Maintaining a Safe Plant

By Joe Kaulfersch, Market Analyst
Pepperl+Fuchs

There is no substitute for proper maintenance – including an initial inspection, verification and periodic inspections and repairs – in plants where there is a danger of fire or explosion. This is true for both safety and economical management reasons.

While there are many guidelines for safe plant maintenance, years of experience have helped us develop some rules which we believe are essential for keeping a plant, its personnel and the surrounding community out of harm’s way. These are not to replace applicable industrial safety standards, but to provide a general guide for safe plant maintenance.

The first thing to keep in mind is that it is important to permit only authorized personnel to repair explosion-proof apparatus, because equipment must not be serviced under power.

There are three types of inspection/maintenance activities: initial inspection, programmed maintenance and apparatus failure and repairs.

First, 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 itself, all field elements, all associated apparatus, and the agreement between safety parameters and interconnected apparatus.

Plant Capability
In verifying plant capability, ensure that protection methods are compatible with the hazardous locations and gas groups present, and 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 not risk of abrasion or cuts; there is no formation or accumulation of condensation; the explosion-proof enclosures have a reliable external ground connection; any mixed protection methods are authorized for the specific type of hazardous location and lastly, there is adequate space to easily remove the lid during maintenance.

Intrinsically safe installations should verify that all electrical conductors conform to the installation standard; intrinsically safe circuit conductors are not mixed with nonintrinsically safe circuit conductors; 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 and/or regulation loops are clear; barrier input terminals are not confused with the output terminals and lastly, 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 ƒq 1.2 V; Imax ƒq 0.1 A; Pmax ƒq 25 mW; Emax ƒq 25 ƒ¶J) or covered by an authorized protection method for the specific installation zone. It is also important that the method is 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 are always present on the lids and/or the labels.

In the case of intrinsically safe apparatus or simple electrical devices, here are the points to verify: Devices should be certified for their hazardous location and the gas groups correspond to the flammable mixtures present. They should have a surface temperature classification compatible with the flammable mixture present (e.g., T6).

Apparatus should be exclusively connected to intrinsically safe circuits, in the manner specified by the certification documents, especially for the eventual connection to alarm safety apparatus, and the safety parameters of the connecting cables have been respected.

They should be powered or interconnected with associated apparatus having safety parameters compatible with the apparatus and the proper protection method. They should have a grounded enclosure (or the risk of electrostatic discharge is not present) and are installed with proper environmental protection. They should 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 should not jeopardize the tightness of the enclosure against water and corrosive atmospheric elements.

Capability of All Associated Apparatus
All associated apparatus must also have capability ensured. To do this, several points must be verified. The devices must be exclusively installed in a protected nonhazardous location, unless using other protection methods that are suitable for the hazardous location. The certification type should be compatible with the intrinsically safe circuit to which they are connected, according to the hazardous location and the gas group. The apparatus must have safety parameters compatible with the connection cable and the intrinsically safe apparatus to which they are connected. Separation and identification of the intrinsically safe circuit must exist along the entire connection route, as required by the standards. The devices must have correctly rated and installed internal fuses and that 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 in connection with other apparatus, even if these apparatus are certified as safe. Take special care 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 field elements connected.

Programmed Maintenance
Programmed maintenance is an extremely valuable tool. It prevents waste, downtime and the deterioration of apparatus. In addition to the inspection and repairs, your maintenance program should include a record noting the maintenance performed, the date and the results.

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 deformations, 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 and guarantees a good ground connection.

Keep in mind during calibration verification that you cannot test your apparatus without power. Therefore, take caution to eliminate the presence of flammable mixtures (determined by on-site testing with an explosion meter) and perform the calibration with the power on. During the subsequent operations, continuously verify the absence of flammable mixtures with the meter. Connect 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. Verify the calibration per manufacturer’s instructions and adjust if necessary. Reconnect the original connections. Close the lid and verify its tightness.

Periodic inspections for intrinsically safe apparatus can be performed without a plant shutdown. Verify that situations of real or potential dangers are not generated for the following reasons:

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.

Apparatus used for the test 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 the procedure. 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 true. Actually, a more dangerous situation could develop. 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 the 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 less than or equal to 1½; 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 are respected.

Calibration Verification
With intrinsically safe circuits, maintenance activities are performed while the power is on (usually 28 V maximum voltage). However, when you have circuits protected by barriers, those barriers could be permanently short-circuited. This could cause the fuse to blow, rendering the barrier useless. Always ensure that the barriers are not short-circuited and all apparatus are safely connected, because in other apparatus, there is still a risk of improper contact remains. When possible, remove the apparatus (particularly easy with plug-in type connectors) and proceed with verification in a lab. When it is impossible to remove the apparatus, you should 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 where they are used. After completing verifications and calibrations, restore input and output conductors with extreme care.

Apparatus Failure and Repairs
Decreasing downtime for repairs is key to reducing maintenance costs. Smart planning during design will help to reduce your downtime and make repairs easier. To do this, select modular and plug-in type field and control room instrumentation and
ensure adequate spare parts to permit immediate substitution.

When operating problems or machine failure occur in an explosion-proof instrumentation plant, it’s vital as soon as possible to determine the cause of the problem, isolate and determine the problem area and substitute or repair malfunctioning components.

Even though the procedure is the same to determine the causes of malfunction and identify failed apparatus in explosion-proof plants as it is in standard instrumentation plants, there is always danger of fire or explosion. Therefore, keep these points in mind: 1) Do not perform connections not shown in the plant’s schematic, unless the risks relative to safety have been analyzed, 2) Do not use test instruments not certified for use in the same hazardous location and gas group as the circuits to be analyzed. 3) Isolate the part of the plant where repairs must be performed. Consider the effect of the tests on the interconnected circuits. 4) 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
Repairing an 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 coats which increase the isolations that are unobtainable 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 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 and/or terminals for armor ground connections
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). Do not substitute fuses unless they are substituted with others of identical type (rapid, medium lag, etc.) and nominal current. Do not substitute main or signal transformers, or certified modules unless they are substituted 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 the component terminals in order to distance them from the printed circuit board. Take care in verifying the repaired card or component, to ensure the complete efficiency of all intrinsically safe protection. If soldered, take care to accurately clean the printed circuit board and restore protective coatings.

It is often difficult for every manufacturer to supply all 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.

Proper maintenance, accurate repairs and periodic inspections are essential for the economical management and safety of your plant when there is danger of fire or explosion. In addition to the previous maintenance criteria, always follow applicable safety standards—tell the control room operators when you are working in an area, 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 your staff and operators to work safely while developing the capability to handle emergency situations is the key to continuous safe operations. Never substitute any of these protection characteristics without having complete documentation and a thorough knowledge of intrinsic safety and the way in which it is used in any particular apparatus.

For more information:
www.am.pepperl-fuchs.com
sales@us.pepperl-fuchs.com
Ph: 330-486-0002