Water Hammer Phenomena – Basic Theory

Part II Water Hammer Phenomena – Basic Theory

2.1 Pressure transient at valve following instantaneous valve closure without friction

Water hammer is caused by a pressure or shock wave that travels through the pipes, generated by a sudden stop in the velocity of the water, or a change in the direction of flow. If the pipe is suddenly closed at the outlet, the mass of water before the closure is still moving forward with some velocity, building up a high pressure and shock waves.

When the valve is instantly closed, the fluid immediately adjacent to the valve is brought from initial velocity V0 to rest by the impulse of the higher pressure developed at the face of the valve. As soon as the first layer is brought to rest, the same action is applied to the next layer of the fluid bringing it rest. In the manner, a wave of high pressure is visualized as travelling upstream of the flow at a wave speed of “a”.

When the pressure wave reaches the end of the flowing water, at a distance L, all the water has been in stationary state. The wave then reflects from this end and returns down the pipe, reaching the valve at time t=2L/a. it is again damping again and again. All the kinetic energy of the water has been transformed into the energy of a wave, which bounces back and forth. The regular arrival are like the blows of a hammer, hence the phenomenon is called water hammer.

Consider a control volume within the pipe and neglecting friction and minor losses, take the momentum in the flow direction and simplifying the expression, we receive an expression for the pressure head rise at the valve,  (for liquid in metal pipes, ), and this equation is known as the Allievi Expression. By using this equation, we are able to predict the pressure transient at the valve shown in above diagram.[pic 1][pic 2]

[pic 3]

2.2 If we consider the friction effect and mechanism of friction recovery, the pressure transient will take a different form as shown in below diagram.

Effect of Friction:

1. The steady state pressure head at the valve is considerably lower than the reservoir head.
2. In systems where the frictional losses are high, neglect of frictional effects can result in serious under-estimation of pressure rise following valve closure.
3. For a long pipeline, the effects of friction may contribute to larger part of pressure rise following valve closure.
4. As flow is retarded, the frictional head loss is reduced along the pipe and the pressure head at the valve increases towards the reservoir valve.

Mechanism of Friction Recovery:

1. As each layer (L1) of fluid between the valve and reservoir is brought to rest by the passage of the initial positive wave, a series of secondary positive wave is transmitted towards the valve, resulting in the full effects being felt at time (2L1/a).
2. As the flow reverses in the pipe during time 2L/a to 4L/a, the opposite effects is recovered at the valve because of the re-establishment of a high friction loss.  The variation due to friction recovery are shown by line AB for point 1, and line CD for point 2.

[pic 4]

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1. Experiment on Water Hammer

3.1 Purpose of experiment

The purpose of this experiment is to Investigate/ demonstrate the characteristics of  pressure surges/ Water Hammer Phenomena in Fluid System under various Operating Conditions, and to study the water hammer pressure transient characteristics and to check the validity of the theoretical equation for the water hammer pressure rise in a pipeline when a valve is closed rapidly.