Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/190260
TURBINE LUBRICATION Figure 1. The results from a mesh blockage particle counter revealed that a large amount of contaminants were released into the lubrication system after 12 hours of operation. down to acceptable limits, but when the turbine speed was increased to synchronize the generator to the power grid, another spike occurred. Plants generally do not monitor particle levels as equipment is placed back into service. When a turbine is at normal operating speed, the minimum oil film thickness between the turbine rotor and the bearing surfaces is in the order of 0.002 inches for journal diameters from 8 to 16 inches. The minimum film thickness increases to 0.010 inches for the largest journal bearings with 30-inch diameters. However, when the turbine is on turning gear, this oil film thickness is only a few microns thick at best. As the speed increases, so does the film thickness between the rotor and bearings. If a large mass of particulate gets into a journal bearing when the film thickness is only microns thick, the particulate matter can scar or damage the journal and/or bearing profile, which in turn affects the fluid dynamics within the bearing. This often leads to overheated bearings and possibly to bearing failure. To control particulate matter, it is better to use filter element sizes than ISO charts, since ISO charts allow a few large particles, which can lead to journal bearing failure. Understanding what is occurring provides the best way to identify and assess risk. Once a risk is identified, actions can be taken, consequences evaluated, mitigations and contingencies developed, and an implementation plan pursued. There are many ways to reduce risks when bringing large equipment back into service. A review of vibration data and other parameters can tell a lot about how the equipment performs when going from cold to normal operating conditions. It also will inform you of what is needed to extend the health of the equipment or increase the operating efficiency. Perhaps the most versatile and cost-effective method to help reduce the risk to equipment is to have a kidney-loop filtration system available. This type of system can be connected to a tank or reservoir to support bringing the equipment back into service. Connecting a kidney-loop filtration system to a tank or reservoir can help reduce the risks when bringing equipment back into service. The filtration unit should be sized to process the contents of the tank or reservoir several times per hour. In addition, the unit can be equipped with a cooler for summer months when thermal loads on the plant limit equipment operation. Along with filtering and cooling, a kidney-loop system can include a dehydrator, which provides an excellent way to extract water from the oil before placing the equipment on turning gear. This will reduce the amount of water that gets pushed through highpressure differential regions of the oil film in the journal bearing and reduce the risk or amount of flashing, which causes damage and erosion of Babbitted and non-Babbitted surfaces. Keep in mind that the expense of an auxiliary filtration skid can be quickly offset by a reduction in maintenance costs. In fact, the cost of replacing a damaged bearing or by keeping plant equipment running during peak summer months could easily justify reducing the risk of repeat failures. For example, a 10-percent power reduction at a nuclear plant during peak summer temperatures would pay for auxiliary filtration systems on most challenged equipFigure 2. This graph illustrates the changes in oil particulate during the startup of a turbine. The dash lines provide the lSO 4406 "alert" level for specific particle sizes. ment within days. 30 September - October 2013 | www.machinerylubrication.com