Every failure disrupts the proper operation of the system and entails costs associated with purchasing the necessary parts and carrying out repairs. Of course, in most situations, a fault is a matter of chance. However, there are many risks that can be avoided through proper care and planning. In the case of water supply installations and all types of water systems, one of the primary risks is the occurrence of so-called water hammer. Research shows that this phenomenon is responsible for nearly 80% of all failures occurring in water supply network pipelines not equipped with protective devices. But what is water hammer? What causes it? And what can be done to prevent it? We answer these and other questions in our article.
Water hammer – the theoretical foundations of the phenomenon
Water hammer is a term describing a sudden increase in pressure caused by an unplanned, rapid change in the flow velocity in a pipeline, forced to occur over a short period. The name of the phenomenon comes from the characteristic sound produced during the process, which resembles hammering. According to the specialist definition, water hammer is “caused by the inertia of the mass of liquid moving through a pipeline, whose flow velocity undergoes a sudden change.” Moreover, “a sudden change in the velocity and volumetric flow rate of the liquid causes a local change in the share of kinetic and potential energy in the total energy of the cross-section, expressed as an increase or decrease in pressure in the flow. Under conditions of very rapid deceleration of the flow, there is a sharp decrease in kinetic energy, causing an increase in potential energy, which manifests itself as a significant rise in pressure.”
Depending on the nature of the phenomenon, we may be dealing with positive water hammer (an increase in pressure) or negative water hammer (a sudden drop in pressure). Regardless of its type, water hammer can cause many problems, including:
- destruction of the pipeline,
- damage to the pump,
- engine damage,
- damage to check valves,
- interruption of the flowing water stream.
What causes the problem?
What causes the negative phenomenon known as water hammer? There may be many causes, but practice shows that the two main factors are:
- a situation in which the check valve on the pipe running from the pump is installed more than nine meters above the water surface level,
- a situation in which the check valve installed on the pipeline running from the pump is leaking, while another check valve installed above the level of the previous one remains tight.
Other causes that may lead to problems include:
- failures such as a ruptured pipe, sudden pump blockage, a twisted-off stem or stripped gate-valve thread, and vibrations of components subject to deformation,
- cavitation caused by excessive water flow velocity, a large difference between the pressure head and the liquid’s vapor pressure at a given temperature, closing the gate valve, or a high location of a network point,
- accidental events, including a sudden interruption in the electricity supply, a change in the initial flow conditions, a change in nodal outflows, or a change in the water level in initial or intermediate network tanks,
- improper system operation, manifested by the sudden shutdown of all pumps supplying the same pipeline, improper closing or opening of gate valves, and improper maintenance and repairs,
- vibrations associated with uneven pump operation or the operation of float valves.
However, this is not the end of the list. Another possible cause of the problem may be the use of unsuitable solutions, such as inserting a new pipe with a smaller diameter into an old pipeline (resulting in an increase in the flow velocity of the liquid). It is worth remembering that any modifications can lead to unplanned damage, so new solutions should be selected according to the characteristics of the system being analyzed.
How can it be prevented?
Importantly, water hammer and the problems associated with it can be prevented. Some methods include:
- selecting the correct cross-sections and protective devices, choosing suitable valves and gate valves, and preventing resonance,
- attempting to eliminate water hammer by directly changing the system configuration, including modifying the route, pipe lengths, diameters, pipeline materials, connections, or bypasses,
- introducing various devices and mechanisms into the system that cause liquid to be discharged from the pipeline or stored in a certain space outside the system; this involves using hydropneumatic tanks, hydropneumatic tanks with a differential orifice installed at the outlet, air-release valves, or safety valves.
