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Jun-2018

Wireless instruments in corrosive and hazardous locations

Wireless instruments introduce a range of benefits in corrosive and 
hazardous process areas.

SHEIKH RAFIK MANIHAR AHMED and KULEESHA MALIK
Fluor Daniel India
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Article Summary
This article explains the difficulty, time and expense involved in installing and maintaining a traditional instrument wiring system or single wiring method and shows how multiple wiring methods/protocols along with wireless instruments can avoid those problems. In general, an instrument wiring system (including cables, junction boxes, conduits, termination racks, cabinets, enclosures, cable tray, tray support system, 
multi-cable transits [MCT] and marshalling panels) contributes significantly to the plant’s total instrumentation costs. The article focuses on different impacts during the implementation of multiple wiring methods/protocols along with wireless instruments on the same project in order to prevent, minimise or control the risk of corrosion and chemical attack on an instrument wiring system or cable infrastructure to meet the plant’s requirements..

The availability of multiple wiring methods/protocols provides numerous benefits over single type of wiring methods/protocols including reduced wiring expenses and marshalling panels, fewer power supplies and safety barriers, smaller equipment room, less documentation and design effort, fewer input/output (I/O) channels in the distributed control system (DCS) and programmable logic controller (PLC) cabinets, improved self-
diagnostics and remote diagnostics, increased functionality and transparency, more information from the field level, easier extension during operation, feasibility of maintenance, and minimal impact of late changes, workmanship and materials.

In a multiple wiring methods/protocols system, it is not a matter of counting the number of I/O signals and figuring out if they are 4-20 mA analogue or 0-24 Vdc, or if they are input or output; it is about selecting the right IO type and wiring protocol to achieve process functionality by reducing effort and cost. The article details different designs, engineering, deliverables and cost impacts during the implementation of multiple wiring methods/protocols along with wireless instruments on the same project.

Oil and gas, petrochemicals, chemicals and other process industries often face problems of electrical downtime, electrical service disruption, erratic instrumentation system behaviour, and loss of I/O cards in DCS and PLC.

Increased resistance across the connection and contactor results in false data transmission through the instrument data cable to the system due to an open or intermittent connection of the power or instrument data cables. Wet, salt-mist, dirty, dust contaminated or corrosive vapours increase the risk of corrosion and chemical vapour attack on the metals used in wire terminal connections, switches and contactors. Wiring connections can also be affected by creep corrosion and metallic dendrites due to the reaction of corrosive vapours with the metal of a connector at high or variable humid conditions. Intermittent terminations of wire connections in instrument wiring are expensive and time consuming problems that are not permissible in today’s automated plant. Temperature and humidity control is the single most important factor affecting corrosive rates when corrosive vapours are present in the environment.

There are many available plans of action designed for safety in process installations. The simplest method to prevent, minimise or control the risk of corrosion and chemical vapour attack is to keep instrumentation out of the area altogether, or to make the area less hazardous or less reactive with controlled humidity and temperature through process improvements. However, when instruments must be installed in corrosion prone areas (such as waste gas treatment and sulphur handling areas), all the instrument wiring system (including cables, junction boxes, conduits, termination racks, cabinets, enclosures, cable tray, tray support system and MCTs) must be installed and maintained to appropriate standards to prevent deterioration of the metals used.

Generally, metals used in wiring terminals have platings or coatings for protection against corrosion and chemical attack. These sacrificial or barrier layer coatings over wiring terminals metals are intended to be consumed during a corrosion reaction or to provide a protective seal. They are effective as long as the metal in wire terminals is undamaged and impermeable. The expense of protecting wired 4-20 mA conventional, HART, Fieldbus, Modbus, Profibus or Profinet instruments in a corrosion prone environment or hazardous area is significant. Wiring for field instruments (such as transmitters with or without display, switches and control valves) requires cables, junction boxes, conduits, termination racks, cabinets, enclosures, cable tray, tray support system, MCTs and marshalling panels. If the instrument is source input or four-wire, it must have separate power wiring. All of the wiring system must meet the requirements of protection that the circuit is certified for. Installing an instrument wiring system can be quite expensive and additional operational maintenance expenses are required on preventing, minimising or controlling the rate of corrosion to ensure the protection level is being maintained.

Typical corrosion related issues found in a wiring system during inspection in a corrosion prone environment are shown in Figure 1 and include:
• Incorrect or damaged cable gland
• Damaged cables
• Corroded terminals, switches and contactors
• Corrosion across the connection and contactors, resulting in increased resistance
• Corrosion fatigue failure of terminals.

A wireless infrastructure could possibly avoid these instrument wiring system problems, corrosion related inspections, and planned/unplanned maintenance.

Effects of corrosion on wiring systems
All instrument wiring systems or cable infrastructures have a tendency to undergo a corrosive reaction with a variety of industrial gases/vapours such as sulphur, chlorine and their compounds. A harsh environment requires detailed engineering design of appropriate corrosion control measures to avoid unnecessary electrical downtime, planned/unplanned maintenance and inspection costs. An instrument wiring system or cable infrastructure affected by short/long term exposure to corrosion displays a variety of  typical behaviour patterns:
1. Electrical shutdown or total disruption of electrical services due to short/open circuits
2. Unexplained loss of I/O section of a circuit
3. I/O cards lost in DCS and PLC, data reading and transmission problems, and erratic behaviour
4. Terminals, leads, switches and electrical contractors become corroded which reduces the conductivity of the metals
5. Connection of the data/power cables inside junction boxes located in the field, and marshalling of system cabinets located in local equipment/control rooms are affected, so resistance increases across the connection and contactors, resulting in false data transmission
6. A combination of a corrosive environment and vibration assists in the formation of corrosion fatigue cracks in terminals leads, leading to wiring system failures.

In addition, non-corrosion related stress in a wiring infrastructure may impact measurement quality, such as crushed cables, excessive length, mechanical fatigue, poor cladding, cable routing complexity, routing between moving components, and weight carrying.
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