Dear Friends,

During LOPA study generally credit of "1" is given for Safety Valves for normal service (PFD = 1E-2) and "0.5" for a Single Transmitter executing safety function (PFD = 1E-1).

Please explain what credit is given for following different cases for Safety Valves and Transmitters..

1. Two Safety Valves, each having 100 % relieving capacity (both in service), installed on the same nozzle of equipment.
2. Two Safety Valves, each having 100 % relieving capacity (both in service), installed on separate independent nozzles of equipment.
3. Safety Valve installed in severe service.
4. Transmitter installed in severe service.

Also, please provide if you have any book suggesting various credits and their rules/guidelines to be followed during LOPA Analysis.

Thanks in advance,