In the
close fluid container where the fluid is injected into the container by means
of a force pump, the pressure in the container could reach an unsafe level. The
pressure operated control system is designed to limit the pressure in the
container up to a safe level. Once the pressure exceeds the safe level the
force pump should stop injecting the fluid in the container. One means of the force
pump pressure up to the safe level is by means of pressure switch PS installed
in the container. see figure (A).
(A)
Close container using the POCS. An example is used in a jetmatic pump.
For an open
fluid container, a level pressure switch LPS shown in figure (B) is
installed at the lower part of the open fluid container. The LPS with
associated control circuit will limit the height of the fluid being pump into
the open container at a pre-determine level.
(B) An open
container using the POCS.
Other
application of POCS using the pressure switches PS and LPS are found in air
compressor and high pressure steam tank. In an air compressor, a motor air
pumps air into the tank container. When the air pressure inside the tank is
about to reach the unsafe pressure level, the motor air pump will stop.
(C)
Control and power circuit of POCS.
In high
pressure steam tank, water is boiled to produce a steam. The steam is entered
into a steam tank. When the pressure is about to reach a dangerous level, the
fire in the furnace that boils the water is cut off.
Control Operation of the Close Tank Container
When the
water is closed container drops at low level, see figure (A), the pressure of
the low trapped air TA inside the tank is lo. This pressure called the cut in
pressure (CI) is not sufficient enough to push to the left and close the
normally open pressure switch PS. With switch PS open, the relay (A-B) Figure
(C) is de-energize.
Normally
closed contact (11-12) is closed to maintain contactor C (A-B) and pilot light
PL at energized container. With contactor C (A-B) energized, its contacts
(1-2,3-4,5-6) will closed to energized
and start the motor pump MP to pump water into the close container.
As the
water in the container rises the trapped air TA is compressed and its pressure
is increased. A certain pressure called the cutout (CO) pressure is reached
where it is sufficient to push to the left and close the pressure switch PS.
With switch PS close, the relay R (A-B) will be energized and it will open its
contact R (11-12) is de-energized. Contacts C (1-2,3-4,5-6) will open to stop
the motor pump MP for pumping water into the close container.
Control Operation of the Open Tank Container
When the
water in an open container drops at certain low level, the water pressure at
the level pressure switch LPS also drops to a point where it will not enough
push to the left, the contact LPS (1-2). Contact LPS (1-2) is therefore open.
The low water pressure is called the cut-in (CI) pressure. With the switch LPS
(1-2) at open position, relay R (A-B) is de-energized. Contactor C (A-B) is
energized and the motor pump MP start pumping water into the open tank
container.
As the water rises up, the water pressure at the
LPS is increase. A certain pressure called cut-out (CO) pressure is reach where
it is sufficient to press the LPS switch contact LPS (1-2) to close position. With the switch LPS
(1-2) closed, relay R (A-B) is energized; contactor C (A-B) is de-energized;
and the motor pump MP stops pumping water into the open tank container. At this
low pressure, the LPS contact LPS (1-2) is maintained at open position.
Back to Previous Article:<--- ALTERNATE OPERATION OF TWO MOTOR PUMPS
0 comments:
Post a Comment