What is the difference between ESD & PLC?

ESD- Emergency shutdown... it is for shutdown the total plant during or in-case of any emergency like,high pressure accured in process , instrument air failure, or any utility failure. PLC- programmable logic controller.. by implementing logic or program's , we can achieve the ESD by using PLC so ESD is a kind of logic, PLC is a hardware. 

Both systems can be described as control systems, both of them are used to control process conditions under certain circumstances. Both of them are programmables using the dedicated software, the programming language can varry from a simple ladder logic up to a detailed statement language depending on the manufacture and the application.

The main difference between both systems is the SIL rating for the application that will, for sure, the SIL rating of the hardware itself.

The ESD is used for SIL3 or higher rated loops requiring more safety requirements, this will affect the hardware structure, so the MTBF of the ESD hardware will be lower and you will find the hardware redundancy everywhere starting from the power passing by processor, communication and ending by I/O cards. Example for manufactures are: Triconex and Hima.

The PLC is used for simple applications that doesn't require a high SIL rating, the hardware is much more simpler and it can be used on simple applications and normally the cost is much more less than the ESD one. Example for manufactures are: Siemens

ESD is Emergency Shutdown system and PLC is Programmable Logic Controller.

ESD system used for shutdown a particualr process or entire plant in the case of emergencies.TRICONEX ESD system is one of the best and used mainly in SAUDI ARAMCO plants.

PLC are control system used for control the process by logic Ladder Diagram by use of Gates (AND,OR etc).

ESD system gives more safety to the plant and expensive on the other hand PLC is for controlling the process and having less cost when compare to and ESD systems.

ESD is a special logic that shuts the plant down in case of any real risk.

PLC is the hardware that controls all process.