Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/190260
TURBINE LUBRICATION becomes rougher (the bearing surface becomes damaged), then the risk of rotor and bearing surface contact increases. This relationship between the surface roughness and the film thickness is important in the consideration of elastohydrodynamic lubrication of a turbine bearing. A major concern with rough surfaces, including gouges or grooves around the journal or along a Babbitt surface, is that they allow high-pressure lube oil to flow away from the thinnest film areas. This permits increased contact and shear forces on the Babbitt than would occur with smooth, concentric and round journals on a smoothly bored Babbitt surface bearing. This is why surfaces that appear to have minimal damage can still cause bearing failure during startup or slow-speed operation before a fully developed hydrodynamic film with a normal film thickness exists. These images show the buildup of particulate in a turbine oil reservoir that is cleaned periodically. Consider the type of buildup that could occur over the life of the plant. During a plant startup, when a turbine is on turning gear or at lower speeds, the lubrication system may supply lube oil with slugs of particulate matter to the journal and thrust bearings. This is especially damaging to journal bearings because they carry the applied load forces that force the journals into contact with the bearing surface, thus providing a very thin oil film. This form of lubrication is known as elastohydrodynamic lubrication. Because only a very thin oil film exists between the journal and bearing surface, and many of the particles in the oil are larger than this oil film thickness, the bearing surface can be scored as particulate becomes imbedded in the surface of the Babbitt. In some cases, the particulate can score the journal, requiring it to be remachined and/or honed. Water Contamination Water contamination in a lubricating or control system is particularly undesirable because it tends to form an emulsion with the oil. Elevated water content in an oil system can also affect lubricity and induce corrosion. When a lubrication or control system is shut down, the normally wetted regions, such as those in the lube oil drain pipes, become exposed to air. Enlarging the surface area of carbon steel that is subject to ambient conditions in reases the amount of internal corrosion c (rust) a system develops. While a unit is operating, the amount of water in the lubricating or control system is normally maintained within the manufacturer's specification by the plant's oil-conditioning system. These systems often contain some type of demulsifying 28 September - October 2013 | www.machinerylubrication.com agent to remove water. Even if the water content is well above normally acceptable standards, the steel surfaces are usually kept wetted with oil, and corrosion (rust) is limited. However, when the turbine is shut down for weeks or months, water will generally separate from the oil in the pipes and components where the oil resides. If water separation occurs, it can corrode the equipment, support microbiological growth and/or affect the functioning of the control system. Additionally, when drained, large portions of the system normally filled with oil become exposed to the environment (air and oxygen). Obviously, the longer the duration a turbine lubrication system remains secured, the more time the equipment has to oxidize. Particle Contamination Particle contamination not only can cause equipment to wear, but it also clogs lubrication ports, in-line filters and control systems. During operation, the parameters used to monitor oil tend to remain stable, but when the system that is designed to maintain lubrication and keep piping and hardware clear is secured, the rate of oxidation on the exposed surfaces increases. Then, when the unit is returned to service, the newly formed corrosion (rust) on any exposed piping or surfaces often is shaken loose or washed away with the restarted oil flow. Furthermore, where there are pockets in which stagnant oil may This orifice plug from a boiler feed pump turbine have settled, a slug of particulate and coagulating oil can easily form. control has deposits covering the orifice holes. When the oil flow is restarted, this particulate matter is easily pulled into the lubrication flow streams and circulated throughout the lubrication system. One substantial area of concern is the control system. Because of the system's complexity and low-flow velocity within the control piping, the system acts as a collection point for particulate. Biological Growth When a lubrication system is secured for prolonged periods of time, biological growth can occur. Low-flow areas can collect a tremendous amount of sludge over the years. With this amount of growth, a sudden change in the lubricating system's tempera- Biological growth, particulate and separated lubricant additives have collected on this pump (above left). The lube oil cooler on the right has biological growth and adhered lubricant additives.