What is Water Hammer? Water Hammer Effect

What is Water Hammer?

A water hammer is a phenomenon that can occur in any piping system where valves are used to control the flow of liquids or steam. A water hammer is the result of a pressure surge or high-pressure shockwave that propagates through a piping system when a fluid in motion is forced to change direction or stop abruptly. This shockwave is also commonly referred to as a hydraulic shock or hydraulic surge and may be characterized by a marked banging or knocking sound on the pipes immediately after shutoff.

A water hammer can occur when an open valve suddenly closes, causing the water to slam into it, or when a pump suddenly shuts down and the flow reverses direction back to the pump. Since water is incompressible, the impact of the water results in a shock wave that propagates at the speed of sound between the valve and the next elbow in the piping system or within the column of water after the pump.

Formation of Water Hammer

The formation of the water hammer can be understood very well from the diagrams below.

After condensate is formed, the flow inside the pipe has two components, steam, and condensate. The flow velocity of steam is much higher than that of condensate. During such dual phase flow, the heavy condensate which flows at the bottom of the pipe is pulled by high-speed steam. This results in the formation of a water slug which is much denser than steam traveling with the velocity of steam. When this slug is stopped by any abruption like a bend or equipment, the kinetic energy of the slug will be suddenly converted into pressure energy which will create a shock wave in the entire pipework. The pipework will keep on vibrating until this energy is dissipated in the structure.

Water Hammer Effect and the Preventive Measures on Pipeline

The water hammer effect means that when the open valve is suddenly closed, due to the inertia of the pressure water flow, the shock wave of the water flow is generated and then causes damage. This is the “water hammer effect” in hydraulics, also equal to the positive water hammer.

On the contrary, after the closed valve is suddenly opened, it will also produce a water hammer, called a negative water hammer, which also has a certain destructive power, but it is not as large as the former.

Usually, the closing element suddenly sucks into the valve seat when the valve is close to closing, which is called the cylinder blocking effect.

Although the water hammer produces a lot of noise, the real damage is caused by mechanical failure. Because the kinetic energy changes drastically into a static line pressure, the water hammer can break the line or damage the pipe support and damage the pipe joint. For valves, a water hammer can cause severe vibration through the spool, which can cause the failure of the disc, gasket, or packing.

For valves, the way to prevent water hammer is to prevent any sudden pressure changes in the system.

To prevent pressure fluctuations, the valve should be closed at a uniformly varying speed. In some cases, when using the fast-open feature, it may be required to change to an equal percentage characteristic. For control valves, the water flow must be throttled when approaching the seat, an actuator with a sufficiently large output thrust, such as a piston-type pneumatic actuator or a hydraulic actuator, or a special notch on the stroke sleeve of a manually rotating operator.

Setting a type of anti-fluctuation on the pipeline system also reduces the water hammer effect. This can be done by using a pressure relief valve or a buffer tank. In addition, gas can be injected into the system, and gas injection can reduce the density of the fluid and provide some compressibility to handle any sudden fluctuations.

Tips to Avoiding Water Hammer

Avoid fast closing valves: Manual valves should be closed slowly, especially when the pumps are still running. Fast closing solenoid valves should be avoided when other mitigating measures are not installed. For domestic purposes, angle seat valves or manual ball valves that you close slowly are excellent valves to use in drinking water systems.”
Install expansion tanks, water towers, surge tanks, or hydraulic accumulators: These systems capture pressure surges by allowing expansion of the fluid and preventing water hammer.
Reduce operating pressure: Quite often the main supply pressure is not needed across the pipe network. Throttling down the pressure locally will help prevent water hammer.
Install pipes with high-pressure ratings: This does not prevent water hammering but protects against damaging the pipe
Process operating procedure: Start-up and shutdown procedures should consider the effects of the water hammer.
Install blow-off valves and air valves: Air valves help to reduce low-pressure regions which prevent the acceleration of the fluid. Blow-off valves allow the discharge of excess pressure in the flow.