The
PC pump is a very simple pump. There are only
2 parts. A stationary part, the stator and a rotating
part, the rotor. It is a positive displacement
pump without valves, and it delivers a constant
flow.
The PC pump is actually a special case of the
gear pump. It is a spiral gear pump with an external-tooth
gear. running in an internal-tooth gear. The internal
gear (with external teeth) is called the rotor,
and the external gear (with internal teeth) is
called the stator. PC pumps are usually made in
the most basic expression of this principle. The
rotor is a gear with 1 tooth, so it becomes a
single threaded helix. The stator is a gear with
2 valleys so it becomes a double threaded helical
cavity. The stator has twice the pitch length
of the rotor. Refer to Fig. 1 the stator is shown
in cross section on the right side. The rotor
is in the centre. When the rotor is inserted in
the stator as shown on the left side, it will
be located on the periphery of the stator cavity
so that the gears are fully meshed. When the rotor
is rotated to the right, it will describe a circle
to the left as it is forced around the periphery
of the stator.
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the cross-sections: |
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At
cross-section A, there is a fully developed cavity
to the left of the rotor. The cavity on the right
is squeezed off (point x). In other works, a cavity
has been terminated and another is about to begin.
At cross-section B, the stator cavity has rotated
90º to the right and point x is now at the
bottom of the cross - section. The cavity which
was sealed off in cross-section A is now 1/ 2 developed.
The rotor has rotated 180º. At this point,
it is obvious that there are cavities on both sides
of the rotor. Therefore, when the rotor was inserted
in the stator, 2 chains of spiral, lenticular on-lapping
cavities were formed. At cross-section C, the stator
cavity has rotated 180º and the cavity, which
we are following, is at its fullest development.
The cavity on the other side of the rotor has reached
its end and is sealed off. The rotor has rotated
360º. At cross-section D, the stator has rotated
270º and the cavity is beginning to disappear.
The rotator has rotated 540º. At Cross section
E, we are back where we started. The cavity is sealed
off, the stator has rotated 360º and the
rotor has rotated 720º. Therefore, a cavity
is 1 pitch length of the stator, which are 2 pitch
lengths of the rotor. The cavities are sealed because
the stator is usually molded in an elastomer and
there is an interference fit with the metal rotor.
When the rotor is turned to the right, the cavities
spiral up the barrel of the pump without changing
size or shape. This is why the PC pump is an excellent
sludge pump. If a particle is caught between the
rotor and the stator wall, it is pressed into the
wall as the rotor passes, then it is expelled into
the next cavity. Each cavity in
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the
chain boosts the pressure by an equal amount. Therefore,
each cavity is a stage. The pressure rating of a
progressing cavity pump is the pressure at which
the pump is the most efficient. Below that rating,
the pump is less efficient because of internal friction.
Above that rating, the pump is less efficient because
of increased slippage. PC pumps will work at pressures
higher than the pressure rating, but the life is
shortened. It is recommended that PC pumps be run
at 75% of the rated head for long life in non-abrasive
conditions; and 50% of the rated head in abrasive
conditions, if possible. The pressure rating of
a progressing cavity pump is usually close to 500
kPa per stage, but that varies with the hardness
of the elastomer and the geometry of the pump (i.e.,
short pitch vs. long pitch).
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