During the design of a pipeline, the possible risk of water hammer must be examined and quantified, in order to install the necessary protection devices, particularly in pumping mains. If protection devices have not been provided, ductile iron pipes have a safety coefficient which is often useful when faced with accidental pressure surges.
Sources of water hammer
If the flow rate of a liquid in a main is abruptly altered, there is a violent change in pressure. This transient problem, known as water hammer, generally occurs when ancillary equipment is interrupted or operated (pumps, valves ...). Waves of pressure surges and pressure drops sweep through the main. Water hammer can occur equally well in gravity and pumping mains. There are four main sources of water hammer:
- pump starting and stopping,
- closing of valves, fire and sluicing hydrants, etc.
- the presence of air,
- inadequate functioning of the protective equipment.
The pressure surges involved can rupture certain pipes in critical cases, where the safety factors are inadequate. The pressure drops can create pockets of cavitation, prejudicial to pipes and valves, etc.
The BERGERON graph method can be used to determine precisely the pressures and flow rates as a function of time at any point in a pipe subject to water hammer.
There are now computer programs that solve these complex problems.
The protective systems that can be installed to limit water hammer to an acceptable level are varied and must be adapted to suit each situation. They act by slowing the change in fluid velocity or by limiting the pressure surge in relation to the pressure drop. The user must determine the pressure surge and pressure drop envelope created by water hammer and judge, according to the pipe profile, the type of protection to be installed:
– pump inertia impellor,
– pressure relief valve,
– air or “automatic air control” accumulator,
– auxiliary suction,
– surge tank.
The anti-water hammer accumulator is frequently used. It has two functions:
– limit the pressure surge (head loss controlled by a check valve),
– prevent cavitation (accumulator drainage).
In the event of a sudden pump shutdown, the pressure drop is offset by a flow rate provided by draining the accumulator. When the direction of water flow reverses, the energy in the water mass is transformed into a head loss by filling the accumulator through a calibrated check valve.
The pipeline profile is the determinant factor in deciding the tank dimensions. In practice, the minimum pressure drop curve (after installing protection devices) must not fall more than five metres below the actual profile of the main. The surge tank volume can be determined from the PUECH and MEUNIER graphs, or from computer programs.
It should also be noted that ductile iron has a high safety margin:
– surges : PAM allows a 20 % excess over the maximum permissible pressure for transient pressure surges ;
– pressure drops : the joint guarantees a seal against external ingress, even in the case of partial vacuum in the main.