Hazardous Area Electrical Equipment
—photo courtesy of Crouse-Hinds Co.
“Is this area really dangerous? How likely is an explosion? Will explosionproof electrical equipment eliminate the danger? If so, what’s available and how is it built? What are its limitations?
These are questions faced by plant designers planning facilities or processes. Fire officials must face them, also, in judging the hazards they might encounter.
Area classification methods aren’t within our scope here. But we should realize that just because some dangerous chemical is used in a building or process, that doesn’t render an area hazardous. Such standards as NFPA 497, “Electrical Installations in Chemical Plants,” or HP-500 of the American Petroleum Institute may define some parts of a plant as nonhazardous, others as only occasionally risky.
Judgment on local level
The National Electrical Code (NEC) defines the degree of area hazard in general terms, subject to rulings by local code-enforcing authorities. Those people may be building inspectors, fire marshals, or others, each with their own views of acceptable risk. It is up to them to judge whether danger is always present in normal service (division 1 locations), or only infrequently in case of “accident” or “abnormal operation,” which is division 2. (As NFPA 497 points out, it’s incorrect to describe division 2 areas as “semi-hazardous.” No matter how unlikely, any explosion in a division 2 area could be just as destructive as one in a division 1 area.)
The fact that the explosion risk varies in different surroundings has led to several basic approaches in the design and use of electrical equipment. First of all, we distinguish between explosion-proof devices for class I service, involving explosive gases or vapors, and dust-ignition-proof equipment for class II service, involving explosive dusts.
The former type is designed not to prevent explosions, but to isolate them. Electrical arcs—either due to insulation failure, as in most motors, or to normal operation of switches—are expected to occur inside the casing of the device. That casing isn’t gas-tight; it can’t be. So explosions can take place inside.
Casing design objectives
Casing design aims for three results:
- Enough strength to withstand the internal blast without rupture. Such breakage would let flame escape to ignite the surrounding atmosphere.
- Joint clearances, or flame paths, long and narrow enough so that burning gas expelled through joints by the explosion will cool below its ignition temperature before reaching the exterior. Threaded joints (figure 1) are widely used. These form an ideal labyrinth path. Easy and economical to make, they are also easy to inspect for adequate engagement. Precision-ground flat surfaces, bolted together, are also common (figure 2). Gaskets aren’t permissible because they could be ignited or blow-n out in an explosion.
- Low surface temperature so outside gas or dust will not be heated to ignition. If the equipment generates internal heat in normal use, as does a motor, transformer or lamp, and could become dangerously hot through overload, it must have thermal protectors to disconnect it before that can happen.
Although many electrical devices are built for both class I and II use, the dust-ignition-proof capability requires some features not needed in a gas atmosphere. For example, even with a heavy coating, or blanket, of dust on the casing surface, hindering escape of internal heat generated by operation of the equipment , the casing must stay cool enough for safety. Class II motors are the prime example of this. (Class III devices, for use in textile mills where the air is filled with flammable fibers, are similarly designed.)
Maintenance of bearings
Bearings, used not only in electric motors but in the driven machinery, can also be a source of heat. Modern ball and roller bearings give long trouble-free life if properly maintained. However, frequent regreasing isn’t necessarily good maintenance. Dirt or moisture may be forced in along with the new grease. Bearing failure may then occur quite rapidly. Therefore, unlike class I motors, those for class II use need special dust seals.
Many dusts are highly absorbent. They harden up a grease quickly. As the bearing then starts to run hot, grease deterioration accelerates. In a grainhandling facility, the results are often catastrophic.
For example, dust-ignition-proof electrical equipment did not forestall the Halston-Purina disaster in St. Louis January 10, 1962. A fan drive bearing failed, overheating the shaft until parts disintegrated, striking sparks w’hich ignited surrounding dust. The ensuing conflagration killed or injured 43 persons, and did $3 million damage to 12 buildings. Better maintenance—plus a vibration alarm on the machine bearings—could have prevented that.
Last March, the chief engineer of a leading grain processing equipment firm testified before a Congressional committee that “I would judge the major ignition source for dust explosions in grain elevators to be improperly designed bearings.” Speakers at the first international symposium on grain dust explosions in 1977 agreed that overheated elevator bearings are a “major threat.” United States attention was drawn to this problem late that year by four elevator blasts which killed 55 persons.
