PFD Calculation Overview in SIL
The PFD is determined by calculating the PFD for all the components in each safety function, which provide protection against process hazardous event, and combining these individual values to obtain the safety function PFD value. This is expressed by the following:
PFD =Σ PFDSi + Σ PFDLi + Σ PFDAi + Σ PFDPSi
Where;
PFDS is the initiator PFD,
PFDA is the final element PFD,
PFDL is the logic solver PFD, and
PFDPS is the power supply PFD.
”i” represents the number of each type of components that is a part of the specific safety function.
Each element of the calculation is discussed in the sections that follow.
Note: If the SIS is designed for fail safe, de-energized to trip, the power supply does not impact the safety
function PFD, because a power supply failure will result in action taking the process to a safe state. Generally the Project uses “de-energized to trip”, therefore,
function PFD, because a power supply failure will result in action taking the process to a safe state. Generally the Project uses “de-energized to trip”, therefore,
Σ PFDPSi =0 will be used except for some specific instances requiring special consideration. The instruments identified of no effective consequence, (e.g. bleed valve for furnace) are excluded from SIL implementation.
The SIL implementation procedure examines the hardware and software required to implement the loops identified with automatic safety action requirements only.
Typical Pattern In SIL
The general definition of the typical pattern is as follows:
• The loop is combined “Sensor portion” with “Final Element portion.”
• “Sensor portion” is defined as:
I_1oo1 (Initiator is one),
I_1oo2 (Initiator is 1 out of 2),
I_2oo2 (Initiator is 2 out of 2), or
I_2oo3 (Initiator is 2 out of 3) etc.
• “Final Element portion” is defined as:
F_V_1oo1 (Final element is one valve) or
F_V_1oo2 (Final element is two valves and series configuration), etc. for valves.
F_P_1oo1 (Final element is one Pump) or
F_P_1oo2 (Final element is two Pumps), etc. for pumps.
F_K_1oo1 (Final element is one Compressor) etc. for compressors.
Where there exist more than one control logic cause for same effect, (final element), we classify the critical effects with the highest SIL level from the SIL identification report or process condition point of view, and these items must be verified to meet the highest SIL level identified.
Determining the PFD for Initiators (PFDS) inSIL
The procedure for determining the PFD for sensors is as follows:
• Identify each sensor that detects the out-of-limit condition that could lead to the event the safety
function is protecting against. Only those sensors that prevent or mitigate the designated event are included in PFD calculations.
function is protecting against. Only those sensors that prevent or mitigate the designated event are included in PFD calculations.
• List the value of λDU for each sensor as described and value of β.
• List the value TI
• List the value of TIF
• Calculate the PFD for each sensor configuration using the equation with appropriate consideration for
redundancy.
redundancy.
• Sum the PFD values for each sensor to obtain PFDS component for the safety function being evaluated.
Determining the PFD for Logic Solver (PFDL)
The procedure for determining the PFD for logic solver is as follows:
• The actual PFD for logic solver is to be obtained from the Vendor,
• Only in case the Vendor cannot give the PFD, the PFD may be calculated using the same methodology of the sensor and the final element.
For the purpose of the examples used in this specification, the PFD value for logic solver is assumed to be 5.00E-05.
Determining the PFD for Final Elements (PFDA) in SIL
The procedure for determining the PFD for final elements is as follows:
Identify each final element that protects against the out-of-limits condition that could lead to the event the safety function is protecting against. Only those final elements that prevent or mitigate the designated event are included in PFD calculations.
List the value of λDU
List the value of β
List the value of TI
List the value of TIF
Calculate the PFD for each final element configuration using the equations with appropriate consideration for redundancy.
Sum the PFD values for each final element to obtain PFDA component for the safety function being evaluated.