Requirements for Sil 3 System


Thread Starter


what are the requirements for sil 3 system? my hardware and software is sil 3. does we still require dual inputs and outputs to make it sil 3? or with single input and output it is sil 3? kindly also refer some pdfs

kindly explain sil 3 system requirements....
Safety Instrumented Systems (SIS) and SIL (Safety Instrumented/Instrumentation Level) are some very expensive guidelines. The control system used in a SIS may be SIS-capable (at various levels, SILs) but a true implementation of SIS involves a deeply detailed investigation of all aspects of the process (it might be a small process, or a large process with lots of auxiliary equipment--we don't know anything about your application/process) and some estimates (guesses-albeit they are supposed to be educated guesses) about failure rates and how critical the components/process(s) are to machine and personnel safety.

So, it's really difficult to answer your question, except to say that simply having SIS-capable control system does NOT make a process or system a Safety Instrumented System. And, depending on the criticality of the input/output and the level of SIL desired (or required) inputs may need to be dual-redundant or triple-redundant.

Search the World Wide Web for SIS, and you will learn about the concept and the various levels (SILs) and what's involved in determining a required SIL "level" and you will be very surprised about how vague the guidelines are, and how much human estimation is involved in the process (the educated guesses).

This probably doesn't answer your questions, but, again--without knowing a LOT more about the application/process and the equipment involved we can't really say much more about your specific questions. And, many SIS-related documents are available on the World Wide Web for anyone to view and (try to) understand. Sometimes it takes several readings to devine the meaning of some of the documents, and they usually need to be taken in small "chunks" and understood before advancing to the next chunk. And, I've found that some of the documents use words and terms which are not defined until later sections of the documents (which is very frustrating for most people). So, patience and perseverance are required--as with most things. But we just can't really say anything about your specific application and hardware/software without knowing a LOT more than you have provided.
You should not be implementing a SIL 3 system unless you know how to so. This is not a learn as you go type of task. Find someone who is experienced and competent in implementing safety instrumented systems (SIS) and learn from them. Take some courses to learn the basics (ISA, SIS-TECH, exida, etc.), but don't start with a SIL 3 system.

You can start learning about SIS systems by understanding the standards: IEC 61511-1,2,3 and ISA TR84.00.02-09. Throw in IEC61508-1 through 6. The following following books will also be helpful:

Reliability Maintainability and Risk, Dr. David J. Smith

Kenexis Safety Instrumented Systems Engineering Handbook, Kevin J. Mitchell, et. al.

Systems: Design, Analysis, and Justification, 2nd Edition, Paul Gruhn & Harry Cheddie

SIS Design Engineering - Collected Works of Kenexis Marking Their 5th Anniversary (Amazon Kindle, a bargain for $1.00)

Safety Instrumented System Verification, William Goble & Harry Cheddie

And now you have a good start.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis
>kindly explain sil 3 system requirements....

For a SIS (Safety Instrumented System) to meet a specified SIL level requires that all parts of the system meet that reliability level. Therefore your field instruments (inputs) and valves (final elements) must have the same level of reliability as your logic solver. That reliability is obtained via redundancy. 2oo3 is very common for transmitters. 2oo2, double block and vent, is very common for valves. The reliability and failure rate of the whole system and all it's components must be calculated.

Check the web site of some of the leading SIS vendors for more info:

-Rockwell (now own and sell ACS Triplex)
-Emerson (Delta V SIS)

masoud Sadra


To have a SIL 3 SIF, just buying a SIL 3 certified primary element (e.g. transmitter), the Logic Solver and the final element is not enough. You must design and construct your system in a way to meet SIL 3. Pls. note that in addition to PFD (Probability of Failure of Demand) there are other requirement such as Safe Fail Fraction, Diagnostic Coverage (DC), and Architectural Requirement. Consult IEC 61508 & 61511 for details (Hardware fault tolerant sys etc.)

M. Sadra