Investigate maintenance
So the inspector judging any hazardous area—especially class II—should keep a close watch on maintenance quality. Look at electrical equipment maintenance records. Talk to maintenance workers. Be familiar with the provisions of NFPA 70B, “Electrical Equipment Maintenance,” on this subject, and work for its inclusion in local codes. The best electrical equipment design can be rendered dangerous by neglect.
How are the designs made? The principles were laid down in the 1920s. (The first explosion-proof motor came on the market about 50 years ago.) Their confirmation and current testing practices based on them is in the hands of Underwriters Laboratories.
Despite its name, UL has no relation to any insurance organization. No connection has existed since 1917.
Division label only
Although the NEC nowhere says that devices for safe use in hazardous locations must be UL-labeled, that label is universally accepted as approval for such service in division 1 only. Division 2 equipment is not labeled. The obvious danger of contact arcing in division 2 is dealt with by the NEC requirement that such contacts (if present) must be safely enclosed. But again, no label is applied. The user must satisfy himself some other way that the full intent of NEC article 500 has been met.
Years of UL testing have generated design methods to support the construction suitable for division 1, as in figure 2 for example. Similarly, number and strength of assembly bolts is based on known explosion pressures. So is required strength of boxes, covers, or housings.
For any new product, UL must approve the design calculations. If there is any question, a product sample is tested. Once UL approves and production starts, UL inspectors spot-check the factory to make sure the equipment is being properly built. Motors, which may need repairs often during their lifetime, must be overhauled at ULcertified shops for such repair work.
Is this foolproof? No, but the experience record is excellent. The main difficulties, besides maintenance, are:
Photos courtesy of Crouse-Hinds Co.
- Introduction of new hazards for which the design rules haven’t been tried. Nearly 9000 different hazardous substances are listed in NFPA 325M, “Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids.” Those who must select plant electrical equipment may not always find in that information guide the substance they are working with. Chemical technology constantly produces new materials. Moreover, these are often found in complex mixtures rather than singly.
- Well-intentioned ignorance of known hazards. Inspectors will encounter people who have “never done it that way, and never had any trouble.” For example, in 1977 a designer complained to a leading electrical trade magazine that he had never called for explosion-proof motors in construct ing fertilizer plants which “blend potash, ammonium nitrate, and other inert materials,” because “the dusts produced by these operations are not of an explosive or combustible nature. But a local electrical inspector recently stated that the equipment in such plants should be class II, group G-rated . . . We do not agree with that opinion.”
Any finely powdered organic material is a potential bomb until proven otherwise. A New York City chewing gum factory was blown apart in November 1976 bv the explosion of magnesium stearate dust used as a non-stick coating on the gum. Six workers died. This material, never considered dangerous, was not listed as such in the handbooks.
Ammonium nitrate “inert,” or “not of an explosive or combustible nature?” Explosion of that chemical pulverized Texas City in 1947, killing 468 persons and causing $50 million damage. The NFPA has pointed out that “it is especially dangerous to permit contamination of ammonium nitrate with oil,” an easy result if it’s used around motors lacking proper bearing seals.
Alternative designs
There are other ways to design equipment for explosive atmospheres. One approach recognizes that some electrical faults don’t release enough energy to ignite vapors which may be present. This has led to the intrinsic safety class of equipment, in which wiring need not be isolated inside heavy sealed cases because arcs—if they occur—don’t produce enough heat to touch off an explosion. Intrinsic safety apparatus normally is available only for low voltage control of metering circuits.
Another alternative, common in hydrogen atmospheres, is purging—excluding explosive gas from the equipment interior by a steady flow of clean air or inert gas, such as nitrogen, piped in from a remote source. Purged equipment doesn’t need to withstand blast pressure because the internal atmosphere never becomes explosive. Controls must ensure that the equipment can’t be energized if the purging system isn’t working. Loss of purge pressure has to cause shutdwn or at least sound an alarm. Details of such systems, which may be complex, are covered in NFPA 496.
Maintenance is even more important here than on UL-labeled apparatus. Making sure all is in working order involves much more than simple inspection of a motor or a nameplate. Complete checkout is an engineering job.
From this brief summary, it is plain that judging the effectiveness of electrical equipment application to an explosive environment is no simple task, but one calling for “fire engineering” as never before.
