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When evaluating the performance of Safety Instrumented Systems (SIS), two metrics are generally used: one of the more common ones is called the Probability of Failure on Demand (PFD), while the other is PFH. PFH originates from the 1997 version of the international standard IEC61508, and it refers to the probability of (hazardous) failure per hour. In recent literature, the definition of PFH has been revised to the failure frequency (per hour) of hazardous failures. So the question is, under what circumstances should we use PFD, and under what circumstances should we use PFH? PFD yunrun.com.cn/product/2849.html 1. Low-frequency and high-frequency demands: In the basic international standard IEC61508, PFD (which is actually often represented by its average value: PFDavg) is defined as a metric for SIS operating in low-frequency demand mode, whereas PFH is used for high-frequency or continuous demand modes. Demand actually refers to certain events that require a response from SIS. If SIS does not respond in a timely manner, these events could cause damage to equipment, personnel, the factory, and even the environment. So a new question arises: what exactly is a low-frequency demand pattern, and what are high-frequency and continuous demand patterns? IEC61508 also provides some guidance regarding the distinction between high-frequency and low-frequency demand patterns: Low-frequency demand pattern: The frequency of such demands is less than once per year ； High-frequency demand pattern: The demand occurs more than once a year. 2. Boundaries of the pattern: This method of classification is very simple and straightforward, and easy to follow. But the question is, why do we choose once a year as the boundary between low-frequency and high-frequency demand patterns, rather than every six months or every two years? We can find some clues in the failure modes and maintenance strategies of SIS. Although many modern SIS systems now come equipped with self-diagnosis functions, there are still some failures that cannot be detected by these diagnostic tools; they must be identified and resolved through regular functional tests. These so-called Undetected Hazard (DU) failures are the main cause of safety instrument system failures. In the performance calculations for the simplest single-channel SIS (such as a closed valve), PFDavg equals the failure rate of DU multiplied by half of the functional testing period; and half of the functional testing period is actually the average downtime of the SIS. In such calculations, we do not take the impact of demand into account, or rather, we assume that the failed SIS can be repaired before the demand arrives. In other words, the calculation of PFDavg is based, to a certain extent, on an important assumption: that if a DU failure occurs, the probability that the system will be repaired during the next functional test and maintenance is greater than the probability that a new requirement will arise. In the long term, the frequency of functional testing should be higher than that of requirements. In many cases, the testing frequency for SIS is once a year, and this frequency can therefore be used as the highest required frequency to ensure the validity of PFD calculations. Although the actual repair rate for failed SIS is once every six months, an annual rate can be considered a safe and clear figure for distinguishing between high-frequency and low-frequency demand patterns. 3. Meaning of the metrics Additionally, the meanings of the two metrics, PFD and PFH, also differ to some extent. PFDavg represents, from a long-term perspective, the average proportion of time during which the SIS is unable to perform its safety functions; it can therefore also be considered as the system’s average unavailability. PFH has some similarities to another metric commonly used in reliability analysis, namely the rate of catastrophic failure (ROCOF). The PFH of a system at a certain point in time can be regarded as the system’s unconditional failure rate. In the long term, PFH is the average failure rate of the system. Therefore, we can understand why IEC61508 provides two sets of safety integrity level (SIL) standards for PFD and PFH. There are other technical articles on Changhui Instrument Network that discuss related topics, and everyone is welcome to learn more about them. 4. Practical application: In the current mainstream SIL rating calculation software, such as RiskCloud from Shanghai Gelue and ExSILentia from E*DA, it is possible to select the mode type via a dropdown menu. http://yunrun.com.cn/upload/201912/25/201912250027444114.png The image comes from the RiskCloud SIL rating calculation software. http://yunrun.com.cn/upload/201912/25/201912250027589183.png The image comes from the ExSILentia SIL rating calculation software