Water hammer in closed piping systems: causes, effects and solutions

What is water hammer?

Water hammer is a sudden increase in pressure in a pipeline caused by rapid changes in flow velocity. This phenomenon occurs when a moving liquid in a closed system is forced to stop abruptly or change direction. This pressure surge propagates throughout the entire system and is initially amplified, leading to a pressure increase beyond the acceptable limit for the components. What is more, the flow velocity increases significantly and can, under unfavourable conditions, reach speeds of up to 1,200 m/s. 

Water hammer occurs not only with water but also with other incompressible liquids (i.e. liquids whose density does not significantly change under pressure) or with gases. The pressure surge is often accompanied by vibrations or a knocking sound that repeats before the pressure subsides, hence the name.

Although the pressure surges can be clearly heard, they are difficult to measure because the pressure surge builds up and subsides within an extremely short space of time, in the millisecond range. Electronic measuring devices are often not suitable for such measurements, as the pressure surge exceeds the upper limit of the measuring range.

The causes of water hammer

There are several possible causes of water hammer, including the rapid opening and closing of valves, rapid starting or stopping of pumps, too rapid closure of check valves, and sudden interruptions in power supply. The separation of the water column or pressure buffer also causes the phenomenon.

However, how a system is designed also influences the occurrence of water hammer. In systems with large differences in elevation, the risk is particularly high because higher pressures are required to overcome the elevation differences.

System designs that interrupt uniform flow also pose a greater risk. Bends and significant pressure fluctuations should therefore be avoided as far as possible.

The direct effects of water hammer

Water hammer can cause serious damage:

• Leaks at components.
• Deformation or destruction of the valve seat.
• Deformation of check valve discs.
• Broken or bent pump shafts.
• Premature flattening of seals and O-rings.
• Broken impellers.
• Leaky or broken pipelines.

Possible long-term consequences for pipeline systems

In addition to the damage to individual instruments and components, water hammer can often impinge on the service life of the entire system. This risk increases with the intensity of the water hammer. Leakages in the pipelines frequently occur, too, resulting in the system not functioning efficiently for an extended period and resulting in the loss of medium or product. 

How to avoid water hammer

Selecting the right valves during the planning phase

Ideally, the system should be planned in such a way as to minimise the risk of water hammering right from the start. The selection of suitable valves plays an all-important role in the planning phase. System planners should ensure that they select the highest quality valves with the appropriate functionality in order to avoid the possibility of water hammer from the outset. Pneumatically or electrically actuated valves are available in the appropriate variants. Manually actuated valves can also be used. Here is a brief overview of the requirements that valves need to fulfil.

Pneumatically actuated process valves with adjustable opening or closing times

For pneumatically actuated process valves, angle seat valves (also known as Y-valves) or full-way valves (also known as straight valves) are suitable options. It is important to ensure that the valves are configured so that they close against the flow direction of the liquid. This aligns the closing function with the inertia of the medium, naturally preventing pressure surges. Spring-return valves and double-acting valves can both be configured in this manner.

Electromotive valves

Like pneumatically actuated process valves, electromotive valves are also available with suitable housing variants as angle seat or globe valves. Motor valves from Bürkert possess the following features that have a favourable impact on the issue of water hammer:

• Rigid drive train that is non-reactive and independent of media pressure. This means that pressure fluctuations or surges in the medium are not transferred to the valve position. Compact valves (Types 3280/3281/3285) are among the recommended options here. 
• The specific mechanical design of these valves, in contrast to pneumatically actuated process valves, does not require spring loading and offers high shock and vibration resistance.
• The positioning speed of Bürkert motor valves can be precisely adjusted via a function in the software. This means valves can be deliberately set to open or close slowly, thus preventing unnecessary pressure increases in the system and minimizing associated impacts.

Electromotive 2/2-way diaphragm valve (open/close)

Type 3323

• Safety position through energy storage
• Adjustable driving force
• Diagnostics functions and fieldbus connection
• Wetted surfaces in Ra ≤ 0.38 µm…1.6 µm (optionally electropolished)
• Available in nominal diameters DN 06…DN 100

Diaphragm valves

In the field of pharmaceutical and biotech production, pneumatically or electromotively actuated diaphragm valves can also be used. This type of valve, unlike seat valves, can be used irrespective of the flow direction, but is subject to the same design criteria (kV value, maximum pressure, etc.) as seat valves. It should be noted that with diaphragm valves there can be a temporary external leakage during water hammer events (not only over the weir), which occurs only during the brief moment of the water hammer. In ideal circumstances, provided the diaphragm is not damaged, the valve can continue to fulfil its normal functioning.  

2/2-way diaphragm valve with pneumatic stainless steel actuator (Type ELEMENT) for decentralised automation

Type 2103

• Valve body and diaphragm are available in various materials and variants
• Wetted surfaces from Ra ≤ 0.38 µm...≤ 1.6 µm (optionally electropolished)
• Available in all common connection sizes and variants

Optimisation options for existing systems

In existing systems, there are various approaches to retroactively reduce the risk of water hammer. This type of system-wide adjustment should only be considered by a process engineer with a comprehensive understanding of the entire process. 

Possible measures include:

• Replacing individual valves with ones that have adjustable opening and closing speeds.
• Installing flow restrictors and pipeline stages to reduce the volume flow rate of the liquid through the valve, ensuring slower valve closure. 
• Slowing down the closing speed of pneumatic process valves by restricting the flow of the pneumatic air supply or exhaust of the pneumatic actuator. 
• Use of electromotive actuators with adjustable opening and closing speeds.
• Limitation of speed changes in the system.
• Deployment of double-acting actuators with appropriate control unit.

Conclusion

Water hammer is a potentially serious problem in pipeline systems that can cause damage to system instruments and pipelines. The correct selection of valves, adjusting the closing speed, installing flow restrictors or check valves, and other system adjustments by experienced professionals can prevent water hammer in most cases.

Do you have a problem with water hammer? Are you looking to prevent it?

The fluidics experts at Bürkert will advise you during the planning phase and are also available to assist you in selecting suitable components.

Contact us now to learn more