Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/147079
Oil-Mist Systems A typical dual-line metering valve comes with a piston pin indicator. Its movement can be visually checked through a transparent cover cap or monitored with a proximity switch. (courtesy Lincoln Industrial) the flow from the valve. Unlike the progressive divider valve, the dual-line dispensing valves can be arranged in parallel so that they are independent of each other. Nevertheless, the indicator pin in dual-line dispensing valves does not cover the feed line. Single-Line Parallel Systems A single-line parallel system independently dispenses lubricant through injectors. Lube point blockage or a faulty injector would not affect the system as a whole. One or several blocked lube points would not be detected easily at the system level. Most single-line injectors are also fitted with an indicator pin, but again this pin does not provide confirmation of lubricant flow at the lube point. While it is possible to use a proximity switch to monitor the functioning of an injector, this is still one step away from flow confirmation at the lube point. Oil-mist lubrication systems are utilized extensively in large rotating equipment where temperature fluctuation is relatively small. Oil mist is generated through a venturi or vortex with passing compressed air and is carried through pipes to classifier fittings at lube points where it is condensed into oil droplets for lubricating bearings. Since an oil-mist system has pressure in the pipeline and the oil-mist flow is relatively difficult to qualify, monitoring this type of system is challenging. Air/Oil Lubrication Systems A centralized oil-circulation system (courtesy SKF) A more recent development in high-speed bearing lubrication is the air/oil system. In this type of system, liquid oil is injected from a positive-displacement pump directly into the air stream at a specific time interval. The compressed air stream then propels oil as droplets through the feed line to lubricate the bearing. Flow confirmation sensors have been developed for this type of air/oil flow. One such device is called an oil-streak sensor, which can be used to monitor the continuity of the oil flow in air/oil systems. Lubricant Flow Confirmation An example of a single-line parallel lubrication system (courtesy Lincoln Industrial) Oil-Circulation Systems When applications require extracting heat from bearings while providing lubrication, an oil-circulation system generally is chosen. In these systems, a large volume or high flow of oil is pumped through the system. Excess oil is directed to a returning reservoir and filtered before re-entering the lubrication system. If necessary, the system can be expanded with progressive divider valves or fitted with flow restriction to properly distribute oil flow. Most oil-circulation systems are equipped with a flow meter for flow confirmation. The flow may also be monitored with an optical laser flow transducer over a transparent tube section. Although many centralized lubrication systems have a built-in indicator to verify that the lubricant is being delivered properly at the dispensing valve, this does not guarantee that the lubricant flow reaches its target point. The most reliable way of ensuring proper lubrication performance is to confirm the flow at the lube point. The flow of lubricant in centralized lubrication systems can be divided into two types: intermittent and continuous. For intermittent flow, the lubricant can be either oil or grease, while continuous flow is usually for oil, oil mist or air/oil. A number of sensors are available to achieve flow confirmation, including flow meters, no-flow switches, flow-monitoring AC switches, inductive flow switches, thermistor flow switches and magnetic-field flow switches. Flow Meters A lubricant flow meter or flow transducer is very similar to a fuel meter at a gas station. Flow meters that have a pair of oval gears fitted with a magnet and a reed switch are the most widely used in industry. These types of flow meters require a downstream evaluation unit or pulse meter to monitor the flow. The evaluation unit has upper and lower limits that can be set and monitored. For example, the lubricant volume through the meter should be within 10 to 12 cubic centimeters in 60 minutes. If the lubricant volume in the specified time interval exceeds the limit, a warning or alarm will be displayed or communiwww.machinerylubrication.com | July - August 2013 27