Safeguard plant safety

The worst thing that can happen at your plant is that an employee suffers a serious injury from a preventable accident. Follow some proven maintenance pointers to keep operations safe.

By Joe Kaulfersch, Pepperl+Fuchs

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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.

Programmed maintenance
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).

Explosion-proof apparatus
Explosion-proof apparatus
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).

Point-to-point wiring
Point-to-point wiring
Figure 3. Point-to-point galvanic isolators simplify system design and installation in hazardous location applications.

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