- Buyer's Guide
It is well-known that the low-cost way to solve a problem is to avoid it in the first place. This is certainly an option for many lubrication-related problems in industry. All that is required is a plan and a little bit of will power. A simple but effective plan should include at least three steps: prevention, detection and correction. This article takes a brief look at each.
Particle contamination is possibly the most well-known form of lubricant contamination. Reducing particle counts significantly extend the life expectancy of equipment. For example, by reducing contamination levels from ISO 21/18 to ISO 14/11, the life of a 50-gpm pump may be extended by a factor of seven. Particle contamination can occur from ingress from the surroundings, improper cleaning of the bearing housing during maintenance cycles, or corrosion products from the high water content in the oil.
Properly specified housing components, including oilers, seals and vents, can be effective in preventing contamination (Figure 1).
For many years, constant level oilers have been essential in maintaining oil levels. Most of these were vented to the surrounding atmosphere, which can lead to contamination ingression to the housing sump. By switching to a nonvented oiler, ingression can be significantly reduced.
Bearing housing seals, more often recognized as isolators, are producing positive results in reducing oil leakage and contaminant ingression. Labyrinth-type isolators are the most widely used on horizontal pumps. Labyrinth seals allow increased internal pressure created by normal pump operation to vent through the seal. Because the rotor and stator are not in contact, venting can occur without creating wear. Consequently, labyrinth seals have proven to be effective at reducing and sometimes eliminating contamination ingression through the shaft seal.
Lip seals can also prevent contamination. But because lip seals are a contact type of seal, they require more frequent replacement to ensure close tolerance operation. Because the surfaces of the rotor and stator are in contact, the contact area gap will increase with time and use, and contamination will increase.
Magnetic seals have also proven to be a good choice for preventing damage to bearings due to contamination. The rotor and stator are designed to hold together magnetically, with contact between the two surfaces, much like a traditional lip seal.
The life of a lubricant is significantly reduced when exposed to high operating temperature conditions. The rate of oxidation for a lubricant doubles with every 18-degree Fahrenheit increase in temperature. This can be significant when considering pump operating temperatures are frequently near or above 140°F. By simply lowering the operating temperature of the oil to 122°F, a 50 percent reduction in the rate of oxidation would occur, doubling the effective life of the oil. The most basic methods to reduce (or maintain lower) oil operating temperatures are:
Understanding and maintaining the proper quantity of lubricant is perhaps the easiest means of increasing lubricant life and effectiveness. The lubricant sump volume may be gauged by observing the oil level at the side of the element of a stationary bearing. Rolling element bearings should not be submerged more than one-half the diameter of the rolling element (ball) at the deepest point of submersion. Applications with shaft flinger rings are more dependent on the shaft speed than the depth of submersion, though depth of submersion is still an important factor. Applications with slinger disks are less susceptible to problems of overlubrication because they are attached directly to the rotating shaft. However, they have been known to cause misfeeding of constant level oilers through the creation of hydraulic currents in the oil sump. Proper selection and mounting often eliminates this condition.
Evaluation of lubrication sampling and testing procedures is recommended. Routine oil sampling and changes can be costly and time consuming. Every piece of equipment has its own unique set of circumstances including age, operating environment, process fluid, speed, operating mode, temperature and history. There are a number of products and recommended practices available to help increase the efficiency of oil sampling, testing and maintenance.
As a rule, viscosity is the most important lubricant property to monitor. A change in viscosity can be an early indicator of contamination and degradation (Figure 2).
Technology is available that will automatically sample and monitor change in the viscosity of oil right at the pump. More commonly, oil is sampled from the pump and sent to an oil analysis lab for evaluation. By understanding what causes a change in viscosity, fault conditions including change in the base number (BN), acid number (AN), oxidation, incompatible fluids and contamination can be identified through additional testing.
Looking at the oil used in a piece of equipment is not a reliable method of determining whether contamination is present or the oil has exceeded its useful life. Color can often be misleading. However, there are simple ways to use visual indication as an early warning of degrading conditions. View ports mounted in the housing can be used for both quantity and quality checks. Blotter testing is a simple, low-cost visual indicator that can offer useful indication of lubricant health change that would prompt more sophisticated analysis.
There is a variety of lubrication-monitoring devices available, depending on the specific problem. As an example, a widely accepted method of measuring water in oil is the Karl Fischer titration test. While useful and available at most labs, this method is usually considered impractical for the average process pump; however, a proper crackle test performed onsite can be an effective screening test for moisture.
There is also an online monitoring technology (Figure 3) for measuring water contamination by percentage, or saturated relative humidity (SRH).
Just as in the air we breathe, this saturation point, or dew point, varies by temperature, hence the relative aspect of SRH. The percent moisture in a system may not change, but if the sump temperature drops below the dew point (saturation relative to temperature), and moisture condenses, then the new form of moisture can have greater negative impact on the machine components. For example, 500 ppm at 150°F may be safe or below the saturation point, but the same 500 ppm at 75°F will be above the saturation point, increasing the risk from moisture contamination. This safe-moisture limit measures strictly in parts per million (ppm). It varies from oil to oil, but is universal in terms of SRH, and can be measured only by sensors specifically designed for this purpose.
Housing Configuration - Maintaining the Headspace
The best way to prevent problems in the system is to ensure it is closed. Through proper configuration of nonvented oilers, housing seals, expansion chambers, vent replacements, desiccant dryers and monitoring devices, the pump housing environment can be maintained nearly effortlessly. For example, by installing a closed system consisting of a nonvented oiler and desiccant oil dryer, oil changes were reduced from every six weeks to every three months. At an estimated cost of $30 per oil change, the payback on the installation cost of $72 was realized in less than eight months (with two desiccant cartridge replacements). Additional cost savings may be realized through increased mean-time-between-maintenance (MTBM) and reduced oil disposal costs. Improper configurations of bearing housings are common and can contribute to high levels of oil contamination. For example, tests have revealed that when a vent is used with certain types of labyrinth seals, the ingression rate for airborne contaminants may increase, as much as 10 times when compared to those applications without a vent.
Water contamination of oil can cause several problems relative to oil contamination or degradation. Because each type of oil has its own safe level of water before damage can occur, the common practice of measuring parts-per-million (ppm) is not conclusive. There are significant differences between oils, beginning with mineral and synthetic bases. Additive packages, commonly referred to as ad-pacs, can also make a difference in how much water an oil can hold before phase separation occurs and free water forms. Temperature also plays a major role in how much water oil can hold. Damaging levels of water, or free water, begin to occur in some mineral-based oils between 400 and 500 ppm at 140°F. Free water may form at 200 ppm at 125°F in the same oil. By the time water becomes visible, damage is already occurring to both the oil and the surfaces of the equipment and components (Figure 4).
Therefore, it is important to know the saturation point of oil at a given temperature to begin to determine a target for effective lubrication maintenance.
The headspace above the lubricant in a wet sump configuration can be used to help control contamination of the lubricant. With the proper tools, a useful strategy and a little diligence, the contamination-prone space above the lubricant can be isolated, monitored and managed effectively. Headspace moisture monitors enable quick and easy assessment of this space, for either small sump capacity pumps or for large hydraulic and circulating systems.