Ordering Information

WATER HAMMER

The following brief equations summarize the hammer effect and are followed by an example of water hammers destructive forces. The following equation determines the maximum pressure change that occurs during a fluid hammer. The equation assumes that the piping is inelastic.

WHERE:

P is the change in pressure resulting from the fluid hammer (pounds per square foot)

r is the fluid density (pound mass per cubic foot)

c is the speed of sound in the fluid (feet per second) v is the change in velocity of the fluid (feet per second)

g is the gravitational constant (32.2 feet per second)

E is the bulk modulus of the fluid media (listed in PSI but must be converted to PSF)

k is the ratio of specific heats (k = 1.4 for air)

R is the specific gas constant (foot pounds per pound mass per degree Rankine)

T is the absolute temperature in Rankine

Example of waterhammer occurring in typical house piping. Assuming you have one inch water piping, how much of a change in pressure will be created from a waterhammer?

Assume that the water is flowing in 10 gallons per minute and the temperature is about room temperature (70°F). A 1 inch schedule 40 pipe has an internal area equal to 0.00600 ft2.

Fluid velocity V = Q/A = 10 gpm (1/448.83 gpm/cfs)/.006ft2 = 3.71 ft/sec.

Where Q is the flow rate, and A is the internal area in the pipe.

In this example, a 1 inch pipe with a flow rate of 10 gpm had a hammer effect resulting in an increase in pressure of 243 PSI above normal operating conditions. Considering normal city water pressure of 50 PSI, most end users would select a sensor of approximately 100 PSI full scale to be on the safe side. A 100 PSI sensor usually has an over pressure of 200% associated with it, meaning it will be able to withstand 200 PSI. Now the hammer increases the system from 50 PSI to 293 PSI (50 + 243), which is overpressurizing the sensor and causing damage to it. Most end users are puzzled as to how a system that is supplied with only 50 PSI is capable of producing over 200 PSI. After reading this article it should be evident that fluid hammers are complex phenomena with simple alternatives in the protection of instrumentation and the piping system.

PROPERTIES OF

WATER AT ATMOSPHERIC PRESSURE

Temp.	Density	Density	Kinematic Viscosity	Viscosity	Surface Tension	Vapor Pressure	Bulk Modulus
°F	lbm/ft³	slug/ft³	lbf-sec/ft²	ft²/sec	lbf/ft	Head ft	lbf/in²
32	62.42	1.940	3.746 EE-5	1.931 EE-5	0.518 EE-2	0.20	293 EE3
40	62.43	1.940	3.229 EE-5	1.664 EE-5	0.514 EE-2	0.28	294 EE3
50	62.41	1.940	2.735 EE-5	1.410 EE-5	0.509 EE-2	0.41	305 EE3
60	62.37	1.938	2.359 EE-5	1.217 EE-5	0.504 EE-2	0.59	311 EE3
70	62.30	1.936	2.050 EE-5	1.059 EE-5	0.500 EE-2	0.84	320 EE3
80	62.22	1.934	1.799 EE-5	0.930 EE-5	0.492 EE-2	1.17	322 EE3
90	62.11	1.931	1.595 EE-5	0.826 EE-5	0.486 EE-2	1.61	323 EE3
100	62.00	1.927	1.424 EE-5	0.739 EE-5	0.480 EE-2	2.19	327 EE3
110	61.86	1.923	1.284 EE-5	0.667 EE-5	0.473 EE-2	2.95	331 EE3
120	61.71	1.918	1.168 EE-5	0.609 EE-5	0.465 EE-2	3.91	333 EE3
130	61.55	1.913	1.069 EE-5	0.558 EE-5	0.460 EE-2	5.13	334 EE3
140	61.38	1.908	0.981 EE-5	0.514 EE-5	0.454 EE-2	6.67	330 EE3
150	61.20	1.902	0.905 EE-5	0.476 EE-5	0.447 EE-2	8.58	328 EE3
160	61.00	1.896	0.838 EE-5	0.442 EE-5	0.441 EE-2	10.95	326 EE3
170	60.80	1.890	0.780 EE-5	0.413 EE-5	0.433 EE-2	13.83	322 EE3
180	60.58	1.883	0.726 EE-5	0.385 EE-5	0.426 EE-2	17.33	313 EE3
190	60.36	1.876	0.678 EE-5	0.362 EE-5	0.419 EE-2	21.55	313 EE3
200	60.12	1.868	0.637 EE-5	0.341 EE-5	0.412 EE-2	26.59	308 EE3
212	59.83	1.860	0.593 EE-5	0.319 EE-5	0.404 EE-2	33.90	300 EE3

Inline pulsation damping devices can dramatically aid in protecting the piping system and instrumentation used to monitor flow, pressure and temperature. Eliminating leaks especially where a vacuum is created which draws air into the piping system can cause a hammer effect during start up and rapid opening and closing of valves.