Hydraulic shock is the term used to describe the momentary pressure rise in a piping system which results when the liquid is started or stopped quickly. This pressure rise is caused by the momentum of the fluid; therefore, the pressure rise increases with the velocity of the liquid, the length of the system from the fluid source, or with an increase in the speed with which it is started or stopped. Examples of situations where hydraulic shock can occur are valves, which are opened or closed quickly, or pumps, which start with an empty discharge line. Hydraulic shock can even occur if a high speed wall of liquid (as from a starting pump) hits a sudden change of direction in the piping, such as an elbow. The pressure rise created by the hydraulic shock effect is added to whatever fluid pressure exists in the piping system and, although only momentary, this shock load can be enough to burst pipe and break fittings or valves.
A formula, which closely predicts hydraulic shock effects is:
p = v
where p = maximum surge pressure, psi
v = fluid velocity in feet per second
C = surge wave constant for water at 73°F
*SG = specific gravity of liquid
( If SG is 1, then p = VC )
|Example: A 2" PVC schedule 80 pipe carries a fluid with aspecific gravity of 1.2 at a rate of 30 gpm and at a linepressure of 160 psi. What would the surge pressure be if a valve were suddenly closed?
From table 1: C = 24.2
p = (3.35) (26.6) = 90 psi
Total line pressure = 90 + 160 = 250 psi
Schedule 80 2" PVC from the table on page 18 has a pressure rating of 400 psi at room temperature.
Therefore, 2" schedule 80 PVC pipe is acceptable for this application.
NOTE: The total pressure at any time in a pressure-type system (operating plus surge or water hammer) should not exceed 150 percent of the pressure rating of the system.
Table I - C-Surge Wave Constant
|Sch. 40||Sch. 80||Sch. 40||Sch. 80|
Proper design when laying out a piping system will eliminate the possibility of hydraulic shock damage.
The following suggestions will help in avoiding problems:
- In a plastic piping system, a fluid velocity not exceeding 5 ft./sec. will minimize hydraulic shock effects, even with quickly closing valves, such as solenoid valves.
- Using actuated valves which have a specific closing time will eliminate the possibility of someone inadvertently slamming a valve open or closed too quickly. With pneumatic and air- spring actuators, it may be necessary to place a valve in the air line to slow down the valve operation cycle.
- If possible, when starting a pump, partially close the valve in the discharge line to minimize the volume of liquid, which is rapidly accelerating through the system. Once the pump is up to speed and the line completely full, the valve may be opened.
- A check valve installed near a pump in the discharge line will keep the line full and help prevent excessive hydraulic shock during pump start- up.