Method Compliance Assessment are specialists in Prior Use assessment of legacy SIF components as per IEC 61511.
Why assess devices?
Each device installed in a Safety Instrumented Function (SIF) must be suitable for use at the target Safety Integrity Level (SIL) required of the SIF. The only route available to end users to demonstrate the suitability of legacy components is defined within IEC 61511 – and this is known as Prior Use (PU) Assessment.
What is Prior Use?
IEC 61511 defines requirements for selection of devices based on PU as:
“documented assessment by a user that a device is suitable for use in a SIS and can meet the required functional and safety integrity requirements, based on previous operating experience in similar operating environments”.
The main focus of the PU evaluation is to provide statistically valid evidence that the rate of dangerous failures of the device is sufficiently low.
What are the key requirements for Prior Use?
61511 specifies the following evidence that should be provided within a PU assessment:
Evaluation of the manufacturers quality, management and configuration management systems
This step evaluates the capability of the manufacturer to consistently manufacture a reliable device. The following are examples of types of supporting evidence:
- An independently verified Quality Management System;
- Design and manufacture to an industry recognised product standard;
- Traceability between reported failures and modifications;
Adequate identification and specification of the devices
The desired functionality required from the device (e.g., environmental performance) should be defined. This is then compared with the device’s capability and performance to confirm that it is fit for purpose.
Demonstration of the performance of the devices in similar operating environments
The following data relating to device performance is required to justify that the historical performance of the device is suitable for the SIF:
- Recorded historical evidence of device hours in use;
- Estimate of the frequency of demand of the safety function;
- Number of failures of the safety function, categorised as safe/dangerous, detected/undetected.
To assure confidence in the completeness of the information, evidence of a formal system to collate and interrogate the data should be provided.
The volume of the operating experience
The following examples of evidence could be used to demonstrate an acceptable volume of operation:
- Unit-hours of operation achieved over the lifetime;
- Process conditions during use;
- The different applications where the component has been used.
How is Prior Use recorded?
The evidence, calculations and conclusions for the prior use assessment should be formally documented and subject to the control of a management of change procedure. In the case of a change being made to the device, evaluation of the continued validity of the evidence must be justified.
InstMC Precision Magazine
This article was featured in the June edition of the InstMC's Precision Magazine.
- IEC 61511, Functional safety – Safety instrumented systems for the process industry, 2016.
- Chemical and Downstream Oil Industries Forum (CDOIF), Guideline - Demonstrating prior use of elements of a safety instrumented function in support of EN 61511, V4.
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