When water pipes are banging: What water hammer in the piping is all about

In the early hours of 2 January 1998 the residents of Fifth Avenue in New York City were woken up by a loud bang: A 100-year old main water line of 48 inch diameter had burst, resulting in several hundred thousand litres of water pouring down the street within record time, completely destroying long stretches of Fifth Avenue. In addition, flames about two-floors high shot out of a gas leak.

On 4 July 2009 a voltage loss affecting the entire city of Hamburg led to the abrupt stop of all pumps in 14 waterworks. When the pumps were started up again, a total of 16 damaged water lines burst, causing immense damage.

These are only two examples of events of one and the same cause – uncontrollable pressure surges in water pipes (water hammer). This phenomenon has been known since antiquity. As early as one hundred years before Christ water hammer in Roman lead and stone pipes was reported. However, it wasn't until 1887 that the first detailed experiments and calculations were conducted by the Russian mathematician Nikolai Joukowsky. The term Joukowsky surge has become established for this physical event. So, what causes such a pressure surge and how can possible damage be prevented right from the planning phase?

Hit hard: Where can water hammer occur and what causes it?

Generally speaking, water hammer can occur in any pipe transporting a liquid, e.g. water, chemicals, thermal oils, also liquid foods such as milk, or beverages. 

It is usually assumed that liquids are incompressible, meaning that unlike gases they cannot be compressed. However, this is not fully correct. Liquids are actually minimally compressible, albeit not as much as gases. Pressure surges cannot occur in “open pipes”.

What causes water hammer?

A pressure surge occurs whenever the flow velocity of a liquid in a pipe changes, i.e. when the flow is accelerated or decelerated. Let’s look at an example: When the tap in a guest toilet is fully opened, the flow in the pipe can easily reach 20 litres per minute – regardless of the pipes being comparatively small in diameter as regulated by the DIN standard for such installations. The result: The flow velocity of the water in the pipe can rise to 4 metres per second and more. When the valve is then closed quickly, the water volume builds up upstream of the valve. The pressure rises and the so-called pressure wave front expands in the opposite direction of the original flow – similar to a billiard ball hitting the edge and bouncing back. At the same time the pressure on the output side of the rapidly closed valve is reduced. This pressure reduction also expands, this time in the direction of flow.

The developing pressure wave in the pipe opposes the original direction of flow until it meets another reflection point, e.g. a sudden change in cross-section, a tee or a lift check valve. The surge is then reflected again, moving to and fro in the system until the pressure wave dies out.

Even lower velocities can cause severe damage by surge pressure when a sudden change in velocity occurs. For private households, water hammer is comparatively insignificant due to small diameters and lengths.

What velocities can such pressure waves reach?

The velocity is very high, depending on a number of factors, one of which is the temperature. However, the more decisive factor is the pipe material. Depending on the material, wave propagation speeds of up to 1200 m/s can be reached. (For comparison: the velocity of sound in dry air at 20 °C is “only” 343.2 m/s). The wave propagation velocity can be accurately calculated using this formula:

What influential factors are to be considered?

An equation developed by Joukowsky can be used as a first approach to calculate water hammer:

∆pJou = ρ · a · ∆v

∆pJou = Joukowsky surge = pressure change in a liquid [N/m2]

∆v = velocity change

ρ = density of the liquid [kg/m3]

a = wave propagation velocity in liquid-filled pipe [m/s]

When assuming simplified values for the acceleration due to gravity and the wave propagation velocity, this results in:

∆hJou = (a / g) · ∆v ≈ 100 ∙ ∆v

∆hJou = change in pressure head [m]

g = acceleration due to gravity [m/s2]

The closing times of the valve play a key role.

An abrupt change in velocity leads to maximum pressure changes whereas a slow change results in significantly smaller pressure amplitudes, providing an option to possibly prevent impermissible pressures.

In simple terms: When the liquid flow is stopped or accelerated, the kinetic energy in the system is converted into pressure energy. Since both the liquid and the piping are slightly compressible and expandable, they can actually absorb a fraction of the energy. The time for the surge pressure to die off depends on the characteristics of the pipe such as its material and the thickness and roughness of its interior walls as it is especially the energy being converted into friction that results in the waves fizzling out. As a rule of thumb:

• The faster the flow in a pipe is stopped or accelerated, the stronger the pressure rise or pressure drop
• And: The faster the liquid was moving prior to its deceleration, the stronger the pressure rise or pressure drop.

