Page 43 - Instrumentation and Measurement

3.4.2 Control of Liquid Level

Figure 3.13 shows a control system used to control the level of liquid in a tank using a float-operated
pneumatic controller and Figure 3.14 shows a block diagram of the system.

When the level of the liquid in the tank is at the required level and the inflow and outflows are equal,
then the controller valves are both closed. If there is a decrease in the outflow of liquid from the tank,
the level rises and so the float rises. This causes point P to move upwards. When this happens, the
valve connected to the air supply opens and the air pressure in the system increases. This causes a
downward movement of the diaphragm in the flow control valve and hence a downward movement
of the valve stem and the valve plug. This then results in the inflow of liquid into the tank being
reduced. The increase in the air pressure in the controller chamber causes the bellows to become
compressed and move that end of the linkage downwards. This eventually closes off the valve so that
the flow control valve is held at the new pressure and hence the new flow rate.

If there is an increase in the outflow of liquid from the tank, the level falls and so the float falls. This
causes point P to move downwards. When this happens, the valve connected to the vent opens and
the air pressure in the system decreases. This causes an upward movement of the diaphragm in the
flow control valve and hence an upward movement of the valve stem and the valve plug. This then
results in the inflow of liquid into the tank being increased. The bellows react to this new air pressure
by moving its end of the linkage, eventually closing off the exhaust and so holding the air pressure
at the new value and the flow control valve at its new flow rate setting.

FIGURE 3.13 Level control system.
FIGURE 3.14 Level control system.

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