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[deltaChe]

I am process engineer in the consulting company and trying to prepare a cause effect in a tabular
matrix format for our client.
Could any master in Process/I&C field help me some problem on below?
First, I have to introduce a little bit about part of the cause effect diagram knowledge which I understand.
Cause & effect is something like relationship between initator event such as signals from equipment start/stop
fuction, hand switch of solenoid valve, mnual valves for block or bypasses of SIS valves and final control elements
such as valves open/close, euqipment on/off.


N1. I am wondering how to decide which components like senesers, valve, solenoids, valve limit switches etc
required to be consider to be put in the cause part.(I understand that interlock description must be refered for doing
this cause & effect diagram). Is any refernce papers or book could be recommand for self-learning or any
must not or/ msut done experience may share with? I am confused now is that sometimes we deal with
on-off valve limit swiches be used as permissive inititotr in safety interlock but sometimes is not.

N2. MOV (motor operated valve) seems not used in SIS. But I am not sure. Could any master explain this relationship
betweem SIS and cause effect.

Thank you very much.

[Eprocess]

Dear hswang2,

Designing the Safety Instrumented System for an industrial plant is a serious challenge. Any tiny mistake in this task can lead to catastrophic accidents. This is not a one man job and depending on the size of the plant, it takes lots of time and energy. The lead designer must have a thorough knowledge of process and safety. I once had this problem and I did not accept the responsibility because it is absurd to commence a task without having enough resources in hand. Some clients do not understand the importance of this job. For your first question, if you do not have any other choices so pick one the following approaches:

1. Set a reasonable Probability of Failure on Demand(PFD) figure for each equipment in your plant(this is determined by the nature of your plant), then determine the Safety Integrity Level for each Equipment by using any of available methods ( LOPA, Risk matrix …) without adding any SIS devices. When you did this, look how far you are from the desired PFD you set for your plant equipments. Now if the calculated PFD is higher the acceptable level (which it sure will be) then consider adding protective layers to reduce this figure. Try to design an optimum system adding too much layers will make your plant complicated and expensive. You can reduce PFD by
- Designing an inherent safe plant
- Adding mechanical protection (Psv, rupture disk,…)
- Adding an independent Safety Instrumented System parallel to DSC (Sensors, Transmitters, CPU and final elements)
- Improving the redundancy of SIS and the system architecture
- …
Each time you add a protective layer go back and check the PFD if it is below the desired amount then you are OK.
Cause and effect it is nothing but the interpretation of the SIS system (ESD). You should determine the number of ESD levels you need, for instance specify the scenarios that will lead to an overall shut down and write down the shutdown procedure in each case, then illustrate it in cause and effect that the other could understand you intention. The other levels of ESD can be set for less serious cases which will not result in overall shutdown and can be handled by corrective measures. I insist this is not a task to be performed by a junior engineer. Now you see why I told you this is not a one man job!
2. Find the P&IDs of a similar plant and try to understand the concepts of the SIS for the plant, and use it as an initial pattern, but beware not to rely on this approach since it is not safe at all. Do it in classical way and just do some cheating.
I Do not get your second question, MOVs are the most routine devices used in SIS.
Hope it helps