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| 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.
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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.
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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).
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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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