Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1222953
22 | March - April 2020 | www . machinerylubrication.com As wet mist is applied to the equipment with an air-flow velocity of 24 feet per second, this same air must also be vented from the bearing housing to allow the mist to move onto and across the bearings to provide lubrication. e flow of oil mist into the bearing housing creates a slight positive pressure (above atmospheric pressure) inside the bearing housing, which prevents the intrusion of airborne contam- inants. e flow of oil mist supplies continuous lubrication to the bearings and passes onto a vent point. e excess oil mist must be contained and collected below the equipment for reuse or disposal. is method is an excellent means of lubrication for bearings with speeds of 500 to 3,600 revolutions per minute (rpm), and is preferred for bearings operating in the 10,000 to 15,000 rpm range where splash lubrication is ineffective. Oil mist is also considered best practice by the hydrocarbon processing industry and is recommended within the specifications of the American Petroleum Institute (API). Verifying That Oil Is Reaching Each Bearing e reclassifier, which controls the amount of lubricant being applied to each bearing, is sized using the same calculations for rotating equipment bearings. Reclassifiers can create back pressure throughout the mist feed lines. Plugging in the lines causes high pressure, while a broken line will result in low pressure. An oil mist generator monitors the pressure, using high- and low-pressure alarms, as well as the mist density for under- and overlubrication of the equipment. Installation of the piping system should be closely monitored for cleanliness. Once the system has been installed, it should be blown with air for eight hours for additional cleaning. After startup, only one thing can plug the system's reclassifiers — wax from paraffin-based oils. e waxing problem is well known to most oil mist users. Synthetic lubricants or high-grade, wax-free mineral oils are commonly used in these systems. Should waxing occur, all the reclassifiers can become plugged within an hour, causing numerous alarms to sound in the control room. With modern-day oil mist systems, plugging of reclassifiers is a rare occurrence when quality lubricants are used. Suitable Equipment for Oil Mist Oil mist is recommended for most types of rolling-element bearings operating between 500 to 15,000 rpm. e most common applications are pumps and motors. Approximately 30,000 pumps in the U.S. refining industry are lubricated with pure oil mist, along with an equal number of motors. Lubricants Used in Oil Mist Systems A diester-based oil is preferred for oil mist systems but is not required. All major lubricant manufacturers have mineral and synthetic oils that can work well in these systems. An ISO viscosity-grade 100 oil is recommended for most systems except where temperatures commonly fall below 32 degrees F. In cold-climate systems that operate at temperatures below 32 degrees F for long periods of time, most users drop down to an ISO viscosity-grade 46 or 68 oil. When selecting a lubricant, contact other oil mist users in the area to see which lubricants are being used and how they are performing. All oils are not equal. Most are blended for sump or splash lubrication. With oil mist, the oil is broken down into micron-sized particles, which may allow the additive package to fall out of suspension if the oil is not properly blended. Be careful with mineral oils that have a paraffin base. Too much paraffin can plug the reclassifiers in the system during hot summer or cold winter temperatures. erefore, always select paraffin-free oils and never use automotive oils in oil mist systems. Minimum/Maximum Operating Temperatures Oil mist does not have a high or low operating temperature. However, oil mist generators will have a low ambient temperature of minus 40 degrees. For hot climates, air coolers are included in the units to prevent heat damage to the electronic components. After the oil mist is generated, it becomes the same temperature as the header pipe. e outside temperature has no effect on the oil mist particles that are in suspension within the piping system. Pure oil mist is often used on process pumps that have tempera- tures of 600 to 750 degrees F. Rolling-element bearings seldom operate at temperatures higher than 200 degrees F. Keep in mind that oil mist does not support combustion and is well below the limits for flammability. It also is below the Occu- pational Safety and Health Administration's allowable limit of 5 milligrams per cubic meter of air for an 8-hour period. With a closed-loop system, the amount of oil in the air is far below the allowable limit. System Reliability Oil mist systems have no moving parts, so there is nothing to wear out. According to mechanical engineering expert Heinz Bloch, an oil mist generator is 99.99 percent reliable. e two requirements for the generation of oil mist are clean instrument air and clean oil. Electricity is not necessary for system operation. MAINTENANCE AND RELIABILITY Figure 2. The gas blower's original oil console. (Ref. Hydrocarbon Processing Magazine)