Water hammer, or hydraulic shock, is the momentary increase in pressure inside a pipe caused by a sudden change of direction or velocity of the liquid in the pipe. Water hammer can be dangerous because the increase in pressure can be severe enough to rupture a pipe or damage adjoining flow control equipment.
Physical Origins of Water Hammer
Water, an incompressible substance, has three types of physical energy when flowing through a pipe: pressure, kinetic, and potential. Pressure energy relates to the water's static or standing energy, kinetic energy relates to the water's velocity, and potential energy relates to the water's relative elevation (potential change in pressure) in the pipeline. All forces of pipe friction aside, the combination of pressure, kinetic, and potential energy will remain constant at all points throughout the length of the pipeline.
The physical law of conservation—i.e., energy can't be created or destroyed—dictates that changing the magnitude of the kinetic energy by changing the fluid velocity will have a direct effect on the magnitude of pressure or potential energy elsewhere in the pipe. If the velocity of the liquid decreases (decrease in kinetic energy), the fluid pressure could increase (increase in pressure energy).
Water hammer most commonly occurs when water is flowing through a pipe and a pump is stopped or a valve is quickly closed. The sudden halt in fluid flow in the pipeline will produce a pressure spike in the form of a shock wave, which will travel back and forth down the piping system, reflecting back from the end of the pipeline. The maximum velocity of this shock wave will be equal to or less than the speed of sound for that specific fluid.
For example, water at 70°F has a sound wave velocity of more than 4,800 ft/sec (1,463 m/sec). However, the dampening effect by which the material the pipeline is made from affects and dictates the pressure wave's terminal velocity. The shock wave will continue to oscillate back and forth down the pipeline, between the closed valve on one end and the end of the line, until the energy is absorbed and the pressure soon equalizes in the pipeline.
If a pump stops or a valve rapidly closes while the fluid is in motion, two things can happen:
- The line velocity moving toward the valve disc will rapidly decelerate, impose a sudden high (positive) pressure force, and thus create a propagating pressure wave.
- The opposite side of the valve, where line velocity was flowing away from the valve, will attempt to sustain the line velocity by working off of its kinetic energy. As the fluid attempts to maintain the line velocity, the pipeline will experience a sudden (negative) pressure force or vacuum, where the fluid will try to separate as the line velocity continues to move away from the valve. This is called column separation.
Column separation is a vacuum pocket in which the vacuum pressure is continuous through the downstream end of the pipeline. Column separation creates negative pressure, which can be sufficient to cause the pipe to collapse, especially when the pipeline is located on a slope, with the water flowing downhill. Valve placement is important to maintaining pipe integrity; therefore, a valve should never be placed at the top of the hill without a downstream vacuum breaker valve close by. Otherwise, a valve that's closed at the top of the hill is more likely to result in negative pressure, which can cause the pipe to collapse.
A slow-closing isolation valve on pump control is more effective at eliminating water hammer compared to check valves, which prevent flow reversal but at a cost. The rapid-close action of a check valve will never be able to stop flow reversal before the pumping action ceases. Therefore, the slight flow reversal that suddenly stops against the back side of a check valve disc could produce a significant level of water hammer. Using external air or hydraulic cushions to slow the closing motion of the check valve offers only some benefit. The best way to eliminate water hammer on a pump control application is to use a control valve that will proactively close on command rather than reactively close when sensing flow reversal.
The slight flow reversal that suddenly stops against the back side of a check valve disc, such as this 36-inch check valve used in a low-service raw water pumping application, could produce a significant level of water hammer. The best way to eliminate water hammer on a pump control application is to use a control valve that will proactively close on command rather than reactively close when sensing flow reversal.
The sequence of operation for a pumping application includes pump startup and shutdown. The pump startup procedure is as follows:
- The pump control valve is fully closed, and the pump is commanded to start.
- The pump starts, and pressure between the pump and the closed valve will be equal to or slightly greater than the downstream static pressure of the pipeline.
- A pressure sensor determines that the pipeline pressure and pump pressure have equalized; it sends a command to the supervisory control and data acquisition (SCADA) system to open the pump control valve.
- The pump control valve begins to open at a rate determined (through surge analysis) to not produce a pressure spike in the pipeline from an excessive flow surge.
- The valve opens fully, without producing a pressure spike, and the pump continues its pumping obligation.
The following steps make up the pump shutdown procedure:
- After the pumping obligation has been satisfied, the SCADA system starts the pump shutdown sequence first by closing the pump control valve.
