National
Drinking Water Clearinghouse
West Virginia University
P.O. Box 6064
Morgantown, WV
26506-6064
Tech
Brief
Water Hammer
by
Z. Michael Lahlou,Ph.D.,
Technical
Assistance Consultant
Summary
Water hammer refers to fluctuations caused by a sudden
increase or decrease in flow velocity. These pressure fluctuations can be
severe enough to rupture a water main. Potential water hammer problems should
be considered when pipeline design is evaluated, and a thorough surge analysis
should be undertaken, in many instances, to avoid costly malfunctions in a
distribution system. Every major system design change or operation change—such
as the demand for higher flow rates—should include consideration of potential
water hammer problems. This phenomenon and its significance to both the design
and operation of water systems is not widely understood, as evidenced by the
number and frequency of failures caused by water hammer.
 Photo Caption-Figure 1 -Illustration of Water Hammer,A column of water acts like a frieght train suddenly stopping when a outlet valve is suddenly closed. Source:Pickford,John.1969.Analysis of Water Surge.Gordon and Breach Science Publishers. |
What is water hammer?
Water hammer (or hydraulic shock) is the momentary increase in pressure, which
occurs in a water system when there is a sudden change of direction or velocity
of the water. When a rapidly closed valve suddenly stops water flowing in
a pipeline, pressure energy is transferred to the valve and pipe wall. Shock
waves are set up within the system. Pressure waves travel backward until encountering
the next solid obstacle, then forward, then back again. The pressure wave’s
velocity is equal to the speed of the sound; therefore it “bangs”
as it travels back and forth, until dissipated by friction losses. Anyone
who has lived in an older house is familiar with the “bang” that
resounds through the pipes when a faucet is suddenly closed. This is an effect
of water hammer.
A less severe form of hammer is called surge, a slow motion mass oscillation
of water caused by internal pressure fluctuations in the system. This can
be pictured as a slower “wave” of pressure building within the system.
Both water hammer and surge are referred to as transient pressures. If not
controlled, they both yield the same results: damage to pipes, fittings, and
valves, causing leaks and shortening the life of the system. Neither the pipe
nor the water will compress to absorb the shock.
Investigating the Causes of Water Hammer
A water transport system’s operating conditions are almost never at a
steady state. Pressures and flows change continually as pumps start and stop,
demand fluctuates, and tank levels change. In addition to these normal events,
unforeseen events, such as power outages and equipment malfunctions, can sharply
change the operating conditions of a system. Any change in liquid flow rate,
regardless of the rate or magnitude of change, requires that the liquid be
accelerated or decelerated from its initial flow velocity. Rapid changes in
flow rate require large forces that are seen as large pressures, which cause
water hammer.
Entrained air or temperature changes of the water also can cause excess pressure
in the water lines. Air trapped in the line will compress and will exert extra
pressure on the water. Temperature changes will actually cause the water to
expand or contract, also affecting pressure. The maximum pressures experienced
in a piping system are frequently the result of vapor column separation, which
is caused by the formation of void packets of vapor when pressure drops so
low that the liquid boils or vaporizes. Damaging pressures can occur when
these cavities collapse.
The causes of water hammer are varied. There are, however, four common events
that typically induce large changes in pressure:
1. Pump startup can induce the rapid collapse of a void space that
exists downstream from a starting pump. This generates high pressures.
2. Pump power failure can create a rapid change in flow, which causes
a pressure upsurge on the suction side and a pressure downsurge on the discharge
side. The downsurge is usually the major problem. The pressure on the discharge
side reaches vapor pressure, resulting in vapor column separation.
3. Valve opening and closing is fundamental to safe pipeline operation.
Closing a valve at the downstream end of a pipeline creates a pressure wave
that moves toward the reservoir. Closing a valve in less time than it takes
for the pressure surge to travel to the end of the pipeline and back is called
“sudden valve closure.” Sudden valve closure will change velocity
quickly and can result in a pressure surge. The pressure surge resulting from
a sudden valve opening is usually not as excessive.
4. Improper operation or incorporation of surge protection devices can
do more harm than good. An example is oversizing the surge relief valve or
improperly selecting the vacuum breaker-air relief valve. Another example
is to try to incorporate some means of preventing water hammer when it may
not be a problem.
