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 IEC 61511 indicate the great …
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, …
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 …
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 …
SIS Life Cycle – IEC 61511 The image shows the main phases of the SIS Life Cycle (Life Cycle of Safety Instrumented System), as well as the technical disciplines that lead each part. Analysis Phase In this phase, a rigorous analysis of the hazards of the process is carried out, comparing the probability that a …
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 …
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 …
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 Safety Functions. Each Safety Instrumented …
Compliance with IEC 61511 What do we have to do to be compliance with IEC 61511? This Standard introduces the concept of Safety Life Cycle and, therefore, the answer is associated with the fulfillment of each phase of this cycle. Our intention is to explain it, in a simple way, for the not so expert. …
