60 Industrial Products Source : November 2014

Simply SIS

by Shivendra Kapoor

SIS has been existing since decadesbut it gained much required attentionafter catastrophic industrial disasterslike Bhopal gas tragedy, Piper Alpha,BP Texas refinery, etc. and introduc-tion of international standards likeISA 84.00.01, IEC 61508 and IEC61511. However, the real problem thatis still prevalent in the process indus-try is in correct understanding andapplication of this three letter word‘SIS’ and associated terms SafetyInstrumented Function (SIF) andSafety Integrity Level (SIL).

This article attempts to explain thesubject in a very simple way withoutdigging too much in the technicalaspects or quoting verbatim from in-ternational standards.

Before we begin with SIS, we need toknow that we are dealing with ‘func-tional safety’ and not just any safe-ty. Functional safety is a part of over-all safety and is concerned with thecorrect functioning of the SIS.

Astrologer and SIS

In a layman’s language,a SIS can be comparedto an Astrologer. Amaz-

ing thought! Isn’t it? Let’s see how.

An astrologer predicts future basedon our horoscope by analyzing plan-etary permutations and combinationsand estimates the likelihood of a par-ticular event that may happen in ourlife.

A SIS, as a minimum, consists of asensor (like a pressure transmitter tomeasure pressure), a logic solver (likea PLC that does some logical calcu-lations) and a final control element(like a valve). This entire loop con-sisting of a sensor, a logic solver anda final element is called a Safety In-strumented Function (SIF). One SIScan have one or many SIFs. ReferFigure 1 that depicts a SIS havingone SIF.

No one is perfect and so an astrolo-

ger cannot always be 100% accuratein his predictions as this is depen-dent on many factors like accuracy ofbirth chart, etc.

Similarly, there is always a probabil-ity that a SIF may fail to perform itsintended function (to protect anequipment) when there is an actualdemand from the process and this isdependent on many factors. This iswhat we all try to find out i.e. theProbability of Failure on demand(PFD) of the SIF. This exercise is car-ried out when we are conducting aSIS workshop session.

So, we have a SIS that comprisesone or many SIFs and each SIF willhave a Safety Integrity Level (SIL)assigned to it.

A SIL is a measure of the performanceof the SIF just as we measure theperformance of an astrologer by thenumber of correct predictions hemakes when we ask a question (un-

Fig 1: A single SIF

E-mail: shivendrakapoor@cholams.murugappa.com

The intent of this article is to assist everyday working engineers, designers, operators, manufacturers, thirdparty safety consultants and anyone involved in the ‘functional safety’ business to have a correct understandingof the basics of a Safety Instrumented System (SIS).

Industrial Products Source : November 2014 61

Company Newsderstand this as a demand placed onthe astrologer). There are only fourdiscrete SIL levels i.e. one, two, threeand four with SIL four being the mostdifficult to achieve and SIL one theeasiest.

That’s all about SIS in a nutshell andwe do just this when we say we areconducting a SIS workshop. Easy,isn’t it?

The SIS Workshop

What do we exactly do in a SIS work-shop? We need tounderstand this asit is the most fea-red workshop wh-en compared withother process safe-

ty workshops like: HAZID, HAZOP,What-If, QRA, EERA, HFE, etc.

After HAZard and Operatibilitystudy (HAZOP) or any other ProcessHazard Analysis (PHA) study is con-ducted the identified hazards that

have risks associated with them arerecorded in a report with necessaryrecommendations to prevent or miti-gate the consequences of a particu-lar hazard. This forms the startingpoint and one of the key input doc-uments to conduct the SIS workshopby a SIS chairman or facilitator.

The SIS study workshop is primarilysub-divided into two main sessions:

i) SIS assessment/classification/as-signment (Target SIL value determi-nation)

ii) SIS verification (Verifying if thetarget SIL is achieved)

The SIS workshop is a very system-atic study and is carried out per theSafety Life Cycle documented in IECstandard. IEC 61508 is the umbrellastandard and has seven parts andcan be used by any industry. TheIEC 61511 is the process sector spe-cific standard and can be used byany process industry like: oil and

gas, refining, chemicals, non-nuclearindustry, etc.The details of SIS assessment andSIS verification are beyond thescope of this article and shall be cov-ered in future articles.

Author’s Brief Bio:Shivendra Kapoor, is an Instrumen-tation Engineer and a Certified Func-tional Safety Professional from TÜVSÜD. He has +11 years of processindustry experience covering a va-riety of sectors like: Oil and Gas,Mines and Minerals, and has wor-ked in best EPC companies in Indiaand abroad. Shivendra is also afreelance writer at heart and hasmany short fictional and non-fiction-al stories published internationallyand in publications like Times of In-dia. He is currently employed withChola MS Risk Services in Mumbaiand is involved in various aspectsof functional safety.