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.
Capability of all associated apparatus. This also must be carefully checked. Verify: The devices are installed exclusively in a protected nonhazardous location, unless other protection methods suitable for the hazardous location are used. The certification type is compatible with the intrinsically safe circuit to which they are connected, according to the hazardous location and the gas group. The apparatus have safety parameters appropriate for the connection cable and other connected intrinsically safe apparatus. Separation and identification of the intrinsically safe circuit exists along the entire connection route, as required by the standards. The devices have correctly rated and installed internal fuses and an external protection device (i.e., isolator breaker) is present on the main power line.
Agreement between the safety parameters and the associated apparatus. Keep in mind that any apparatus certified as safe can actually become unsafe if used with other apparatus, even if these apparatus also are certified as safe. It is essential to ensure that the integrity of the devices is not compromised when working with connections. Don’t let a false sense of security prevent you from checking all circuits and connections to every device. Be sure to study the schematic drawing thoroughly. Always use the interconnected apparatus documentation to verify that all connections are specified and permitted by the certification and that parameters derived by the interconnection remain compatible with values that are characteristic of the cable and the connected field elements.
A program of regular inspection and repair is an extremely valuable tool. It prevents waste, downtime and the deterioration of apparatus. It should include records noting the maintenance performed, the date and the results.
Explosion-proof apparatus. In conducting periodic inspections of explosion-proof apparatus, it’s essential to verify that all explosion-proof lids are tightened, there are no signs of deformation, cracks or corrosion in the flanged joint, tightening lid thread and pipe union, the enclosure’s external grounding terminals are tightened, and the grounding conductor is integral as well as guarantees a good ground connection (Figure 2).
|Figure 2. Checking the tightness of lids and terminals and for corrosion and deformation is essential.|
Keep in mind that you cannot test your apparatus without power. Therefore, take steps to eliminate the presence of flammable mixtures (determined by on-site testing with an explosion meter) before performing the calibration with the power on. During subsequent operations, continuously use the meter to verify the absence of flammable mixtures. Connect the calibration instrument to the input/output connection according to apparatus instructions. Verify the absence of flammable mixtures with the explosion meter and restore the power. Check the calibration per manufacturer’s instructions and adjust if necessary. Reconnect the original connections. Close the lid and verify its tightness.
Intrinsically safe apparatus. Periodic inspections for intrinsically safe apparatus can be performed without a plant shutdown. However, to avoid situations of real or potential danger, ensure that:
• Instruments or verification connections do not cross the protection resistors of the barriers.
• Ground connections are not interrupted while intrinsically safe circuits are powered or are connected to other powered circuits.
• Apparatus used for the test are certified and suitable to operate in a hazardous location with explosive mixtures present and do not introduce dangerous voltages or currents in the circuit.
When possible, remove your apparatus from the plant to verify and safely test them in a nonhazardous location. Plug-in type connectors simplify this. The maintenance procedure in a hazardous location should be limited to disconnection and removal of apparatus and part of the connections, calibration adjustment of the apparatus, the use of permitted and specified test apparatus and other permitted or specified maintenance activities.
Although it may appear that there is less danger when inspecting or repairing apparatus in a nonhazardous location, this is not necessarily true. Indeed, a more dangerous situation could develop — yet, often less care is taken as a result of the nonhazardous classification. A faulty process in a nonhazardous location can generate an explosion in a hazardous location through an interconnected circuit.
For circuits protected by diode safety barriers, verify that: each ground conductor barrier is properly tightened and maintains a total resistance up to the isopotential ground point ≤ 1 ohm; safety circuits (measured by nonrepetitive samples) are isolated from other ground points, and the isopotential point to which they are connected is according to the standard; and the separating distances of safety terminal blocks and conductors are respected.
For circuits protected by galvanic isolation barriers, verify that the separating distances of safety terminal blocks and conductors satisfy requirements (Figure 3).
|Figure 3. Point-to-point galvanic isolators simplify system design and installation in hazardous location applications.|
With intrinsically safe circuits, maintenance activities are performed with the power (usually 28 V maximum voltage) on. However, when there are circuits protected by barriers, those barriers could be permanently short-circuited. This could cause a fuse to blow, rendering the barrier useless. So, always ensure that the barriers are not short-circuited and all apparatus are safely connected and have no improper contacts. When possible, remove the apparatus (particularly easy with plug-in type connectors) and check them in a lab. When it is impossible to remove the apparatus, disconnect input/output conductors and, after identifying them, connect them temporarily to the isopotential ground, or, if already grounded, keep them isolated and anchored to the free terminals of a supporting terminal block. Then connect a calibrator to the input and a calibration indicator to the output. Both must be certified for the division and gas group in which they are used. After completing verifications and calibrations, restore input and output conductors with extreme care.
