First stage : Operating pump loses power suddenly, but pump blades still rotate due to inertia. However, rotating speed will decrease and consequently the flow rate and lift are reduced. However, pump motion and flow at this Time are still along the normal direction.

Second stage : Pump lift decreases continuously due to power loss, and water flows backward once the lift becomes lower than the static pressure inside the pipe. However pump blades are still rotating, and backward pressure (negative pressure) collides directly with pump outlet pressure (positive pressure), generating series of unstable pressure and causes internal friction and collision. This is called water hammer effect. At this time, pressure inside the pipe escalates and backward water imposes obstructing force on the rotating pump blades, which reduces the rotating speed and flow rate rapidly.

Third stage : Pump rotating continues to drop. At the next instant to blades stopping, the on-going backward water rotates the blade, causing reverse rotation.

How to avoid water hammer effect
In order to avoid water hammer effect, at the first stage mentioned above, the check valve should be closed at an appropriate speed to prevent the friction and collision enerated from positive and negative pressure inside the pipe.
After stage two, negative flow rate generates, the check valve gate should be closed completely at this time.
When backward flow pushes the gate and generate another impact, the water hammer arrester can absorb unstable pressure above the gate.

  Silent check valve :
Prevents the backward fluid inside the pipe. When pump is off, the gate can be closed at appropriate peed, thus avoid collision between positive and negative pressures. Noise can be prevented.

Water hammer arrester :
Absorbs unstable pressure due to sudden closing of gate and tranquilizes the fluid impact inside the pipe.

Pressure Relief valve :
Releases pressure above the safety limit and water hammering effect from pump shut-down.

Noise Recognition :
  Check valve :

High pitch sound comes from malfunctioning check Valve, which is similar to that of hitting an iron pipe with a hammer.

  Water hammer absorber :
  Noise coming from a malfunctioning water hammer is low and booming, with continuous sounds and pipe vibration.

If only the check valve is installed (and
water hammer arrester is mitted),sudden backward pressure will be imposed on the check valve gate when pump is off and the check valve is closed. This generates another impact (water hammer effect).

The graph at left can be a reference for layout of check valve, pressure relief valve and water hammer arrester in ordinary water lift pipeline.
When water lift pump is off, the silent check valve can close the gate at an appropriate speed. Water hammer arrester can absorb pressure imposed by back flow. If the ressure
is above the safety limit, pressure can be released by pressure relief valve to ensure the normal function of water lift pipeline.

If ordinary swing check valve is used, the Gate closing will need backward flow when pump is off. The backward flow might hit the pump blades and in some serious
cases, the blades will be damaged by shearing force.

In case of high lift straight pipe or long-distance straight pipe, the check valve should be avoid to be placed in series when loss in water head is low (see graph at left)
(1) When pump is off, upper check valve still has sudden backward pressure, which will be imposed upon the gate and generates. Another water hammering impact.
(2) The water hammer effect is an unstable pressure wave that transfers back and forth inside the pipe. It's highly probable that another check valve will be reversely pushed open, causing more unstable pressure impact.
In this case, pressure relief valve can be used to release the increased pressure and protect the pipline.

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