9.11 Water hammer protection
Water hammer protection
When is water hammer protection equipment necessary? Extra protection equipment is necessary when the pump installation would otherwise be damaged by the resultant excess pressure or where there is risk of cavitation, caused by negative pressure in the line. Installations subjected to repeated water hammer must be dimensioned to withstand fatigue.
Detailed water hammer calculations are complicated and costly. This obviously raises the question as to when a detailed investigation is necessary. For certain installations simple estimations can give an indication as to the magnitude of water hammer.
Valve closing time
Figure 9.11a Pump installation with slow-closing valve.
The pump starts and stops against a closed valve. The pressure changes at the valve in connection with closing can be approximated by:
where
L = pipe length (m)
v = initial flow velocity (m/s)
g = acceleration due to gravity (9,806 m/s²)
T = valve closing time (sec.)
a = speed of wave propagation (m/s)
The above formula assumes a linear valve closing curve, i.e. linear reduction in flow in relation to time.
Pump stoppage with non-return (check) valve
The pressure changes with non-return valve in connection with pump stoppage are approximated by:
(Note! Does not apply to delayed non-return valve)
Figure 9.11c Auxiliary values for equation 9.11b
Example:
Following data applies to a pump installation with non-return valve; pipe length L = 1700 m, delivery head H = 22 m and flow velocity v = 1,3 m/s in the pipeline.
Estimate the magnitude of pressure surge in the event of pump stoppage.
L = 1700m → K =1,1
H/L = 22/1700 → C = 1
Then according to equation 9.11b
±ΔH = 37m
Pump stoppage without non-return (check) valve
The installation is the same as in figure 9.11a. The pump stops because of power failure. The slow-closing valve remains open.
If hL / H > 0,2
then Ht max ≤ H
where
hL = pipe system head loss (m)
H = pump delivery head (m)
The size of the pressure drop (-ΔH) is estimated according to equ. 9.11b.
Pressure head lines
Figure 9.11d Checking pressure head lines.
The rules of thumb give no regard to the shape of the pressure lines along the pipeline. The lines shown in the figure are based on estimates derived from experience. It is, however, possible to conclude that there are definite risks of cavitation in the case of the pipeline shape as represented by the continuous line, The chain-dotted pipe-line routing eliminates this risk. It then remains to be shown whether or not Ht max at the valve is acceptable.
By means of these simple estimations it is possible to judge many non-critical cases usually having short pipe-lines and low flow velocities. In cases of doubt it is advisable, as the next stage of investigation, to refer to the sizing diagrams which are published in technical literature etc. These cover more thorough calculations with regard to pump stoppage without non-return valves for certain simple pump installations and also quote figures for Ht max and at mid-point in the pipeline. If there is still cause for doubt then the only solution is to carry out a detailed computer calculation using the characteristic method.
Water hammer protection equipment
Water hammer is avoided during normal operation by ensuring that load changes are carried out sufficiently slowly. Starting and stopping of pumps is carried out against closed valves or by some other means of control. Extra protection equipment is therefore usually installed in order to protect the pump installation against power failure.
One large group of protection equipment attempts to eliminate the basic causes of water hammer, the altogether too rapid flow reduction resulting from a power failure.
Figure 9.11e Surge tower (column).
A surge tower of sufficient diameter provides good protection against water hammer. In the event of a power-failure the non-return valve closes and isolates the pump from the pipeline. Liquid from the surge tower flows out into the pipeline and prolongs the retardation process, weakening the negative pressure wave. The tower refills when the flow in the pipeline has changed direction. The mouth of the tower can be throttled to increase dampening in the system. A disadvantage of the surge tower is that it must be at least as high as the maximum pressure head at the non-return valve. This problem can be avoided by means of an air chamber.
Figure 9.11f Air chamber.
During normal operation the air pressure in the air chamber is equal to the pump delivery head. When pumping stops liquid flows out of the air chamber and the air-pressure reduces. The greater the volume of air the smoother the process. The system can be suitably dampened by throttling the chamber at the flow inlet. A disadvantage with the air chamber is the necessity of providing a compressor to replace the air which is dissolved in the liquid and carried away in the pipeline.
Figure 9.11g Flywheel
The retardation of the pump rotor is also affected by the moment of inertia of the rotating parts. By fitting the pump with a flywheel it is possible to prolong the “run-out” time. The method is most effective for high speed pumps and moderate pipe sizes. A disadvantage is increased starting difficulties.
Figure 9.11h By-pass line
For low or negative static heads there is a risk of negative pressure on the “pressure side” of the pump. The pressure drop across the pump is reduced by means of a by-pass line.
The other main group of protection equipment is intended to eliminate water hammer at a somewhat later stage of the transient process.
Figure 9.11i Surge tank.
Negative pressure with the associated cavitation risks can be avoided by placing a surge tank at critical points. In the event of negative pressure, liquid flows from the tank into the pipeline. The tank is refilled during normal operation via a separate level controlled filling line.
Figure 9.11j Valve for air intake
An alternative method is to let air into the pipeline when the pressure falls below atmospheric pressure. The pipeline must be ventilated, however, prior to re-starting. In installations having large static delivery heads it is often the excess pressures which occur upon reversal of the liquid column and in bringing it to rest, which are most critical. These pressures can be reduced with the aid of control valves.
In order not to worsen the initial wave, negative pressure wave the controlled by-pass valve remains closed, in principle, until the flow at the pump is reversed. It then opens rapidly, thereby relieving pressure in the pipeline and ultimately closes slowly.
Figure 9.11k Controlled valve in by-pass line.
The controlled valve in the main line should, in principle, remain open until the flow at the pump reverses. It then closes rapidly to its optimum position, the ideal characteristics of which is to retard the reversed flow with optimum distribution of pressure drop between the pump and valve.
Figure 9.11l Controlled valve in main line.
Figure 9.11m Effects of reverse lock.
Negative pump speed can be hindered by means of a reverse lock or brake. The reversed pump flow is then retarded more smoothly and the pressure surge is dampened. The risk of high negative “runaway” speed is eliminated.
Apart from the pure pressure surge dampening characteristics, the choice of protection equipment is also affected by other factors. Liquid characteristics, maintenance and procurement costs are examples of such factors. Each case must be judged on its own merits as to the most suitable choice of protection equipment.