Tank Level Control in PLC

Design a PLC program to control the liquid level in a tank. A level transmitter is used to measure the tank level into a standard current signal from 4 to 20mA as illustrated in the below process diagram.

Tank Level Control in PLC

PLC Program Logic

1. The agitator motor is to be started and stopped by a pushbutton station.
2. The solenoid valves are to be open (Energized) when the agitator is running.
3. Pump N0.1 starts at the 75% liquid level and stops at the 25% level. The pump will run only if the agitator is running.
4. On a high-level alarm (Tank 90%), an alarm light will come ON and stay on even if the tank level drops. An operator must press the reset button to turn off the light. On the high-level alarm, both the solenoid valve will also close (de-energize).
5. A low-level alarm light shall be provided (Tank 10%), similar to the high level, and reset using the same pushbutton.

Scaling Tabular Column:

PLC Programming for Tank Level Control

PLC Program Description

Rung 0000:

Start/Stop PB latched with memory B3:0/0.

Rung 0001:

B3:0/0 enabled to turn on agitator (O: 0/0), Solenoid valve 1 (O:0/1) and Solenoid valve 2 (O:0/2).

Rung 0002:

Scaling block is used, Level transmitter gives output in mA (N7:0), to convert that into PLC range (N7:1), and scaling block is used.

Refer to the above tabular column for a respective range of values.

Rung 0003:

According to the program logic, we need to turn on the PUMP (O:0/3) at tank reaching 75%

And the pump still is in ON condition up to 25%, so comparator blocks are used to get this condition.

Agitator motor contact also connected in series as mentioned in program logic.

Rung 0004:

Memory coil connected to turn ON pump motor output.

Rung 0005 & 0006:

High-level alarm should turn ON when tank reaching 90% and should not turn off until alarm reset (I:0/2) is pressed.

Greater than or equal block is used to compare the real scaling value(N7:1) and register value.

Rung 0007 & 0008:

Low-level alarm should turn ON when tank reaching 10% and should not turn off until alarm reset (I:0/2) is pressed.

Less than or equal block is used to compare the real scaling value(N7:1) and register value.

Conclusion

We can use this example to understand the programming logic in Allen Bradley Programmable Logic Controller (PLC).

Author: Hema Sundaresan

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