Oilless pneumatic systems have been in the norm for some time because they don’t require lubricators, oil, and periodic maintenance. But the fact remains that the majority of pneumatic systems are lubricated.

That’s because many users deem the total savings from lubricated air exceed the cost of installing and maintaining the lubricators. Lubrication helps reduce friction between sliding surfaces, and that improves efficiency and permits higher operating speeds. It also reduces wear, which ultimately means longer component life and less maintenance. Moreover, lubrication in pneumatic systems can reduce both internal and external leakage around valve spools, cylinder rods and pistons, and air motor and rotary actuator vanes, rotors, and housings.

That said, a major drawback to lubricated systems is they discharge oil mist when the air exhausts. And that can bring the unwanted attention of regulatory authorities.

Discharge regulations

The Occupational Safety and Health Administration (OSHA) Air Contaminants Standard 1910.1000 mandates employees’ exposure limits to toxic and hazardous substances. For oil mist, compressed air exhaust may not contain more than 5 mg/m3 (about 4.3 ppm) on a time-weighted average basis. The standard also details how to calculate total risk when a worker’s exposure varies throughout the day.

Likewise, the National Institute for Occupational Safety and Health (NIOSH) recommends short-term exposure limits of no more than 10 mg/m3, defined as a 15-minute exposure that should not be exceeded at any time during the workday.

In addition to oil-mist limits, regulations govern noise levels of compressed air exhaust. OSHA Standard 1910.95 on occupational noise, in short, limits a worker’s average noise exposure in an eight-hour day to 90 dB, and less than 15 minutes at 115 dBA.

Obviously, most every manufacturer strives to keep its plant safe for workers. Failing to comply with the standards can result in considerable costs and headaches for a company. In addition to potential fines and penalties, operating outside the recommended limits can bring requirements for respirators and hearing protection for employees, ongoing monitoring of noise levels and air quality, annual company-paid medical check-ups, and onerous paperwork and record keeping. And that’s not to mention the unpleasant prospect of a government inspector lurking behind your back.

Two-in-one solution

In a nutshell, routing noisy and oily compressed air into plant can be a real problem. Fortunately, there’s a simple solution to the problem — air exhaust mufflers that reduce noise  and also limit emissions of oil and dirt. These products go by a number of names, including coalescing mufflers, coalescing silencers, filter silencers, and reclassifiers.

Regardless of name, manufacturers state that these products easily bring pneumatic systems into compliance with government regulations. Internal geometry of the devices reduces air velocity and baffles for audio damping take care of noise; filtration takes care of the oil. But just any filter-muffler may not suffice. A standard filter-muffler has a porous element to trap solids entrained in the compressed air stream, but it is not designed to trap oil vapors.

A coalescing muffler, on the other hand, operates on the same principles as a coalescing filter. As air flows through the coalescing element, oil particles are captured by three different mechanisms: direct interception, inertial impaction, and diffusion. In direct interception, oil particles simply collide with and are trapped by filter fibers. With inertial impaction, the element’s turbulent air stream throws oil particles against fibers, which trap the oil. Diffusion causes the smallest particles to vibrate and collide with each other - and eventually the element’s fibers - which trap the oil.

However, because these devices are on the exhaust side of a pneumatic system they are exposed to sudden shock loading. This means their oilremoval capabilities may not be as consistently good as those of a pure coalescing filter.

Nonetheless, a good coalescing muffler typically gives 2 ppm or less oil discharge in normal operation. (Without coalescing mufflers, contamination may exceed 50 ppm in a plant that is tightly closed for operation in cold weather.) Most also have a port for draining collected oil, and elements that can be changed without having to replace the entire muffler.

Whatever the choice, mufflers that remove oil from the exhaust air should be considered - especially if air exhausts near workers. It is not uncommon for coalescing mufflers to remove 99.97% of the entrained oil. Even if oil-free compressors are used, exhaust mufflers can reduce noise to OSHA-acceptable levels and ensure that exhausted air is clean.