Soils 456 Notes Chapter 8

1. Water hammer

 

3 Water hammer in pipe lines occurs when flowing water is suddenly stoped. Because water is comparatively incompressible the water is unable to absorb the shock which is then transferred to the pipe. The intensity of the shock wave pressure varies with the modulus of elasticity of the pipe material and the velocity of the water just before the sudden stoppage of flow.

P= (4700 Vw/144g )*((E/ (E+294,000 d/t))^0.5)

where:
P = excessive pressure in psi
V= velocity of flow ft/sec
w=unit weight of water 62.4 lb/ft^3
d= diameter of pipe in inches
t= thickness of pipe wall in inches
g= acceleration due to gravity 32.3 ft sec^2
E= modulus of elasticity of the pipe material in psi

E= 10,000,000- psi for aluminum
E= 30,000,000 psi for steel
E= 400,000 psi for plastic pvc

Example for 1 ft/sec velocity and 1.25 pipe
P= 4700 * 1*62.4/144*32.2 * (400,000/(400,000+294000 * 1.66/ 0.14))^0.5= 63.2 * .32=20.3psi

see spreedsheet

 

4. Rules

1. pressure rating of pipe two times the pressure in the pipe

2. surge pressure should not exceed 28% of the pipe class rating. or operating pressure at 72% of pipe pressure rating .

3. Valve size should be same size at pipe or flow range should match flow in pipe.

4. Have air and vacum rellief valves. As an air slug escapes velocities increase and then decrease causing hammer.

5. Valves flow rates

Compare flow rating of valve vs flow rating of pipe.

6. Pressure relief valve.