Comparison of PFD calculation

Safety systems are be used in a wide range of technical application. Beside the availability of such systems the safety aspects, e. g. PFD and PFH figures, must be observed. Especially the calculation of these figures requires the use of standards. Worldwide are standards available for this calculation. The newest standard is IEC 61508. This standard is worldwide accepted. Another standard, which is used since years, is ISA-TR84.0.02. In this standard a safety calculation can be performed without using MTTR and common cause failure. Since the introduction of the standard IEC 61508 a lot of discussion concerning the PFD-number appears in the industry. The reason for that discussion is the way of calculation this numbers. This contribution will compare both calculation-methods. Introduction In the process industry is the use of safety related controllers and systems increasing by regulative measures. For the validation of applications of those systems specific figures of the failure rates are used. VDE 0801 part 1 to 7 "Functional safety, Safety related systems" has been recently the state of the art for national and international standards (also known IEC 65A/179/CDV, Draft IEC1508). It describes the procedures and the calculations of complex electronics and microcomputers for safety related applications. After the introduction of the IEC/EN 61508, a common national and international standard was created that describes/specifies generic safety related systems. Today in various publications exist different ways of calculating the PFDfigures and availability-figures. Some parts of them are based on the ISATR84.0.02 (1998) and the therein described equations. To get reasonable analysis related to the safety and the probability of failure rates, it is required to do the comparisons on the same base. Basically there is a differentiation in the failure analysis between safe and dangerous failures. Further more the safe failures are differentiate in • safe detectable • safe undetectable. Safe failures are failures, which have no effect to the safety function of the system, either detected nor undetected. At dangerous failures this situations is not valid. These failures lead at their occurrence to a dangerous situations in the application, that can lead under certain circumstances up to massive risk for human life. These failures are differentiate as well in • dangerous detectable • dangerous undetectable. When the safety related system is designed properly the system reaches the safe state at detectable dangerous failures. For this cases the safety related system is able to bring the complete system or the plant in the safe state. The critical state is caused by the undetectable dangerous failures. In their occurrence there is no possibility in any safety related systems to detect them. They can exist in the systems until the systems will be shut down. Or in the worst case they can be present without possibility to be detected and any knowledge of the user up to the system hazard.