ME508 Safety Instrumented Systems Assignment

Assignment Help Electrical Engineering

Reference no: EM132672256

ME508 Safety Instrumented Systems - Engineering Institute of Technology

Case study objective:

In this case study, you will carry out SIL verification using Fault Tree Analysis (FTA) technique. You will be utilising an FTA software for this purpose. The Fault Tree Models need to be generated based on the given description of the plant and its safety shutdown system.

Case description: Reactor High Temperature Trip

Please refer to the following figure showing the arrangements of a Safety Instrument Function PAHH-3001 with a target SIL = 3 in low demand operation (PFD 10-3 to 10-4) and a target proof test interval (Ti) of 24 months.

The Oil-Gas separator vessel receives Wet Crude through Nozzle N1. Gas is separated and exits through Nozzle N2 under back-pressure control. Liquids exit the vessel from Nozzle N3 under level control. These control loops are not shown to keep the figure less complicated. The vessel is rated at a Design Pressure of 26.5barg, shown as the Set Point of Safety Valve connected to Nozzle N11. To further protect the vessel from potential high receiving pressures at inlet, PT-3001 A/B/C are provided to sense the vessel pressure and to trigger an automatic shutdown through closing of Emergency Shutdown Valve ESDV-3030, installed upstream of Nozzle N1, thus isolating the vessel from potential high inlet pressures. The Emergency Shutdown System is a PLC based system and is set at 23barg to trip the ESDV-3030 close at this set point.

For the top event of PAHH-3001 Failure on Demand, develop a Fault Tree Analysis using downloaded FTA software (e.g. Reliotech Top Event FTA Express - a Fault Tree Analysis software, freely downloadable).

Questions:

a) Model the Top Event, i.e. failure of PSHH. Show the FTA evaluation diagrams generated from the FTA software (clear screenshots are acceptable) showing the PFD figures and full details of the Models used for each element and for each sub-system.

Note: PFD is also the measure of unavailability of a safety function. If a demand to act occurs after a time, the probability that the safety function has already failed is represented by the unavailability of the function.

Hint: The top event, i.e. PSHH action will fail if any of the three sub-systems (Input, PLC or Output). The pressure transmitters (input sub-system), Logic Solver, and ESDV form the three inputs of an (OR) gate that outputs the Top Event, i.e. failure of any of the sub-systems will lead to the top event. The input sub-system is a 4-input (OR) gate, fed from the output of 3 (AND) gates and One (1) Basic Event (CCF). The 2oo3 pressure transmitter arrangement is to be modelled as three separate (AND) Gates, Transmitter A (AND) B, B (AND) C, A (AND)

C. The Logic Solver and ESDV are two separate Basic Events feeding the 3-Input OR gate, whose output is generating the Top Event (Failure of the system). The pressure transmitters, PLC and ESDV are to be modelled with "MTBF" models, whereas the CCF is an "unrepairable" model. The FTA software does not accept repeat of the exactly same name in more than one Logic Gate. Hence, you will have to use slightly different names for repeated transmitters, for e.g., "PT-3001A", "PT-3001-A".

b) Confirm, or otherwise, that the SIF reliability calculations match the target SIL for the initial proof test interval (this is mission time within the FTA software). Show the PFD contributions for each of cut set evaluated in the FTA software.

c) Adjust the mission time and re-run the calculation to identify the optimum (longest) mission time that achieves the SIF reliability target to be within the SIL PFD range. What is the longest mission time (proof test interval) for the SIF that will just meet the SIF SIL target given the reliability of the SIF components provided? Show the PFD contributions for each of cut set evaluated in the FTA software.

d) Considering that practical proof-test intervals are in multiples of 6 months, what proof test interval is necessary (recommended) to be made standard for this SIF loop, for meeting the target performance? Show the output of the software for the recommended PTI.

e) Study the quantitative impact of reducing the CCF to 5% on the SIF performance. Provide the results along with the model and minimum cut sets.

f) Recommend minimum three (3) measures to qualitatively reduce CCF. Explain each measure in brief.

Attachment:- Safety Instrumented Systems.rar

Reference no: EM132672256

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