What is Water Hammer Surge?
We are all familiar with the sound. We suddenly turn the water off in our shower or kitchen and hear a banging coming from the wall. No, your house isn’t haunted, it’s just the water bouncing back and forth through the pipes. This is called water hammer and it happens in large industrial pipes the same way it happens in your home. Not only can the sound be annoying, but it can cause serious damage to your piping. These pressure spikes can be so forceful that they can rupture pipes entirely.
Water hammer is just one type of phenomenon generally known as hydraulic transients. Unlike most transient fluid analysis, these circumstances are controlled by the small compressibility of liquids. Water hammer may be the most well known example, but others include pulsating flow and cavitation.
Cause of Water Hammer
When a liquid is flowing it contains a large amount of energy in the form of momentum. When the flow is suddenly stopped, all that energy must go somewhere and it does I the form of a pressure wave. This wave propagates from the original location at the stoppage all the way upstream to the source. From there it is reflected back and the wave “bounces” back and forth slowly dissipating. A huge pressure increase occurs at the location of this wave. If the fluid were truly incompressible and the pipe truly inelastic, this pressure rise would be infinite. The small compressibility of the liquid, as well as the elasticity of the pipe, limit the true pressure rise to a finite value. For this discussion, we will assume the stoppage occurs when a valve is closed. Keep in mind though, any change to distribution of fluid can cause a hydraulic transient. This includes the starting/stopping of a pump, a relief valve opening, a sudden valve closure, and possible others.
Max Water Hammer Pressure Rise Calculation
The maximum pressure rise due to the water hammer can be calculated using the Joukiwski formula.
The pressure wave velocity for thin walled pipes is given by:
The numerator in this equation gives the wave velocity for a perfectly rigid pipe, and the denominator corrects for wall elasticity. If the pipe is thick-walled as opposed to thin walled, the D/b term must be replaced by:
This maximum pressure surge will occur when the downstream valve closes before the pressure wave can travel to the pipe inlet and back.
If the valve is not fully closed when the pressure wave returns, the spike in pressure is reduced due to cancellation effects between the direct and reflected waves.
The most effective way to ensure a safe operating environment is to perform a surge analysis to identify weak spots and predict possible effects given a range of different scenarios. This analysis will allow to better develop emergency scenarios and maintain a safe operating environment.
Transient Analysis Software
Due to the complexity of the calculations, it is recommended to use a software package. Some of the most common packages are as follows.
With a primary focus on water, wastewater, and storm water systems, Bentley’s OpenFlow HAMMER software allows you to locate trouble spots and develop appropriate control strategies to prevent potential catastrophic incidents.
AFT Impulse allows you to perform analysis on a range of liquids to determine possible transient conditions and their effects. Impulse also has optional additional add-on modules for handling settling slurry and pulsation frequency analysis.
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