PRESSURE OPERATED CONTROL SYSTEM (POCS)

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.

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