In every hydraulic systems, when sudden speed variations arise to the transported fluid, impulsive pressure waves are generated which propagate through the pipe submitting every passed components to additional stresses.
The trigger could be the simple closure of the valve, the starting
of a pump or the intervention of a check valve.
The rate of change of speed has effect on the entity of the peak of overpressure.
To avoid the damaging of the system is necessary to sizing up correctly the pipes and the components to withstand at the impulsive stresses which happen during the phase of operation.
Selection of the kind of fluid
Density of the fluid at 20°C
Selection of the type of pipe
Young's module of material
Selection of pipe dimensions
Dimensions of the chosen pipe
Length of the pipe
Intervention time of the valve to the bottom line:
Fluid speed in the pipe (v):
Common values for speed values.
|Peak of overpressure (waterhammer)||kg/cm2|
|Total peak pressure||kg/cm2|
|Propagating speed of the shock wave (celerity)||m/s|
|Critical period (phase time) Tf||s|
In this chart is shown the trend of the peak of overpressure (Delta P) referred to the intervention time of the valves (v m/s), below the phase time (Tf) the overpressure does not increase.
The phase time varies related to the propagating speed of the shock wave which in turn is influenced also by the temperature of the pipe and of the fluid (they have effects of their elasticity modulus), in this calculation we consider 20 20°C.
To reduce the peaks is also possible to install some dampers, also called shock absorbers for waterhammer. In commerce there are of different dimensions, for bigger systems is also possible to build, in these cases they are called air box.