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[Leopard_nett]

Hi everybody,
I had a question regarding LOPA for a high demand mode scenario.

- In a low demand mode, every thing is clear IPLs along with the associated PFDs are determined and using the formula: f_final= f_{initiating event}x PFD₁xPFD₂x...XPFD_n
Then RRF would be eaual to:(f_final/ f_Tolerable) and the SIL requirement is determined through comparing the PFD (i.e. 1/RRF) with the table given in the IEC 61511,

-Now in high demand mode based I do not know how use the above mentioned procedure, in the CCPS LOPA book it is mentioned:

f_final= (2x test frequency of the IPL₁)x PFD₁xPFD₂x...XPFD_n

Here I can not understand the following points:

- how can we calculate the RRF here and use it to calculate the required SIL, especially that the SIL criteria in the high demand mode are defined based on PFH (propability of failure per hour) and not based on PFD;

-To my understanding PFD in a high demand mode has no meaning, we have to be able to consider some how the PFH for the IPLs to get at the end a PFH value, or?

- to describe better my problem, a hypothetical case: Suppose there is vessel, having a PSV, and PIA (pressure indicator alarm), here the vessel is designed to be filled 2000 times a year by high pressure natural gas. at the entrance there is a self-regulator having a failure rate 0.5 a year.
This means: f_{initiating event}=1000 /year, and our IPLs would be the PSV and the Alarm (+its associated operator action),

This case is a high demand mode case, could you please tell me how I can perform the LOPA in this way and determine the SIL requirement (assume f_Tolerable=1E-4/year)

Thanks a lot in advance.

P.S: any worked example of LOPA in high demand mode is greatly appreciated.

[robbiejustice]

Hi, I can't help but I am struggling with the same issue. Did you manage to understand this? If so have you got any worked examples or good reference material?