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SIL Understanding
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from the instrument department...
SIL 2 or SIL 3 transmitter
Posted by reza heydari on 7 May, 2006 - 2:13 pm
Please tell me what's difference between SIL3 transmitter and SIL2 transmitter,it is better that i
ask how a usual transmitter can modifies to, for example SIL2 or SIL3?Reply to this post...
Posted by DaveMH on 8 May, 2006 - 9:43 am
The difference between a transmitter certified for use in SIL3 applications and a SIL2 certifiedunit is that the SIL3 unit will have satisfied more onerous requirements with respect to its design,
in particular reliability and failure modes.
If you are talking about IEC61508/61511 SILs...
Note that the use of SIL certified devices, be they PLCs or transmitters etc., does not itselfguarantee that you have satisfied the requirements for the safety function. It is the safety function
itself (e.g. trip of plant item r items) which carrys the requirement for a specific integrity level.
Therefore the whole of the system for that specific function (from sensor(s) to final element(s))
must be designed with the performance requirements and management requirments that comewith the integrity level specified.
Use of certified devices does make the process of putting together your safety claims/argumentseasier and more traceable.
If you are unfamiliar with the design practices using the safety standards I suggest you get a
specialist in who has a verifiable track record.
Hope this helps
Reply to this post...
Posted by Anonymous on 21 May, 2006 - 3:09 pm
A safety certified transmitter is different from regular transmitter as it has additional certification
pertaining to IEC 61508.
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It does not differ in operational functionality but has documented failure modes with
identification of lambda, SFF and finally has a defined reliabilty.
You may achieve SIL3 risk miigation even by SIL2 transmitters used in 1oo2 or 2oo3
configuration as you meet the required Pfd(avg).
Even uncertified hardware can be used in SIL2/3 application if you can get safe and dangerousfailure rates through OREDA or other reliability handbooks, achieve desired Pfd(avg) or
compensate through enhanced loop inspection/maintenance frequency (though not desirable)as
guided by IEC 61511.Reply to this post...
Posted by boshra on 31 March, 2009 - 5:59 am
Thanks a lotReply to this post...
Posted by Neil Munro on 8 May, 2006 - 3:12 pm
Reza,
There is not a quick an easy answer, but I will try...
The SIL (Safety Integrity Level) is a measure of the probability of failure on demand. In otherwords will the thing work when I want it to ! The higher the SIL number the less chance it has of
failing when you need it, so a SIL3 device has a lesser chance of failure on demand, than a SIL2
device.
SIL levels are more applicable to safety systems and normally are stated for systems rather than
single devices. SIL levels are increased by adding more devices to do the same thing(redundancy) e.g. if you have two tranmitters measuring the same variable and you monitor them
both then you have effectively increased the SIL level of that particular part of the system.
Some manufacturers are now claiming that their devices are certified to a particular SIL level by
adding redundant sensors/electronics etc. in the same device, but they haven't if there is still asingle point of failure!
For more information, try Google with IEC 61508.
Hope that helps!Reply to this post...
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Posted by vahid on 17 August, 2009 - 5:19 pm
Neil Mounro,
Thank you for your reply.Refer to your reply I understand that we have to take into account two parameters:
1)Failure of the device/instrument and its effect on our process
2) the hazard/dangerous events that affect the human safety.
I did not understand the meaning of your sentence "Some manufacturers are now claiming thattheir devices are certified to a particular SIL level by adding redundant sensors/electronics etc. in
the same device, but they haven't if there is still a single point of failure!"
Would you please explain more?
Thanks for your concern.Reply to this post...
Posted by Shailesh C Patel on 9 May, 2006 - 10:33 pm
Dear reza,
Full form of SIL is Safety Integrity Level. As per standard practice you can say that "a"
transmitter is suitable for SIL 2 or SIL 3 loop. If any one component level is SIL 1 then loop will
be always SIL 1, even Transmitter is suitable for SIL 3. Main difference between SIL 2 and SIL3 is Probability of failure on demand per year (PFD). PFD for SIL 2 is >=0.001 to =0.0001 to
7/28/2019 SIL Understanding
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I was surfing for the SIL levels & as per the information SIL are for PFD. As SIL 3 is more
capable & more safe then SIL2. Howeever i would like to know, with whom it has to be certify?
& who are the manufacturers for PLC with SIL compatability.
Regrds,
nirav shahReply to this post...
Posted by Roy on 17 December, 2008 - 9:34 am
EXIDA is the agency certifying for SIL Ratings.Reply to this post...
Posted by W.L. Mostia on 22 December, 2008 - 10:15 pm
"EXIDA is the agency certifying for SIL Ratings."
Exida (http://www.exida.com) is not really a certifying agency and I don't believe that they
would qualify as NRTL. It is a commercial company in the functional safety business. They doprovide failure mode and effect and diagnostic analysis (FMEDA) reports for equipment used in
safety service, which provide failure rates, SIL compliance, and other information in regards to
compliance with safety system standards, as well as other commercial services related to the
functional safety business. These FMDA reports have been used commonly by manufacturer forexisting equipment, which they wish to sell in the SIS marketplace, to provide SIS information to
their customers.
The most common certifying agencies for SIS equipment are the German TUV agencies (there
are several). FM, BASEEFA (British), as well as some other agencies do this but are minor
players in this area. Almost all new SIS equipment will have a TUV approval.
