The number of inputs and outputs to a SIF (safety instrumented function)—including voting—affects the probability of failure on demand (PFD) and the SIL (safety integrity level), assuming the test interval remains the same. The larger the number of inputs and outputs that perform separate functions, the higher the PFD and potentially the lower the SIL. What if the SIF includes all the sensors and final elements that are involved in any trip of a large processing unit like a heater, a reformer, or a distillation column? What if the SIF includes actions to shut down upstream and downstream units? When SIL verification is done for such a SIF, the target PFD and SIL may not be possible to achieve. The temptation may be to add redundancy in sensors and final elements or to reduce the proof test interval in an attempt to reduce the calculated PFD. Frustration may abound as capital and operating costs rise steeply. This paper shows how to use the principles of layer of protection analysis and the information in the process hazard analysis to split up the massive SIF into smaller SIFs that are more manageable. An example is included.

中文翻译：

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“你的 SIF 是不是太过分了？”

假设测试间隔保持不变，SIF（安全仪表功能）的输入和输出数量（包括投票）会影响按需故障概率 (PFD) 和 SIL（安全完整性等级）。执行不同功能的输入和输出的数量越多，PFD 越高，SIL 也可能越低。如果 SIF 包括在大型处理单元（如加热器、重整器或蒸馏塔）的任何行程中涉及的所有传感器和最终元件怎么办？如果 SIF 包括关闭上游和下游装置的行动怎么办？当对此类 SIF 进行 SIL 验证时，可能无法实现目标 PFD 和 SIL。诱惑可能是在传感器和最终元件中添加冗余或减少验证测试间隔以试图减少计算的 PFD。随着资本和运营成本的急剧上升，挫折感可能会比比皆是。本文展示了如何利用保护层分析的原理和过程危害分析中的信息，将海量的 SIF 拆分为更易于管理的较小的 SIF。包括一个例子。