The Risks of Water Hammer

The risks of water hammer range from vibrations in the pump and its breakdown, as well as the valves. It can also lead to the breakage of connections and pipes, as well as the damage of valves, and the collapse of the entire system.

Description of the Water Hammer Phenomenon

When there is any change in the steady-state flow of a fluid within a conduit or a pipe network. Such as the closure of a valve or the stopping of a pump, the sudden change affects the steady-state flow. Waves are generated that propagate at a speed close to the speed of sound in the fluid. Starting from the point where the flow disturbance occurred (the valve or the pump) to the end of the conduit. Or any change in the pipe cross-section or branching. These waves are then partially or fully reflected and return to the original section from which they were launched, to be reflected again, and so on. Until they are damped due to friction, and the fluid settles into a new equilibrium position.

The transition from a steady flow state in the conduit or network to another steady state is always accompanied by the propagation of pressure waves throughout the conduit or network. Leading to a change in the fluid pressure in the conduit. The values of the transient pressures – which can sometimes have destructive effects – depend on the magnitude of the change in the fluid flow velocity in the conduit or network. From the element that caused the disturbance (valve, pump…), and led to the deceleration or acceleration of the fluid.

Protection Methods Against the risks of water hammer

In principle, the conduit or any pipe system can be designed to withstand all the maximum and minimum pressures. That can arise under any possible operating conditions during the project’s lifetime. However, such a design would be economically unviable in most cases. Therefore, it is necessary to follow protective methods that rely on the use of special equipment. Or control procedures aimed at preventing the occurrence of high or low-pressure waves. That can cause serious damage to the conduit or the pipe system.

There are many water hammer protection devices, and the design and working principle of each varies according to the nature of the case for which it is used. There is no single suitable device for all cases and all operating conditions. Therefore, when designing a conduit or a pipe system, a balance must be struck between a set of options and the most appropriate solution for the conduit or system with a suitable economic cost must be selected.

There are several commonly used means of protection against water hammer, and the appropriate cases for their use include the following:

Slow Valve Closure

The rate of valve closure is of great importance in determining the maximum value of the pressure wave generated by the closure. If the valve closing time is short (rapid closure), the pressure at the valve is likely to rise to large values. Which may pose a danger to the pipe.

The optimal solution is to choose a suitable valve closing time so that the maximum and minimum pressure values generated by the closure process are within acceptable limits. This is determined by calculation methods.

Surge Tanks

In cases where it is not possible to control the transient pressure values in the conduit. Or system by adjusting the valve closure process. Or slowing down the pump deceleration, diverting the fluid flow to surge tanks may reduce the deceleration rate and hence the resulting pressure values.

Pressure Vessels

Pressure vessels are used in cases where it is not possible to use open-top surge tanks for economic or technical reasons. A pressure vessel is a container that holds compressed gas (usually air) in its upper part and liquid in its lower part. Pressure vessels are often used as a means of protection against the water hammer resulting from the sudden stoppage of pumps. In this case, the pressure vessel is placed at the pump discharge end, downstream of the check valve.

In the event of a sudden pump shutdown, the pressure at the pump discharge decreases. Causing the air in the tank to expand and push the liquid toward the conduit, thus reducing the severity of the flow rate change in the conduit and the resulting pressure drop. Upon reversal of the flow in the conduit, the check valve at the pump discharge closes. The entire flow is diverted to the tank, causing the air to compress and its volume to decrease. The flow in and out of the tank and the expansion and compression of the air inside it serve to mitigate the resulting maximum and minimum pressure values.

Pressure vessels have several advantages over open-top surge tanks. The most important is that the required pressure vessel size to maintain the maximum and minimum pressures within acceptable limits is always smaller. They can also be installed horizontally and near the pump, which is not feasible for large-volume surge tanks. The main drawback is the need for air compressors to compensate for the air dissolved in the liquid. That requires periodic maintenance of the compressors.

Air intake and exhaust valves

When the pressure at certain points in the conduit drops below atmospheric pressure. This can lead to the separation of the liquid column and then its subsequent re-joining. Which is accompanied by high pressures. In such cases, it may be appropriate to use air intake valves at those locations exposed to low pressures. The function of the air intake valve is to open and allow air to enter the conduit when the pressure at the valve drops below atmospheric pressure. The air intake valve must allow sufficient quantities of air to enter during the low-pressure wave. Without being expelled too quickly once the wave subsides. This ensures a gradual re-joining of the liquid column and mitigates the resulting shock.

Pressure relief valves

In some cases, it may be more suitable to use pressure relief valves for protection against high-pressure waves, instead of using protection or pressure tanks. A pressure relief valve generally contains an orifice closed by a piston resting on a spring or a weighted gate. If the pressure of the fluid flowing in the pipe exceeds a pre-set limit. The maximum allowable pressure for the pipe with an appropriate safety margin. The piston or gate moves, exposing the orifice, and the fluid is discharged, reducing the pressure. Once the high pressure has subsided, the piston or gate returns to its original position due to the spring or external weight.

Additional information about protection against water hammer damage.