WHAT IS THE WATER HAMMER CONCEPT?
Other terms such as fluid hammer, surge, and hydraulic transients are sometimes used as synonyms for water hammer.
Each series of articles are written by pipe flow analysis engineers from Applied Flow Technology. As industry leaders in water hammer and surge analysis, AFT has collected models and data from projects around the world to use as reference materials for published technical papers, case studies, and blogs. Visit www.aft.com for more information on analysis tools.
What is the Concept of Water Hammer?
Water hammer is a broad term referring to the pressure response in a system due to a rapid change in liquid velocity. Though the term “water hammer” includes the word water, this phenomenon is not exclusive to water systems. Other terms such as fluid hammer, surge, and hydraulic transients are sometimes used as synonyms for water hammer.
When water hammer occurs it often causes loud noises and can visibly shake the piping structure. In more extreme cases structural damage may be observed. Closing a faucet quickly and hearing a sound from the plumbing is a simple example of water hammer, though the impact of such an event is minor in scale when compared to the effects that may be observed in an industrial setting.
What Causes Water Hammer?
The liquid velocity change which initiates water hammer can be caused by three fundamental mechanisms:
- 1) Liquid-full systems where there is a planned or unplanned change in equipment or component operation such as pump trips, pump startups, or valves changing position.
- 2) Liquid or vapor systems where there is a rapid phase change which causes a change in volume that accelerates a liquid slug
- 3) Liquid/gas two-phase flow, or liquid flow into an evacuated system where differences in velocity can cause liquid slugs to impact equipment and elbows
In practice, all three of the mechanisms above can be equally detrimental and should be considered. However, only the first mechanism above is commonly addressed during waterhammer analysis. This is primarily due to the relative lack of knowledge on how to analyze the second and third mechanisms. The following topics on this website will primarily address waterhammer occurring in liquid-full systems.
What are the Effects of Waterhammer?
During a waterhammer event a sudden change in velocity causes a pressure wave to be propagated through the system. As the pressure wave is transmitted and reflected following the waterhammer sequence interactions with components in the system can cause further increases and decreases in pressure. If the velocity change is significant, these high and low pressure waves created in the piping network can exceed the maximum or minimum allowable pressures for the system components, resulting in potential damage to piping, fittings, pumps, and the support structures. Depending on the system fluid and location, the nearby population and environment can be impacted as well.
In most industrialized countries, there are strict reporting requirements to authorities when there is a fatality or injury due to water hammer. In cases where there is an environmental impact or a visible and significant impact from the accident (e.g., a release of a toxic, flammable or otherwise dangerous or undesirable fluid, or possibly damage from an excessive amount of water), the facility or pipeline owners are usually required by their authorities to address the accident. There is informed speculation by some specialists that repeated water hammer events over many years can cause fatigue damage and failure. These events rarely get properly attributed to water hammer due to the lack of a clear cause and effect relationship because of the deferred nature of fatigue.
Some documented cases and examples of water hammer related incidents are listed below:
- The US Department of Energy (2006) reported a fatality at Hanford DOE facility (see page 3)
- Nennie et. al (2009) discusses a large oil spill caused by water hammer
- Leishear (2018) discusses fatigue failure from water hammer
- This case study from Ingenero Technologies and AFT highlights damage to mixed bed polishers caused by the valve closure sequence used in the system
Depending on the system there are several water hammer mitigation options available to prevent the situations discussed above. Performing a water hammer analysis can help to determine what operational changes and/or equipment may be effective to avoid the potentially dire consequences of a water hammer event.