Failures of the pump’s bearings are the cause of pump outages and repairs nearly as frequently as failures of the pump’s mechanical seals.
Centrifugal pumps often make use of both rolling element (anti-friction) bearings and sliding element (plain) bearings in their construction. Each of these two groups of bearings can be discovered in small as well as large pumps, as well as single and multiple-stage configurations. On the other hand, sliding element bearings are typically preferred over rolling element bearings for extremely big or extremely high-speed pumps.
Rolling element bearings are typically utilized in the majority of centrifugal pumps onboard vessels. This is due to the fact that the vast majority of centrifugal pumps have a horsepower rating of less than 200 (or -150 kW). Failures of rolling element bearings should therefore be avoided or reduced if possible, as this should be the primary focus of any conversation concerning the extension of pump life.
A rolling element bearing is a device that requires a high level of precision and is a feat of engineering. It is quite improbable that any other mass-produced item is machined to tolerances that are quite as stringent as these. Rolling contact surfaces and geometries are normally maintained to millionths of an inch, whereas boundary dimensions are often kept to tenths of a thousandth of an inch. Because of this obviously important factor, only a negligible amount of surface degradation can be tolerated.
In most cases, fatigue failure rather than wear is what causes a rolling element bearing to fail before its expected lifespan, provided that the working circumstances are satisfactory. The number of stress reversals and the cube of the load that is creating these stresses are the two factors that determine how long a bearing will last under optimal operating circumstances before it fails due to fatigue. As an example, if the load that is being supported by the bearing is doubled, the theoretical fatigue life will be cut in half. In addition, the potential fatigue life is cut in half if the speed is increased by a factor of two.
The friction torque that is generated by a rolling element bearing is fundamentally made up of two different parts. One of these is a function that depends on the design of the bearing as well as the load that is placed on the bearing. The other aspect is determined by the type of lubrication, the amount of lubricant used, and the speed at which the bearing rotates.
It has been discovered that the friction torque in a bearing is at its lowest when there is just enough oil of the appropriate viscosity to produce a film between the contacting surfaces of the bearing. This is just one of the many reasons why oil mist is a superior technique of lubricating. The amount of friction will increase if there is a bigger quantity of oil or if its viscosity is increased. When there is more oil present than what is necessary to keep the rolling parts apart, the friction torque will grow along with the speed.
The following are some of the tasks that are performed by a bearing lubricant:
- To prevent sticking and provide smooth sliding between the cage and the other components of the bearing
- To function as a lubricant for any points of contact between the races and the rolling parts in regions where there is slippage.
- In roller bearings, to lubricate the sliding contact that exists between the rollers and the guiding parts.
- In some instances, to dissipate the heat that has been generated in the bearing.
- To prevent corrosion from occurring on the surfaces that have been thoroughly polished.
- To create a barrier that is airtight and watertight against any outside elements.
Oil is the lubricant of choice for rolling element bearings because of its superior properties. Simply combining oil with soap or other thickeners that aren’t soap-based to create grease is an easy approach to get more use out of the oil and grease can be used for a variety of purposes. When making a grease, the thickener’s primary function is not that of a lubricant but rather that of a carrier.
Grease is used to lubricate a far larger number of rolling bearings than any other type of bearing, despite the fact that only a small percentage of centrifugal pump bearings are greased with grease. The widespread use of grease has been motivated by the possibility of simpler housing designs, less maintenance, less problems with leaking, and better sealing against filth. In addition, the use of grease has led to improved sealing against dust. On the other hand, due to certain restrictions, the use of grease is not permitted. It is not a good idea to use grease in situations where a lubricant must rapidly remove heat. Oil is required for rolling element bearings the majority of the time since the accompanying machine parts that they interact with are lubricated with oil.
There are advantages and disadvantages of using grease to lubricate the rolling element bearings. The advantages are:
- Simpler housing designs are possible; piping is greatly reduced or eliminated.
- Maintenance is greatly reduced since oil levels do not have to be maintained.
- Being a solid when not under shear, grease forms an effective collar at bearing edges to help seal against dirt and water.
