Modern pumps can benefit from a range of lubrication oils, from mineral to synthetic. While characteristics such as thermal stability and oxidation resistance are important to consider, every end user should focus on finding a lubricant with the longest life for a particular application. Extreme temperatures and contaminants can greatly reduce a lubricant's life span.
The wrong lubricant can lead to complications that endanger the pump's performance, such as premature bearing failure. The lubricant's viscosity should ensure appropriate performance at both the lowest startup temperature and in-service, or operating, temperatures. During operation, the life of the lubricant reduces by half for every 12 degrees Celsius above the rated temperature.
Optimizing a pump lubrication strategy can be tricky. The best approach is to combine the latest technology, such as advanced monitoring, with proven traditional methods. The lubrication system should be designed to deliver the right amount of oil at the right time to the right application.
Smaller Pumps for Harsher Applications
End users who face difficult applications and punishing operating conditions often rely on pumps with compact designs and smaller footprints. These modern pumps pose new challenges for lubrication oils and oil systems caused by extreme operating temperatures, higher loads, higher pressures and contaminants. In addition, operators expect higher performance, less downtime and more productivity to decrease costs and improve profits.
Downsized pumps usually require less oil and additives to lubricate and protect the pump components. However, as equipment loads increase, the lubrication oil must endure higher temperatures and quicker oxidation—a chemical process in which rapidly forming sludge shortens oil and pump life. Expensive downtime, repair or replacement costs are the result.
Smaller sizes and tighter tolerances also mean limited oil reservoirs. Oil systems must cycle lubrication more often, with less time to dissipate heat, release foam and remove contaminants. Deposits can form, damaging the pump and blocking filters and valves.
Modern oils should contain high-performance additives. These additives keep the lubrication oil thermally stable and robust enough to ensure it performs more efficiently. Lubricants with additives clean the oil system and carry away heat and contaminants.
Lubrication Challenges
Even with the use of additives, other steps must be taken to protect lubrication systems.
Keeping a modern lubrication system dry is harder than it appears. For many pumps, water can easily penetrate the lubrication oil system through the oil reservoir. Water mist or vapor from any part of a plant can enter the oil reservoir breather, forming condensation in the reservoir when hot-running equipment cools after shutdown.
The cleaner, cooler and drier the lubrication oil is kept, the longer the lubrication oil life and better the pump reliability and performance.
Lubricants oxidize when the oil's hydrocarbons chemically combine with oxygen. Heat and pressure accelerate oxidation. After prolonged oxidation, the oil becomes physically more viscous and chemically acidic, leading to corrosion. The oxidation process usually emits a sour or pungent odor, similar to rotten eggs.
Lubricants in high-temperature pumps often suffer from thermal failure. Darker, more viscous oil is a sign of thermal failure and gives off an odor similar to burned food. When both oxidation and thermal failure occur, the lubricant's performance diminishes and begins to pose a danger to the pump.
Contamination can occur from different sources. Produced materials from wear, oil degradation byproducts, dirt, water, process-related fluids should be considered contaminants. If these contaminants cannot be removed from the system by filtration or dehydration, a more drastic approach could be required—such as changing the lubrication oil.
Viscosity Index
The International Organization for Standardization viscosity grade (ISO VG) of a lubrication oil only reports viscosity at a single temperature. For any lubrication oil, however, the viscosity should be evaluated relative to changes in operating temperature. These changes are called the viscosity index (VI).
Pumps with a wide range of operating temperatures depend on a higher index for consistent performance. The higher the VI, the more stable the viscosity across a range of temperatures. In this way, the VI is as important as the viscosity itself. Unlike other conditions that affect a pump's viscosity requirement, such as bearing design, loads or speeds, the VI is often ignored during the lubrication selection process.
The VI is an empirically derived, unitless number. Traditionally, a lubricant that was similar to paraffinic crude oils was assigned a VI of 100. If the lubricant was similar to naphthenic crude oils, it was assigned a VI of 0. Currently, a lubricant's VI could be from minus 70 to more than 420. Lubricants with a VI in the range of 90 to 200 are commonly used.
The importance of a lubricant's VI can be demonstrated by comparing two well-known ISO VG 100 oils. The first is a mineral oil with a VI of about 100, and the second is a sophisticated synthetic oil with a VI of about 150. For any pump operating at 150 C, the lubrication oil with a VI of 150 demonstrates about 20 percent more viscosity compared with the oil with a VI of 100. The difference in viscosity can result in significant operating and performance advantages.
Though expensive, high-VI lubricants are always preferred, particularly for modern variable-load, variable-speed pumps. This is especially true for applications where the optimum viscosity or VI is difficult to determine. Outdoor pumps are one example, especially when high-temperature pumps (above 200 C) are installed in a climate with a minimum ambient temperature of minus 30 C.
The selection of the pump lubrication oil is based on previously successful operating references and performance observations. These often require a safety margin in terms of a low variation of viscosity. For example, ambient temperature or temperature range could be different site to site. In addition, the ISO viscosity grades usually demonstrate large viscosity steps—for example, ISO VG 32, 46, 68, 100, 150, 220, 320 and 460. These characteristics often require a relatively high VI for any lubrication oil.
Oil Additives
Additives are organic or inorganic compounds dissolved or suspended as solids in the lubrication oil. They typically range from 0.2 to 12 percent of the oil volume, depending on the lubricant and pump.
Additives play the following three basic roles:
- Enhance existing base oil properties, for example, anti-oxidants, corrosion inhibitors, anti-foam agents and demulsifying agents
- Suppress undesirable base oil properties, such as pour point depressants and VI improvers
- Impart new properties to base oils, such as extreme pressure (EP) additives, detergents, metal deactivators and tackiness agents
Additive depletion is a widespread issue in the lubrication industry. An oil analysis can determine the health of the remaining additives in lubrication oil. The analysis can reveal if additives are anchoring to the metal surfaces of the pump, known as metal wetting. When metal wetting occurs, the additives attach to the interior of the casing, gear teeth, bearings and shafts.
Many additives are consumed or chemically depleted while performing their function. The lubrication oil's performance then suffers. When additives are completely consumed, the oil should be changed.