Chemical Processing - February 2008 - (Page 26)

Pump Performance Grease. Easy to apply, grease can be retained within a bearing’s housing and provides extra sealing protection. Depending on the rotational speeds and operating temperatures, relubrication may be required to combat short grease life. When the operating conditions allow, “greased-forlife” bearings, which eliminate requirements for relubrication and related maintenance tasks, can offer an attractive alternative. Oil bath. This option establishes an oil level at the center of the bearing’s bottom rolling element and represents the comparative baseline of bearing friction among the lubrication methods. Best results over time can be achieved using a constant-level oiler. Oil ring. In this method, an oil ring is suspended from the horizontal shaft into an oil bath positioned below the bearings. The rotation of the shaft and ring ings oil from the bath onto the bearings. The lower oil volume in the bearing reduces the viscous friction in the bearing system to allow higher shaft speeds and better cooling. Oil mist and air-oil. Here, oil is atomized and carried by an air stream to the bearing. This approach generates the least amount of friction (allowing rotational speed to be based on the bearing design instead of lubrication limitations) and creates a positive pressure within the bearing housing (fending off invasive contaminants). Regardless of lubrication method, always specify lubricant according to the demands on vertical shafts and resistance to solids, pressure, temperatures, loads and chemical attack. Where pump locations may be dif cult to access, fully automatic delivery systems can be integrated. Sealing the system Bearing seals in centrifugal pumps handle four crucial tasks. They retain lubricants or liquids, exclude contaminants, separate uids and con ne pressure. So, the seals have their work cut out for them. Solid contaminants, depending on particle size, hardness and brittleness, will produce either indentations or wear on the bearing surfaces; water will affect the ef ciency of the lubricant; and contamination in the lubricant can dramatically reduce bearing life. When a seal fails, contamination can in ltrate the bearing area, entering the lubricant and then the bearing. In addition, loss of lubricant from the bearing can lead to dry-running operation, which ultimately can cause bearing failure. The choice of seal for centrifugal pump bearings depends on the unique demands and operating conditions of the application. Keep in mind, though, that the bearing and sealing arrangement represent an integrated system. Dynamic radial seals generally are the best choice for centrifugal pumps. These seals create the barrier between surfaces in relative motion (one usually stationary while the other rotates). Typically, radial shaft seals incorporate: 26 • February 2008 >> Portable condition-monitoring device Figure 2. This instrument provides a range of testing and analysis techniques as well as data recording. Steel or elastomer shell. This is bonded to the sealing material and enables the requisite interference t of the seal in the housing bore to be maintained (as well as facilitates proper installation); and Elastomer sealing lip. Usually installed against the shaft, this provides dynamic and static sealing against the shaft and features a sealing edge formed by pressing, cutting or grinding. Most sealing lips are made from a formulation of nitrile rubber. However, materials speci cally for use with fuels, industrial uids and highly compounded lubricants are available. Seal selection ultimately must be based on a thorough review of application parameters and environmental factors. Particularly in pump applications, for example, seals will be exposed to relatively constant pressure differentials — making pressure seals (in which the seal cavity is pressurized) the preferred choice. Due to the nature of their design and the harsh operating environment, seals usually provide a much shorter life than the components they protect. So, don’t fall into the common but unfortunate habit of scheduling seal replacement only at intervals dictated by requirements of other components such as bearings. Many bearing failures can be prevented if seals are replaced when the rst signs of wear or leakage have been detected. To get an inkling of how long seals should last, consider the amount of contamination a seal will encounter, cycle times, speed and heat. Of course, contamination isn’t the only reason for premature seal failures. Others culprits can include: • poor selection; • improper installation, resulting in leakage; and • change of lubricant, causing adverse reactions in common sealing materials. www.chemicalprocessing.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - February 2008

Chemical Processing - February 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Nanoparticle Safety Raises Questions Take the Pressure Off Vacuum Systems Achieve Optimum Centrifugal Pump Performance Rethink Batch-Manufacturing Alarm Systems Dr. Gooddata Orlando Plant Pioneers HMI Migration Strategy Process Puzzler Plant InSites Equipment & Services Ad Index Product Spotlight/Classifieds End Point

Chemical Processing - February 2008

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