Recently, I was reminded by a customer of the importance of “Inspection” as part of maintenance program for Safety Instrumented System (SIS). When designing SIS, we are very concerned about the type and frequency of functional testing because this is a critical input to Safety Integrity Level (SIL) verification. However, a maintenance & testing program should not ignore the requirements for inspection of SIS equipment. In fact, ANSI/ISA 84.00.01-2004 (IEC 61511 MOD) includes requirements for inspection in Clause 16.3.2.

Each SIS shall be periodically visually inspected to ensure there are no unauthorized modifications and no observable deterioration (for example, missing bolts or instrument covers, rusted brackets, open wires, broken conduits, broken heat tracing, or missing insulation).

It came to my attention that Process Safety regulatory authorities may be including this as just one small element of an overall compliance assessment that includes key safeguards such as the SIS. The key aspects to inspection identified in the standard are “unauthorized modifications” and “deterioration”.

All SIS modifications should be subject to Management of Change (MoC) procedures. Therefore, it is unlikely that a visual inspection would reveal unauthorized modifications, nor should it be relied upon as an element of change management. Deterioration can certainly occur when SIS are subject to harsh environmental conditions, such as temperature, humidity, rain/snow, dust/dirt, vibration, salinity, etc. These can cause deterioration on transmitters, field wiring, junction boxes, conduit, solenoids, process valves, insulation / cladding, etc.

Note; there is no specification in Clause 16.3.2 for the frequency or type of inspection, other than it should be ‘visual’. The frequency and type of a visual inspection should be based on consideration of these mechanisms for deterioration. At a minimum, we recommend that inspection occur during each instance of SIS function testing. This might be annually, or as much as 5 years depending on the specific system. If this occurs at an interval at which the process is also shutdown for a maintenance overhaul, then it may be practical to inspection more SIS equipment with a more rigorous visual inspection. If function testing occurs online, then visual inspection may be more limited.

The maintenance department should also consider the need for special measures to allow for adequate visual inspection for deterioration, such as removing cladding / insulation, opening junction boxes or control cabinets / panels, removing covers on cable trays, providing lift equipment, removing fire proofing on valves, opening instrument access covers, etc.

In situations where environmental conditions are particularly harsh, it may be advisable to conduct visual inspections more frequently than SIS function testing. This might be monthly or quarterly checks for any signs of deterioration of mechanical components, or pluggage of instruments or pneumatic systems. For example, an energize-to-trip application might require more attention to well-maintained mechanical, electrical, or pneumatic or hydraulic services. Inspection for water in an instrument air service would be a type of inspection that might be advisable in this example. Pluggage the vent port of a Solenoid Operated Valve (SOV) might occur if the exhaust filter is not adequately installed or becomes caked with dirt, covered in ice, etc.

At a minimum, you should ensure that procedures for periodic SIS function testing include a step for visual inspection of field equipment including transmitters, field wiring, and final elements. Documentation that this inspection has occurred will be important in the event that your program is subject to either an internal or external audit.

Contact Kenexis if you have more questions about SIS maintenance and testing programs.