Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/190260
TURBINE LUBRICATION By Craig Jennings, TRI Transmission and Bearing Corp. Quality Changes During Startup How Oil E Equipment manufacturers generally provide guidelines on how oil should be maintained for reliable operation. Because components have little tolerance for contamination or oxidation, frequent oil sampling and monitoring are performed while the equipment is online in order to keep the unit operating without any issues. However, many times little attention is paid to the oil quality when the equipment is shut down for a maintenance outage or on standby. This can eventually affect the unit's reliability. For instance, when a steam turbine is down for prolonged periods or for a short three-week maintenance outage, the consequences of not having hot, circulating oil running through the system is usually not considered. In fact, the oil quality when placing a turbine on turning gear or during startup is often not the same as when the unit came offline. Typical turbine bearings are designed for fluid-film lubrication. Most fluid-film bearings are intended for hydrodynamic lubrication. This means an oil wedge is formed in a hydrodynamic bearing to develop Prolonged exposure to water separation and maintain can rust a control system. an oil film between the rotor and bearing. The film thickness is a function of rotor speed, load and oil viscosity. Under fluid-film conditions, an increase in viscosity or speed increases the oil film thickness, while an increase in load or decrease in rotor speed reduces the oil film thickness. 26 September - October 2013 | www.machinerylubrication.com In developing an oil film, the bearing surface geometry and rotor surface are just as important as rotor speed, load and oil viscosity. To establish a stable oil wedge and fluid film between the surfaces, the rotor and bearing profiles must be The congealed mass shown above is an example of particulate matter and degraded oil buildup that can accumulate in a turbine control servo. Left unattended, this condition could eventually cause poor turbine control response and accelerate equipment wear. perpendicular. If either or both surfaces become damaged by large particles being pushed through the bearing, the ability to form an oil wedge is diminished, resulting in a thinner oil film thickness. Reducing the oil film thickness increases the susceptibility of additional damage or producing a rub during coastdown or startup. When the turbine rotor is at normal operating conditions, the bearing is in hydrodynamic lubrication conditions. However, when a turbine rotor is on turning gear or beginning to roll at slower speeds, the journal bearing is in elastohydrodynamic lubrication conditions. These conditions are generally indicated by some metallic friction and wear. Very high coefficients of friction may also be reached. At this point, the oil film thickness between the bearing and the turbine rotor will be at its minimum. The film thickness will increase as either the speed is increased, the lubricant viscosity is increased, the load is decreased or the geometric shapes of the journal and bearing surfaces relative to each other are improved. If the surface