Protecting Hazardous Locations from Explosion

Summary

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.

The need for explosion protection

Piper Alpha was an offshore production facility operated by Occidental Petroleum Limited that exploded and sank in July 1988 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.

Understanding class, zones and groups

The following categories define hazardous (also known as “classified”) areas:

• Class: The type of hazard present. For example, in the United States, Class I denotes gases and vapors; Class II denotes dust.
• Division or Zone: The likelihood that the potential for a fire or explosion exists. Division 1 and Zone 0 or 1 are more dangerous than Division 2 and Zone 2.
• Group: The specific “type” of medium present creating the hazard.

Explosion protection methods

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

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:

• The only practical solution for some applications
• Can protect large volumes, panels and even entire control rooms
• Systems and expertise are readily available
• General purpose equipment can be used in an area where they normally could not (X and Y purge)

Disadvantages:

• Clean air or a protective gas can be expensive and requires regular maintenance
• May take up a lot of real estate or be too large for some applications
• Loss of pressurization could shut down production
• Safety concerns for personnel if nitrogen is used
• Class 1 Div. 2 equipment still required for Y purge

Containment

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:

• Only method available for some applications
• Expertise must be readily available
• Can protect almost any type of device
• Readily available in many sizes and types

Disadvantages:

• Must be meticulously maintained
• Internal electronics not easily available
• Cannot work on “hot”
• Does not prevent explosions

Energy-limiting technologies

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.

• Safest method of explosion protection
• Least expensive to install and maintain
• Only method that you can work on “hot’’
• Takes up less space than other methods
• Only method approved for Zone 0

Disadvantages:

• Not viable for some high-powered applications
• Requires approved field devices in many cases

Final thoughts

While explosion protection methods include exclusion and containment, energy-limiting technologies, especially intrinsic safety, are the most widely used. Intrinsic safety offers the safest, most cost effective, and easiest way to deploy solutions that safeguard process operations.

_{This feature originally appeared in InTech Focus: Control Systems 2022, the InTech Focus ebook for September 2022.}