WATER HAMMER

NAVEEN SUDHARSAN

MT14WRE010

What is Water Hammer?

Water hammer is a pressure surge or wave resulting when a fluid in motion

is forced to stop or change the velocity

A water hammer commonly occurs when a valve closes suddenly at an

end of a pipeline system, and a pressure wave propagates in the pipe. It is

also called hydraulic shock.

Reasons

Immature operation of valve

Sudden stoppage of pump

Power outage

Sudden Closure Of A Valve

Fluid Mechanics with

Engineering

applications by

Joseph B Franzini &

E.John Finnemore

Mechanism

Mechanism

Sudden Closure of the valve

•Fluid lamina near to the valve compressed

•This compression reaches upstream end of the pipe(t=L/a)

At the Reservoir

•Unbalanced condition at the reservoir pressure

•Flow from pipe to reservoir starts as the wave moves downstream

•Reservoir pressure unchanged(2L/a)

Low pressure moves upstream as waves

•Makes the velocity of flow zero again

•But the fluid in the pipe will be expanded(3L/a)

At the Reservoir

•Unbalanced condition

•Due to Negative pressure in the pipe

•Flow towards pipe starts(4L/a)

Fluid Mechanics by

Victor L Streeter, E

Benjamin Wylie & Keith W

Bedford

Water Hammer Equations

Joukowsky (1898) eqn

∆𝑃 = 𝜌𝐶𝑝∆𝑉 Where ∆𝑃 is change in pressure

𝜌 density

𝐶𝑝 is the velocity of wave (1496 m/s at 25°C in water)

∆𝑉 is change in velocity

This equation can be used only for rapid closure( t < 2L/𝐶𝑝)

Water hammer equations for all conditions:

Equation of motion

𝜕𝑉

𝜕𝑡+ 𝑔

𝜕𝐻

𝜕𝑥+

𝑓𝑉 𝑉

2𝐷= 0

Equation of continuity

𝜕𝐻

𝜕𝑡+

𝑎2

𝑔

𝜕𝑉

𝜕𝑥= 0

These two pde are solved using method of

characteristics.

Consider a 500mm dia pipeline, L = 8000 m, 𝐶𝑝 =1000m/s

& v = 2 m/s, a gate valve is closed in 5 seconds.

Calculate the pressure surge. Calculate the force

exerted on the gate.

𝑇𝑟 =2𝐿

𝐶𝑝= 16 𝑠 > 5 𝑠

Joukowsky’s equation may be applied.

∆𝑃 = 𝜌𝐶𝑝∆𝑉

∆𝑃 = 20 𝑋 105𝑁

𝑚2

Area = .196 𝑚2 ≈ 0.2 𝑚2

𝐹𝑜𝑟𝑐𝑒 𝐹 = 𝑃. 𝐴 = 20 𝑋 105𝑋 0.2 = 400000 𝑁

F = 400 𝐾𝑁

Applications

Hydraulic Ram works with the

principal of Water hammer

Leaks can sometimes be

detected using water hammer

The US Navy is conducting field trials for

mine clearing using water

hammer.

Effects of Water Hammer

Damage to pipes, fittings, and valves, or any connected equipment causing leaks and shortening the life of the system

Bursting of pipes can occur if the pressure is high enough

Water hammer can have devastating effects on pump system

Devices for Protection from Water

Hammer

Soft Starters

Slow closing valves

Variable Frequency Devices

Water Hammer Arrestors

Surge Tanks

Pressure Release Valves

Conclusions

Water hammer can cause severe problems in the system if not designed properly. So at the time of designing of the system potential water hammer problems should be considered for thorough analysis. And proper methods or devices are to be used so as to assure the proper functioning of the system and to lessen the effect of water hammer

Water hammer is useful in certain cases as hydraulic ram, mine clearing etc.

References

Fluid Mechanics with Engineering Applications by Joseph B Franzini & E

John Finnemore, The McGraw Hill Companies, 1997

Fluid Mechanics by Victor L Streeter, E Benjamin Wylie and Keith W Bedford,

The McGraw Hill Companies, 2010 reprint

KSB Know how, Vol 1 Water hammer, KSB Aktiengesellschaft, Johann-Klein-Straße 9,67227 Frankenthal

en.wikipedia.org/wiki/Water_hammer