Water Flow Characteristics — Water-hammer

时间：2016-07-09 10:39:12点击：次

When a pipe  contains a  column of moving  liquid, considerable kinetic energy  is stored  in  the  liquid by virtue of  its mass and velocity.  If  the velocity  is suddenly destroyed by  the quick closing of a valve  this energy cannot be absorbed because  liquid  is nearly incompressible. Therefore, an  instantaneous shock is  created which may  represent excessively high pressures. Maximum pressure caused by water-hammer may be calculated with the following formulae:

Pwh =  pressure surge, psi
 V =  maximum velocity change, ft./sec.
gc   =  acceleration of gravity, 32.2 ft./sec.2
K    =   luid bulk modulus, 300,000 PSI for water
d    = pipe inside diameter, inches
b    = wall thickness, inches  
This  is what should be reviewed when a plastic piping system is being considered.
• Speed of opening or closing of regulating type valves.
• Action of pumps starting or stopping.
• Movement of entrapped air through the system.
Understanding  the  concept of water-hammer and designing the system to minimize it is the best possible preventive measure. A  few  tips  to  consider when attempting  to  reduce  the causes of surge  in a piping system are:
• Keep  fluid velocities under 5  feet per second.  (see pages 7.08 ~ 7.11)
• Check  the cycling  time of valves  to prevent abrupt changes  in  flow. Both manual and actuated valving should be checked for speciic closing times.
• Evaluate low at pump start-up and during spin-down. Also determine how much air,  if any,  is  introduced during pump startup.
• Use properly sized vacuum breaker-air relief valves to control the amount of air that is admitted or exhausted throughout the system.
SPECIFIC INSIDE DIAMETERS OF PIPE

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