There is no substitute for proper maintenance in plants posing a danger of fire or explosion — both safety and efficiency demand it. While many guidelines for safety maintenance exist, years of experience have helped us develop some rules that we believe are essential for keeping a plant, its personnel and the surrounding community out of harm’s way. These are not meant to replace applicable industrial safety standards, but to provide general guidance. They cover three key aspects: initial inspection, programmed maintenance and apparatus failure and repair.
It is impossible to overemphasize the importance of a comprehensive initial inspection. Safety procedures and effective preventive measures begin before your plant starts production. Prior to startup or modification of a plant with hazardous locations, ensure that qualified personnel perform a thorough inspection. The inspection should include documentation that verifies the capabilities of the plant, all field elements, all associated apparatus and the agreement between safety parameters and interconnected apparatus (Figure 1).
|Figure 1. An initial inspection should verify safety barriers, wiring correctness and other conformance issues.|
Plant capability. In verifying plant capability, ensure that protection methods are compatible with the hazardous locations and gas groups present and that the technical rules of each protection method are followed. Make certain that all components and electrical lines in a hazardous location are explosion-proof, with continuity up to the exit point of the hazardous location, or are protected with properly installed authorized methods. This means that apparatus are correctly anchored and do not exert force on the connecting pipes, causing cracks or deformation; locking joints conform to standard and are properly tightened and sealed; conductors leading into the junction boxes are firmly anchored with no risk of abrasion or cuts; there is no condensation forming or accumulating; the explosion-proof enclosures have a reliable external ground connection; any mixed protection methods are authorized for the specific type of hazardous location; and there is adequate space to easily remove the lid during maintenance.
For intrinsically safe installations, verify that: all electrical conductors conform to the installation standard; intrinsically safe circuit conductors are not mixed with nonintrinsically safe ones; conductors of different types of intrinsically safe circuits are properly separated and isolated from each other; diode safety barriers have an isopotential system with plant grounds conforming to the applicable standard; nonhazardous-location apparatus directly powered with voltages larger than 250 V are not present, unless protected and certified for that use; shields of conductors associated with diode safety barriers are connected to the isopotential ground; reciprocal and toward-ground isolation of all intrinsically safe circuits conform to the standard; markings specifying different measuring or regulation loops are clear; barrier input terminals are not confused with the output terminals; and installation conforms to the manufacturer’s control drawing.
Capability of field elements. Managers, supervisors and other pertinent personnel must confirm that all apparatus installed in hazardous locations are free from risk of fire or explosion by verifying that the devices are connected with simple devices (Vmax≤ .2 V; Imax ≤ 0.1 A; Pmax ≤ 25 mW; Emax≤ 25 µJ) or covered by an authorized protection method for the specific installation zone. The method must be compatible with the gas groups and temperature class.
In the case of explosion-proof apparatus, verify that: all apparatus are installed in Division 2, or Division 1 with certain restrictions; certification is compatible with the gas group and surface temperature class of the flammable mixture present; explosion-proof points are not damaged; all openings toward the outside of the enclosure are used or properly sealed with explosion-proof caps; all removable lids are integral, closed and tight; all threaded junctions are integral and protected against corrosion; each enclosure has an efficient, properly tightened external ground connection; and warnings stating that the power should be shut down before opening appear on the lids or the labels.
For intrinsically safe apparatus or simple electrical devices, verify that: Devices should be certified for their hazardous location and the gas groups corresponding to the flammable mixtures present. They should have a surface temperature classification compatible with the flammable mixture present (e.g., T6). Apparatus should be connected exclusively to intrinsically safe circuits, as specified by the certification documents, especially for the eventual connection to alarm safety apparatus, with connecting cables that adhere to their safety parameters. Units are powered or interconnected with associated apparatus having safety parameters compatible with the apparatus and the proper protection method. They have a grounded enclosure (or no risk of electrostatic discharge) and are installed with proper environmental protection. They do not have points of the circuit grounded or poorly isolated toward ground (only if connected with diode safety barriers without galvanic isolation) with the exception of the equipotential ground connection on the barrier. The eventual loosening of cable clamps or conductor input joints will not jeopardize the tightness of the enclosure against water and corrosive atmospheric elements.