Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/108300
Pump Lubrication Tips for Better Process BY HEINZ P. BLOCH The overwhelming majority of process pumps in industry comply with the standards of the American National Standards Institute (ANSI) or the American Petroleum Industry (API). However, neither of these two standard categories mandates any one particular type of lubrication. Grease lubrication, as well as lubrication by liquid oil or atomized oil (oil mist), or even pressurized pure oil are used in various process pumps. Generally, grease is chosen for ease of lube containment, but it is limited to relatively small ANSI pumps. The larger pumps and virtually all API-style pumps are oil-lubricated, but lube type selection and the designs that apply lube to bearings can vary. Lubricant types are categorized as either mineral oils or synthetic oils, and they certainly differ in cost. Also, lubricants need to remain relatively clean in service, and lube change intervals must be optimized. In short, important choices and decisions will need to be made. Therefore, guidance from reliability improvement and cost points of view will be necessary. Oil Rings Interact with Lubricant Viscosities Viscosity is by far the most important property of lubricants applied to process pump bearings. In general, thicker viscosity oil films will protect rolling-element bearings better than oils with thinner viscosity. For process pumps with rolling-element bearings, ISO Grade 68 (VG 68) lube oils will allow higher operating loads than VG 32. The problem is that oil rings or slinger rings that are optimally designed to apply VG 32 will not work equally well with the more viscous VG 68. Oil rings typically operate best in an as-designed speed range with closely maintained depth of immersion, ring concentricity, shaft system horizontality and surface roughness of contacting Figure 1. Oil rings can have a tendency parts. For long life to malfunction if they contact stationary housing parts. and low frictional losses, different bearings are sometimes best lubricated with different viscosity lubricants. But what if these bearings share the same bearing housing? Reliability-focused pump users will need an optimization strategy, and lube-related knowledge will help. Fortunately, ISO Grade 32 synthetic lubricants exhibit oil film thickness and strength properties close to those of VG 68 mineral oils. Therefore, suitably formulated VG 32 synthetic oils are preferred by reliability professionals for many process pump services. An excellent point in favor of synthetic VG 32 can also be made for many pump drivers, such as the steam turbine bearing housings shown in Figure 2. The sleeve bearing at the drive end of this small turbine was optimally lubricated with a VG 32 lubricant, whereas the rolling-element bearing at the governor end (the thrust-loaded bearing) would survive longer if VG 68 oil could be applied. Because of their superior properties, VG 32 synthetic lubricants incorporate the viscosity needed for reasonable oil ring performance as well as oil film thickness and tenacity characteristics desired for bearing life extension. Here are a few general guidelines worth considering: • Using a mineral oil would generally require oil to be changed every six to 12 months. With a clean, premium-grade synthetic lubricant, you would typically extend oil change intervals to at least 24 months. Incorporating the advanced bearing housing protector seals shown in Figures 3 and 4 and guarding against contaminant entry at breather vents could make 36-month change intervals possible. • ISO Grade 32 mineral oils are often considered too "thin" for pump bearings. They rarely qualify for long-term, risk-free use in pumps equipped with rolling-element bearings in typical ambient conditions. But simply switching to ISO Grade 68 mineral oils will be risky for bearings that depend on oil rings for lube application. • Appropriately formulated with the right base stock and with proprietary additives, ISO VG 32 synthetics are quite acceptable from film strength and film thickness points of view. In fact, the performance of some ISO VG 32 synthetics duplicates that of ISO VG 68 mineral oils. These superior ISO VG 32 synthetics excel by simultaneously satisfying the requirements of sleeve and rolling-element bearings. www.machinerylubrication.com | January - February 2013 www.machinerylubrication.com | July - August 2012 | 11