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What is Waterhammer?

Waterhammer (also known as surge) occurs when fluid velocity is changed by actions such as valve position changes and planned or unplanned pump trips. Little guidance exists in codes and standards, and accidents are more frequent than we would like to admit. It is the purpose here to summarize existing knowledge and practice on waterhammer, discuss the abilities and limitations of commonly used calculation methods, provide warnings on what may happen when systems experience phenomena such as transient cavitation and liquid column separation, and give some high-level guidance on how to solve surge issues in pumping systems.

Waterhammer is fundamentally the same phenomenon across all industries which need to transfer fluids. However, depending on the nature of the fluid (benign, toxic, flammable, biologically active, etc.) and nature of the application (high pressure, proximity to people, remotely located such as in Space) different concerns and strategies are involved. It is essential that engineers take proper precautions in their design and operations to ensure safe operation of pumping systems.

Waterhammer Overview

Waterhammer is a broad term that encompasses fast pressure transients as a result of a rapid change in liquid velocity. The liquid velocity change can be caused by three fundamental mechanisms:

1. Liquid-full system where there is a planned or unplanned change in equipment or component operation
      a. Examples:
          i. Pump trips and starts
         ii. Valve closing or opening

2. Liquid or vapor system where there is a rapid phase change which causes a change in volume which then accelerates a liquid slug
      a. Example:
         i. Condensation of a vapor which creates and/or augments a liquid slug

3. Liquid/gas two-phase flow where differences in velocity can cause liquid slugs to impact equipment and elbows
     a. Examples:
         i. Oil and natural gas near extraction points and put into a common pipeline before separation
        ii. Starting a pump into an evacuated, air-filled line 
        iii. Air trapped in storm water systems where free surfaces exist and liquid is accelerated due to movement of the air

The second and third types of waterhammer are important but not as well understood as the first. The analysis methods and tools for these types of waterhammer are typically complicated and expensive. Merilo (1992) offers a voluminous discussion of the first and second types of waterhammer mostly relevant to the power generation industry. See Klaver et al. (2018) for a discussion of the third type of waterhammer in storm water systems. This current paper focuses on the first type of waterhammer in liquid-full systems.

Note that in the English language the word “surge” is an alternate and synonymous term for waterhammer.

Applicable Codes & Standards for Waterhammer

Unfortunately, very little guidance exists from codes and standards on waterhammer. In practice, engineers are expected to use judgement and experience. Here are two excerpts from ASME piping code:

ASME B31.4: “Surge calculations shall be made, and adequate controls and protective equipment shall be provided, so that the level of pressure rise due to surges and other variations from normal operations shall not exceed the internal design pressure at any point in the piping system and equipment by more than 10%.”

ASME B31.3: “In no case shall the increased pressure exceed the test pressure used under para. 345 for the piping system.” And “Occasional variations above design conditions shall remain within one of the following limits for pressure design: Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 33% for no more than 10 hr at any one time and no more than 100 hr/yr or 20% for no more than 50 hr at any one time and no more than 500 hr/yr.”

Accidents & Safety Issues

It is the authors’ experience that the vast majority of waterhammer accidents go unreported. In most industrialized countries there are reporting requirements to the authorities when there is a fatality or injury. 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. Finally, there is informed speculation by some specialists that repeated waterhammer events over many years can cause fatigue damage and failure which never gets properly attributed to waterhammer due to the lack of a clear cause and effect relationship because of the deferred nature of fatigue. 

For examples of the preceding, see U.S. Department of Energy (2006) for a discussion of a fatality, Nennie et al. (2009) for a discussion of an environmental impact, and Leishear (2018) for a discussion of fatigue failure from waterhammer. See Karney (2018) for a discussion of many issues in municipal water systems. 

Even in cases where there is no impact to human or environmental safety, waterhammer can cause damage to piping, pumps and other equipment, pipe supports and insulation. This can lead to significant expense to system owners due to repairs and loss of production during system downtime.
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