Surge and water hammer

Water hammer or pressure surge is the sudden rise or fall in pressure caused by an abrupt change in the fluid velocity within the pipe system. The usual cause of these flow changes is the rapid closing or opening of valves or sudden starting or stopping of pumps such as during a power failure. The most important factors which influence the water hammer pressure in a pipe system are the change in velocity of the fluid, rate of change of the velocity (valve closing time), compressibility of the fluid, stiffness of the pipe in the “hoop” direction, and physical layout of the pipe system.

An approximate relationship for the maximum conceivable pressure increase at a given point in a straight pipeline with negligible friction loss can be calculated from the formula:

Δ H = (wΔv)/g

where:

Δ H = increase in pressure, m

w = surge wave celerity, m/s

Δ v = change in water velocity, m/s

g = gravity acceleration, m/s²

The surge wave celerity of FLOWTITE pipes is in the range 340 – 620 m/s, depending on pressure class and stiffness class (and to some extent diameter). For steel and ductile iron it is in the range 950 – 1050 m/s, i.e. between 1.5 and 3.0 times higher than for FLOWTITE pipes, with corresponding increase in surge pressure.

In addition to this, due to its extra high initial strength (see AWWA M45), FLOWTITE pipes have a surge pressure allowance of 40% of the nominal pressure, i.e. a pipe may be operated at PN allowing surge pressures up to 1.4 x PN.

The potential for damage in FLOWTITE pipe systems due to water hammer is thus considerably lower than for stiffer pipes. See more about water hammer and surge wave celerity in the technical characteristics.