- The surge analysis determines the minimum time needed to close the pump control valve—without producing a pressure spike or column separation—and the pump control valve starts to close. Valve closure is performed while the pump continues to run. If the pump were to stop running before the valve were fully closed, flow reversal would occur and cause unnecessary damage to the pipeline in the form of a pressure spike. The flow reversal could also backspin the pump, thus causing mechanical damage that could have been avoided. Note that the rates at which the valve opens and closes must be independent of each other and field-adjustable. To this end, electric operators use fixed gearing to develop the mechanical advantage required to operate a valve. Thus, it will have only one (fixed) rate of operation for both opening and closing. This approach isn't recommended for controlling water hammer.
- As the pump control valve approaches its fully closed point, a switch located on the pump-control-valve position indicator is tripped; this is the signal for the SCADA system to command the pump to stop.
- The pump control valve closes, and the pump stops the flowing water, preventing pressure surges. This approach can be used for all types of pump control applications.
How to Minimize Water Hammer
A surge analysis can be performed on the piping system to determine the proper amount of time required to open and close a pump control valve. But sometimes pressure surges occur when a valve is manually closed too fast in the water distribution network. You can't perform surge analysis on everything, so the best way to address pressure surges is to use isolation valves with slow-closing (taking a high number of turns to close) or characterized closing actuators.
The following formulas will help you determine how slowly to close a valve. The proper open and close rates of a valve can be calculated and practiced to ensure safe and proper pipeline operation.
To calculate the correct valve-closing rate, first calculate the critical closing time of the pipeline being analyzed. Use this equation to find this time in seconds (s).
L = length of pipe in feet
a = water hammer wave velocity (ft/sec)
Maximum Head Pressure.
Next, determine the maximum head pressure due to water hammer, in excess of the initial head or initial pressure energy, in feet (h).
a = estimated or derived water hammer wave velocity from equation 1
V = initial pipeline velocity (ft/sec)
g = acceleration due to gravity (ft/sec 2 )
Optimal Time to Close Valve. You have calculated the excess pressure due to water hammer (in feet; this is also considered the change in kinetic energy) and the critical time in which the pressure wave will travel from the valve to the end point of the pipeline and return (in seconds). You can now use equation 3 to determine the desired or adjusted time required to close the valve in order to not exceed a satisfactory net pressure in the pipeline.
H = net excess head due to water hammer at the valve in feet (this value is chosen as the “not to exceed” additive head pressure)
s = critical closing time (sec), from equation 1
h = water hammer pressure above initial pressure (ft), from equation 2
t = adjusted valve-closing time (sec)
This is an example of how to determine water hammer and solving for valve closure times. Say that you have 1,800 feet of ductile-iron pipeline running at 8 ft/sec, with initial pressure at 60 psi.
Using equation 1, determine the critical closing speed and wave velocity for the pipeline under review.
s = critical closing time (sec)
L = 1,800 (length of pipe in feet)
a = water hammer wave velocity (ft/sec)
General wave velocities can be used to estimate the critical closing time. Water hammer wave velocities (ft/sec) are provided for the following materials:
- Polyvinyl chloride: 1,000–2,000
- Ductile iron: 3,200–4,500
- Steel: 2,000–4,000
If a precise wave time is necessary, the accepted approach to determining water hammer wave velocity (a) can be derived using the following equation:
K = fluid bulk modulus for water (300,000 psi)
E = modulus of elasticity of the pipe being analyzed (psi)
d = diameter of pipe being analyzed (in.)
t = wall thickness of pipe being analyzed (in.)
For this example, we use the water hammer wave velocity a = 3,300 ft/sec.
s = 2L/a
s = 2 × 1,800/3,300 = 1.1 sec
Next, use equation 2 to solve for the maximum head pressure due to water hammer.
h = aV/g
a = 3,300 ft/sec (from equation 1) for ductile iron
V = 6 ft/sec
= 820 feet of excess head pressure due to water hammer (820/2.31 ft/psi = 355 psi)
H = 46 ft
s = 1.1 sec, from equation 1
= 20 sec or longer to close the valve and not exceed an extra 46 ft of head pressure
Remember that the pressure that forms when you close the valve in the above calculation is additive (initial pressure energy + change in kinetic energy). So the initial line pressure must be added to determine the final line pressure. Initial 60 psi line pressure will increase to 80 psi after the valve closes.
If ductile-iron pipe (rated to 150 psi) were used in this example, it would be fine under normal conditions, but the pipe could rupture if the valve were to be closed more quickly.
Solving for H and t = 3 sec,
= 300 ft of extra head (130 psi), could have a damaging effect on this pipeline.
Other Causes of Water Hammer
Water hammer can also be caused by other factors, including the following:
- Rapid pump startup can induce a pressure spike due to pipe friction or air pockets in the pipeline.
