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• What is the Waterhammer Concept?
• What is Wavespeed
• Instantaneous Waterhammer Equations

• Inherent vs. Installed Valve Characteristics
• Inherent Valve Characteristics
• Installed Valve Characteristics

• Instantaneous Transients
• Effective Closure Time
• Instantaneous Waterhammer Sequence
• Communication Time: Pipe Period
• Common Causes of Waterhammer

• What is Waterhammer Mitigation? 
• Equipment-based Mitigation: Relief Valves and Air Valves
• Equipment-based Mitigation: Check Valves

• Waterhammer Codes: ASME B31.3 and  ASME B31.4
  
• Considerations for the Worst-Case Scenario
  
• When the Joukowsky Equation Fails

• Part 1: Inherent Valves
• Part 2: Installed Valves

Water hammer is a general term for pressure waves resulting from a sudden change in fluid velocity, such as during a valve closure or pump transient event.

Since water hammer is caused by sudden changes in fluid velocity, avoiding these sudden changes similarly avoids water hammer. Many cases, however, have unavoidable velocity changes, requiring surge be mitigated instead.

There are two general approaches to address water hammer. The first option is to avoid the water hammer surge by changing operation. As a simplified example, if a valve closes in 1 second and causes unacceptable water hammer, the valve could instead be closed over 10 seconds to reduce the water hammer effects. However, often the cause of a water hammer event is outside of an engineer’s control, who instead must focus on mitigating the event. This can be addressed by designing relief systems, strengthening surrounding piping and supports, or sizing equipment such as arrestors to mitigate the unavoidable surge.

Water hammer is caused by a sudden change in fluid momentum resulting in a change in the fluid’s pressure. Most often the concern is pressure increase, or surge, when stopping or slowing a fluid.

Water hammer is the result of conservation of energy. When a fluid’s velocity changes suddenly, the sudden change in momentum requires a change in the fluid’s pressure. If flow is suddenly stopped, the kinetic energy of the fluid is converted into internal energy, resulting in a sudden pressure increase known as surge. Any event which can suddenly change a fluid’s flow (pump start-ups, trips, relief events, valves opening and closing) has the potential to cause a water hammer event.

Absolutely! The sudden pressure rise from waterhammer can exceed pipe design limits and create large forces on the piping as pressure waves travel through a system.

Sudden pressure increase is often the primary focus of waterhammer analysis. Pipe design pressures should have sufficient margin above peak waterhammer pressure instead of only considering the normal operating pressure of a pipe. This ensures pipes do not burst during a transient event, potentially contaminating an area or creating unsafe conditions. The supports on pipes may also be impacted since large pressure differentials along a pipe can create significant forces on a structure.

Hydraulic transient phenomenon has many names: waterhammer, water hammer, hydraulic surge, hydraulic shock, etc. While the names may seem interchangeable, it is important to consider their implied meaning.

The word ‘waterhammer’ comes from the hammer sound created during a transient event. Often ‘waterhammer analysis’ may be narrow in scope only to look at high pressures generated from events like valve closures. The scope of transient analysis is far more nuanced, and these transients will happen to fluids other than water too! Low pressure events are just as dangerous as ‘surge’ high pressure events. It can be difficult to put a tidy name on such a messy phenomenon, so whether it is water hammer, waterhammer, surge or shock, ensure there is a clear explanation of the cause and effect of a transient event instead.

Arrestors and gas accumulators acts as sources and sinks of flow during a transient, smoothing rapid changes from unavoidable transients like pump trips and emergency shutdowns.

Ideally, waterhammer events can be completely avoided with proper design and considerations. However, some transient events are determined by outside forces instead of by an engineer’s design. These events, such as pump trips from power loss and emergency shutdowns, must be mitigated instead, dealing with the generated pressure change to minimize its effects. In these cases, equipment such as gas accumulators and arrestors can act as sources and sinks of flow during a transient, smoothing the otherwise aggressive change in velocity and accompanying change in pressure. However, improper sizing has the risk to exacerbate a transient event.