prEN 17955 Industrial Valves: the draft standard

March 2023. Draft standard EN 17955 “Industrial valves - Functional safety of safety-related valves and actuators” is released, which could partially change the way functional safety is evaluated for all mechanical components of automated valve packages and actuators that are generally used in safety instrumented systems.

Application and publication of the new EN 17955

This standard applies to single components (e.g., valves, actuators or mechanical parts of solenoid valves) or to assemblies and interconnecting parts (e.g., gears, adaptors or brackets).

 

The expected publication date of the standard is May 2024.

 

The purpose is to standardize aspects of evaluation and certification of purely mechanical final elements and to make the failure rates obtained for very similar components consistent.

How it differs from IEC 61508

Although IEC 61508 refers directly to electrical/electronic/programmable electronic components, until now mechanical components have also been analyzed considering only this standard.

From January 2024, if the date is confirmed, the requirements of EN 17955 will also need to be considered.

 

EN 17955 provides a method for determining all relevant factors to be fully considered and thus meet the specific needs of product users.

The basic prerequisite for the application of this standard is knowledge of the intended use.

This standard describes a system to avoid systematic faults conforming to the targeted Safety Integrity Level.

Pre-announced changes

With the release of EN 17955 we will see a number of concepts take shape, including:

• Utilization Rate

A new parameter is introduced by the standard (compared to IEC 61508) that determines the classification of a device between Low Demand Mode and High Demand Mode. Basically, a device could have a higher demand rate than once a year, but still fall into Low Demand and have a low utilization rate. A typical application in this respect could be for components used exclusively by SIS with demand rates between 2 and 10 times per year.

 

• FMEDA

The FMEDA will no longer be needed to determine failure rates for standard valves. In fact, failure rates will be given in tables within EN 17955. The FMEDA will only be required for innovative components.

 

• Field returns

Standard data presented by EN 17955 can be improved against evidence of field returns or with a specific FMEDA.

 

• Safe Failures

These are not mentioned in the standard because there are none for valves and actuators for closing functions.

 

Here are the pre-announced changes. We will return to this topic as soon as the official version of the new EN 17955 is available. In the meantime, we will keep in touch.