The IEC 61511 standard requires that the user specify extremes of environmental conditions as part of the safety requirements specifications package. This requirement is actually not a safety requirement but a general requirement for operation of all instrumentation and control devices, but its inclusion in the list of safety requirements specifications implies that the associated information must be available in an SRS documentation package. The standard specifically calls out the requirement to document environmental extremes in two locations. In section 10 SIS safety requirements specifications, in clause 10.3.1 that standard states that the safety requirements specifications shall include:
The extremes of all environmental conditions that are likely to be encountered by the SIS shall be identified. This may require consideration of the following: temperature, humidity, contaminants, grounding, electromagnetic interference/radiofrequency interference (EMI/RFI), shock/vibration, electrostatic discharge, electrical area classification, flooding, lightning, and other related factors;
In general, Kenexis recommends that SRS General Requirements are documented in a relational database format, where the general requirements for a table with multiple fields including:
1) Requirement Group
2) Requirement Subgroup
3) Requirement Title
4) Requirement Subtitle
5) Requirement Text
SIS Specifiers have several options for presenting data regarding environmental extremes. The environmental extremes can be specified at the individual devices level, but are generally specified at the system level to minimize information repetition. Environmental extremes are generally specified once in a general design document sometimes referred to as a Basic Engineering Design Questionnaire (BEDQ) or Basic Engineering Design Document (BEDD). This single document contains all of the environmental extreme data, along with other information that define the physical location and environmental conditions of the plant. For the purposes of SRS, it is often most efficient to simply refer to this external document as an SRS requirement.
Proposed SRS General Requirement Text – Option 1 – External Document Reference
Requirement Group: Environmental Conditions
Requirement Title: Environmental Conditions
Requirement: SIS Detailed Design Contractor shall coordinate with Equipment Vendor(s) and SIS Owner/Operator to ensure that all SIS equipment is suitable for the extremes of all environmental conditions to which that equipment is exposed. The environmental conditions that shall be considered shall include: temperature, humidity, contaminants, grounding, electromagnetic interference/radiofrequency interference (EMI/RFI), shock/vibration, electrostatic discharge, electrical area classification, flooding, and any other relevant related factors. Expected extremes of environmental conditions are included in the site Basic Engineering Design Document (BEDD).
Another option is to list the numerical data directly in the SRS requirement text. This will result in some data duplication, as these requirements will also be listed in the BEDQ/BEDD documents, so ensuring that data inconsistency does not occur with rigorous management of change will be important. If the actual numerical values of extreme environmental conditions are entered directly in as SRS general requirements, they can all be lumped together into a single requirement entry, or each number might be entered in as a separate record.
Proposed SRS General Requirement Text – Option 2 – Numerical Values in a Single Record
Requirement Group: Environmental Conditions
Requirement Title: Environmental Conditions
Requirement: The SIS shall be designed to withstand the following extremes of environmental conditions: Field Instrument Ambient Maximum Temperature – 120 F Field Instrument Ambient Minimum Temperature – -40 F
Proposed SRS General Requirement Text – Option 3 – Numerical Values in Multiple Record
Requirement Group: Environmental Conditions
Requirement Subgroup: Field Instruments
Requirement Title: Ambient Temperature
Requirement Subtitle: Maximum
Requirement: 120 F
Requirement Group: Environmental Conditions
Requirement Subgroup: Field Instruments
Requirement Title: Ambient Temperature
Requirement Subtitle: Minimum
Requirement: -40 F
While Option 2 has the advantage of being more compact, Option 3 is more readable, and because each data item is a separate record in the SRS database, filtering, searching, and selective reporting are possible with Option 3.