Compressed Air Best Practices - September 2008 - (Page 50) | 09/08 Compresse Air Industry FOOD PACKAGING & PROCESSING pressed | BY DOCTOR VACUUM It is a fact that oil-sealed vacuum pumps are prone to oil carryover. Residual oil makes its way through internal separation systems and is found at the discharge of these types of pumps. If oil accumulates in great enough quantities, it becomes an environmental issue or safety hazard. It is therefore the responsibility of vacuum pump owners to maintain clean work areas. However, this is not always easy to do given the dynamics of how vacuum pumps operate within everchanging vacuum systems. In addition, some pump designs do an excellent job of attenuation while others allow for significant contamination. Oil carryover is easy to spot. If a vacuum pump is exhausting directly into the local area, there will be an oily film on the vacuum pump itself, on interior walls and on close proximity equipment. Some vacuum pumps will actually “smoke” oil from the exhaust port. This is an extreme condition that was more common historically. New separation designs have reduced the number of these occurrences but they are still to be found in some applications. Where there is a discharge pipe venting vacuum pump exhaust to an outside wall or ceiling, there will be oil dripping down the exterior wall or pooling on the roof. Oil can also be leaking from pipe joints and dropping onto the floor or back onto the vacuum pump. The mechanics of why oil is present at the exhaust of a vacuum pump is not difficult to understand. Most separation systems can effectively remove discharge oil contamination down to 1 ppm to 4 ppm (parts per million). This may seem to be an insignificant amount, but consider what happens over time. A vacuum pump that is running 24/7 will be discharging oil continuously in small amounts. Eventually, the buildup becomes a problem. A good example of this is a 40-horsepower (hp) vacuum pump operating continuously at 20” HgV. At a carryover rate of 3 ppm there will be approximately one quart of oil per month that makes its way past the separation system. That is more than three gallons of oil per year that will end up somewhere — on the roof, on the walls, out to the environment or on top of production equipment. Some separation systems are so loose that carryover is measured in pints per day. Solving this issue is not always easy and other factors must be considered. The type of oil used will have an effect on the carryover rate. Some oils vaporize more easily than others and if oil particles are small enough, they will travel right through most separation media with the exhaust air. The discharge temperature of the vacuum pump also plays a critical role in many applications. With some oils, a higher discharge temperature will increase the carryover rate. If discharge temperature is lowered, separation will be better but there is also the risk of water (or other 50 www.airbestpractices.com http://www.airbestpractices.com
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