- With grease lubrication, leakage is minimized where contamination of products must be avoided.
- During start-up periods, the bearing is instantly lubricated whereas with pressure or splash oil systems, there can be a time interval during which the bearing may operate before oil flow reaches the bearing.
The disadvantages are:
- Extreme loads at low speed or moderate loads at high speed may create sufficient heat in the bearing to make grease lubrication unsatisfactory.
- Oil may flush debris out of the bearing. Grease will not.
- The correct amount of lubricant is not as easily controlled as with oil.
When it comes to high-speed bearing functioning, making the right choice between a grease and an oil is of the utmost importance. To ensure that the bearing is properly lubricated, an elastohydrodynamic oil film needs to be created and kept between the spinning components of the bearing. Lubricating oil needs to have a viscosity that is high enough to survive the given speed, load, and temperature conditions in order for there to be an adequate buildup of an oil film that is capable of carrying an adequate amount of load.
To fulfill these requirements in the most effective manner, it is most likely advisable to use a different lubricant for each particular application. When it comes to the relatively simple rolling element bearing care required by the ordinary centrifugal pump, it is generally possible to reduce costs by stocking only a select few grades of lubricant. This will allow the pump to operate more efficiently. It has long been the common suggestion of many bearing manufacturers that the minimum base oil viscosity be maintained at 70 Saybolt Universal Seconds (SUS) or 13.1 centistokes (cSt). It was applied to the majority of types of ball bearings and some roller bearings in electric motors with the understanding that the bearings would operate close to their published maximum rated speed, that naphthenic oils would be used, and that the viscosity would not drop below this value even at the maximum anticipated operating temperature of the bearings. This was done in order to ensure that the bearings would be able to withstand the high speeds that would be required of them.
When applying greases to lubricate pump bearings, it is important to take the appropriate safety precautions. Because of the high shear rate that these lubricants have, bearings will overheat if they are lubricated with soft, long-fibered types of greases or oils that are extremely heavy. This will result in increased churning friction at higher speeds.
High temperatures are another side effect of using an excessive amount of lubricant.
It is possible to significantly lessen the heat-generating effects of lubricants by employing oils with sufficient film strength but low viscosity, as well as channeling or semi-channeling greases. This is one of the benefits of using these types of lubricants. The capacity of these greases to “channel,” or be pushed aside by the spinning ball or roller elements of a bearing, and to lie basically inert in the side cavities of the bearing or housing reservoir is the primary source of the advantages that these greases offer. Channeling greases are typically described as having a “short-fibered” structure and a buttery consistency, both of which contribute to the lubricant’s low shear rate. Even if a lubricant is supplied that has a slightly higher viscosity than the application required, this low temperature helps a functioning bearing to create a temperature equilibrium. This is true even if the application requires a viscosity that is somewhat lower. When there is more fluid friction, the temperature of the lubricant rises, and it stays at that level until the viscosity is decreased to the appropriate level. It is important to note, however, that the use of short-fibered greases in applications that are sensitive to vibration but do not include equipment rotation can cause damage known as “false brinelling.”
In general, greases are employed in situations where oils cannot be utilized. This includes areas where sealing does not occur or is insufficient, unclean environments, inaccessible locations, areas where oil pouring or splashing cannot be permitted, and areas where “sealed-for-life” lubrication is sought.
The majority of bearings experience premature failure due to static overload, wear, corrosion, failed lubrication, contamination, or overheating in their early years. It is possible to eliminate skidding, which is another common source of bearing difficulties, by guaranteeing that the bearing will always be loaded. Skidding refers to the incapacity of a rolling element to remain in rolling contact at all times.
Better bearing specification procedures have been proved to eliminate the vast majority of difficulties caused by static overload, as demonstrated by actual operations. By selecting, using, and storing lubricants in the appropriate manner, additional issues such as wear, corrosion, and lubricant failure, as well as contamination and overheating, can be avoided. The viscosity of the oil and the presence of moisture contamination are the key concerns, and in general, lubricants with a higher viscosity are favored.
