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What is Water Hammer?

Water hammer (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 water hammer, 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.

Water hammer 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.

Three Types of Water Hammer

Water hammer 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:

Liquid-full system where there is a planned or unplanned change in equipment or component operation such as pump trips and starts or valve closing or opening
Liquid-full system where there is a planned or unplanned change in equipment or component operation such as pump trips and starts or valve closing or opening
Liquid-full system where there is a planned or unplanned change in equipment or component operation such as pump trips and starts or valve closing or opening

Codes & Standards for Water Hammer

Unfortunately, very little guidance exists from codes and standards on water hammer. 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

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 water hammer events over many years can cause fatigue damage and failure which never gets properly attributed to water hammer 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 water hammer. 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, water hammer 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.

Training Options

There are a few options to choose from when discussing training with your engineering representative:

Lunch & Learn (Free): Schedule a 1-hour discussion with an AFT engineer to educate your team on water hammer and mitigation options. This can be a virtual call, or if your team has office capability, AFT will be happy to send lunch to you for up to 5 people while you take the call and learn more about water hammer and surge analysis.

Water Hammer Analysis (Paid) Training for you or your team. AFT Impulse was created to be an intuitive tool for everyday engineers. But there is still a learning curve. Once you learn it, you will join the hundreds of specialized engineers around the world who use AFT Impulse to design and troubleshoot their piping systems. Your training can be at the AFT office in Colorado Springs, Colorado, US, or on location at your office. Virtual training options are available but are not advised. Learning how to analyze water hammer is more impactful in a direct-contact environment where questions and conversation can flow freely.


Consulting can range from project assistance to complete project analysis.

Flow Expert Pack: AFT offers prepaid hours in increments of 5, 10, and 20 hours that are flexible for your use. The most common uses are for team training or detailed modeling or analysis support for a specific project.

Full Project Consulting: Purple MountainTechnology Group specializes in surge consulting projects. They have worked on hundreds of surge projects in almost every industry. All projects are kept confidential and results are delivered in a presentation-ready format for you to share with your team.


Use the form below to request more information about training and consulting options.

If you are not located in the US or need languages other than English, AFT has 32 partners that serve 80+ countries. 

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