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Institute of Measurement and Control Functional Safety Conference 2016
Challenges in Achieving Safety Instrumented Function Response Time for a Fast-Acting Process
Page 2
Challenges in Achieving Safety Instrumented Function Response Time for a
Fast-Acting Process
Author Block: K. Rampaul, A. Barnwell, S. Sookhoo
Keywords: Process safety time, response time, SIF and fast acting.
Date: 16-September-2016
Abstract
There has been significant emphasis on achieving functional safety certification for devices used as part
of safety instrumented functions, particularly in terms of product testing and documentation of failure
rates. The response time of a safety function is as crucial to the ability to act on demand as are device
failure rates. Functional safety standards include details of achieving target Safety Integrity Levels based
on probability of failure on demand, hardware fault tolerance and systematic capability. However, the
guidelines on device response time requirements are not as defined.
Using an example, this paper illustrates the challenges faced in providing suitable instrumentation for a
fast-acting process. It discusses the design approach, findings from consultation with suppliers, and
investigation of standards to find a solution.
Determination of process safety time can be a rigorous exercise, and the guidelines for process safety time
design margins vary. Even though response times are included in safety manuals, there are devices for
which the response time can only be estimated and no written guarantee provided. This creates an issue
for functions which are both safety-critical and highly time-dependent. Furthermore, accuracy can affect
the response time of an instrument, particularly in a fast-acting process.
This paper draws on various references to emphasize the importance of process safety time consideration
in safety instrumented function design. It discusses the information that is currently available, and
highlights potential gaps in achieving required safety instrumented function response times.
Introduction
Achieving functional safety is critical in ensuring that due diligence has been followed in the design of
safety systems. It is important to show that all Safety Instrumented Functions (SIFs) are in compliance
with industry standards and recommendations, as much as is practicable. This is commonly demonstrated
using in-depth analysis of the Probability of Failure on Demand (PFD) of the individual and combined
components of the SIF, and identification of Hardware Fault Tolerance (HFT) and Systematic Capability
(SC).
To achieve functional safety, however, the Process Safety Time (PST) is also a critical factor. It is defined
as the time period between a failure occurring in the process or the basic process control system (with the
potential to give rise to a hazardous event) and the occurrence of the hazardous event if the safety