Sil

[najmi78]

Hi,
I need information about SIL studies?????.Can any one help me with that i`ll be really obliged.

Regards,

Salman

[sri2cool4u]

Mr.Salman,

Welcome to the group. Could you please be specific on what you are exactly looking for in SIL Studies ? Like Standards, Methodologies etc ., so that people can help your specific need.

Thank you

[najmi78]

Well i need to know the following things:
1-What is SIL
2-Where and when are they carried out
3-Methodologies
4-if possible some example of SIL studies

Regards,

Salman

[cosili]

Hy Najmi,
Please read some topics abouth SIL studies in general posted by me some month ago.
After that, if is necessary I am able to help you more.
Regards,
Costin

[cosili]

The concept of safety integrity levels (SILs) was introduced during the development of BSEN 61508 (BSI 2002) as a measure of the quality or dependability of a system which has a safety function – a measure of the confidence with which the system can be expected to perform that function. It is also used in BS IEC 61511(BSI 2003), the process sector specific application of BS EN 61508 ways.
In particular they recognise that many such functions are only called upon at a low
frequency / have a low demand rate. Consider a car; examples of such functions are:
· Anti-lock braking (ABS). (It depends on the driver, of course!).
· Secondary restraint system (SRS) (air bags).
On the other hand there are functions which are in frequent or continuous use; examples of
such functions are:
· Normal braking
· Steering
The fundamental question is how frequently will failures of either type of function lead to
accidents. The answer is different for the 2 types:
· For functions with a low demand rate, the accident rate is a combination of 2 parameters
– i) the frequency of demands, and ii) the probability the function fails on demand (PFD).
In this case, therefore, the appropriate measure of performance of the function is PFD, or its reciprocal, Risk Reduction Factor (RRF).
· For functions which have a high demand rate or operate continuously, the accident rate is the failure rate, λ, which is the appropriate measure of performance. An alternative measure is mean time to failure (MTTF) of the function. Provided failures are
exponentially distributed, MTTF is the reciprocal of λ.
These performance measures are, of course, related. At its simplest, provided the function can be proof-tested at a frequency which is greater than the demand rate, the relationship can be expressed as:
PFD = λT/2 or = T/(2 x MTTF), or RRF = 2/(λT) or = (2 x MTTF)/T
where T is the proof-test interval. (Note that to significantly reduce the accident rate below the failure rate of the function, the test frequency, 1/T, should be at least 2 and preferably ≥ 5 times the demand frequency.) They are, however, different quantities. PFD is a probability– dimensionless; λ is a rate – dimension t-1.

[cosili]

Dear Najmi,
Fault tree analyses is the best method to obtain PFD and of course the SIL level.
Also there is a lot of dedicated tools for SIL evaluations..but also I NEED IT.
Regards,
Costin

[adelnifoo]

Dear Najmi,
Fault tree analyses is the best method to obtain PFD and of course the SIL level.
Also there is a lot of dedicated tools for SIL evaluations..but also I NEED IT.
Regards,
Costin

Dear cosili,
I'm an HSE engineer in a gaz company, at this yime i'm doing a study on "SIL assessment of a detction gaz system". I had already do a database on this system and on its failures along three years up to now.
I'm calculating Its MTBF and Its MTTR but the question is : can λ be equal to 9 and more or is it an error on my job ? (this value is abtained on such gaz detector which commonly fail, half of time it's fail)
please can you help me to solve this problem
I used IEC 61508 & ISA 615XXXX (I dont remember the code exactly).

[cosili]

Hy adelnifoo,
In general, lambda is the law of exponential distribution, and express the number of failures per hour or per year.
In general, lambda is relatively small values of the order 0.00001 or less.
I do not think that the calculated values are correct.
Can you send how you calculated the values of lambda and law repertitie what you used for gas detectors.
Regards.

[sri2cool4u]

Adelnifoo,

First of all you should be clear on what failure you are talking about. Fail to detect? Faulty failures etc

Let me think in this way, see whether i align with your thoughts

Are you saying the detector fails 9 times in 3 years ? and so your lamda (failure rate in hours) is 9 / (8760*3) per hr - 3.425E-04/hour which sounds a bit higher, but still can be. As general failure rate of hydrocarbon detectors fall in the range of 1E-05/hr (critical failures), but this varies depending on the type of detectors.

Basically I would think, u cannot derive lamda with a data of three years (cosli correct if I am wrong), this may end up with anomalous figures if your system is in initial installation period (due to installation trouble / trips), or final deterioration period. Usually lamda value is the figure of failure (as explained by cosli) during the normal operational period of an equipment under maintenance regime (remember the bath tub curve of equipment failure). So if your equipment is in either installation or degradation stages, I would not recommend you to take this as failure rate.

Secondly what is value 9 represents ? Detected or Undetected? If any of those, does it represent the Dangerous failure of Equipments ? I believe detectors generally result in faulty failures a lot due to external conditions (dust, mist etc) and also depends on detectors. Generally 90% of the failures of detectors are non-critical, 5% critical and 5% spurious trips.

You need to answer these for yourself before finalizing your failure data.

Regards