special section: electrical test equipment
Calibration in hazardous areas
Hazardous areas have many ratings and types of devices; what test equipment do you use in an area?
By Heikki Laurila
What is a hazardous area? A hazardous area is an area (inside or outside) that contains, or may contain, flammable substances. The flammable substance may be a liquid, gas, vapor, or dust. The area may contain a flammable substance all of the time, most of the time, or only in specific situations, such as shutdowns or incidents.
In such a hazardous area, an explosion or fire is possible if all the three conditions of the “explosion triangle” are met. These three conditions are fuel (a flammable substance), a source of ignition (or heat), and oxygen (air). The explosion triangle is often presented as a triangle, therefore the name.
How to prevent an explosion
If we keep in mind the explosion triangle and think about how an explosion can be prevented, we come to the conclusion that one or some of the three elements of the explosion triangle must be eliminated. Many times, it is not possible to eliminate the flammable substance, so the oxygen (air) or the source of ignition has to be eliminated. It is also often impossible to eliminate the air. Therefore, the most practical solution is to eliminate the source of ignition, spark, or heat.
In the case of electrical calibration equipment, it can be specially designed for use in hazardous areas. There are many ways to design electrical equipment suitable for hazardous areas, and this topic will be discussed later. Often calibration equipment has been designed so that it cannot provide enough energy to cause the source of ignition, spark, or heat.
Some of the first hazardous areas were discovered in the early coal mines. Both the coal dust and the methane absorbed created a hazardous area, because they are flammable substances. As the lighting in early mines was created by candles and torches, there was also a source of ignition. This caused many accidents.
Later on when the electrical equipment (lighting, tools) began to be used in the mines, there were accidents caused by the electrical equipment (sparking or heating). Eventually standards were developed to guide the design process in order to prevent sparking and heating of electrical equipment. These were the first “intrinsically safe” electrical equipment and led the way to the actual standards for equipment used in hazardous areas today.
Typical industries with hazardous areas
There are many industries that can have hazardous areas. Some plants have large hazardous areas, while some have only small parts classified as hazardous areas. Typical industries include chemical and petrochemical industries, offshore and onshore oil and gas, oil refining, pharmaceutical, food and beverage, energy production, paint shops, and mining.
Remember that the flammable substance may be a liquid, gas, vapor, or dust, so there are surprisingly many different industries with some areas where these substances may be present during normal operation or during a shutdown. Even some seemingly safe industries may have hazardous areas.
In plants, all areas classified as hazardous areas should be clearly marked for identification with the Ex logo.
Flammable and combustible liquids
What are flammable and combustible liquids? Generally speaking, they are liquids that can burn. Some of these liquids are present in many working places. They are gasoline, diesel fuel, many solvents, cleaners, paints, or chemicals.
Flashpoint and autoignition (also known as kindling point) temperatures are also often discussed. The flashpoint is the lowest temperature of a liquid that gives sufficient vapor to form an ignitable mixture with air. With a spark or enough heat, it will ignite. The autoignition temperature is the lowest temperature at which a liquid will ignite even without any external source of ignition. The most common flammable and combustible liquids have autoignition temperatures in the range of 572°F to 1022°F (300°C to 550°C). However, there are liquids that have an autoignition temperature even below 392°F (200°C).
Based on their flashpoint, liquids are classified as flammable or combustible. Flammable liquids may ignite at normal working temperatures, while combustible liquids burn at higher temperatures. Often 100°F (37.8°C) is considered the temperature limit. Flammable liquids have a flashpoint below 100°F, and combustible liquids have a flashpoint above 100°F.
To be more precise, flammable and combustible liquids themselves do not burn; it is the vapors that burn. Or actually, it is the mixture of their vapors and air that burns. There are also limits of the concentration within which the mixture can burn. If the mixture has too small of a concentration (too lean) it will not burn, and also if the concentration is too high (too rich) it does not burn. The limits are known as lower and upper explosive limits (LEL and UEL).
It is good to remember that some liquids may have a pretty low flashpoint. For example, gasoline, which is flammable, has a flashpoint as low as about –40°F (–40°C). It gives enough vapors at normal environmental conditions to make a burnable mixture with air. Combustible liquids have a flashpoint way above normal environmental conditions, so they have to be heated before they will ignite.
Finally, it is important to ensure that the equipment is suitable for the environmental conditions where it will be used. For example, the safe operating temperature of the device must match the temperature at which the equipment is used in a plant. In wet and dusty conditions, the protection rating of the equipment casing needs to be considered; this can be ingress protection (IP) or NEMA classified. Different protection techniques may require different classifications on the casing.
It is also important to remember that the casing of some Ex equipment is made out of nonstatic (semiconducting) material to avoid building up any static electricity. Depending on the classification, there are limits to the size (static) of stickers that you can put on the device. For example, Group I equipment for Zone 0, with gas Group IIC, is allowed to have a plastic sticker with a maximum area of only 4 cm2 (0.6 inch2). It is important to keep that in mind before attaching any identification stickers on Ex equipment.
[Note: There is more detail about intrinsic safety in this month’s Automation Basics article, “Intrinsic safety 101 – hazardous locations.”]
There are different kinds of hazardous areas, zones, and divisions.
Using non-Ex calibration equipment in a hazardous area may be possible, but it requires special approval from the safety personnel in the factory. Frequently, this also involves the use of safety devices, such as personal portable gas detectors, to be carried in the field during the work. Correctly using rated Ex-equipment is easier, as it does not require any special approvals. Naturally, the Ex-rated calibration equipment must be suitable for the hazardous area where they are used.
The Ex classification of calibration equipment also has many different criteria and classifications. Selecting the suitable calibration equipment for your hazardous areas is important for safety.
Example of test equipment marking
Here is an example of Ex-rated calibration equipment. The example is an intrinsically safe process calibrator, model Beamex MC5-IS. This is a multifunctional portable process calibrator, which can be used in a hazardous area.
The product has the Ex marking Ex II 1 G, Ex ia IIC T4 Ga (Ta = –4 °F to 122°F [–20 °C to 50°C]). That marking is also shown in the picture as written in the front face of the device. This is the breakdown of the Ex marking code and a look at what it means in practice:
International/North American legislation and differences
There are two different standardizations specifying the hazardous areas and the classification of the equipment used in hazardous areas. One is the international IEC standard and the ATEX directive used for international and European legislation. The second is the North American legislation.
ABOUT THE AUTHOR
Heikki Laurila (firstname.lastname@example.org) is product manager for hardware products at Beamex Oy Ab, Finland, where he has worked for 25 years. Beamex provides products and services for the calibration of process instrumentation (www.beamex.com).