Fundamentals of explosion-proof motors and pumpsKevin J. Harrigan | January 12, 2021
Motors and pumps for hazardous, potentially explosive locations have a different set of selection criteria than for non-explosive use cases. These types of environments are common in oil and gas, chemical processing, refinery, mining and milling environments. These are locations where combustible gas, vapors or airborne particulate may surround or even ingress into the motor housing. Since many pumps are driven by electric motors, the same considerations apply for pumps in these types of environments.
Generally, motors create an explosion in these environments by either a malfunction, such as an arc from winding insulation or power supply damage, or by generating enough heat during operation that it reaches the auto-ignition temperature for the combustible material present.
These types of applications require an “explosion proof” motor construction. Explosion proof motors have specific design requirements that will prevent an internal malfunction or explosion from cascading into a larger blast.
A strengthened, sealed enclosure is typical for explosive proof electric motors. This can prevent the ingress of most combustible particulate matter, such as coal dust or wood shavings, as well as varying amounts of liquid. However it can be difficult to isolate the internal components of the motor – where the windings and electrical connections reside. In case these motors do explode, the enclosures include flame paths that will dampen flames and temperatures before reaching the ambient atmosphere.
Another important consideration is the surface temperature of the enclosure during normal operation cycles. These surfaces must remain well below the ignition temperature of the ambient combustible. During operation, the motor windings are the hottest component, regularly exceeding more than 100° C versus other motor components. This temperature increase is known as temperature rise.
Manufacturers integrate types of magnet wire, ground insulation, varnish, lead wire insulation, tapes, strings and sleeves to compensate for temperature rise, which is assigned a NEMA temperature class rating, which states a permissible maximum operating temperature. The standard assumes a 40° C normal operation temperature, and supplies 15 °C of tolerance as well as a ‘hot spot’ allowance. Altogether, this ensures components used meet a minimum duty cycle, which helps streamline selection for combustible environments.
|NEMA temperature class||Maximum operating temperature|
In Europe, CE-ATEX regulates the manufacture and use of low-voltage AC motors in hazardous environments. Once certified by a certified third-party lab, applicable motors will be marked “CE Ex” to note the motor’s compliance.
ATEX classifies application environments by equipment groups and myriad subcategories. A product code will include all groupings to which the motor belongs.
The majority of motors will apply to Group II.
· Group I: strictly mining and subsurface applications.
· Group II: all surface applications.
· Category 1: combustible materials are continuously present during normal operation
· Category 2: combustible materials have a high likelihood of occuring
· Category 3: combustible materials are unlikely, but possible, to be present during normal operation
· Zone 0: continuous presence of combustible gas
· Zone 1: highly likely presence of combustible gas
· Zone 2: unlikely but possible presence of combustible gas
· Zone 20: continuous presence of combustible dust
· Zone 21: highly likely presence of combustible dust
· Zone 22: unlikely but possible present presence of combustible dust
· D: suitable for ignitable dust particulate
· G: suitable for gaseous combustibles
· A: low flammability gas
· B: medium flammability gas
· C: high flammability gas
· d: flameproof
· eb: increased safety
· ec: non-sparking
· t: enclosure
· eb: increase
· Ia: intrinsically safe
· p: pressurized
· q: quartz filled
· m: encapsulation
In the U.S., explosion-proof motors must be reviewed by UL, an OSHA-approved independent testing lab. Testing criterion is outlined by UL 674: Electric Motors and Generators for Use in Hazardous Locations. Its certification and coding structure are similar to CE-ATEX, and will typically be affixed to the motor’s identification plate.
UL certifies motors to different classes and divisions, suitable for specific applications and material exposure.
Class I locations have combustible vapor or gases present. The motor design presumes the vapor or gas will enter the motor enclosure, so its enclosure is designed to contain a motor explosion from setting off a larger blast. Class I motors have the strictest design requirements, including flame paths, an enclosure that withstands an internal explosion and a surface temperature that stays below the auto-ignition temperature of the gas.
These motors are divided into Division 1 and Division 2 applications. The former assumes the vapor or gas is continuously present; the latter is for locations where an explosive material would only present due to a separate equipment malfunction, such as a pipeline leak.
The materials themselves are further classified by explosive pressure and ignition temperature.
· Group C: acetaldehyde, cyclopropane, diethyl ether, ethylene, isoprene
· Group D: acetone, ammonia, benzene, butane, ethane, ethanol, gasoline, methane, propane, styrene
Class II locations contain a combustible dust that will pervade via light air currents. When compared to Class I, engineering a motor to exclude dust and debris is easier than excluding gas or vapor. The goal of this motor enclosure is to prevent the combustible from ever reaching an ignition source in the motor, and also keep surfaces below the auto-ignition temperature.
As with Class I, this category is further divided into divisions and classes. Division 1 locations contain combustible dust under normal conditions; Division 2 locations will not contain the dust unless an error or malfunction has introduced the dust to the environment.
· Group F: coal
· Group G: corn, nylon, polyethylene, sugar, wheat, wheat flour
These locations have the presence of combustible fibers, such as textile strands or wood pulp, typically as a manufacturing byproduct. Class III location materials typically settle quicker than the gaseous and fine particulate materials of Classes I and II, but are still a safety risk if left to accumulate on motor surfaces.
The fibers of a Division 1 location are used in manufacturing, or are the result of a subtractive process. Division 2 locations have these materials on-site for any reason other than manufacturing.
Explosion proof motors and pumps protect workers, assets and productivity in any industrial process where combustible materials are present. Electric motors are ubiquitous in their abilities, necessitating design adaptations to ensure they are safe to use in these types of applications.