Explosion proof and intrinsic safety 101


Source: Clean Earth Technology, Inc.

Portions of the standard Spill Buster system operate in various “classified” flammable or explosive vapor zones or areas as defined in the US under the NEC codes. The location of the equipment on the site must be designed properly; or the equipment must be enclosed in appropriate enclosures, to prevent ignition of these vapors should the failure of a component occur. Since this is such an important topic we will present a few basics in this manual to help you or your customers understand the meaning of the various terms defining safe use when planning or installing our equipment. Please note that what we have written here is not meant to supersede anything that may be written in detail in the NEC codes or applicable local codes. It is your responsibility and the responsibility of the Authority Having Jurisdiction to study and follow these codes.

The Magnum Spill Buster system may need to be partially or completely located in areas that are classified as “Class 1, Division 1, Hazardous Atmospheres” as defined in the NEC code section 500. A Class 1, Division 1, safety rating is the highest rating in the US for flammable and combustible zones. There are several methods to insure that electrical systems are safe to operate in these areas. The Standard Spill Buster system can be modified when required to utilize two of these basic methods to achieve safe operations in these atmospheres per NEC 2011 electrical codes: the Intrinsically Safe method, and the Explosion Proof Enclosure method.

The sections below provide a brief explanation of these two methods and the different ways in which they make an electrical system safe for operation in Class 1 Div. 1 areas.

Intrinsic Safety
Intrinsic safety (IS) is a protection technique for safe operation of electrical equipment in hazardous areas by limiting the energy available for ignition of explosive well gases. The concept of (IS) circuits was originally driven by some devastating coalmine explosions in Europe due to “firedamp” gases being ignited by “electrical shorts” in the wiring of early communication devices such as the electric telegraph.

In signal and control circuits that can operate with low currents and voltages, the intrinsic safety approach simplifies circuits and reduces installation costs over other protection methods. High-power circuits such as electric motors or lighting cannot use intrinsic safety methods for protection.

One of the most common methods for protection is to limit the current by using multiple series resistors (assuming that resistors always fail open); and to limit the voltage with multiple Zener devices to ground (assuming diodes always fail shorted).

Approval standards for intrinsic safety barriers require that the barrier maintains approved levels of voltage and current to the specified components. This is accomplished by preventing ignition of the protected device and stopping sparking of damaged wiring to the protected components.

An example of an Intrinsically Safe (IS) certified Zener barrier is shown here on the left. Signals going to an electrical device located in a classified zone are wired into one end of the modular barrier and then wired out into the classified zone. These can be purchased off the shelf and are certified Intrinsically Safe by various 3rd body testing laboratories.

Explosion Proof Enclosures
Explosion Proof (EXP) enclosures are not intended to seal the electrical equipment they contain from the entry of explosive atmospheres that may be outside the enclosure. Instead, they prevent any explosion that might occur within the enclosure from initiating a secondary fire or explosion in the surrounding area outside of the enclosure. Since they are not tightly sealed, liquids and gases may flow into the enclosure.

EXP enclosures are usually made of heavy cast aluminum or stainless steel and are of sufficient thermal mass and mechanical strength to safely contain an explosion should ignition of the flammable material occur within the housing. These enclosures are typically produced with wide, flat, flanges but may not have a gasket between those flanges. Should an explosive mixture ignite inside the enclosure because of an electrical short or an overheated component, the design of the flanges cools down any hot gases escaping between them enough to prevent the ignition of the external combustive atmosphere.

This is important to understand because it means that by design these enclosures are neither completely air tight nor waterproof. The EXP enclosure that can be used with the Magnum’s control box includes a window to allow the operator to see the instrument panel. We strongly recommend that these enclosures be mounted in an upright position and raised off of the ground to prevent possible internal flooding. If the housing must be at ground level, and especially if it is laid flat with the control panel window in the horizontal position, a protective cover should be used to keep rain from leaking inside the enclosure and flooding the control box electronics.

