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Surge pressure in pipelines, or Water Hammer, is commonly associated with runaway conditions in hydraulic turbine systems as a result of the loss of load or a line break, or with rapid closing of a dischage valve. Transient flow conditions can occur during any start-up, shut-down, speed adjustment or breakdown, and can create surge pressures and severe stress on the system.  Perhaps the most common cause of water hammer is the rapid closing of a discharge valve.  Typically the pressure increase upstream of the valve is the greater concern, but pressure loss downstream can also result in pipe collapse or cavitation.

Calculations to determine the magnitude of the pressure change are complex,  but with a few calculations, some conservative values can be estimated and many potential problems can be avoided.  In order to calculate the effects of water hammer, several system conditions must be known in order to determine approximate values for the magnitude of the pressure surges, including pipe diameter, wall thickness, material, length, flow rate and maximum head.

First the speed at which a pressure surge rate travels through a pipeline must be determined.

for water, a _w = 4660/ (1+(K/E)*(D/t))^^1/2, where:

a _w = Wave velocity (ft/sec)

K = Bulk Modulus of liquid (psi)

• K (water) = 0.3 x 10 ⁶ psi
• K (oil) = 0.25 x 10 ⁶ psi

E = Modulus of Elasticity (psi)

• E (steel) = 29 x 10 ⁶ psi
• E (Cast Iron) = 15 x 10 ⁶ psi
• E (Concrete) = 3 x 10 ⁶ psi
• E(Plastic) = 0.4 x 10 ⁶ psi

D = Inside pipe diameter (psi)

T = Pipe wall thickness (in)

L = Length of pipe (ft)

Next, the critical time (T _cr) that it takes for the pressure wave to travel to the end of the pipe and return to the starting point can be calculated:

T _cr = 2*L / a _w

To prevent water hammer and excessive pressure surges, the velocity needs to be changed slowly until the energy in the pressure wave is absorbed by pope friction.

The minimum recommended valve closing time should be 15 times larger than T_cr.

The surge pressure (in feet) can be calculated as follows:

h = a*(V1-V2)/g,

where V1 and V2 are the velocities of the water in the pipeline before and after valve closing, or change in flow rate.

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