When it comes to the mechanical design of reliable centrifugal pumps, there are a few things that need to be taken into consideration in addition to the numerous conceivable elements that could have an effect on bearing life. Bearings with precision tolerances are less tolerant of off-design pump operation than deep-groove Conrad-type radial bearings with slack internal clearance.
Because of their sensitivity to misalignment, ball bearings require careful mounting in order to remove this potential source of failure. The amount of misalignment allowed cannot be higher than 0.001 inch for every inch of the shaft’s length. It doesn’t matter what kind of cage a bearing has if it’s working in an improper alignment since it will eventually fail.
In conclusion, it is very important to remember that bearings should always have a trace amount of lubricant, which might take the form of oil or grease depending on the application. If this is not done, there will be damage to the bearing surfaces, which will shorten the life of the bearing. It is possible to prevent this damage by performing thorough cleaning and applying fresh lubricant as directed. Typically, there is a significant amount of time that passes between cleaning and re-lubing the bearings.
It is not difficult to determine whether or not a bearing requires oil. Check the indication for the level of oil at the site. Grease presents a unique challenge, as it is impossible to know when additional grease should be applied to a bearing. This is because the grease that is already there in the bearing does not abruptly lose its ability to lubricate it. These qualities diminish progressively with the passage of time. In order to identify when more grease should be added, prior operating experience, often known as history, is a helpful guide. The intervals are determined by the characteristics of the grease, the dimensions and configuration of the hearing, the working speed, the temperature, and the relative humidity.
It is recommended that the grease in the bearings of significant process pumps be changed once every 12 to 18 months. Because the lubricating capacity of grease naturally decreases with the passage of time, doing this will make the operation and maintenance of the pump more trustworthy.
If water or moisture is able to enter the hearing chamber, the intervals at which the bearings should be cleaned and re-lubricated should be reduced to a greater degree. Rain, hose-downs, pumps positioned under dripping equipment, mist, fog, and condensation are some of the other environmental factors that can contaminate bearings. Inadequate, damaged, or improperly filed shaft seals, as well as the breather cap and the lubricant oil fill cap, could all serve as potential entry points.
In most cases, grease is injected by means of a port known as a zerk (or zirk) fitting, or alternatively, the bearing end cover or housing cap must first be removed. Always remember to open the drain or expel port before injecting grease, whether you are doing so mechanically or hydraulically. Under the force of the pressure, the old grease will be expelled by the new grease. Also, don’t forget to close the drain port when you’re finished. The amount of grease that needs to be added is determined both by the size and design of the housing as well as the size of the bearing. The bearing should be completely saturated with grease, and the housing should be filled approximately 25% of the way. Overheating can occur when there is an excessive amount of grease.
To clean off the old grease from the bearing internals and the housing, do the following:
- Remove as much as you can by hand.
- Use a warm kerosene flush to clean the bearing and the housing.
- Next, a flush with mineral oil of SAE 10 viscosity should be performed. If the old grease has caked up and become solid, you should do the following:
- Soak the bearing and housing in heated kerosene.
- Use your hand to slowly rotate the bearing clockwise.
- Give the bearing a final cleaning with some clean degreasing solution or kerosene.
- Once more, rotate the bearing’s outer race by hand while imparting a light axial and radial stress to the balls and races. Do this while keeping the bearing at room temperature.
- Repeat the process of soaking and rinsing as many times as required until the earring rotates freely and smoothly.
After all of the old grease has been removed from the housing, it should be cleaned, and then it should be carefully inspected to ensure that there is no damage. If the bearing is not damaged in any way, it can be reinstated in the machine after being repacked with new grease of the appropriate sort and consistency, or it can be put away for use at a later time. Wrap the bearing entirely with wax or oilpaper and place it into a storage box so that it can be preserved for later use.
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Source and Bibliography:
- Know and Understand Centrifugal Pumps – Larry Bachus & Angel Custodio
- Youtube video training credit – SKF Group; JAES Company
- Photo credit: chiefengineerlog; ACOEM USA; Metcar