In recent years, SIL certification has been increasingly specified in valve procurement technical documents. The following summary addresses key concepts, parameters, and implementation pathways.
1. What SIL Measures
SIL (Safety Integrity Level) classifies the probability of failure on demand of a safety instrumented function. For SIL 2, the average probability of failure on demand (PFDavg) is required to be between 10−310−3 and 10−210−2.
SIL assessment focuses on the reliability of performing a required action upon demand, rather than on general product quality. For a single valve, the certification obtained is a "SIL capability certificate," which documents critical failure data including λDU (dangerous undetected failure rate), λDD (dangerous detected failure rate), and SFF (safe failure fraction).
2. Core Parameters for Achieving SIL 2
SFF (Safe Failure Fraction) and HFT (Hardware Fault Tolerance): According to IEC 61508‑2, hardware architectural constraints are applied jointly. For Type B components (which include valves, actuators, and positioners), a single channel (HFT=0) requires an SFF in the range of 60 % to 90 % to support SIL 2. If the SFF is below 60 %, a single‑channel architecture is not permitted for SIL 2. When a redundant architecture (HFT≥1) is used, even an SFF within the 60–90 % range can support higher SIL levels.
Diagnostic Coverage: Increasing diagnostic coverage, for example through Partial Stroke Testing (PST), can convert some "undetected" dangerous failures into "detected" ones. Provided that the PST frequency is at least ten times the proof‑test interval, a single‑channel Type B element with SFF > 60 % can be used in SIL 2 applications.
3. Common Technical Pathways
Three primary approaches are used to achieve SIL 2 certification:
Path A: Redundant Solenoid Valves (1oo2 in series) – An additional solenoid valve of the same model is installed in series with the existing one. The valve remains open if either solenoid is energized, and closes only when both are de‑energised. This option has relatively low initial cost and is suitable for first‑time certification, with a typical timeline of 3 to 6 months.
Path B: Partial Stroke Test (PST) Module – A PST module is added to the positioner or actuator, automatically moving the valve 10 % to 20 % of its stroke and then returning it, without full closure. This converts undetected failures into detected failures. It offers a balanced cost‑benefit ratio and generally requires 6 to 12 months.
Path C: Redundant Valve Arrangements (1oo2 or 2oo3) – Two or three independent valves and actuators are used, with separate air supplies and electrical power, and physical separation. Any single valve closure can isolate the process. This approach addresses higher safety demand scenarios and typically takes 12 to 18 months.
4. Certification Considerations
Certification Bodies: Reports issued by TÜV, exida, BV, SGS, and other internationally recognised organisations are generally accepted by major projects.
Certificate Validity: The certificate must remain valid throughout the project construction period; expiration can affect acceptance.
Preparation: Availability of documents such as FMEDA reports, safety manuals, and ISO 9001 quality management system documentation directly influences the certification timeline and costs.
5. Common Tendering Queries
When a tender specifies SIL 2 but does not clearly define the required certificate type, a SIL capability assessment report from an accredited body, including failure data and calculation methodology, should be provided.
For SIL 3 applications, a single‑channel architecture is insufficient; a 1oo2 or 2oo3 redundant solution is required.
Summary
SIL certification serves as a compliance prerequisite for market access in many process industry sectors. Selecting the appropriate certification pathway and solution based on the product portfolio and target project requirements is a practical approach. For project‑specific queries or selection assistance, consultation with the technical department is recommended.
