9.10 Water hammer
Water hammer
Water hammer are also known as pressure surge in pump installation is caused by change in flow and especially by rapid flow changes. Rapid flow changes occur during:
- rapid valve movement
- starting and stopping of pumps
- special situations
Since the shock of water hammer can reach considerable magnitude there is risk of damage to the pump installation. Apart from the pressure oscillations which occur with normal variations, the prediction of unintentional operational conditions must also be considered. Accidental pump stoppage caused by power failure (power cut) is one such situation which a pump installation must reckon with sooner or later. An estimation of the magnitude of the line shock must therefore be carried out for every installation.
Water hammer caused by cavitation in the pipeline is difficult to estimate. If the absolute pressure somewhere in the pipeline is allowed to fall below the vapour pressure of the liquid, vapour is formed in the pipeline. The original column of liquid divides itself into two which, with different speeds and separated by a growing vapour bubble, flows on through the pipeline. Eventually the two columns of liquid begin to approach each other causing the vapour bubble to implode, as the liquid columns reunite, resulting in a large pressure surge.
The magnitude of the water hammer shock depends upon the instantaneous difference in velocity of the two columns of liquid at the moment when they reunite, which, in the worst case, can be greater than the steady flow velocity in the pipeline. Cavitation in pipelines must, therefore, be avoided which also substantiates the necessity for controlling Htmin. The risk of unacceptable negative pressures are greatest at high points, downstream of valves and pumps and in long suction lines.
The following situations are especially susceptible to water hammer:
- Cavitation due to heating or insufficient cooling in thermal installations.
- Trapped air-pockets; pump start against closed or nearly closed valves in an insufficiently ventilated system.
- Automatic re-start of pump, which rotates backwards after a power failure or after reverse flushing.
- Resonance effects due to periodical disturbances caused by valve oscillations, displacement pump surge etc.
Pressure head lines or hydraulic grade
In order to obtain a complete picture of the distribution of pressure throughout a pump installation, use is made of so-called hydraulic grade lines. By definition for pressure head Ht:
Ht = pressure head above the pump installation’s lower liquid surface (m)
PI = static pressure above atmospheric (Pa)
ρ = density of the liquid (kg/m³)
g = acceleration due to gravity (m/s²) 9,086N
z = level above the installation’s lower liquid surface
Figure 9.10a Illustration of pressure head lines (hydraulic grade lines) for a pump installation.
This method of representation presents the pipeline inner positive pressure as the difference between the Ht -line and the shape of the pipeline. The figure also shows a high point where there is negative pressure. Parameters which should be observed are the prevailing inner positive and negative pressures as regards dimensioning for stress and margins of safety against the risk of cavitation. The cavitation risk is greatest in pumps, valves and other installation components having restricted sections together with high points in the system.
Figure 9.10b Pressure head lines for closing valve.
The hydraulic grade lines for steady flow take on different appearances for different operating points. The extreme value for the Ht-line often occurs, however, in conjunction with load changes associated with a transient (time dependent) process.
When closing a valve the pressure increases in front of the valve. Increasing pressure gives rise to a pressure wave, which travels through the pipeline at the speed of wave propagation “a”.
The magnitude of the speed of wave propagation “a” ≅ 400 m/s for water in steel pipe and “a” ≅ 300 m/s for water in plastic pipes, see also Chapter 7, Properties of liquids>>>, section Wave propagation speed. The pressure wave is reflected at the end of the pipe as a negative pressure wave, returns to the valve, is reflected again and so on. In this way a process of pressure oscillation is set up in the pipeline and continues, even after the valve is completely closed, until the oscillations are dampened out by friction. The pressure head lines Ht max and Ht min connect the highest and lowest positive pressures which occur at different points along the pipeline during the oscillation process.
The greatest pressure variations are to be found in the valve and occur when the valve is closed rapidly. The greatest amplitude of pressure surge is:
ΔPmax = ρ * a * v
where
ΔPmax = max pressure variation (Pa)
ρ = density of the liquid (kg/m³)
a = speed of wave propagation (m/s)
v = velocity of flow before valve begins to close (m/s)
For water in steel pipes (ρ = 1000 kg/m³, a ≈ 1100 m/s) and a flow velocity of 1 m/s creates, for rapid valve closing, a water hammer pressure of 1,1 *106 Pa = 11 bar = 110 m.
The pressure process at the valve cannot be affected before the pressure wave has been reflected at the end of the pipe and returned to the valve. This time is referred to as the pipeline reflection time and is designated tr.
tr = 2 * (L/a)
where
tr = pipeline reflection time (sec.)
L = pipe length (m)
a = speed of wave propagation (m/s)
If for example the length of pipe is 1100 m, then tr = 2 sec. All valve closing times ≤ 2 sec. thus give maximum water hammer pressure.
For a plastic pipe ΔPmax = 3*105 Pa ≈ 30m is obtained for valve closing times ≤7.36. Increasing the closing time reduces the magnitude of water hammer and very slow closing valves result in no line shock at all.