Water hammer

Joukowsky-Pressure-Shock-01

Blown expansion joint

Water hammer pressure

Water hammer (also known as hydraulic shock) is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (a momentum change). This phenomenon can commonly be observed in both plumbing and hydraulic systems when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.

Causes

The cause of water hammer is a sudden change in the flow rate of a fluid. When a valve closes quickly, it causes a rapid change in the velocity of the fluid, which in turn leads to a sudden increase in pressure. This pressure wave can travel through the system at high speed, causing vibrations and noise. In some cases, the force of the water hammer can be strong enough to cause pipes to burst or fittings to become detached.

Effects

The effects of water hammer can range from a loud banging noise to serious damage to pipes, valves, and fittings. The intensity of the water hammer can depend on several factors, including the speed of the fluid, the length and diameter of the pipe, and the speed at which the valve is closed.

Prevention and Mitigation

To prevent or mitigate the effects of water hammer, several strategies can be employed. These include:

• Installing air chambers or water hammer arrestors at strategic points in the plumbing system. These devices can absorb the shock wave created by the sudden stopping of water.
• Gradually closing valves, especially large ones, to reduce the rate of change in fluid velocity.
• Using variable speed pumps that can slow down before stopping, reducing the chances of creating a pressure surge.
• Designing piping systems with smooth changes in direction and securing pipes properly to reduce movement.

Mathematical Analysis

The mathematical analysis of water hammer involves the use of the Joukowsky equation, which relates the change in pressure to the velocity of the fluid and the speed of sound in the fluid. This equation is a fundamental tool for engineers designing systems to withstand the effects of water hammer.

Historical Context

The phenomenon of water hammer was first observed and documented in the 19th century with the advent of steam engines and the widespread use of pipe systems for the distribution of water. Engineers and scientists such as Nikola Tesla and Lorenzo Allievi have contributed to the understanding and solutions for managing water hammer.

See Also

• Hydraulics
• Piping
• Fluid dynamics
• Pump