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Red for Danger


The throaty growl emanates from the back rooms of service stations and machine shops, from factory rooftops, refineries and process plants, from cargo ships at berth and airplanes on the ground. You hear the murmur in dentists offices, at auto body shops and around construction sites. Air compressors are so ubiquitious as to be almost invisible, but they rumble assertively to life at the flip of a switch, ready to supply vast quantities of air for chilling, drying, blowing and hammering.

By some estimates, up to 40 percent of a manufacturing plants electrical cost can go to its air compressors. Thats a good reason to pay attention to lubrication and compressor efficiency. But a more compelling reason is safety. Whether an air compressor is used to fill tires, charge oxygen tanks, drive tools, aerate water treatment plants or support blast furnaces, it holds everything you need for a fiery accident: oxygen, an ignition source (the heat of compression), and a source of fuel – the lubricant used to oil and cool the unit.

The majority of these beasts work via positive displacement, forcing air into a confined chamber using a reciprocating piston, rotary vane or a rotary screw to compress it, and then releasing the air in directed bursts. When it comes to lubrication, there are two main designs for compressors: those that use lubricants in the compression chamber to improve component life, and those that exclude it in order to keep the air oil-free.

The outlet air from an oil-injected compressor will contain some the lubricant in the form of vapor or mist, albeit in very tiny amounts. Manufacturer Atlas Copco notes that modern lubricated piston and screw compressors use oil very frugally. The air may contain as little as 3 milligrams of lube per cubic meter, and multi-stage filters can reduce this further if needed.

In an oil-free compressor, the air chamber is fully sealed off from the lubricant to ensure an air supply that is free of any oil content. This high-quality air may be required for food and medical processes, for chemical plants and other applications. However, the oil-free units still require lubrication and cooling for other moving parts, and in case of a damaged or worn seal, some lubricant can be drawn into the compression chamber.

In either type of air compressor, when lubrication is neglected, safety experts say it can result in a sudden and lethal explosion. That was the case in 2005, when a supervisor in a California car wash died after the air tank he had just switched on exploded with great force. Testers later found the compressor had been oiled with the wrong type and viscosity of lubricant, resulting in an oily residue and carbon deposits in the supply line and combustion products in the air tank. Under the heat of compression, this carbon buildup probably ignited the oil vapor in the tank and resulted in the explosion, investigators said.

Another example was seen aboard the containership CanMar Spirit. As she was preparing to dock at the port of Montreal in 1999, an air compressor explosion occurred which sent steel shards flying in all directions. The vessels duty greaser, who had just engaged the compressor, was struck and later died. Investigators again blamed poor maintenance practices; among other problems they found the vessels pressure-relief valves and non-return valves were clogged with an oily, sooty residue which prevented them from operating.

According to Kevin McKenna, product technical advisor with ExxonMobil Lubricants & Specialties, the list of fires, explosions and sudden detonations in air compressors dates back at least a century, yet this sad history continues to repeat itself. Most realize that in case of a fire or explosion, the lubricants present can feed the flames. But under some conditions the lubricant itself contributes to the detonation, he warned.

There are a number of mechanisms that cause concern in compressed air systems, McKenna told the Society of Tribologists and Lubrication Engineers annual meeting in Las Vegas earlier this year. One of the first papers describing an air compressor explosion involving lubricants was published in 1911, the Bixby, Okla., industrial lube expert noted.

Then in 1925, the publication Safety Engineering highlighted the danger of explosion, he said, and in 1934, following a methane gas explosion in the mining industry, Germany enacted the first safety regulation covering air compressors. Recognizing that heated and compressed air were a danger in conjunction with a ready fuel source (like the devices lubricant), German regulators limited the air outlet temperature from compressors to 160 degrees C (320 F). That limit was further lowered in 1963 to 140 C (284 F), following another compressed air explosion in Germany, a powerful blast which killed 19.

McKenna pointed to other incidents, including the Reed Pipeline explosion in 1940, which caused severe property damage and destroyed a 40-mile pipeline. Another air compressor, at a urea plant in 1970, detonated with such force that an intake valve weighing almost 10 pounds was flung 400 yards away.

One of the first steps in understanding these incidents is to become familiar with the terminology, McKenna said, including auto-ignition temperature, fire point, combustion, explosion and detonation.

When it comes to air compressor concerns, fire, explosion and detonation are at the top of the list, he stated, and contrary to what many believe, detonation doesnt need an ignition source. Its so quick and fast that its hard to contain.

Of the two, detonation is more forceful than explosion, and can travel at speeds faster than sound. Detonation force can reach 1,000 to 5,000 times the initial pressure, McKenna noted. So, where can you put a relief valve, to help control that? This kind of incident can create major, shrapnel-type damage.

Compressed air tends to expand at very high velocity when suddenly decompressed, amplifying the outward force. Another reason these eruptions can be so destructive is that the force of detonation doesnt like to take turns, McKenna pointed out. If a detonation begins inside a piece of equipment or piping, and reaches a T or elbow turn, the direction of the force tends to go straight – blasting through the obstruction and out into the workspace like a cannon shot.

Besides reducing efficiency, deposits and residue can limit lubricant flow and lead to overheating. In an oil-flooded rotary screw air compressor with proper lubrication, if the lubricant is flowing correctly you get cooling. But without cooling, what happens? The compressor efficiency begins to drop, and the air discharge temperature quickly climbs, McKenna said. He pointed to one case where air compressor efficiency dropped under poor lubrication conditions to 37 percent or less, and the air discharge temperature went from 68 F at the start to 1,364 F at discharge.

Do your regular maintenance, he urged. Choosing the right lubricant will help reduce concerns as well. For example, the National Petrochemical & Refiners Association recommends that mineral oil compressor lubricants used in the process and refining industry be limited to operations below 150 degrees C (300 F), which in normal circumstances may be fine. But what about excursions into higher temperature ranges? Synthetic lubricants, with a higher auto-ignition temperature, can safeguard against unexpected highs, McKenna declared.

Other reasons to use synthetics, he said, include their higher oxidation resistance and deposit control, which is especially important in reciprocating compressors. Most OEMs, especially in the United States, recommend synthetic-base types of lubricants for their higher temperature capabilities, McKenna said. But he also acknowledged that some operators of oil-flooded rotary screw types dont follow that guidance, due to cost.

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