An explosion is an exothermic, or heat-releasing, reaction of a substance at a high reaction rate. It requires the presence of an explosive mixture/atmosphere and an ignition source, plus an extraneous cause that triggers the explosion (Figure 1). The potential for an explosion is present where electrical equipment and potentially explosive mixtures coexist. Protection and/or mitigation must be incorporated to prevent explosions. Approved methods that provide protection from ignition in potentially explosive areas must be designed into electrical equipment and processes.
Explosion protection methods include exclusion, containment, and energy-limiting technologies. A common form of the latter is known as intrinsic safety. We’ll look at these methods for protecting various types of hazardous areas in process plants with the help of Jesse Hill, process industry manager at Beckhoff Automation LLC.
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Piper Alpha was an offshore production facility operated by Occidental Petroleum Limited that exploded and sank in July and killed 165 people. Workers were performing maintenance on a pressure relief valve, and they didn’t finish before the end of the shift. They put a temporary seal in place, and that communication didn’t get to the next shift. There was a gas leak, which hit an emission source and caused the explosion. The economic impact was almost $2 billion.
In , BP’s Texas City Refinery was the second-largest oil refinery in the state and the third largest in the United States. On March 23, an explosion occurred when a vapor cloud of natural gas and petroleum ignited and violently exploded at the isomerization process unit and killed 15 workers, injured 180 others, and severely damaged the refinery (Figure 2). Factors contributing to the explosion included serious issues with items of safety-critical equipment that had not been resolved prior to the startup: an inoperative pressure-control valve, a defective high-level alarm in the raffinate splitter tower, and a defective sight glass used to indicate fluid levels at the base of the splitter tower. Also, the vital splitter tower-level transmitter had not been calibrated.
These and other examples highlight the importance of explosion protection planning. Every plan involves assessing the risk to be found in a given area, and that requires an understanding of the definitions for hazardous classes, zones, and groups.
The following categories define hazardous (also known as “classified”) areas:
Codes and standards in North America are NFPA 70: National Electrical Code (NEC) and CSA C22.1-: Canadian Electrical Code. In North America, there is an additional hazardous area, which is divided into Division 1 and Division 2. Throughout the rest of the world (e.g., ATEX), Division 1 is divided into two zones, with Zone 0 being more dangerous than Zone 1 (Figure 3).
Engineers designing electrical equipment and processes for use in hazardous areas have an abundance of explosion protection methods at their disposal. These methods are exclusion, containment, and energy-limiting technologies (Figure 4). Examples of exclusion methods are oil immersion, purge, and pressurization. Containment includes explosion-proof or flame-proof enclosures. Energy-limiting technologies include non-incendive and intrinsic safety. Each method has advantages and disadvantages.
Exclusion involves removing the fuel source from the ignition source. Examples of exclusion methods are encapsulation/oil immersion, purge, and pressurization. Encapsulation is an explosion protection concept whereby electrical equipment that could potentially cause an ignition is encapsulated within a compound or resin to prevent contact with an explosive atmosphere.
Oil filled is a type of explosion protection applied to an electrical apparatus so that all internal parts that are capable of igniting a flammable atmosphere are completely immersed in oil so they cannot come into contact with those atmospheres. Sand/powder/quartz filled is a method of explosion protection in which electrical equipment capable of ignition is in a sealed enclosure filled with quartz or glass powder particles. “However, the most prevalent exclusion method in North America is purge/pressurization,” said Hill.
Before an enclosure with equipment not rated for that area is energized, it must be purged. A purge involves subjecting the enclosure to five times its volume of inert gas like nitrogen or air to purge any hazardous gases out of it, then maintaining a slight positive pressure so hazardous gases cannot get in. Applications where dust is present can’t be purged, but protection comes from performing a meticulous cleaning, then applying positive pressure.
The three types of purge systems are X, Y and Z, according to Hill. A Type X purge system reduces the classification within the protected enclosures from Division 1 to non-hazardous. Generalpurpose equipment can be operated within the protected enclosure. A disadvantage of X purge is that if the positive pressure air supply is lost, there must be an automatic shutdown in place.
A Type Y purging system reduces the classification within the protected enclosure from Division 1 to Division 2. All equipment used within the enclosure must be Division-2-rated. A Type Z purge system reduces the classification within the protected enclosure from Division 2 to nonhazardous. General-purpose equipment can be operated within the protected enclosure. With Y and Z purges, if purging air pressure is lost, an audible alarm must be activated, but an automatic shutdown is not required.
The purge and pressurization method has advantages and disadvantages.
Advantages:
Disadvantages:
Containment allows the fuel source to reach the ignition source, but it contains any explosion, thereby preventing a catastrophic event. Explosion proof or flameproof technologies are the only methods of protection that don’t prevent explosions. They actually allow it. There are many applications for explosion proof containment, and materials are readily available. Many distributors seem to be always nearby with conduit, rigid conduit, and explosion proof enclosures.
According to Hill, gases are allowed inside explosion proof enclosures. A necessary defined gap is present in the flange (Figure 5). If and when gases penetrate the enclosure and there is an arc or spark, there will be an explosion as hot gases and pressure try to escape the enclosure.
“The gases will try to escape via the conduit run because it’s the biggest opening,” said Hill. Explosion proof seals are poured at the conduit (and other) openings, which prevent the hot gases and pressure from escaping through that path. For these reasons, explosion proof installations must be properly designed, installed, and maintained.
