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Air Compressors

June 12, 2007 |

Air compressors are easy to ignore. Compared to your other equipment, they don’t cost a lot, they only do one thing and they’re so easy to operate that many contractors forget to look after them.

But a diligent equipment manager or owner should take care to treat these machines well. Otherwise the two primary and most expensive components, the engine and the air end, can fail prematurely. These are designed to last the full lifecycle of a compressor, but replacing them can put a painful dent in your cost of ownership, not to mention stop whatever work you had going until you can find a rental unit or new machine.

To find out more about what an air compressor maintenance best practices regimen would look like we talked to Don Holston, field service manager for Ingersoll-Rand compressors, and Butch Goodwin, rental shop supervisor for Cowin Machinery in Birmingham, Alabama. We used Ingersoll-Rand’s most popular compressor, the P185WJD, as a point of discussion, but most of the advice they gave us applies to diesel-powered compressors of all sizes.

Neglect is easy
Compared to the big equipment on a construction site an air compressor can seem insignificant. “They tend to get run over, beat up, scratched and sandblasted,” Holston says. “All of that has an effect. After a few years the machine probably looks terrible, even though the engine is far from worn out.”

Holston estimates 185-cfm and smaller machines see something less than 500 hours of use a year. The bigger machines can see 600 hours or more. A typical lifecycle – the point at which the engine or air end needs rebuilding – can stretch out to 10 years or more.

Oddly enough it’s the machines that don’t log many hours in a year that have a greater risk of a shortened lifecycle. This is due in part to neglect, but also to the fact that infrequent use is hard on a machine.

“With the machine going through heat up and cool down cycles you get moisture condensing in the compressor oil system,” Holston says. “Things like your separator element will develop condensate and promote bacteria growth in the element.” Left to sit for days or weeks with no activity, diesel fuel tanks also develop condensation and contaminants.

Machines that sit idle tend to get limited attention and field expedient maintenance. “The worst thing contractors do is put bad fuel or used oil in them,” Goodwin says. “They’ll grab whatever fuel is handy. Or if the low oil indicator light comes on, some contractors will just drain the crankcase of another machine and top off the compressor with whatever’s quickest.”

Dirty business
This is flirting with disaster. “Water and dirt in the fuel are very bad on injection pumps,” Goodwin says. “That means you have to replace your filters more frequently. If you don’t, the injection pumps can be very expensive to replace. Just pulling and calibrating an injection pump is expensive.”

Likewise water and dirt in the oil is hard on the engine bearings and crankshaft. Dirt that gets into the lubrication system of the air end is equally bad. “The screws that compress the air operate under very tight tolerances,” Goodwin says. “When you get dirt in there, it acts like a grinding compound wearing out the surfaces.”

Defensive maintenance
Other critical areas for dirt and contamination control are the air filters for the engine and the air end. On its rental units, Cowin Machinery puts a seal and a decal on both filters with instructions not to break the seal. “Every time you open up an air filter you have a tendency to knock dirt into the engine,” Goodwin says. “And then you knock the filter element against something to try and dislodge the dirt. This damages the edge of the filter and makes it harder to seal.”

Another precaution Cowin takes is to add a safety element inside the air filters of units that don’t come with one. “It’s just a small safety air filter inside the bigger air filter,” Goodwin says. If something punches a hole in the big air filter the safety element prevents the engine from inhaling a fatal dose of dust. Many of the larger compressors come with a safety element installed. But given the abuse and neglect rental machines get, Cowin outfits all its compressors with them.

Operator error
Carelessness is one of the biggest problems with compressors. Holston cites the third wheel on a jack as an example. “You see a lot of people who don’t jack those up high enough for travel purposes,” he says. “Then they hit a pothole and tear them off.” Holston says it’s not uncommon for contractors to lose a third wheel every year or so.

Hasty or improper shut-down procedures can also accelerate wear on an engine. “You’re supposed to close the service valve, which causes the machine to idle back, and let it run three to five minutes to cool down and then turn the switch and walk away. Then the machine will automatically relieve all the air pressure in it,” Holston says. “What we find is that a certain number of operators shut it down, turn the switch off and then open the service valve, which causes it to blow down instantaneously. When they do that it causes the oil to foam and come up into the element.”

Sudden shut downs can also degrade turbochargers, Holston says. “If they don’t idle it for a few minutes and let it go through a cool down, you lose oil pressure,” he says. “But the turbo is still running at full speed, so you have a tendency to carbonize the oil in the turbo, causing it to lose its bearings.” Water-cooled turbos are less prone to this than air-cooled versions, but neglecting the cool-down period could require you to replace at least one, and possibly two, turbos over the life of the compressor.

Variable maintenance intervals
Another aspect of compressor maintenance that trips up less diligent contractors is the fact that routine maintenance schedules are different for the engine and the air end. “You change the engine oil and filter at a different time than you change the air end oil and filter,” Holston says. As a result one of the two changes tends to get ignored or forgotten, usually the first. “On the newer machines we are getting closer to having the same maintenance intervals,” he says. “It’s a lot easier when you get in and can do it all.”

With Cowin’s rental fleet compressors, this problem is attacked several ways. Using a 500-hour oil usually ensures the machine will come back to the shop in time for its oil changes. The company also sticks to a strict maintenance schedule, and when in doubt, filters and fluids are changed sooner than scheduled, rather than later, Goodwin says. Another precautionary practice Cowin uses is to put a maintenance decal with information and scheduled service dates on all its compressors.

Applications and air demand
The uses an air compressor are put to don’t exert a wide range of demands on the engine. The engines are designed for continuous duty and spend most of their time operating at high idle, says Ingersoll-Rand’s Don Holston.

Even if you select a compressor that’s too small for the job, the end result won’t be so much accelerated engine wear as not enough pressure on the tool end. A compressor engine works hardest when the you have a constant air demand that equals the compressor’s rating, Holston says. Sandblasting, with its constant, high volume/high pressure demand for air, is probably the most demanding application. Crack cleaning with open air nozzles and concrete sawing are also high-stress applications.

A lot of cycling where the engine ramps up to meet demand and then idles down quickly can also be hard on the rubber diaphragm inside the inlet loader. Eventually the diaphragm develops a hole or tear. Instead of idling back it will continue to build up pressure and pop the safety valve rather than close the intake to the compressor. Diaphragms may need to be replaced every two to three years if the compressor is going through a lot of cycling, Holston says.

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