Pumps & Systems, April 2013
While conceptually simple in design, reliable pump operation requires a focus on precision maintenance. However, even simple tasks—such as inspections, level checks and oil changes—can have a profound effect on pump life.
Precision lubrication for pumps requires that the correct lubricant be installed in the right volume and maintained in suitable condition. Lubrication precision maintenance (PM) programs for these systems should be designed around these requirements, but sometimes execution can be difficult unless these items were considered during the design process.
Important accessories are commonly excluded from equipment designs to reduce costs, but the negative effects on maintenance costs and equipment reliability can be drastic. Fortunately, this negligence can be corrected with simple equipment modification in the field.
Pump modifications for precision lubrication can be divided into three categories:
- Contamination control
- Condition monitoring
Most oil-lubricated pump bearings use a bath/splash lubricant application method that presents challenges for precision lubrication. One challenge is that bath lubricated bearings are sensitive to proper oil level, which is half-way up the bearing element, directly in the center. The difference between perfect lubrication and drastic under-lubrication is typically less than an inch of variance in oil level. High oil levels can also cause problems in the form of aeration, excess heat and increased energy consumption.
Many pumps are installed with no effective way to visually inspect the oil level. Instead, they rely on a level plug that requires a technician to remove the plug and insert a probe or add oil until it pours out. This practice can lead to gross contamination of the lubricant. It can also be cumbersome, time consuming and potentially unsafe. In many cases, the oil level does not get checked at all, leading to pump failures caused by undetected, low oil levels.
A good lubrication PM program requires frequent oil level checks and other visual inspections to verify a proper level. To efficiently execute these tasks, all oil-lubricated sumps should be fitted with a visual level inspection device such as a column-type level gauge or a bull’s eye sight glass. These devices should also be properly marked with both high and low levels and possibly running/idle levels. If a column type sight glass is used, good practice is to connect the sight glass vent to the case vent to avoid erroneous readings.
A constant level oiler is also a good option. When properly installed and maintained, the oiler will add make-up oil as needed and maintain the proper oil level at all times. The inclusion of a constant level oiler does not affect the need for a level indicator, which is still required to ensure proper function of the oiler.
The difference between perfect lubrication and drastic under-lubrication is typically less than an inch of variance in oil level. A constant level oiler, similar to the one shown here, is a good option.
Oil is added directly through a filtration unit, further cleaning it and preventing atmospheric contamination by keeping the reservoir sealed.
Another common problem with oil lubricated pumps is getting the oil into the reservoir without contamination. Typical fill ports are small and often require the use of a funnel, which can lead to gross particle contamination of the oil. Proper application methods combined with the appropriate fittings easily eliminates this problem and increases efficiency and safety. By removing fill and drain plugs, replacing them with quick connect fittings and using high quality fluid transfer equipment, oil can be pumped and filtered directly to the sump without exposing it to atmospheric contaminants.
The single largest factor that determines the service life of oil-lubricated bearings is the level of lubricant contamination, whether water, particles or both. The main goal of any precision lubrication program should be the aggressive pursuit of oil cleanliness.
Two primary strategies are possible for controlling contamination in lubricated systems. The easiest and most cost effective is contamination exclusion. The other is contamination removal.
For most oil-lubricated pumps, exclusion is the only reasonable option since they typically do not employ circulating pumps, and offline filtration equipment is usually too large to use with these systems while they are operating. Therefore, contamination exclusion methods should be carefully considered. Most contamination comes from two, easy-to-control sources: new oil additions and the air the sump breathes.
To prevent new oil contamination, it should be filtered prior to use using quick connects with hand pumps or portable filtration units. The breathing issue is easier to address when the problem is understood. Many maintenance professionals may not stop to consider that wet sump applications, such as pumps, breathe significant amounts of air as the temperature in the head space changes. They should remember that this air contains millions of damaging particles that are much smaller than what can be seen with the naked eye. Much of the moisture ingression suffered by pumps actually enters as humidity, which can precipitate when the temperature drops.
Breather plugs are easily replaced with inexpensive breathers that efficiently filter particles as small as two microns and absorb moisture down to approximately 20 percent relative humidity. This method of controlling contamination is often referred to as head space management and usually requires simple modifications to effectively execute.
The final contamination control is seal quality. The use of inexpensive seals often leads to oil leaks and the ingression of contamination. Using high quality mechanical seals will prevent the vast majority of these problems when properly installed. If process fluid migrates down the shaft from the pump to the bearing housing, consider using a collar on the shaft to deflect the fluid. When gross moisture contamination is likely, a bottom, sediment and water (BS&W) bowl provides an effective way to inspect for free water and drain it with the included drain valve.
BS&W bowls provide an effective way to inspect for and remove gross water contamination.
Some maintenance personnel may argue that routine visual inspections are the most important type of PM. However, the most significant problems associated with the lubrication of these systems cannot typically be observed without instrumentation.
Oil analysis is unique in its ability to serve as a proactive condition monitoring tool. Rather than simply focusing on the symptoms of problems and serving as an early warning for bearing failures, oil analysis is ideal for proactive condition monitoring to detect the root causes of machine failure, so they can be eliminated. The most common root causes of wear, and ultimately failure, in oil lubricated bearings is particle contamination, moisture contamination or using the wrong oil or one that has degraded beyond the point of usefulness.
Oil analysis is perfect for identifying and quantifying all these conditions. Using viscosity testing, acid number, fourier transform infrared (FTIR) and elemental analysis, end users can determine if the correct oil is in the machine and determine its fitness for use.
Particle counters, when used correctly, can indicate the exact level of particle contamination in many size ranges. Certain moisture tests can give an accurate measure of moisture contamination. Detecting and eliminating these problems dramatically extends the life and increases the availability of these assets.
In addition to proactive functions, oil analysis can be a sensitive tool for identifying bearing faults far in advance of a failure. Unfortunately, many users of oil analysis are not aware of these benefits for a number of reasons, but the most prevalent is poor sampling methods.
Some of the most important tests require good sampling practices to provide any real value. Sampling from drain ports typically yields poor results and can lead to unnecessary corrective action. To take a representative sample, machines should be sampled while in operation, and for most pumps, this method leads to a dangerous drop in the oil level. Traditionally, those who attempted to take “good” oil samples used the drop-tube method, which requires the insertion of a flexible tube into the oil sump. This method is cumbersome and can also provide poor results.
The biggest problem is the inconsistent location of the sample extraction point and the likelihood of contaminating the tubing. To take high-quality, repeatable samples, proper sampling hardware should be installed. The proper type of hardware for most pumps is a minimess-style sample valve with a pitot tube. The minimess is a normally closed check valve—similar to a hydraulic diagnostic valve—that requires an adapter to open it.
For wet sump applications, such as pumps, these devices are available with a pitot tube that can be cut to length and bent to facilitate sample extraction from the desired location. Because most pumps have numerous drain or side ports, these devices can be easily installed. When used properly, the devices provide highly repeatable and valid sample data. They also make sampling easier, safer and more efficient than other sampling methods.
Proper sample valves provide consistent valid data and make sampling easier.
Design for Precision Lubrication
Precision lubrication is difficult for the average pump given the typical configuration. However, with minor modifications it can be relatively simple. A myriad of inexpensive devices are available for inspection, contamination control, lubricant application and oil sampling. Most of these devices can be easily installed.
The most efficient way to incorporate these items is to specify them during the design and installation phase, but retrofits in the field are usually easy as well. Precision lubrication requires using the right oil, in the right place, in the right amount, at the right frequency, in the right condition and with proper equipment design. P&S