Although the meticulously designed gap has normal clearance during equipment operation, they are made to expand when there is an explosion. There are specific torque ratings for the enclosure bolts. If not closed correctly, the gap may become slightly wider, which will cause a problem.
The explosion proof method has advantages and disadvantages.
Advantages:
Disadvantages:
The “intrinsic safety” technique of energy-limiting explosion protection is universally accepted and applied worldwide as the preferred method of protection in potentially explosive atmospheres. The objectives of intrinsic safety are to limit current, limit voltage, and limit stored electrical energy.
Intrinsic safety is the safest form of explosion protection, the least expensive to implement, and the easiest method to deploy. It is the only method of explosion protection approved for Zone 0, the most hazardous area recognized by ATEX, IECEx, and NEC (Article 505). Zone 0 is considered to be “continuously” hazardous.
“Intrinsic safety is required to withstand two electrical faults and remain safe. It also is inherently safer for personnel, as its energy limiting principle typically only allows up to 30 volts or 100 mA to the hazardous area,” said Hill.
The two types of intrinsic safety devices are Zener barriers and galvanically isolated barriers. Zener barriers are relatively inexpensive passive devices. They don’t modify signals. They only limit the energy so no signal conditioning is required. The Zener diodes load the energy and shunt any overvoltage to ground, which means it is important to have an approved, intrinsically safe ground. Galvanically isolated barriers are active devices that also can perform signal conditioning. They are application specific and require no intrinsically safe ground.
Figure 6 shows a basic circuit design of a Zener diode barrier. The circuit includes an energy source, fuse, resistor, or Zener diode that shunts any overvoltage to the intrinsically safe ground.
Hill says an intrinsically safe ground must be separate from other grounding, and it must be a minimum of 12 AWG. It must be labeled as intrinsically safe, and it cannot exceed 1 Ohm. ANSI/ ISA recommended practice also recommends that there be redundant intrinsically safe grounds, which can simplify testing but also cuts the resistance in half. The resistance must be less than 1 Ohm.
Hill says a big advantage of intrinsic safety over explosion proof is that safe area wiring practices can be used. No rigid conduit and no cord seals are required. “When it comes to intrinsic safety, I.S. stands for intrinsic safety. But when it comes to wiring, it stands for identification and separation,” he said.
Intrinsically safe wiring must be identified. Light blue is the universal color for intrinsic safety. If that is not practical, wiring must be labeled as intrinsically safe every 25 feet. In addition to identification, the wiring must be kept separate from non-intrinsically safe wiring. They must be separated by 50 millimeters or 2 inches.
The intrinsic safety method has advantages and disadvantages.
Advantages:
Disadvantages:
What Is Explosion Proof?
The concept of explosion proof does not mean a product can withstand any external explosion. For a product, such as explosion proof junction boxes to be classified as explosion proof it would have to be able to contain any explosion originating within the housing due to any sparks from within and prevent igniting vapours, gases, dust or fibres in the air surrounding it.
For safety reasons, when equipment is based in hazardous areas, it is important to use high quality and certified explosion proof equipment as this would greatly reduce the risk of explosions and fire to personnel and industrial property.
There are two directives when dealing with ex-proof equipment .
Further explanation and details for these certificates can be found from another blog entry titled “What are ATEX and ICEX Certification?”
There are altogether six types of area classification guide to classify explosion-proof environment/equipment:
New Number (IEC)
General rules
EN--0
Oil immersed
Ex “o”
EN--6
Pressurization
Ex “p”
EN--2
Power filling
Ex “q”
EN--5
Flameproof enclosure
Ex “d”
EN--1
Increased safety
Ex “e”
EN--7
Intrinsic safety
Ex “i”
EN--11
Non sparking
Ex “n”
EN--15
Encapsulation
Ex “m”
EN--18
For more information, please visit explosion proof control panel.
IEC stands for International Electrotechnical Commission, a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic, and related technologies – collectively known as "electrotechnology".
Place of use
Group
Areas
Representative gas
Mines susceptible to firedamp
I
Gaseous Mines
Methane
Surface industries
II A
Surface
Propane
II B
Ethylene
II B + H2
Ehtylene, Hydrogen
II C
Hydrogen, Acetylene
Temperature class
T1
T2
T3
T4
T5
T6
Maximum Surface
450oC
300oC
200oC
135oC
100oC
85oC
What does “IP Rating” stand for and what does it mean?
IP Ratings is a short form for Ingress Protection (IP) mark, IEC standard . This system was started by the International Electrotechnical Commission or short form IEC.
IP Ratings are normally comprised of two digits that is commonly used for electrical items such as current day mobile phones or mechanical enclosures such as explosion proof enclosures. This rating would be able to tell people immediately the level of protection that the product has based on the table further below.
The minimum requirement for Explosion-Proof protection is IP54.
This standard has an emphasis on increased safety which in turn for products to be able to be granted this standard, they must be able to withstand excessive temperatures as well as sparks inside and outside of the enclosure.
Common uses of this standard are for products such as terminal boxes, control boxes and light fittings.
This standard focuses on resistance to flame within the enclosure. The enclosure needs to be able to safely contain any explosion and the accompanying pressure that develops and prevent any fire or sparks from escaping the enclosure and into the surrounding explosive environments.
Equipment that has this standard can be products such as control equipment or transformers.
Supermec mostly caters to Ex “d” and Ex “e” types of protection. The table below shows the advantages and disadvantages of the two:
You may read more about Ex “d” vs Ex “e” in another blog post here.
Summary
If you are looking for more details, kindly visit explosion proof lighting fixture.