National
Drinking Water Clearinghouse
West Virginia University
P.O. Box 6064
Morgantown, WV
26506-6064
Ask
the Experts
“For those working with water distribution
systems, water hammer is a very serious problem. What steps does your system
take to prevent water hammer from occurring?”
Bigger Systems Mean More Troubles
What
we are seeing is the trend toward bigger and bigger water distribution systems.
It’s becoming easier to find a good source or treatment process and then
pipe the water long distances, than to deal with multiple water treatment plants.
However as these rural water systems grow, they tend to use more pipe and higher
pressures in order to effectively reach the ends of their distribution grid.
This definitely opens the door for water hammer problems.
One way to prevent (or lessen) water hammer is to start and stop high service
pumps in such a manner that it does not become an issue. Either Variable Frequency
Drives (VFDs) or “soft starts” allow the RPMs
(revolutions per minute)
of the motor to slowly increase (on start) and decrease (on stop). This prevents
that rapid change in energy inside the distribution system, which results in
water hammer. With the increased use of solid state circuitry and SCADA
(supervisory control and data acquisition)
systems, operators are becoming more familiar (comfortable) with this type of
technology; thus opening the door for the increased use of VFDs. Another potential
benefit of this technology may be the longer life of pump motors and reduced
overall electrical costs.
Rodney L. Coker
Tribal
Utility Consultant
Indian Health Service
Pressure Reducing Valves Are a Solution
To reduce
the possibility of water hammer, the City of Auburn, New York, uses pressure
reducing valves in the sections of the city’s distribution system that
typically experience pressures above 115 psi (pounds per square inch). In the
past, we’ve had problems with people using hydrants and shutting them down
improperly. Now, the Fire Department receives training in the proper use and
operation of hydrants. We have also established regulations that forbid the
use of hydrants without permission. If permission is granted, the operator of
the hydrants is given a brief lesson in the operation of the hydrant so as not
to cause a “hammer” in the system. These regulations also allow us
to have better control over backflow or cross-connection problems,
as we do not allow the hydrant to be used without a backflow preventer and meter
attachment.
Frank J. DeOrio
Director of Municipal Utilities
City of Auburn, New York
Fire Hydrants Can Be a Problem
The city of Greeley, Colorado, is very fortunate in that the topography of our
service area allows for the majority of the water to be fed by gravity. The
system has two water treatment plants. The main plant at Bellvue runs year-round,
and feeds the city some 37 miles away through three transmission lines—completely
by gravity. These lines do have 25 two-way air relief valves that allow us to
take the lines out of service and refill them, without any distribution system
problems due to entrapped air. The second treatment plant at Boyd Lake is a
peaking plant.
This pumped line goes directly to a 15 million gallon storage reservoir that
acts as its own surge-suppressor. There is also a surge-suppression tank located
at the pump station. The pumps all have soft-start motors and discharge valves
that open slowly through solenoid controls, to prevent hammering the pipeline
when the pumps are called for. The biggest problem our system faces with water
hammer is firefighters and contractors closing fire hydrant valves too quickly.
Unfortunately, that you cannot protect against completely.
Nelson Yarlott
Chief Plant Operator
Greely, Colorado
Sudden Closure Is the Main Culprit
Sudden closure of a control valve or stopping of a pump produces excess pressure
in a pipeline. Water hammer, or hydraulic transient, refers to pressure fluctuations
caused by a sudden increase or decrease in flow velocity.
According to the Washington State Water System Design Manual, “There are
a variety of ways to provide surge control. Methods include: open surge tanks,
pressurized surge tanks, surge anticipator valves, vacuum relief valves, regulated
air release valves, optimizing main size and alignment, electric soft start/stop
and variable speed drives for pumps, electric interlocks to prevent more than
one pump from starting at the same time, slow opening and closing valves, and
increasing the polar moment inertia of the rotating pump/motor assembly. A combination
of methods may be necessary and care must be taken in the design so that the
addition of a protection device does not cause a secondary water hammer equal
to or worse than the original design could cause.”
Reliability of the surge protection facility is also important. Where appropriate,
redundancy should be provided for essential equipment, such as vacuum relief
valves. Adequate alarms should be provided on surge tanks and similar components
to give operators early warnings. Consideration should be given to preventing
the pumping system from operating if the surge protection facilities are not
operable.
In Washington State, transmission mains designed to operate at velocities greater
than 10 feet per second must include a hydraulic transient analysis in conjunction
with a hydraulic analysis. Many of the computer programs designed to perform
hydraulic analysis are capable of performing transient analyses. The Peninsula
Light Company owns and/or manages very small water systems. Most of these systems
are groundwater wells with hydropneumatic (i.e., pressure) tanks that are not
only sized to reduce the frequency of pumps cycling on and off, but act as surge
protectors.
Surge control valves can also be installed between the pump discharge flange
and the check valve. The surge control valve is fully open when the pump is
started and passes sufficient flow in the fully open position. Once the pump
is running at full speed, the surge control valve slowly closes while the hydraulic
head slowly increases. Likewise, it slowly opens prior to the pump stopping.
Any new pump that is installed with greater than 15 horse power is required
to have a soft-start not only for control of water hammer, but less electrical
load impact.
In a few of our systems that have storage tanks, the booster pumps are controlled
by float switches rather than a surge suppressor. We also have a few systems
with fire hydrants. Staff are trained in how to properly open and shut these
valves. Unfortunately, we still have occasional “unauthorized” use
of fire hydrants. We are placing signs on the hydrants in the hopes of deterring
non-fire related use. Although, we’ve been mostly concerned about the potential
of cross-connections.
Lisa Raysby
Water Department Manager
Peninsula (Washington) Light Company
Got a Question?
Do you have a question you would like our experts to answer?
If so, please contact Kathy Jesperson at kjespers@wvu.edu or Mark Kemp-Rye
at mkemp@wvu.edu.
You also may call (800) 624-8301 or (304) 293-4191.