Title Pump Fundamentals 6.3 MB 127
##### Document Text Contents
Page 1

javascript:;

Page 126

Water hammer (pressure surge): If in systems with long discharge lines,(e.g. in industrial and municipal

water supply systems ,in refineries and power stations) the pumped fluid is accelerated or decelerated,

pressure fluctuations occur owing to the changes in velocity. If these velocity changes occur rapidly , they

propagate a pressure surge in the piping system, originating from the point of disturbance ; propagation

takes place in both directions (direct waves),and these waves are reflected (indirect waves) at points of

discontinuity ,e.g. changes of the cross sectional area ,pipe branches, control or isolating valves, pumps or

reservoir. The boundary conditions decide whether these reflections cause negative or positive surges.

The summation of all direct and indirect waves at a given point at a given time produces the conditions

present at this point.

These pressure surges, in addition to the normal working pressure ,can lead to excessive pressure and

stresses in components of the installation . In severe cases such pressure surges may lead to failure of

pipe work, of fittings or of the pump casings. The minimum pressure surge may, particularly at the highest

point of the installation ,reach the vapor pressure of the pumped liquid and cause vaporization leading to

separation of the liquid column. The ensuing pressure increase and collision of the separated liquid column

can lead to considerable water hammer .The pressure surges occurring under these conditions can also lead

to the failure or collapse of components in the installation.

For the maximum pressure fluctuation the JOUKOWSKY pressure surge formula can be used:

Δp = ρ . a . Δv

Where ρ = density of the pumped liquid

a = velocity of wave propagation

Δv = change of velocity of the flow in the pipe.

The full pressure fluctuation corresponding to the change of velocity Δv occurs only if the change of

velocity Δv takes place during the period.

t ≤ reflection time tr = 2.l /a

where l = distance between the nearest discontinuity (point of reflection ) and the point of disturbance .