Water Hammer not due to Closing of Valve

By TK NG

            Water hammer is experienced when water in motion is forced to stop abruptly, such as closing of a valve at the end of a pipeline.  However, there are cases where the cause of water hammer is due to other reasons.

2.         An example is the water hammer found at the roof pipe elbow (EB1) of a water supply system shown in Figure 1 when the pump is stopped. Water continues to flow into the roof tank for a short while due to inertia although the water pump is no longer in operation.  Water hammer then appears at EB1 but not the non-return valve at the pump discharge.

Figure 1

3.         The water hammer phenomenon in this case stems from the inertia/momentum of the large volume of water inside the long length of horizontal pipe run at roof.  When the pump stops running, gravity acts against the upward inertia of those vertical columns of water inside pipe risers A and B, causing the upward movement of the water columns to slow down quickly.  For the volume of water inside the long horizontal pipe run at roof, its inertia towards the right is countered by the gravity of the short water column inside riser B but the effect is minimal.  On the other hand, water column inside riser A quickly comes to a standstill when the non-return valve at the pump discharge closes.  This would lead to a lower than atmospheric water pressure at the pipe elbow EB1 to act against the right-ward movement of the horizontal length of water, bearing in mind that the pressure at the roof tank pipe discharge is atmospheric.  Cavitation would occur at EB1 (see Figure 2) when the local water pressure drops below its own vapour pressure.

Figure 2

4.         The right-ward movement of the horizontal length of water would finally come to a halt and move back towards the elbow after bursting of the vapour bubbles formed under cavitation.  As the non-return valve has already closed, the reverse flowing volume of water would just hit on the elbow to cause water hammer.

5.         There are two different ways to resolve the issue –

(a)

Reduce the water flow rate of the pump to an extent such that the inertia/momentum of the volume of water inside the horizontal pipe run towards the right side is not large enough to cause cavitation at the elbow upon pump stops operation; or

(b)

Install a water hammer arrestor at pipe elbow EB1.

However, method (a) above is not always possible if the pump flow rate cannot be reduced to the required extent due to whatever reasons.

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