What is Water Hammer?

Water hammer, also known as hydraulic shock, occurs when fast-moving water suddenly hits an obstruction in a piping system, like a bend or valve. It is defined as a sudden increase in pressure due to the abrupt halt or change in direction of fluid motion.

An example of water hammer occurs during the startup of a long steam line when steam and condensate mix. In daily life, it happens when we suddenly open or close a water tap, causing a sudden increase in pressure.

Misunderstood Concept about Water Hammer

Many thermo-hydraulic phenomena are often mistaken for water hammer, such as hydraulic and thermal shocks. Water hammer typically occurs due to a lack of awareness and poor operation and maintenance practices. As the saying goes, “prevention is better than cure.”

Occurrence of Water Hammer

When steam leaves the boiler, it travels a distance before reaching its point of use, like a steam turbine or heat exchanger. During this journey, the steam loses heat and starts to condense. During plant startup, the rate of condensate formation is very high because the system starts from a cold state.

During the operation these droplets of condensate starts building up along the length of steam pipe net work and thus forming a solid slug of condensate as shown in the given

Condensation creates water droplets that gradually build up along the pipe, forming a solid slug. When this slug encounters an obstruction, like an orifice, valve, or bend, it suddenly stops. The kinetic energy of the slug then converts to pressure energy, which the pipe network must handle.

Impact of Water Hammer

It is necessary to understand the serious impact of water hammer in equipments used in the plants. Given below example clearly explains the destructive nature of water hammer:

• For Saturated steam recommended velocity is 25 to 35 meters per second
• For water in a pipe network recommended velocity is 2 to 3 meters per second

When water hammer occurs, condensate slug is dragged by steam and thus the water slug travels with a velocity equal to that of steam which is ten times more than that of water velocity. Thus water hammer is always associated with very high pressure.

Factors help in avoiding water hammer

Steam system is very complex and dynamic, thus avoiding water hammering is a challenging task. But with the help of following the best engineering practices its occurance can be easily surmount by adopting:

• Proper inclination should be provided in the steam lines in the direction of flow.
• Steam trap installation at regular interval and that too at the lowest points. Installation of the steam trap at the lowest points ensure the removal of condensate from the system.

• Pipe sagging results in formation of condensate in the piping network and may result in increasing the chances of water hammering. Thus steam pipes should be properly supported to avoid any sagging.
• Standard start up procedures are required for cold start of the plant. Operators should be properly trained to take care of the openly of isolation valve slowly.
• Proper sizing of the drain pockets, to ensure that the condensate should not jump or pass it easily. The purpose of drain packet should not be defeated to collect all the condensate and pass the same through the trap.
• Type of reducers shall be eccentric instead of concentric reducers.

Water Hammer in Power Plants

Water hammer occurs when water, accelerated by steam pressure or a low-pressure void, is suddenly stopped by impact on a valve or fitting, such as bend or tee, or on a pipe surface. Water velocities can be much higher than the normal steam velocity in the pipe, especially when the water hammer is occurring at startup.

When these velocities are destroyed by impact, the kinetic energy in the water is converted into pressure energy, and a pressure shock is applied to the obstruction. In mild cases, there is noise and perhaps movement of the pipe.

More severe cases lead to fracture of the pipe or fittings with almost explosive effect and consequent escape of live steam at the fracture. Fracturing of pipes or steam system components can propel fragments that can cause injury or loss of life on it.