Most common errors in the SIS

Most common errors in the SIS The aim of this article is to review some of the most common errors in the SIS (Safety Instrumented System) that can serve as a guide and reason for reflection for all those involved in its design, implementation and maintenance. Both IEC 61508 and […]

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Systematic Failures

Systematic Failures IEC 61511 defines systematic failures as those “related to a “pre-existing” fault, which can only be eliminated by removing the fault by a modification of the design, manufacturing process, operating procedures, documentation or other relevant factors”. The peculiarity of systematic failures is that they are “hidden” and may […]

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Partial Valve Stroke Test

Partial Valve Stroke Test (PVST) The Partial Valve Stroke Test is used in the Safety Instrumented Function (SIF) to diagnose valve failures without having to stop the process. The problem of the shut-off valves in low demand SIFs is that they remain in a position (open or closed) for many […]

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Transmitter diagnostics

Transmitter Diagnostics In the previous post we have explained in a brief way what are the failures of transmitter that affect the calculation of PFDavg / PFH and MTTFS of the Safety Instrumented Function (SIF). We will go deeper into the transmitter diagnostics and explain it with some example. Recall […]

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Transmitter Failures

Transmitter Failures (4-20 mA) In the following table we see an example of the types of transmitter failures and which we should consider when verifying the Safety Instrumented Function (SIF). The rates in the example correspond to the Rosemount model 3051 pressure transmitter. Type of failure Example of Failure Rates […]

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Logic Solver

Logic Solver (PLC versus relays) The Logic Solver is the SIS subsystem where the logic of the safety functions resides, that is, what are the interlocks of the sensors and how they act on the final elements. We do not always refer to a Safety PLC when we talk about […]

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Key Parameters

Key Parameters As previously seen, to design the SIS (“Safety Instrumented System”) according to IEC 61511, several requirements must be met throughout its life cycle. Compliance with the SIL level assigned to each SIF (“Safety Instrumented Function”) is one of these requirements, and is performed by calculating the Average Probability […]

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Architectures

Most used Architectures In the following table we show the most used architectures when designing Safety Instrumented Functions (SIF). LogicChannelsHFT (S)HFT (A)ObjetivesUsed for1oo1100 sensor, actuator1oo2210Safetysensor, actuator2oo2201Availabilitysensor, actuator2oo3311Safety+Availab.sensor, PLC1oo2D210Safety with high diagnosticsPLC HFT(S)= Hardware Fault Tolerant for Safety HFT(A)= Hardware Fault Tolerant for Availability Recall that the logic of the sensor and […]

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Safety Instrumented Function

Safety Instrumented Function The Safety Instrumented Function, whose abbreviation is SIF, is a protection layer whose objective is to achieve or maintain a safe state of the process when a specific dangerous event occurs. The SIF is implemented in the SIS (Safety Instrumented System) which is normally composed of several […]

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