Another example of the Explosion Proof methodology would be the manner in which electrical wire is fed through piping. Per the NEC electrical code spelled out in section 500, cabling fed through piping usually utilizes tapered pipe threads. The code requires a minimum engagement of six threads; and since it is tapered, the spaces between the threads get tighter as the pipe is tightened up. There still is the potential for vapors to leak through these threads, but once again any hot gases escaping through the threads are cooled enough to prevent a secondary explosion.

A third example of EXP methodology would be the use of a multi-conductor cable feed through. The feed through fittings are once again designed to be strong enough to contain an explosion. These fittings leave sufficient room to provide for stripping the outermost wire jacket and enough room for separating the individual wires from each other. A powdered mixture is then mixed together with water and poured in through an access plug to completely surround each conductor. A common material used is a compound from Cooper Crouse-Hinds called Chico®, which cures to a cement-like consistency. In the event of an internal explosion, any hot, explosive gases escaping through the hardened Chico are cooled before entering the explosive atmosphere outside.

Flammability and Combustible Liquids Definitions 101
When using a Spill Buster system it is important to understand the difference between the definition of flammable liquids and combustible liquids. All flammable and combustible liquids will vaporize into flammable gases at certain temperatures and pressures. The temperature point at which a liquid turns to a gas vapor is referred to as the “flashpoint” of the liquid. The determination of the flashpoint for each flammable or combustible liquid is done by performing what is commonly known as the “closed cup” flashpoint test.

It should be mentioned that flashpoint was selected as the basis for classification of flammable and combustible liquids because it is directly related to a liquid’s ability to generate vapor, i.e., its volatility. Since it is the vapor of the liquid, not the liquid itself that burns, vapor generation becomes the primary factor in determining the fire or explosive hazard.

Furthermore, the liquids vapor pressure is an important factor and is a measure of a liquid’s propensity to evaporate. The higher the vapor pressure, the more volatile the liquid and, thus, the more readily the liquid gives off flammable or explosive vapors.

The results of flashpoint testing have been used to help regulating bodies determine the temperatures at which potentially combustible liquids turn into a vapor, creating a flammable or explosive gas.

Hazardous locations are further defined by means of the class/division system and have been formulated by the NEC, CSA, OSHA, and the National Fire Protection Association (NFPA). These definitions are as follows:

Flammable Liquids
Flammable fluids are defined as liquids having closed cup flash points below 100°F (37°C). Flammable liquids are referred to as Class I liquids.

  • A class IA flammable liquid is a liquid with a flashpoint below 73ºF (22.8ºC) and a boiling point below 100ºF (37.8ºC). An example of a class IA liquid is n-pentane, since its flashpoint and boiling point are 56ºF (49ºC) and 97ºF (36ºC), respectively.
  • A class IB flammable liquid is a liquid with a flashpoint below 73ºF (22.8ºC) and a boiling point at or above 100ºF (37.8ºC). An example of a class IB liquid is acetone, since
  • Its flashpoint and boiling point are 0ºF (18ºC) and 133ºF (56ºC), respectively.
  • A class IC flammable liquid is a liquid with a flashpoint at or above 73ºF (22.8ºC) and
  • Below 100ºF (37.8ºC). An example of a class IC liquid is turpentine, since its flashpoint
  • Lies in the range from 95 to 102ºF (35 to 39ºC).

Combustible Liquids
Combustible fluids are defined as liquids having closed cup flash points at or above 100°F (37°C). Combustible liquids are referred to as Class II or Class III liquids.

  • Class II liquids – flash points at or above 100°F (37.8°C) and below 140°F (60°C).
  • Class IIIA liquids – flash points at or above 140°F (60°C) and below 200°F (93.4°C).
  • Class IIIB liquids – flash points at or above 200°F (93.4°C).

An example of a Combustible liquid is the Lamplight® Ultra Pure Red Paraffin Lamp Oil that CET uses for testing new and repaired units before shipment. This material has a flashpoint of 250F (121C) making it a class III liquid. Furthermore, its low toxicity makes it a safe testing fluid due to its high ignition temperature and the lack of chemicals that are listed in the CERCLA Hazardous Substance list. CET uses a red colored fluid but it also can be found in clear, or colored green or blue.

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