Key points
Instrumentation for hazardous areas come in many different forms with varying types of protection classification depending on the operating environment, this article highlights reductions in hazardous area inspections requirements choosing a WirelessHART device over a 4-20mA HART device.
Simplify hazardous area inspections by using WirelessHART instruments.
When instruments are required to be installed in hazardous areas, there is a requirement that all wiring must be installed and maintained to the relevant standards to prevent an ignition of the explosive environment.
The costs of protecting wired 4-20mA HART instrument in hazardous areas is not just simply the installation cost but also ongoing inspections and remedial works that will inevitably follow. This can often be overlooked where the designer is not the plant operator or maintainer.
WirelessHART devices certified to the relevant protection level for the hazardous area can reduce the duration and complexity of inspections due to the reductions of equipment installed in the hazardous area, such as the wiring, junction boxes, cables and IS barriers needed with a traditional wired installation.
WirelessHART process transmitters (Figure 2) can be powered by an IS power module, which eliminates the need for power and signal wiring. Such power modules are field interchangeable in the hazardous location.
For example, a refinery in Mexico was having problems with its cooling towers. The wired cooling tower instrumentation was old and most measuring devices were out of service.
Measurements were difficult because the process environment was corrosive to wiring, mainly due to hazardous chemical vapours. As a result, these areas were poorly instrumented. Consequently, control was poor, operations were inefficient and the towers required a great deal of maintenance.
Installing wired instruments was deemed too expensive because of the cost of equipment and its supporting infrastructure, and because the corrosive environment would degrade the instrumentation and raise maintenance costs. Required inspections would add further costs, making wired instrument installation impractical.
Instead, the refinery installed 122 Emerson wireless sensors and transmitters in enclosures to protect the instruments from the environment. Because these instruments are rated for use in the area, no special protective measures were required.
Inspection requirements
4-20mA HART field transmitters require 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.
Maintenance (Figure 3) is required to ensure that the protection level is being maintained.
IEC 60079 covers the use 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 that the interval between following inspections shall not exceed three years without seeking expert advice. Typical inspections are done annually.
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.
There are four grades of inspections defined in 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 3) which identifies, without use of access equipment or tools, defects which would be apparent to the eye, such as missing bolts or damaged cables.
3. Close inspection − defined as an inspection which 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 without opening equipment.
4. Detailed inspection − defined as an inspection which encompasses everything covered by a close inspection and identifies defects which are only apparent by opening the enclosure and using tools and test equipment. Detailed inspections can find loose terminations, incorrect grounds missing seals in glands.
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]’). Inspecting all the equipment in the circuit can be difficult due to cable routes and general access issues when the plant is running that may require permits to access hazardous areas.
Once the inspection is done records have to be updated and any remedial works have to be scheduled and carried out before the circuit can be signed off as compliant.
Cutting costs with WirelessHART
Adoption of WirelessHART equipment can have significant impact on both CAPEX (capital expenditures) and OPEX (operating expenses).
In the CAPEX phase, designs using a wireless device can achieve cost savings. By removing the glands, wires, conduit/tray, field junction boxes and marshalling cabinets − wiring component costs are reduced, and further savings are realised by eliminating associated installation drawings and equipment lists that are required for maintenance and installation.
The cost of installation must also be considered, as not having to install equipment required for wired instruments reduces labour costs, scaffolding etc. and speeds up installation and commissioning times.
WirelessHART also reduces OPEX, especially the cost of hazardous area inspections. This is due to not having to have supporting infrastructure between the device and the host system for the measurement circuit.
Only the instruments themselves have to be inspected and possibly the wireless gateways if they are installed in the hazardous area. In many instances, these gateways can be installed outside of the hazardous area, further cutting costs.
This is especially important when the inspection is on the critical path of a plant outage. By reducing the volume of hardware to be inspected and maintained, outage duration due to inspections can be reduced or inspections taken off the critical path.
In comparison the costs of inspecting and maintaining wired 4-20mA HART or fieldbus process instruments in a hazardous area is very high because of the need to support the instruments with an infrastructure including power supplies, wires, and conduit and marshalling cabinets.
All of these items need to be protected from igniting a flammable or explosive atmosphere, all of this equipment needs to be inspected on a regular basis, and repairs are often required to keep the entire system safe.
Using WirelessHART eliminates wiring infrastructure, and reduces required inspection tasks. In risk reduction eliminate and reduce offer the best protection over inspect and control.
