Figure $1$: Boiler Drum System

Based on the safety requirement you are assigned to design a SIS for the process.

a$)$ Identify the required elements of the SIS.

b$)$ Using the ISA guidelines, determine the required configuration of the SIS to meet the

desired SIL of the safety function $($use the simplified equations$)$ and calculate the PFDavg

for the system. Assume the test interval $($TI$)$ is equal to $2$ years.

Assumptions:

i$.$ All inputs and outputs in the example are assumed to be part of the same SIF.

ii$.$ The SIF is designed as de$-$energize to trip and will go to a safe state on loss of power.

iii. Redundant AC power supplies $(2)$ are provided external to the system.

iv$.$ All redundant devices are assumed to have the same failure rate.

v$.$ The logic solver is a PES with output redundancy to prevent unsafe failure of an output

and has an external watchdog circuit. The MTTF for logic solver provided by

manufacturer is $100.$