Water hammer

Changes in the flow regime of a fluid contained within a pipe often lead to sudden variations in pressure. These variations will be greater if the fluid is a liquid, and if the change in flow rate has been abrupt. Compared with the steady state, pressures can reach excessive values.

Water hammer is the term used to describe the pressure variations caused by a sudden change in the flow of a liquid inside a pipe.

Water hammer can have a variety of causes, but is a frequent occurrence when starting up or shutting down a hydraulic system, for example, when a valve is closed quickly or a pump is shut down quickly.

The unsteady operation of a hydraulic system, even in very exceptional cases, should always attract the attention of the design engineer. These phenomena can have unfortunate consequences, such as burst pipes and damage to equipment through which the fluid flows.

It is therefore vital to anticipate and study these transient phenomena in order to reduce their effects through the use of special devices and the correct dimensioning of the various components of an installation.

Although its application remains limited, this article will first deal with the very simple theory of "mass water hammer", which can sometimes be used in certain hydraulic installations.

The properties of pressure waves in deformable pipes will be developed. Unsteady phenomena, for which the study of pressure wave propagation is indispensable, are often referred to as "wave water hammer", as opposed to "mass water hammer". In everyday language, they are simply referred to as "water hammer".

The fundamental equations, translating the unsteadiness of a flow, can be directly processed on a computer, but the numerical results obtained in this way do not have the merit of accurately describing the physical phenomenon. This is not the case for Bergeron's graphical method, which will highlight the nature of water hammer. The principle of this method will first be developed for wave water hammer, then extended to pumping stations with their water hammer protection devices.

On the other hand, Bergeron's graphical construction has the disadvantage of becoming confusing when hydraulic installations are complex. Computer-aided design has, of course, replaced graphical construction with a variety of methods. We'll take a closer look at the one most commonly used.