Pipe material as one decisive factor for possible pressure surge damage

Cast iron for instance is a rather brittle material and particularly prone to damage. Other more expandable materials such as plastic can absorb pressure surges better without rupturing. (See the influence of the modulus of elasticity in the above equation.) They too can become damaged, just like valves, sprinkler heads and pipe adapters. Water hammer can also have detrimental effects on pipe supports and pump foundations.

Pretty strong: What are the possible consequences of surge pressure?

The examples in the introduction highlight that surge pressure may have immense consequences. For instance, the short-time dynamic load in a 12 mm copper pipe can exceed 60 bar – no specialist tradesperson would subject a domestic water system to pressures this high when performing a leak test.* Also, the pressure may drop down to vapour pressure, possibly resulting in damage such as:

• Pipe rupture and valve damage
• Loosening of pipe fasteners
• Damage to pumps and their foundations
• Deformation of plastic and thin-walled steel pipes
• If the pressure drops, air or contaminated water can be sucked in via flanged connections and sleeve joints, gland packings or leaking points.
• The lining of pumps (cement mortar lining, plastic) in pipes may flake off. 
• Separation of the water column and macro-cavitation (separated water columns colliding or a separated water column hitting a closed valve)

In comparison, the “normal” side effects of pressure surges seem rather harmless: vibrations as well as banging, rattling or hammering noises being emitted from the water pipe. And even that may be bothersome at night.

So, how can pressure surges be prevented?

Damping surge pressure: How can water hammer be prevented?

The bad news is: From a purely physical point of view pressure surges cannot be prevented at all. However, they can be contained within permissible limits. And this starts in the planning phase. A specialist plant designer should have a fair amount of knowledge regarding fluid mechanics and the challenges when selecting water and heating systems. The following parameters are critical in preventing water hammer:

• The piping layout
• The piping length
• The pump’s moment of inertia (plus motor, coupling, pulley, etc.)
• The pipe material and dimensions
• Other important parameters are the flow rate, fluid handled and water levels.

In the end, the only one who can protect a system against water hammer is someone who knows how to accurately calculate the pressure developing in a piping system, who can assess these calculations correctly and who knows how to select the above-mentioned parameters accordingly.

In addition, there are some active protective measures that can be taken to spare a system from pressure surges:

• By using a frequency inverter a pump’s speed can be controlled precisely, providing protection against the pump stopping too quickly, for example.
• A so-called soft starter prevents excessive pressure peaks when starting up and stopping a variable speed pump.
• Opening and closing the valves correctly is another crucial factor: The slower a valve is closed, the lower the surge pressure. This can be achieved by hydraulically actuated butterfly valves, for example.
• Pressure relief valves prevent the occurrence of excess pressure in the piping. Vacuum safety valves installed downstream of the valve allow air to enter the system if necessary in order to prevent a vacuum.
• A larger diameter reduces flow velocities. This also applies to the maximum possible velocity change following abrupt deceleration.
• The pipe material markedly influences the amplitude of the wave propagation velocity.
• An accumulator contains a compressible “air cushion”. Similar to a balloon the air cushion absorbs the energy generated by a pressure increase.

In any case the following factors should be considered in addition: the quantity of pumps, the conditions for normal stopping and for stopping in the event of a power failure, and the risk of deformation, material fatigue and clogging.

Taking pressure off the piping: Summary and conclusion

Pressure surges in the piping occur whenever the flow velocity of a liquid changes more or less abruptly, for example when a pump is stopped or a valve is closed. As the pipe walls are of little elasticity the pressure increase caused by the liquid accumulating can only expand in one direction: axially, opposing the direction of flow. Radial expansion is restricted by the pipe wall and/or only permitted to a minor extent. The pressure wave therefore expands in the system and may have drastic consequences: banging noises in the piping, damage to the valves, leakages, and even major pipe ruptures. 

A number of measures can be taken to prevent such pressure surges: from proper planning and selection of the entire system, installing slowly closing valves and mechanical water hammer dampers right through to electronic control modules ensuring suitable starting and stopping of pumps. It is important to bear in mind that there is no general solution – every system requires its own individually tailored protective measures. Vast knowledge is necessary to plan a heating system in which surge pressure is prevented. KSB will be pleased to support you with the selection, for example with KSB EasySelect, our selection software for all applications.

* https://www.ikz.de/detail/news/detail/druckstoesse-im-trinkwassernetz/