Chemical plant equipment including tanks, pressure vessels, piping, rotating equipment, vent systems, and safety instrumentation should be identified and categorized into different degrees of hazard potential. Classification systems could be simple or very complex. A complex system could be a matrix of increasing severity ratings on one axis and the frequency of occurrence on the other axis. A simple identification system could consist of three priorities such as:

• Critical Consequence Class 1. Containment equipment or the critical instruments serving that equipment whose failure would result in uncontrolled releases of dangerous materials, situations resulting in accidental fires or explosions, reportable environmental releases including closing of nearby highways or "shelter in place" for community members, personal injury, death, or major property or major production loss.

  Tanks, pressure vessels, piping or rotating equipment containing highly hazardous or flammable chemicals are easily identified by experienced operations personnel as "Class 1" equipment. These containment systems must be inspected in some effective manner on a priority basis.

  Critical instruments assigned a "Class 1" include those necessary to avoid a failure which may cause the perils listed above or instruments which fail to inform of upset conditions which may result in perils. Testing of these instrument systems may be mandated by regulatory agencies, in-house technical safety review committees, HAZOP studies, or designated as critical by operations supervisors. All of these shutdown systems and alarms must be prooftested in accordance with a proper schedule. [8]

• Serious Consequences Class 2. Equipment or the...