What is the relationship between SFF and PFD?

2022-12-12

Well not much. They both need the failure rates of the equipment to be calculated but that is where it stops.

The SFF or safe failure fraction is a property of a safety device used in a safety function. The PFD is a property of the complete safety loop. The SFF is a ratio, the PFD is a probability. They represent two completely different things.

The SFF is a parameter that helps understand the fail-safe design and the built-in diagnostics of a piece of equipment. The higher the SFF the more fail-safe and/or the more diagnostics there is. The SFF is one of the parameters that help decide the required architecture of a subsystem of a safety loop.

The PFD is the probability that the safety function does not work when we want it to work. The PFD confirms that the design architecture and configuration of the loop meet the probability requirements of the required SIL level or risk reduction needs.

You need both parameters to achieve SIL.