Equipment calibration in hazardous locations

Tuesday, 16 September, 2008



Striking a match in an environment that contains combustible gas is very dangerous — personal injury and property damage are likely consequences. Improperly calibrating an instrument in this hazardous environment can be almost as dangerous. For example, hydrocarbons in mines, oil refineries and chemical plants are flammable. Leaks, abnormal conditions and fluid accumulation in vessels and pipes may allow hydrocarbons to be present such that the flame could ignite the hydrocarbons with disastrous results.

Combustible fuels, such as natural gas, are used in all industries, including agriculture, food, pharmaceuticals, power generation, pulp/paper and even retail. Even seemingly safe water treatment systems use combustible materials such as chlorine in their processes, so certain areas of a water treatment plant may well be considered hazardous.

Similarly, certain areas of food plants, such as reactors that hydrogenate oils, may also be hazardous. Therefore, it is important for plants to examine their processes and identify hazardous locations so that the proper instruments are selected, installed and maintained in accordance with practices that are appropriate for the hazard.

Equipment requirements in hazardous locations

Protection requirements for hazardous locations vary according to the type of material present, frequency of the hazard and the protection concept applied. The intensity with which various vapours can combust is generally different. Groupings as defined by AS/NZS 60079.10:2004 in order of decreasing ignition energy (with example gas) are:

  • Group IIC — Acetylene
  • Group IIB+H2 — Hydrogen
  • Group IIB — wEthylene
  • Group IIA — Propane

The hazardous area classifications in order of decreasing frequency are:

  • Zone 0 — Flammable material present continuously
  • Zone 1 — Flammable material present intermittently
  • Zone 2 — Flammable material present abnormally

Intrinsic Safety (IS) is the most common protection concept applied to calibrators that are used in hazardous locations. In general, the IS concept is to design the calibrator such that it limits the amount of energy available so it cannot ignite a combustible gas mixture. Adding the applicability of IS designs to various hazards in the previous table yields:

  • Zone 0 ia — Flammable material present continuously
  • Zone 1 ia, ib — Flammable material present intermittently
  • Zone 2 ia, ib — Flammable material present abnormally

In addition, a hot surface temperature on a device can cause ignition. Temperature classes limit the maximum surface temperature between 450°C (T1) and 85°C (T6).

Calibration solutions for hazardous locations

Instruments designed to measure flow, level, pressure, temperature and other variables in hazardous locations are generally used to monitor and control the process. In some applications, it is practical to remove these instruments and calibrate them on the workshop with a calibration test bench. Often, however, this is not the case, which means that many instruments are calibrated in the field. Many calibrators, such as portable Beamex instruments, are specifically designed to operate safely in rugged environments and hazardous locations, compliant to AS/NZS 60079.10.

AMS Instrumentation & Calibration
www.ams-ic.com.au

 

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