Safety Systems And Difference Between Safety PLC & Conventional PLC.

INTRODUCTION TO SAFETY SYSTEMS:

What is safety?

Safety is nothing but the condition of being protected from or unlikely to cause damage, danger, injury or harm.

Why safety system in industries?
A safety system in a industry, Is a system that provides an independent and predetermined emergency shutdown path in case a process runs out of control.
If a process runs out of control, Then it may cause:-
*Damage To People/Environment.
*Loss Of Equipment/Production/Money.

Figure No.-1

Figure(1), Shows some few disasters happend because of system failures in the industries.
System failures are furtherly classified into two types:
1)Hardware Failure.
2)Systematic Failure.
Hardware failures are caused by the stress occurred in the hardware component.
Such stress can be of different types. Some of the few are:
*Heat
*Chemical Corrosion.
*Humidity.
*Vibration.
*Electrostatic discharge
*Operational and Mainatainance errors.
Systematic failures are related to errors in software design. These can be solved by reprogramming the software.
Such type of disasters[as shown in figure(1)] can be avoided by using SAFETY INSTRUMENTED SYSTEMS(SIS) in the industrial processes. It is configured or programmed to automatically respond to certain processes and reduce the loss occurs due to it.

Some of the basic applications of safety systems in industries are:
*Emergency Shutdown.
*Safety Shutdown System.
*Safety Interlock System.
*Burner management.
*Fire and Gas detection.

Basic Fundamentals Of SIS

As shown in the figure, When the process is within the range of normal behaviour the basic process control system will control the process.
When the process reaches the unstable state, then the operator will takes action to control the process.
If the process value continues in the unsafe region and trip level is reached, Then the SIS executes an Emergency Shutdown Action, Preventing the process from exceeding the safe levels.

Safety Instrumented Systems:
The Safety Instrumented Systems(SIS) plays a major role in proving in providing a protective layer around industrial process systems. Whether called an SIS, Emergency Shutdown System or Safety Shutdown System. Its purpose is to take process to a "safe state" when pre-determined set points have been exceeded or when safe operating conditions have been exceeded.

An SIS is comprised of safety instrumented functions(SIFs) with sensors, logic solvers and final control elements.

Figure No.-2

 Figure(2), Shows the function of SIS(red) and Basic Process Control System(blue) in the process.

Every Safety Intrumented System(SIS) has one or more Safety Instrumented Functions(SIFs) and each affords a measure of risk reduction indicated by the Safety Integrity Level(SIL)

Safety Integrity level:
SIL is the assumption to what an extent can a process be expected to perform safely? And in the event failure, to what an extent can the process be expected to fail safely?
These questions are answered through the assignment of a target safety integrity level(SIL).
It is incorrect to call a particular safety system as SIL1 or SIL2.  Either than it is correct to say a particular safety system is "suitable for use in a SIL1 or SIL2 Environment.
There are four levels which have three important properties.
1) Applicale to the overall safety function.
2) The higher the SIL, The stricter are requirements.
3) Applicable to technical and non-technical requirements.

SIL Levels

Figure No.-3

The SIL selcetion is based on the risk posed by the hazard.

Safety Instrumented Function(SIF):
IEC61511 defines SIF as a identified safety function that provides a defined level of risk reduction or SIL for a specific hazard by automatic action using instrumentation. An SIF is made up of sensors, logic solvers, final elements that act in concert to detect a hazard and bring the process back to safe state.

Basic Architecture of SIF

There are two types of SIFs, The first is a safety instrumented protection function, which is safely instrumented function opearting in the Demand mode. The second is a safety instrumented control function, which is a safety instrumented function operating in the Continuous mode.

       Anatomy Of A SIF    

Some of the few examples of safety instrumented functions(SIFs) are,
* High pressure in a vessel opens the vent valve. The specific hazard is overpressure of the vessel. The high pressure is detected by a pressure sensing instrument, and logic(PLC) opens a vent valve, bringing the process to a safe state.
* High temperature in a furnace can cause tube rupture shuts off firing to furnace. The specific hazard is tube rupture. Instrumentation automatically causes a main fuel trip that removes the heat, bringing the system to a safe state.
* Flame-out in an incinerator can lead to a release of toxic gas causes process gas feed to be shut-off. The specific hazard is a flame-out. The automatic instrument protective action is to close process gas feed to the incinerator Which stops any toxic gas release, bringing back the system to a safe state.
* Flame-out in an incinerator can cause fuel gas accumulation and explosion causes a main fuel gas trip. The specific hazard is a flame-out. The automatic instrument protection action is a main fuel gas trip, which cuts off the fuel and prevents fuel gas accumulation, bringing back the system to a safe state.

SIS Is An Combination Of One or More SIFs 

PLCs As A Safety Systems In The Industrial Applications:

Safety Programmable Logic Controllers(PLCs) are special purpose controllers taht are used to provide critical control and safety applications for automation users. These controllers are normally an integral part of safety instrumented systems(SIS) which are used to detect potentially dangerous process situations. If such a situation occurs, The SIS is programmed to automatically take action to bring back the process to a safe state. There is a serious question though,
What is the difference between the Safety PLC & Conventional PLC?
Why shouldn't a Conventional PLC be used in Critical Control and Safety Applications?
A Saftety PLC was specifically designed to accomplish two important objectives,
1) Do Not Fail(redundancy that works well) but if that cannot be avoided.
2) Fail only in a predictable safe way.
Many special design considerations are taken into account. A safety PLC will emphasize internal diagnostics, A combination of hardware and software that allows the machine to detect improper operation within itself. A safety PLC will have redundancy to maintain operation even when parts fail. A safety PLC will have extra security on any reading and writing via a digital communication ports.
A safety PLCs also differs from a conventional PLC in that the saftey PLC is typically certified by third parties to meet rigid safety and reliability requirements of international standards.
Special electronic circuitry, Careful diagnostic software analysis and full fault injection testing of the complete design insure that safety PLCs are capable of detecting over 99% of potentially dangerous internal component failures. A Failure modes, Effects and Diagnostic Analysis(FMEDA) is conducted on the design, Indicatin how each component in the system fails and how the systme detects the failure.
Tough international standards for software apply to Safety PLCs. These standards demand special techniques to avoid complexity. Extensive analysis and testing carefully examines operating systems for task interaction. This testing includes real-time interaction, such as multi-tasking and interrupts. Special diagnostics called "program flow control" and "data verification" are required.
Program flow checking insures that essential functions execute in the correct sequence. Data verification stores all critical data redundantly in memory and checks validity before use.
There are certainly many similarities between a Safey PLC and Conventional PLC. Both have the ability to perform logic and math calculations. Both typicaly have input and output(i/o)modules that provide them with the ability to interpret signals from process sensors and actuate final control elements. Both will scan inputs, perform calculations and writes output. Both typically have digital communication ports. But the Conventional PLC was not initially designed to be fault tolerant and fail safe. That is the fundamental difference between the Safety PLC and Conventional PLC.
The realization of many users that Conventional PLCs cannot be depended upon in critical protection applications creates the need for Safety PLCs. The standards are high for Safety PLCs design, manufacture and installation.