Failure Probability

Failure Probability Is it very important to calculate very precisely the average probability of failure “PFDavg” of a Safety Instrumented Function? The short answer is “NO“, and we will try to explain it below. The first thing to say is that Functional Safety is much more than meeting the probability of failure. IEC 61511 defines […]

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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 never manifest or be detected,

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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 months, without knowing if they

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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 that the “Fail High” and

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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 per hour (FITS) Notes Fail

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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 the Logic Solver, because the

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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 of Failure on demand (PFDavg

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Most used Architectures In the following table we show the most used architectures when designing Safety Instrumented Functions (SIF). Logic Channels HFT (S) HFT (A) Objetives Used for 1oo1 1 0 0   sensor, actuator 1oo2 2 1 0 More Safety sensor, actuator 2oo2 2 0 1 More Availability sensor, actuator 2oo3 3 1 1

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