Finding Practical Solutions
The surge pressure must be incorporated with the operating pressure in the
design of the pipe. The recommendations and requirements regarding allowances
for surge pressure are given in the American Water Works (AWWA) standards
and manuals for water supply practice, and vary depending on the type of pipe
used. The following are some tools to reduce the effects of water hammer:
Valves
Water hammer often damages centrifugal pumps when electrical power fails.
In this situation, the best form of prevention is to have automatically-controlled
valves, which close slowly. (These valves do the job without electricity or
batteries. The direction of the flow controls them.) Closing the valve slowly
can moderate the rise in the pressure when the downsurge wave—resulting
from the valve closing—returns from the reservoir.
Entrained air or temperature changes of the water can be controlled by pressure
relief valves, which are set to open with excess pressure in the line and
then closed when pressure drops. Relief valves are commonly used in pump stations
to control pressure surges and to protect the pump station. These valves can
be an effective method of controlling transients. However, they must be properly
sized and selected to perform the task for which they are intended without
producing side effects.
If pressure may drop at high points, an air and vacuum relief valve should
be used. All downhill runs where pressure may fall very low should be protected
with vacuum relief valves. Vacuum breaker-air release valves, if properly
sized and selected, can be the least expensive means of protecting a piping
system. A vacuum breaker valve should be large enough to admit sufficient
quantities of air during a downsurge so that the pressure in the pipeline
does not drop too low. However, it should not be so large that it contains
an unnecessarily large volume of air, because this air will have to be vented
slowly, increasing the downtime of the system. The sizing of air release valves
is, as mentioned, critical.
Figure 2-Sudden Valve Closing
 (a) Valve in open position. (b) Valve
is shut. A pressure wave moves upstream with velocity “a”.
At the same time water still enters the pipe with (c) Wave
front continues upstream until it reaches the end taking time T a to
reach there. The time 2L/a is known as the (d) If
the total quantity of water that enters the pipe during this time 1/2
mu is Delta V then because it is moving with velocity V0 , |
Pump
Pump startup problems can usually be avoided by increasing the flow slowly
to collapse or flush out the voids gently. Also, a simple means of reducing
hydraulic surge pressure is to keep pipeline velocities low. This not only
results in lower surge pressures, but results in lower drive horsepower and,
thus, maximum operating economy.
Surge Tank
In long pipelines, surge can be relieved with a tank of water directly connected
to the pipeline called a “surge tank.” When surge is encountered,
the tank will act to relieve the pressure, and can store excess liquid, giving
the flow alternative storage better than that provided by expansion of the
pipe wall and compression of the fluid. Surge tanks can serve for both positive
and negative pressure fluctuations. These surge tanks can also be designed
to supply fluid to the system during a downsurge, thereby preventing or minimizing
vapor column separation. However, surge tanks may be an expensive surge control
device.
Air Chamber
Air chambers are installed in areas where water hammer is encountered frequently,
and are typically seen behind sink and tub fixtures. Shaped like thin, upside-down
bottles with a small orifice connection to the pipe, they are air-filled.
The air compresses to absorb the shock, protecting the fixture and piping.
Conclusion
Water hammer will continue to challenge engineers, operators, and managers
of water systems because it is associated with systems that cannot be exactly
defined due to the size and length of the water distribution system with ondulating
profile or the lack of definition of the system components such as valves
or pumps. There is a need for a more practical approach while research continues
to provide better descriptions of the physics of water hammer and for useful
computational solutions including those basics.
Where can I find more information?
Kroon, J. R., M. A. Stoner, and W. A. Hunt. 1984. “Water Hammer: Causes
and Effects.” Journal of the American Water Works Association.
76: 39–45.
National Drinking Water Clearinghouse. 2001. “Ask the Experts.”
On Tap. Vol. 1, Issue 3: 10–11.
Parmakian, J. 1963. Waterhammer Analysis. Dover Publications.
Sharp, B.B. and D. B. Sharp. 1996. Water Hammer: Practical Solutions.
New York: Halsted Press.
Weis, F. 1996. “Dispelling Common Misconceptions about Water Hammer.”
Water Engineering and Management. 143: 24–30.
Wood, D. J. 2002. SURGE2000 Software. (Modeling water hammer in pipes and
a wide range of hydraulic and surge protection devices are addressed). Civil
Engineering Software Center, University of Kentucky Lexington, KY.
About the Author
Z. Michael Lahlou holds a doctorate in environmental and natural resource economics and a master's in civil and environmental engineering.Formerly the technical assistance coordinator for NDWC,Lahlou now resides in Huntington Beach ,California.