Each item of hardware for use in the Safety Instrumented System (SIS) must have an accompanying safety manual provided by the manufacturer. This manual contains all the necessary information required by the Contractor to ensure the device is fit for its intended purpose. Below are key data points that should be included in the safety manual, many of which may not be found in a typical user guide or installation manual:
- Device’s Useful Life: Specifies the operational lifespan of the device, after which it must be replaced or refurbished to an as-new condition.
- Proof Test Procedures: Includes one or more proof test procedures, including partial valve stroke test procedures for components of valve sub-assemblies.
- Proof Test Coverage: Defines the level of fault detection achieved by proof test procedures and partial valve stroke tests.
- Mean Time to Restore (MTTR): Indicates the average real-world time taken from fault detection to full restoration of the safety function. If the device has internal diagnostics, the manual should specify whether MTTR includes the time for diagnostics to detect faults.
- Random Hardware Failure Rates: Provides statistical failure rates associated with random hardware faults.
- Common Cause Failures: Identifies potential causes of common cause failures or common mode failures.
- Common Cause Factor (Beta, β): Specifies the factor to be applied in probabilistic failure rate calculations.
- Environmental Limits: Lists storage and implementation conditions, including temperature, humidity, electrical noise, electromagnetic interference (EMI), vibration, dust, offshore conditions, etc.
- Survivability in Extreme Conditions: Defines how the device performs in extreme conditions, such as fire resistance.
- Application Constraints: Specifies operational limitations such as allowable process fluid types, power supply limits, operating temperature and pressure, and mounting orientation.
- Valve Tightness Specification: Defines the required tightness level, typically in terms of ANSI class I to VI.
- Response Time: Specifies the expected or guaranteed response time of the device.
- Accuracy: Details the accuracy specifications for analog devices.
- Maintenance Procedures: Outlines required maintenance and servicing guidelines.
- Internal Diagnostics: Includes information on detectable faults, diagnostic methods, and the device’s response when faults are detected.
- Configuration and Security: Provides details on programming configurable parameters and securing them against unauthorized modifications (e.g., password protection).
All the information provided in the safety manual is essential at different stages of the functional safety lifecycle to ensure the effective and reliable operation of the SIS.
References:
Functional Safety from Scratch by Peter Clarke
