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PLC

Definition: “PLC is a digitally operating electronic system

designed for use in an industrial environment which uses a programmable memory for the internal storage of instruction for implementing specific functions to control various types of process”.

In the earlier days, the equipment was operated by Electro-mechanical Relay mounted panel

The PLC replaced Relay mounted panels

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PLC

The advantages of PLC over the Relay Logic are:

- Less cabling

- Less space requirement

- Very High flexibility

- High reliability

- Easy diagnostic

- Very fast response time

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PLC

PLC in Refinery PLC System Architecture Processor Architecture Signal Flow In PLC Software Diagnostics

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PLC In Refinery

Mainly Allen-Bradley make Others PLC Siemens, Mitsubishi The Siemens PLCs are installed in the Sulfur

and PP plant. The Mitsubishi PLCs are installed for the Access

Gate Control Allen-Bradley having PLC 5 and SLC family In PLC 5 family, the most populated models in

Refinery are PLC 5/40 and 5/60

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PLC in Refinery

In the SLC family, main models are SLC 5/02, 5/03 and 5/04

PLC and SLC Controls Operation of the Compressors, Heaters, Filters, F&G, Analyzer Safe Guarding System and Other package units

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Processor Architecture

The main parts of PLC are:

1) CPU

2) Memory

3) Inputs

4) Outputs

5) Power Supply

6) Programming Terminal

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Processor Architecture

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PLC System Architecture

Input to the PLC are mainly of two types. - Digital Input: Proximity Switch, Pressure switch,

Temperature switch etc. - Analog Input: 4 to 20 mA signals of Pressure,

Level, Temperature, Flow transmitters. Output form the PLC going to I/P converter of

the valve, Variable speed drives, Relay, Lamp indication, Hooter, etc.

The Digital and Analog input signals comes to the Digital and Analog input card respectively

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PLC System Architecture

The Digital and Analog outputs are coming from the Digital and Analog output cards respectively.

This cards are installed in Chassis called Remote I/O Chassis

The Remote I/O chassis is connected with PLC thru Belden 9463 (blue Colour)

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PLC System Architecture

Why Remote Chassis are Required? Remote I/O adaptor Card is Required for the

Communication with the Processor. It always remains in the first slot of the Chassis

The Power Supply card in the Chassis is required to supply power to the complete chassis

The 110Vac power supply is required for Power Supply card

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PLC System Architecture

The Chassis contains the processor is called Local chassis (Particularly Primary)

The processor is fixed in first slot The Second slot normally contains BCM

card The Third slot normally contains BEM card In the last slot , Power supply card is

installed

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PLC System Architecture

There are two Local chassis containing Processor.

One processor is in primary mode and the other is in secondary mode

The processor is having node address which can set by switch settings on the card

The primary processor is connected on the DH+ network

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PLC System Architecture

The primary processor appears as node no “n” in the DH+ network and the secondary processor appears as “n+1” in the network

The KF2 module is connected on the network KF2 Module Converts DH+ data into RS-232

Signals which is compatible with AB Integrator in DCS architecture

The KF2 and AB Module are act as link between PLC and DCS

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PLC System Architecture

The Panelview is used for monitoring the data of the process as well as controlling the operation

It is connected to the 2B channel of the processor

The baud rate used for communication is 57.6 K Programming Terminal for Viewing, Modifying

and Configuring the PLC Programming

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Signal Flow In PLC

The Field Signals are connected with I/O cards in RIO chassis

Processor taking data from RIO chassis thru Remote I/O link and stores in I/P image tables / memory

The data is being updated on every scan of the PLC

The data is processed in Processor according to the program written

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Signal Flow In PLC

The result is transferred to the output cards in RIO Chassis.

The Output will be in the terms of 4-20mA or Contacts.

The Processor updates the data on every scan, The scan time is in terms of milliseconds (averagely 40 mS)

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Signal Flow In PLC

The scan of the processor means processor doing input scan, then program scan, then output scan, service communication and housekeeping. The time taken for completing this activities once is call scan time

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Signal Flow In PLC

Processor Memory can be divided into two parts

One part contains data files which having All Input/Output status and intermediate flags

Other consists of program files in which ladder program has written

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System Redundancy

Why Redundancy is required Each PLC 5/40 having redundant processor It is Hot Standby redundancy The BCM cards installed in the Processor

chassis are handling the changeover and important data transfer

The Power Supply cards for I/O chassis are redundant

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System Redundancy

Redundant DH+ Network for communication with the DCS (A and B network for DCS)

The BEM cards are installed for increasing the no. of communication channels available for networking.

Redundant 1770-KF2 modules for communications with DCS

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System Redundancy

Both the PLC executes Application program

Primary processor controls the I/O, but Secondary can monitor only

The Switch over between processor is doing by BCM and BEM card

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PLC 5/40 Details

The front face showing Status LEDs, Communication Ports, Keyswitch, Battery and EEPROM installation

The LEDs are for (from the top) Battery status, Processor status, Force status and Channel 0 communication status

Apart from that, Each port (1A,2A,1B,2B) having their own status LEDs

The steady green shows healthy, Red for fault

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PLC 5/40 Details

1A and 2A Channels are used for Serial data transfer to DCS on DH+ Network

2B used for Panelview/MMI on PC/Redundant I/Os

1B used for RIO communication Channel 0 is for RS 232/ RS 422

Communication with the PC

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PLC 5/40 Details

Lithium battery is for retaining the program in Processor when it is Powered OFF.

EEPROM is to provide memory backup when processor memory corrupted

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PLC Software

The program can written in Ladder Logic, Sequence function charts or Structured text format

We have used Ladder Logic The Programming package is RsLogix5

(Windows based) and 6200 series Software (DOS based)

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PLC Software

The Software package can load into PC / Laptop

The PC requires KT card and the Laptop requires PCMK card

If KT/PCMK not available, then Processor can communicate on COM port of PC/laptop Serially thru CH0 on processor

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PLC Software

The RsLinx software is also required for the Communication over the network and serially

The PLC program is protected with 4 different level access passwords

In our complex, there is no password for the Reading the program online

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Man-Machine interface

The Operator interface monitor for Allen-Bradley PLC is called Panelview

The Panelview can - Display process graphics - Display various digital indications - Display various Analog signals from

transmitters mounted in field - Start/Stop of Motors - Varying Set points

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Man-Machine interface

The Panelview can

- Controller Auto/manual mode change

- Sequence start command

- Acknowledgement and Reset of Alarms

- History of Alarms

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Diagnosis

There are status LEDs on the processor face The first LED is battery status indicator, if it glows

RED, then Either the battery is low or missing If the Second LED is steady green, processor is

healthy If it is flashing red, the program downloading is

under progress or major fault, for steady red, hardware fault

The force glows steady amber when any I/P or O/P are forced in the program

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Diagnosis

If Communication LED flashing green, the Channel 0 communication is working

Any of the channel status LED glowing steady green, then that channel is healthy

If it glows flashing Green, bad communication

When it is red, adaptor fault

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