Forum Topic

Friday 07 December 2007 4:00:18 am

I want to some more about how to maintain the Safety instrumented system when it is running.
After evaluating the SIL of the SIS. we may be find that the PFD of SIS can't meet the demands. Then what should we have to do?
I just think that we can change some parameters related to PFD,such as failure, common causes, diagnostics, and periodic test intervals, to decrease or increase the PFD. so we get new SIL.
if you have some suggestions, please tell me .thank you .

Thursday 20 December 2007 9:40:29 pm

The PFD is a design parameter. That means that we evaluate the PFD when the design of the safety function is complete.

If we the put the safety function live and use it in real operation we need to make sure that the assumption we used during design are still correct after 1, 2... 10 years. In other words if you assumed that the failure rate of a certain valve is X but in a reality it is 10 times worse, then you should redo your calculations with the "new" data and see that you are still SIL compliant....

Tuesday 25 December 2007 4:06:21 am

Thank you for giving me a help. I mean that SIL is suitable for safety function at the first step, but we may find it decrease because of some reasons. Then besides changing the SIS, which methods could we do?
I think about this and we could do this in theory. We can give some methods to maintain the SIS by changing the some Parameter related the PFD.

Monday 28 January 2008 1:06:20 pm

The PFD is a design concept/parameter not an operational parameter. Once designed the PDF starts to increase over time due to the failure behaviour. You can play with the design parameters to influence the PDF but you cannot do that any more once the SIS in installed in th field. Once installed in the field there is only 1 parameter that can help you, periodic testing...

Friday 08 February 2008 11:35:47 pm

I am really interested on safety systems and I am preparing to be a professional safety certified but unfortunately this is something new here and there is not background and projects are executed without serious risk analysis and no one be care about to supervise or be responsible of safety systems during design, commissioning even operation. Our costumers just asking for TMR´s and they think that it means SIL3 without consider the SIF, periodical test and kind of this.
So that I want to know how do you perform safety projects in your country? And where can I find information related?
and of course tips to get enough cuotatiin in my test
Thanks

Tuesday 11 March 2008 3:50:39 pm

Risk analysis is not country depend nor industry dependent. Either you do it well or you do not. The case you describe just demonstrates the lack of knowledge of the client. In theory risk analysis is simple:
- Identify the hazard
- Determine the risk associated with the hazard
- Determine whether risk reduction is necessary.

These steps you can do with many different kind of techniques, e.g., FMEA, FTA; HAZOP, LOPA, Risk matrix, risk graph, event tree analysis, and so on....

I suggest to read any book from CCPS.