Inspecting instruments installed in hazardous locations
A difficult, time-consuming, and expensive task is made much simpler with wireless instruments to relay information from flow, pressure, temperature, level, and other field transmitters.
A major trend in process industries is the increasing use of wireless flow, pressure, temperature, level, and other field transmitters. While these instruments are widely used to provide inexpensive, easy-to-install measurements in unclassified areas of process plants, they can also be used in hazardous locations (Figure 1). This article explains the difficulty, time, and expense involved when installing and maintaining traditional two-wire or fieldbus transmitters, and shows how wireless transmitters avoid almost all those problems.
Many strategies exist for safety in process installations. The simplest strategies to minimize risk in a hazardous area are to keep instrumentation out of the area altogether, or make the area less hazardous through process improvements.
But when instruments must be installed in hazardous areas, all of the wiring must be installed and maintained to the relevant standards to prevent an ignition of explosive environments. The costs of protecting wired 4 to 20 mA and fieldbus instruments in hazardous areas are significant.
Field transmitter wiring requires wiring, conduits, cable trays, field junction boxes, and marshalling cabinets. If the instrument is 4-wire, it must have separate power wiring. All wiring systems must meet the requirements of IEC 60079 for the type of protection that the circuit is certified for. This could be "EX ia" for intrinsically safe, "Ex d" for explosion-proof, or one of the other types of protection permitted.
Installing these wiring systems can be quite expensive, but costs don't stop with installation as initial and periodic maintenance are required to ensure that the protection level is being maintained. IEC 60079 covers inspection of electrical equipment in hazardous areas. IEC 60079-14 requires that an initial inspection must be carried out when the equipment is first installed. IEC 60079-17 says the interval between inspections shall not exceed three years without seeking expert advice.
Although complete inspections have to be done at least every three years, the grade of inspection and the interval between inspections should take into account the type of equipment; that is, some devices may need to be inspected more often than every three years depending on the environmental conditions of the installation. Continuous and visual inspections, as defined below, should be done on a regular basis, or whenever maintenance work is done in the area. Close and detailed inspections may require opening enclosures and using specialized tools so such inspections can be done during shutdowns or when the area is safe.
There are four grades of inspections defined in IEC 60079-17:
1. Continuous supervision—defined as frequent attendance, inspection, service, care and maintenance of the electrical installation by skilled personnel who have the knowledge and skills to maintain the equipment in accordance with IEC 60079-17 Clause 4.5.
2. Visual inspection—an inspection (Figure 2) that identifies, without use of access equipment or tools, defects that would be apparent to the eye, such as missing bolts or damaged cables.
3. Close inspection—defined as an inspection that encompasses aspects covered by a visual inspection and identifies defects that are apparent only by the use of equipment and tools, such as loose bolts or damaged cable glands.
4. Detailed inspection—defined as an inspection that encompasses everything covered by a close inspection and identifies defects that are only apparent by opening the enclosure and using tools and test equipment. Detailed inspections can find loose terminations or incorrect grounds.
These inspections include all equipment located in the hazardous area, and any protection device located in the safe area (in the case of "EX ia"). In practice, this is difficult and expensive to achieve. Some wiring systems may be routed in such a manner as to make inspection very difficult. Process plants have internal permitting procedures that must be followed when entering hazardous areas, adding further costs. And there is, of course, the time required to make inspections, to document work done, and to keep required records.
It's important to verify that an installation conforms to a detailed equipment list and circuit diagrams. These and any other documentation listing specific conditions of use must be made available for the inspector. Often the equipment list has not been updated with equipment changes as plant modifications are carried out, due to either failed equipment being replaced with alternatives, or new wiring additions.
If clarification has to be sought regarding the installation and associated documentation, this adds to the time taken to carry out the inspection. Reducing the number of equipment items reduces the burden of keeping equipment lists updated, and wireless instruments are one of best ways to address this issue.
Typical issues found during inspections include:
- Incorrect or damaged cable gland
- Damaged cables
- Non-IS (intrinsically safe) circuits installed with IS circuits
- Incorrect segregation of IS and non-IS circuits
- Wrong zener barrier or galvanic isolator fitted
- No earth on zener barrier
- Non-IS earths connected to IS earth, and
- Equipment not as scheduled.
Table 1 describes the types of inspections typically required for traditional instruments and their associated wiring systems, and as can be seen they are quite extensive. But there is a simplifying solution, one that eliminates wires and their associated equipment.