Understanding Water Hammer

Ohio AWWA State Conference Presentation

September 17, 2004 Dan Barr, PE

What Is Water Hammer?

What Is Water Hammer?

What Is Water Hammer?

Webster says

“A concussion or sound of concussion of moving water against the sides of a containing pipe or vessel.”

Also known as Surge Events or Fluid Transients

Valve Closure

System Dynamics of Valve Closure

Pump Start

Why Should I Care About Water Hammer?

Why Should I Care About Water Hammer?

Service breaks Main breaks and flexible pipes versus rigid

– Plastic pipe has lower pressure rises but could fatigue and potentially fail at pressures under their rated capacity over time.

– Steel, Concrete, and Ductile Iron don’t expand as easily generating higher pressure rises, but lower fatigue problems.

Contamination potential from negative pressures in mains

Instrumentation and Gauge failures from extreme high or low pressures

Control and Alarm issues – PRVs, Pressure based pump controls, etc.

Where and When Should I Be Concerned About Water Hammer? Anywhere flow stops or starts quickly

– Pump stations – Control valves and isolation valves– Hydrants– Rapid and large demand changes– Air release valves– Column separation

Longer Mains and higher velocities Networked mains less likely to have problems

Air Valve Caused Surge

Air pocket

Air Valve Caused Surge

Air is quickly expelled out of valve.

Air Valve Caused Surge

Water hits valve and is immediately stopped.

Air Valve Caused Surge

Secondary transient starts

Water Column Separation

High point in main

Water Column Separation

Low pressure wave arrives causingpotential vacuum conditions

Water Column Separation

High pressure wave arrives laterand forces vacuum pocket closed

Water Column Separation

Secondary transient wave started

Do I Have Problems with Water Hammer? How Can I Tell? Banging Noises and Vibration – Where term water

hammer comes from– Pressure waves bouncing off piping– Check valve slamming

Frequent pipe breaks in the same places especially near pump stations, etc or low or high points in the system.

System measurements – normally transients are too fast for SCADA systems

Potentially minimal symptoms due to infrequent occurrence or no monitoring

What Do I Do About It?

Competent computer analysis to fully determine cause of problem or confirm no problem. Will also establish proper selection of any required corrective measures including locations, timing, and sizing.

Preventive measures – Valve operation including timing– Pump starts/stops including delays, interlocking, and

timing– Proper check valve selection to prevent slamming – Air valves

Air Valves:Controlled Column Rejoining

a lot of air

Low pressure wave hits and valve lets air in quicklyto prevent vacuum.

Air Valves:Controlled Column Rejoining

a little air

High pressure wave arrives and tries to pushout air. Small orifice prevents this and

cushions the pressure wave.

Air Valves:Controlled Column Rejoining

Water comes to a stop with no transient

What Do I Do About It?

Control measures– Surge control valves like pressure relief valves– Surge and air tanks– Pump bypasses– Stronger system design

Questions and Answers