Apparatus failure and repair
Decreasing downtime for repairs is key to reducing maintenance costs (see CP, October, p. 25). Smart planning during design — such as selecting modular and plug-in type field and control-room instrumentation and ensuring adequate spare parts to permit immediate substitution — will help to reduce your downtime and make repairs easier.
Repairing explosion-proof apparatus. When operating problems or machine failure involve explosion-proof plants, it’s vital to determine as soon as possible the cause of the problem, identify and isolate the problem area and substitute or repair malfunctioning components. Always keep in mind that only authorized personnel must be permitted to repair explosion-proof apparatus, because equipment must not be serviced under power.
Even though diagnosing explosion-proof plants involves the same procedures used elsewhere, there is always danger of fire or explosion. Therefore, keep these points in mind:
• Do not perform connections not shown in the plant’s schematic, unless the risks relative to safety have been analyzed.
• Only use test instruments certified for the same hazardous location and gas group as the circuits to be analyzed.
• Isolate the part of the plant where repairs must be performed. Consider the effect of the tests on interconnected circuits.
• Most importantly, do not cross or eliminate the safety protections that are present in the safety barriers and in other parts of the plant.
Repairing intrinsically safe apparatus. Working on intrinsically safe apparatus does not have to compromise the characteristics of intrinsic safety. The most frequent ways to ensure that apparatus are intrinsically safe are:
1. surface distances between the main line and the intrinsically safe circuit, and between two different types of intrinsically safe circuits;
2. protective coatings that increase the insulations to more than those obtainable with distances only;
3. protective fuses on main and signal transformers, and output circuit barriers;
4. signal and main transformers with dielectric rigidity that has been individually tested, and with distances and insulating or isolating materials that are guaranteed;
5. barrier resistors with construction techniques, nominal powers, values and tolerance as per the certification documentation;
6. diode or zener barriers with nominal voltage, tolerance, nominal power and assembly polarity well-defined;
7. optoelectronic coupler that is certified as a component having surface and internal distances and approved construction techniques;
8. electromagnetic relays that are certified with guaranteed surface distances between the coil circuits and the contacts or terminals for armor ground connections; and
9. functional modules, encapsulated or not, that are certified as components that are compatible with the concept of intrinsic safety.
There are some additional points to remember if an intrinsically safe apparatus must be repaired in an emergency situation. Do not modify air and surface distances of the barriers and their components. Do not substitute any component that determines its intrinsic safety (usually marked with shading on the silk screen of the printed circuit boards and on the schematics). Only replace fuses with identical ones (rapid, medium lag, etc.) and nominal current. Do not substitute main or signal transformers, or certified modules unless with identical components supplied by the manufacturer of the apparatus. Take care in reciprocal positioning of the components and in repositioning eventual isolators or spacing collars placed on component terminals to distance them from the printed circuit board. Carefully verify the repaired card or component, to ensure the complete efficiency of all intrinsically safe protection. If soldered, make sure to accurately clean the printed circuit board and restore protective coatings.
It is often difficult for every manufacturer to supply all of the documentation necessary to perform repairs on intrinsically safe apparatus. The safer and sometimes less costly solution is to keep a series of spare cards or modules that permit an immediate substitution of the faulty unit. The defective unit can then be sent to the authorized service dealer where it can be repaired.
Rounding out the program
In addition to the previous maintenance criteria, always adhere to established safety practices — e.g., tell the control room operators when you are working in an area and follow lockout procedures without fail. Safety meetings, held on a regular basis, offer an opportunity to discuss various scenarios and ensure that all personnel are familiar with the correct procedures for each specific hazardous area within your plant.
Implementing effective preventive measures and training staff and operators to work safely while developing the capability to handle emergency situations is the key to continuous safe operations.
Joe Kaulfersch is a market analyst for Pepperl+Fuchs, Twinsburg, Ohio. E-mail him at email@example.com.