- Rapid pump shutdown can act like a quick-closing valve and create a rapid change in flow velocity. This creates a positive pressure surge on the suction side of the pump and a vacuum surge on the discharge side. The worst case of these two surges will be on the discharge side, where column separation could occur.
- Check-valve slam due to incorrect application occurs more often than it should and is costly to fix. So if in doubt, consider using an alternative method to control flow reversal, such as a control valve or automated isolation valve.
- Air pockets can form in the pipeline or where check valves and pneumatic surge tanks are close to each other.
- Water column separation can produce a vacuum in the pipeline and produce water hammer when the column rejoins.
Proper valve operation time can be calculated through surge analysis and thus can completely avoid water hammer.
Reducing or Eliminating Water Hammer
Try to avoid water hammer by taking the following actions:
- Educate and train personnel on the dangers of water hammer and how to avoid it by properly opening and closing valves.
- Use a soft-start or variable-frequency drive on pumps to reduce the possibility of creating damaging water hammer conditions.
- Reduce the velocity of fluid in the pipeline to 5 ft/sec or less to keep the potential of water hammer conditions to a minimum. Increasing pipe size is one approach.
- Use slow-closing valves within the water distribution network. Any multiturn-operated valve (gate, butterfly, or knife) is considered slow-closing. Valves with a handle (quarter-turn operation) are considered quick-closing and shouldn't be used.
- Use cylinder-operated valves for high-service pump discharge applications to provide independently adjustable rates of opening, closing, and emergency closing action. This will eliminate the anticipated surge during pump startup and shutdown as well as the unanticipated potential water hammer upon loss of pump power.
- Use air-release valves along high points of the pipeline to remove trapped air that would otherwise act like air springs in the pipeline. Use air-admitting (vacuum) valves to also curb potential pipe collapse conditions when a valve is closed too quickly.
- Use surge tanks to cushion possible water hammer situations, but they can exacerbate the potential for water hammer when used in proximity to uncushioned check valves.
Avoid Disaster and Control the Operation Rate
Water hammer can be expressed in two ways: (1) as a pressure wave in a pipeline when a downstream valve is closed too quickly, which may cause a pipe to burst, or (2) as high vacuum when the same upstream valve is closed, which may cause a pipe to collapse. The remedy to eliminating both of these potential disasters when closing any valve is to design into the valve the ability to control its operation rate.
Proper valve operation time can be calculated through surge analysis and thus can completely avoid water hammer. Through proper pump station design, the correct valve choice can ensure that when a pump startup or shutdown sequence is initiated, the isolation control valve will be “smart” enough to eliminate surges upon pump startup or water hammer upon pump shutdown.
Electric motor operators provide only a fixed rate of control. Unless they have a pulse feature, electrically operated valves offer little in the ability to adjust to varying (diurnal or seasonal) application conditions. Check valves that have hydraulic cushions can play a role in these applications but only when application pressures and flow velocities permit the use of this type of valve. An operator should verify that the specified check valve can provide the control to minimize or eliminate water hammer.
Unfortunately, check valves that actually do control water hammer will carry a higher head loss burden. Optional valve features that claim to eliminate water hammer, such as external levers and weights or internal springs, will produce more head loss compared to a gravity check that has no ability to combat water hammer. This minimizing difference will be reflected in the respective valve's head loss table. Again, it's important to verify that the feature will perform as required.
As mentioned earlier, one way to eliminate water hammer is by training personnel in the field on how to properly close down a water main; this will also eliminate further damage to fragile infrastructure. This begins by specifying manual actuators that require a minimum number of turns to open; the number should be based on a valve size range (e.g., a 3- to 20-inch valve requires a minimum of 30 turns, a 24- to 36-inch valve requires a minimum of 40 turns). The fewer turns needed to open the valve, the quicker the field operator can close the valve and thus run the risk of water hammer.
- Manual of Water Supply Practices M77, Condition Assessment of Water Mains, 2019, https://awwa.org/store (catalog No. 30077), AWWA
- Manual of Water Supply Practices M32, Computer Modeling of Water Distribution Systems, 2017, https://awwa.org/store (catalog No. 30032-4E), AWWA
- Water System Operations Water Distribution, Grades 1 & 2, 2016, https://awwa.org/store (catalog No. 1943), AWWA
- Water System Operations Water Distribution, Grades 3 & 4, 2016, https://awwa.org/store (catalog No. 1944), AWWA
- Timothy Fallon is senior product manager for Mueller Water Products (www.muellerwaterproducts.com), Atlanta.