Using Pressure and Load-Sensing Instrumentation in Hazardous Locations

Equipment which is certified as being intrinsically safe by an independent testing agency (e.g., FM Global) decreases the risk of death or injury when using it and helps to prevent the loss of buildings and machinery from potential explosions. It can also produce direct savings from lower insurance premiums. Specific areas of intrinsically safe instrumentation addressed in this article are:

  • Summary of what constitutes a hazardous area
  • Explanation of “intrinsic safety”
  • Importance of independent testing and certification
  • Use of pressure and load-sensing instrumentation in hazardous locations

Identifying Hazardous Areas

OSHA and National Electrical Code (NEC) standards concern themselves with the use of electrical equipment in hazardous locations and they offer advice on mitigating the potential risk. Relevant OSHA standards include 1910.307 and 1910.399 and NEC standards include NEC 500 – which is currently being superseded by the internationally harmonized standard NEC 505.

These standards address what OSHA terms, “electric equipment and wiring in locations that are classified depending on the properties of the flammable vapors, liquids or gases, or combustible dusts or fibers that may be present therein and the likelihood that a flammable or combustible concentration or quantity is present.” Even materials that are not normally considered ignitable, such as metals, can burn with rapidity in the right environment. Examples of these environments include petroleum and chemical processing plants, paint-finishing process plants, sugar refineries, and in general, any area where an explosive atmosphere can be built up.

NEC 500 and 505

The normal approach is to categorize potentially hazardous locales by the combustible materials that are likely to be present. The current NEC500 standard, used throughout USA and Canada, possess two classes. The first class (Class 1), is concerned with vapors and gases, whereas Class 2 deals with dust. Several divisions within the classes cover the likelihood of a combustible material being present. Division 1 states refers to an explosive atmosphere being continually present (or likely to be) under normal conditions.

Division 2 covers locations that, according to OSHA, “would become hazardous only in case of an accident or of some unusual operating condition.”

In the newer NEC 505 standard, the Classes and Divisions are replaced by Zones. Zones 0, 1 and 2 refer to gases or vapors while Zones 20, 21 and 22 are concerned with dust. In both, the second digit indicates the risk with 0 signaling a continuous hazard and 2 stating that a hazard is rarely, if ever, present.

Understanding Intrinsic Safety

Many techniques are produced to ensure the safety of electrical equipment under hazardous conditions. Within these, the safest alternative is to use “intrinsically safe” device and instrumentation. This is an approach to prevent and minimize the ignition sources, such as from sparks and hot surfaces.

Equipment such as motors are not automatically suited towards the intrinsic safety approach, therefore, they must be enclosed. However, low voltage instruments can be engineered to be intrinsically safe. If a complex piece of equipment or interconnected devices is to qualify as being intrinsically safe, then every component within the equipment is required to be integrally safe. Designers of process control and instrumentation equipment should bear this in mind.

Another advantage of using intrinsically safe certified devices or equipment is that it allows live maintenance to be undertaken, thus avoiding the need for long shutdown periods, venting and gas clearance. Additionally, fault finding is overly simplified as the circuit tests can be undertaken when the equipment is energized.

Over time, equipment will need to be repaired or replaced, otherwise new instrumentation will need to be retrofitted in a hazardous location. When this occurs, it is crucial that maintenance technicians use intrinsically safe certified components.

However, there are some exceptions. Instrumentation categorized as a “simple apparatus” does not need to necessarily conform to intrinsic safety design principles. To be coined a “simple apparatus”, the safety of the instrument should be immediately demonstrable. Examples of simple apparatus include thermocouples, RTDs and junction boxes.

Independent Testing and Certification

People who use safety-related equipment tend to prefer an independent confirmation of conformance to the relevant standards, and insurers may also insist on it. There are a number of organizations which offer independent testing and certifications. OMEGA is partnered with FM Global, a National Recognized Testing Laboratory (NRTL), to make sure that equipment such as pressure transducers are safe to use in hazardous environments.

As the name suggests, FM Global is a global organization and is recognized internationally. They possess several research centers where they perform product testing and provide product certification services.

Instrumentation sold by OMEGA that meets intrinsic safety standards displays the “FM Approved” mark. Equipment without this marking cannot be used in hazardous environments unless other necessary precautions are taken.

Pressure and Load-Sensing in Hazardous Locations

There is an extensive list of locations that require intrinsically safe devices. The majority of process control applications in industries should use this kind of instrumentation, and this can range from chemical refineries, power generation, oil and gas production, to anywhere with airborne dust, such as silos.

There is a wide range of equipment that can be used, or found, in hazardous locations, including hydraulic and pneumatic systems, natural gas compressors and shock and vibration measurement and testing hardware. One routine task is in the measurement of pressures, be it absolute pressure, vacuum, or barometric pressure.

Filters and pumps, like those found in the production of oil and gas products, require monitoring to prevent and quickly identify any blockage or failure. Also, flow measurements are an essential component of process control procedure across many industries.

Less obvious dust-related environments such as sawmills, grain silos and sugar refineries also qualify as hazardous locations a lot of the time. Load cells are used extensively in these environments to check container weights. As with pressure-sensing, the implantation of intrinsically safe instrumentation is advised.

OMEGA possess a wide range of intrinsically safe pressure sensors, configurable pressure transducers and load cells – including amplified load cells, low-profile load cells and pancake load cells. If the potential application is in a hazardous environment, clarify the individual specifications for FM approval.

Summary

Many industries have very real explosion risks, be it from dust, flammable gases and vapors. NEC standard 500, currently being replaced by the globally-harmonized standard NEC 505, addresses the design of electrical equipment for hazardous environments.

Several preventative methods have been employed to handle the use of electrical and electronic equipment in these environments. However, only intrinsic safety aims to prevent explosions by minimizing the energy produced, therefore avoiding sparks and hot surfaces.

When an intrinsic safety approach is performed, the equipment and instrumentation employed in hazardous locations must be designed correctly and verified by an independent testing authority. OMEGA works alongside FM Global in the verification of all of our intrinsic safety certification needs, thus ensuring that the electrical equipment you will be installing carries an FM approval.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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