William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-Tech Solutions
Reply to this post...
Posted by Anonymous on 21 June, 2009 - 6:22 am
There are vendors who provide SIL certification by FMEDA based on EXIDA report. Most of
the other manufacturers strongly recommend to go with TUV certified instruments. WHich one
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to choose as a user? Is there any catch going for FMEDA certified instruments?
Pl adviseReply to this post...
Posted by wboyes on 22 June, 2009 - 9:29 am
There doesn't appear to be. Exida appears to be a thorough, professional, knowledgeable agencyfor both functional safety and functional security certificaton. So of course, is TUV Rheinland
and so is TUV Nord. All three agencies do this work regularly.
Walt BoyesEditor in Chief
Control and Controlglobal.com
www.controlglobal.comMailto:wboyes [at] putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff
Reply to this post...
Posted by Samir on 23 June, 2009 - 5:51 am
Thanks.
Exida is not a notified party.
1) Still can we use FMEDA certified instruments based on exida for SIL2?
2)Can I accept SIL3 certified transmitter in SIL2 loops?
SIL3 certificate by FMEDA talks about Software certification does not mention anything onsoftware. Hence this question.
Thanks in anticipation
Reply to this post...
Posted by A.ZUBAIDY on 17 August, 2009 - 8:20 am
Hi all
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I have system certifcate as SIL2 (Yokogawa DCS). I want to install the sensor is SIL 3
certificate. Is it Ok? Or i'm going to lose my system certificate?
Reply to this post...
Posted by Girish on 23 June, 2009 - 5:57 am
For SIL2 applications which one to shoose? FMEDA / Exida certificate or TUV? Why?
ThanksGirish
Reply to this post...
Posted byDaveMH on 23 June, 2009 - 5:13 pm
I may have got the wrong end of these last few posts. Apologies if thats the case.
There's a few points I'd like to make.
1) The responsibility for making sure that equipment is appropriate for use lies with the plant
designer and/or operator.
2) What equipment is appropriate depends upon the application. There can be no generic
solution. E.g. better a non-certified device performing the correct function in the actual
environment than a certified one thats not quite right.
3) There is no absolute requirement to use certified devices. In some cases there may not be a
certified device. See 2 above. So devices with an FMEDA only may be OK. It depends on the
application and integrity requirements.
4) An FMEDA report is not a certificate and does in itself not address all of the requirements for
evaluation with respect to the IEC61508 standard e.g. software design for modern instruments.An FMEDA can form part of the documentation of the assessment against 61508/61511. An
FMEDA cannot deal with assessment of software quality a different tool is require hence
(probably) the mention of software.
5) I would say that Exida and TUV reports and certificates are largely equivalent although there
maybe differences in the quality of the underlying work. There are also other organisations
which may be acceptable e.g. Sira in the UK.
6) Whether a transmitter certified for use in SIL3 applications is able to be used in a SIL 2 safety
function depends on the application. Probably from a reliability point of view but thats not the
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only consideration.
In all these things engineering judgement and experience is needed. It sounds like you mightneed some help to make these decisions. Go find it and don't be embarrassed to ask. Nobody
knows everything and peoples safety is the concern here.
Hope this helpsDaveMH
Reply to this post...
Posted by Osama Saied on 9 June, 2006 - 1:06 am
SIL stands for Safety Integrity Level which is a safety level stander approved by TUV, there are
four levels: 0 (the lowest safety requirements) to 4 (The highest level with the lowest probability
of failure). so SIL selection depends on the criticality of the control loop.Reply to this post...
Posted by W.L. Mostia on 18 June, 2006 - 12:00 pm
There are a number of inaccuracies in the above statement.
First, TUV approval does not determine SIL level. SIL level is system based (sensor, logic
solver, final elements), not component based. TUV or any other approval agency certify
components as suitable for use in a particular SIL application, i.e. suitable for use in a SIL 2system, but the SIL of the safety instrumented function (SIF) depends on all of the components
and the installation in the safety trip path as well as their proof test frequencies. Second, there is
no SIL 0 in the standards (S84, 61511, or 61508). Some companies utilize SIL 0 or SIL "a", as ameans of identifying safety systems which are determined not to be required by a risk analysis
(note that this type of analysis is typically a minimum required analysis), which are existing and
retained or new and required for other reasons (These are sometimes called orphaned safetysystems). These types of systems are typically required by company standards to meet certain
requirements in the safety standards but not the complete standard, e.g. S84/61511. Third, SIL is
not intended to apply to control loops (i.e. basic process control systems (BPCS) loops) but to
demand and continuous safety systems. Fourth, "criticality" is a vague term because a protective
system can be "critical" for a number of reasons - safety, environmental, asset protection,operational, etc. SIL is determined by a risk analysis by the residual risk once all other layers of
protection for a specific hazard have been considered. Several of the methods of determining SILare discussed in S84/61511 part 3. Also, note that there are really two SILs, target SIL which is
determined by the risk analysis and achieved SIL which is the SIL of the designed safety
instrumented function as determined by quantitative calculation and architectural constraints. Inthe end, the achieved SIL must meet the target SIL.
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William (Bill) L. Mostia, Jr.
Principal Engineer
WLM Engineering Co.Reply to this post...
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