Valve Magazine - Spring 2011

Figure 2. Expansion joints that have collapsed due to sudden valve closure

Figure 3. Typical pipe burst from over-pressurization

acoustic velocity (speed of sound) of the fluid in the pipe.

The basic pressure transient equation, h = av/g , was developed by Nikolai Yegorovich Joukowski in 1904. In this equation; “h” represents the pressure transient, “a” is the acoustic velocity of the fluid, “v” is the fluid velocity and “g” is the gravitational constant. Small amounts of entrained gas in the fluid can dramatically reduce acoustic velocity. In the case of sudden pump stoppage, some gas may come out of the fluid solution, which, because it reduces the acoustic velocity, will also reduce the pressure spike. This is one of the factors that makes the analysis and prediction of pressure transients very complicated.

SOLUTIONS FOR WATER
HAMMER
Although mixing a gas into the liquid
stream would reduce acoustic velocity,
this obviously is not a solution in many
cases. However, there are other ele-
ments of a piping system that can
absorb part of the pressure surge. Air
pads, surge tanks, valve speed controls
and check valves are potential solutions
for water hammer problems. Under-
standing the underlying root cause or
source of the shock wave will help in
finding the best solution.

SUBSTANTIALLY
REDUCES
MAINTENANCE
COST

HIGHLY VISIBLE
(CHROME YELLOW)
PROVIDING VISUAL
EVIDENCE OF VALVE
ACTIVATION

PREDICTING AND PREVENTING The analysis of pressure transients is complex and cannot be done through a few general equations. However, simple equations can be used to predict the pressure spike using key factors that influence the spike’s potential. These factors include: fluid velocity, the rate of deceleration of the fluid, the speed of closure of valves and the