse. See Figure 4.
Plant systems that are susceptible to this type of water hammer are:
BWR PWR
Moisture Separator ReHeater
Moisture Separator ReHeater
Condensate Condensate
Feed Water Feed Water
Residual Heat Removal Residual Heat Removal
Reactor Coolant System
Steam Generator BlowDownPiping
Steam propelled water slugs may occur in piping which collects condensate upstream of a
closed valve or in low point of normally empty steam lines downstream of closed valve.
This type water hammer often occurs in poorly designed or sagging piping. The opening
of a valve allows steam flow to accelerate the water slug, sweeping it up even if the water
does not fill the pipe initially. Relatively small steam pressure may result in significant
slug velocity, and large forces can result when the slug impacts at an elbow or hits a
restriction in the pipe. See Figure 5.
Plant systems susceptible to this type of water hammer event are:
BWR PWR
Main Steam Main Steam
Aux Steam Aux Steam
HPCI Feed Water
RCIC S/G Blow Down
Reactor Coolant (PZR)
6-Valve Slam
Rapid closure of a check or other type valve or abnormal valve opening or closing events,
such as those caused by actuator failure, result in pressure transients. In addition to the
severe transients from the closure event, column separation and rejoining may occur on
the low pressure side of the rapidly closing valve. This type of water hammer event is not
limited to any particular plant design or system
1-Instantaneous Closure
2-Rapid Closure
3-Slow Closure
If check valve or other leakage occurs in a system with an elevation or head change (of
over 34 feet), a vacuum may be formed after a pump shutdown. This will occur if a keepfull
system is not operating or is inadequate to prevent it. The void at the high point
collapses after pump restart, causing a substantial pressure transient in the water filled
piping.
Plant systems that are susceptible to this type of water hammer are :
BWR PWR
HPCI Safety Injection
RCIC Feed Water
Core Spray Service Water
Service Water Residual Heat Removal
Residual Heat Removal Circulating Water
Circulating Water
8- Transients in NonFlowing Systems
Valve opening or a sudden release from a pipe can also generate a water hammer. In this
case, the fluid flow is away from the standing column or reservoir toward the opening,
i.e., toward the low pressure end. This rapid change can result in a pressure wave in the
standing water which has not yet started to move. This is a variation of the column
separation type event.
There is also the possibility of water hammer in stagnant or “dead” legs where no flow
exists. This is due to a water hammer pressure wave from the main pipe dividing and
traveling down the standing fluid column, or from excitation of an acoustic resonance in
the stagnant leg. The wave can simply rebound in the dead leg, and eventually die out, or
it could excite an acoustic or structural resonance of the pipe causing an intense transient
response.
The steps needed to determine the source of the water hammer are:
1. Examine marks and impact deformation to identify the direction, sequence and
extent of pipe movement.
2. Study local deformations of the pipe that do not have associated external impact to
determine if a water slug occurred.
3. Study the pattern of line distortion and any system damage to determine if a
pressure wave event occurred.
4. Decide if the event was due to a pressure wave or a water slug.
5. By combining the evidence of direction of pipe motion and the type of water
hammer, identify the source or originating point of the water hammer.
6. Investigate the possible water hammer generation mechanism and scenarios at the
source of the event.
Water Hammer Prevention, Elimination and Mitigation
Prevention or elimination of water hammers is the preferred solutions. Mitigation should be
pursued if accommodation to the effects of water hammers must be done instead of
elimination. The mitigation would reduce the effects on the piping system and other
components so that they might be able to withstand the forces resulting from the pressure
transients. Along with mitigation, some redesign or reinforcement of the piping and support
system may still be necessary
Some design and procedural techniques and changes to consider are:
· Analysis, design and construction, including modifications, to reduce the effects of
anticipated water hammers and to mitigate the effects of unanticipated ones.
· Installation of vacuum breakers to replace steam void and prevent vacuum formation.
· Installation of designs that prevent inadvertent drainage and creation of high point
voids.
· Designs which eliminate dead legs and stagnant sections without adequate vents and
drains.
· Ensure that asbuilt
pipe configurations and slopes are correct in order to prevent the
formation of condensate traps or pooling from inadequate draining.
· Finding ways to keep systems water solid or ensuring that techniques already in place
do the job adequately.
· Use of maintenance and operating techniques that prevent the accumulation of steam
in normally waterfilled
systems.
· Use of void detection devices.
· Improved venting and draining procedures.
· Use of appropriate warming techniques and procedures.
· Fill locations and fill rates should be tightly controlled and monitored.
· Eliminate valve slams through the use of controlled closure devices, and ensure that
those devices work properly.
· Select valve actuators which are designed to minimize opening or closure problems
that might result in water hammers.
· Ensure that valve actuators function properly without sticking and jumping, and that
level controls are set to avoid unnecessary repetitive and/or rapid cycling.
· Provide pressure equalization paths and devices that reduce the risks from having
widely variant pressures across a valve or pump prior to operation.
· Provide appropriate and adequate pressure relief valves or schemes.
· Install surge tanks, or surge suppression devices in pump systems where appropriate.
· Prevent reverse rotation of pumps after power failures.
· Prevent automatic or quick restarts of pumps while there is still reverse flow in the
attached pipe.
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