pls explain why we use PLC in Fire & Gas system. what is difference with out PLC in F&G System

pls explain why we use PLC in Fire & Gas system. what is difference with out PLC in F&G System

please explain the functional role of a Programmable Logic Controller in Fire & Gas System. what is the difference between with or without PLC in Fire & Gas System. I need more technical details. this information for prepairing an interview in oil & gas industry

First, I wish you all the best in your interview!

I will assume you are referring to ignition and firing of systems like flares, incinerators, boilers and the like. Personally, I have never worked in Oil and Gas.

The instrumentation (pressure, flame eyes, etc), mechanical piping, and safety/relief valving are important to the integration of control. The architecture of the system around the machine and the environment where it is operating are also factors to consider.

The overall choices for control include:

1. Good old mechanical relays hardwired to instruments and control valves. Using fail-safe circuit techniques, this approach is still not bad in some circumstances. For example, if the location is remote and must be maintained by people with limited technical experience and access to replacement parts. Then simple and visibly understandable controls might be best.

2. Packaged firing control systems can be purchased. See companies like John Zink and Flare Industries, Inc for some descriptions and drawings. The packaged systems can be relay, embedded microprocessor, PLC based. You just follow the vendors design and integrate it into yours – then let it do the work. Doing this can sacrifice some flexibility and communication options. However, it is purpose built and tested by the manufacturer and other customers over time.

3. PLC – programmable logic controllers: This is a huge area for discussion all on its own. The basic idea is simple – a CPU with a real-time operating system, custom programming languages and tools is connected to racks of local and remote Input/Output (I/O) to perform measurement and control actions. An I/O rack has many cards which are then connected with wires to the actual instruments and devices in the field. Usually each card is specialized with 8 or more individual: 120VAC inputs, 24VDC outputs, 4-20mA inputs, etc in most every combination and kind imaginable. Just choose the ones you need. Major manufacturers include

I can only imagine a PLC approach gives you more control over parameters than purely a mechanical solution. Here’s an excerpt form a webpage I located:

PLC-Based Systems
For mid-size to larger applications, the Programmable Logic Controller (PLC) has evolved as a highly intelligent controller. PLCs reliably detect alarms and initiate an orderly system control or shutdown in emergencies.

Their advantages include:
Microprocessor intelligence
Communications and networking (e.g., MODBUS)
Hardware and software self-diagnostics
Lower cost than hardwiring large system