Lubricating And Troubleshooting Centrifugal Pumps
Your site's pumping equipment calls for special treatment when it comes to lubrication. What you don't know about it could break the real 'heart' of your system.
(Author's Note: Much of the information in this series is based on the practical knowledge of real-world lubrication professionals. Once such expert is Mark Kavanaugh, who has over 42 years of experience in large manufacturing operations, and is currently responsible for coordinating the lubrication of thousands of pieces of rotating equipment in a refinery. Mark is certified as a CLS, MTL I and MLA II.)
Pumps are an integral part of any manufacturing plant. Most operations have to move fluids from point to point. Some large end-users, like chemical plants and refineries, have thousands of pumping systems. Pump operation and maintenance should never be taken for granted. Often referred to as the "heart" of a plant, pumps are key com-ponents in a site's overall reliability program. Table I lists various pump types classified as either positive displacement or centrifugal. This article will focus on centrifugals.
Centrifugal pump considerations
The centrifugal pump family includes radial-flow types (the most common) and axial-flow types.
- A radial-flow centrifugal pump uses a rotating impeller to create centrifugal force on a fluid, creating energy that results in flow. The flow is accelerated in a radial, outward motion by the pump impeller into a diffuser or volute chamber and exits into downstream piping.
- An axial-flow centrifugal pump develops flow from the lifting or propelling effect of the vanes on the impeller.
- In a mixed-flow variation of the radial- and axial-flow types, pressure is developed both by centrifugal force and the lifting effect of the vanes.
The two types of radial pumps are ANSI (based on the American National Standards Institute Pump Standard) and API (based on standards of the American Petroleum Institute). Standardized dimensions for ANSI pumps include:
- The distance between the suction and discharge nozzle centerlines
- The height of the pump coupling
- Location of the pump hold-down bolts
ANSI pumps are usually smaller, overhung units for light-duty types of service. API pumps (which meet API's higher standards) are typically used in heavier-duty services than ANSI pumps. There's a movement to combine both ANSI and API Standards into one standard for the U.S.
ANSI pumps have both radial and thrust rolling-element bearings. Typically, the radial bearings are single-row, deep-groove ball bearings. The thrust bearings are either paired angular-contact or double-row angular-contact ball bearings.
API pumps typically incorporate double-row, deep-groove ball bearings and angular-contact ball bearings for thrust load. For larger API heavy-duty pumps, radial bearings are cylindrical—for their greater load-carrying ability—and thrust bearings are paired tapered-roller types for thrust loads.
Sealing for general service is moving more toward bearing isolators or magnetic seals. Lip seals are no longer approved for API pumps, but are still used in some ANSI pumps. For pumping environmentally unfriendly fluids, mechanical seals are used.
Lubrication selection guidelines
Most pumps are lubricated with rust and oxidation inhibited oils (R&O) that also have small amounts of antifoam and demulsifier additives. Some pump manufacturers recommend antiwear (AW) additives, but most use R&O oils.
Viscosity is the most important criteria in the selection of the proper pump lubricant. Table II lists guidelines on the proper viscosity selection for rolling-element bearings.
The most common viscosity used for centrifugal pump rolling-element bearings is ISO VG 68. In some colder climates, ISO VG 32 is used.
Lubrication application methods
The proper application of lubricants is as important as the correct lubricant selection. The methods used are:
- Bath
- Bath with slinger ring
- Circulation system
- Pure mist
- Purge mist
Bath method…
The most common centrifugal-pump lubrication-application system is the oil bath fed by a bottle oiler. In some cases, depending on the amount of oil in the bearing housing, a slinger or flinger may be required to properly lubricate the bearing. Figure 1 depicts an oil bath in a centrifugal pump. When lubricating solely with an oil bath, the oil level should be maintained so it covers half the lower ball. In Fig. 1, the radial bearing is a deep-groove single-row ball type, and the thrust bearings are paired angular-contact ball bearings.
The Trico bottle oiler is the most common method used in establishing an oil bath. An oil sight glass made of acrylic is attached to the bottom of the oiler reservoir. This type of sight glass is useful in observing the color of the oil—and, in turn, helping to determine when to drain water and debris from the bottom of the reservoir.
Figure 2 shows how a bottle oiler maintains the oil level in an oil bath. As the oil level drops in the reservoir, the oiler feeds additional oil and breaks the seal in the bulb, introducing air, which allows oil to flow from the bulb. When the correct level is reached, the seal stops the oil flow.
Best practices when using a bottle oiler are as follows:
- Correctly set the level adjuster (spider). (I have seen some people throw them away.)
- Mount the bottom oiler on the correct side of the pump. The direction of rotation from the bottom of the shaft should be toward the bottle. This will prevent overfilling of the reservoir, resulting in high oil temperatures that can lead to bearing failure.
- The bottle should be mounted straight and not cocked or tilted.
- A standpipe (but preferably a breather) should be attach-ed to prevent overfilling due to the venturi effect of air currents passing over an opening.
- Fill the bottle when the oil level gets down to one-third. Constantly filling the bottle will introduce excess oil in the reservoir.
- Fill the bottle nearly full, but drain from an opening in the bottom of the reservoir. If no air bubble is seen, the line from the oiler to the reservoir is plugged and must be cleaned (not an uncommon problem). Drain enough oil to be 75% full. This important (but frequently over-looked) step, automatically resets the bath level to the correct depth.
If the pump design doesn't allow the oil to cover 50% of the bottom ball, an oil ring—preferably a flinger—should be installed. Ensuring that the oil ring/flinger is immersed at the correct oil level is crucial. API Standard 610 2.10 under lubrication recommends the level to be from .12 to .25 inches from the lower edge of the flinger, or above the lower edge of the bore of the oil ring. Pumps using oil rings can suffer the following lubrication problems:
- Slow ring rotation and insufficient lubrication from too high of an oil level
- Bouncing on shaft, likely caused by too low of an oil level
- Running against one of the bearings because the pump is out of level. (This could starve the bearing on the opposite end, elevate oil temperatures and introduce oil-ring wear particles to the bearing from rubbing.)
Check for oil rings that have become out of round. If they are found to be more than .010 out of round, replace them.
Looking at Figs. 3 and 4, can you detect any problems?
Oil-mist methods…
(Oil mist was discussed in detail in the March/April 2012 article on lubrication of electric motors. ) An oil-mist system produces a mist that can travel 600 feet and supply hundreds of lube points. It consists of oil particles <3 microns that go through a reclassifier and deposit a very thin oil film. The refining and petrochemical sectors have used this technology for 50+ years.
There are two types of oil-mist: pure and purge. Pure mist is applicable to ball bearings in pumps and electric motors. Purge mist is suited to any type of bearing. As with motors, oil-mist lubrication of pumps offers a number of benefits:
- Bearings running significantly cooler (15-30 degrees F)
- Clean once-through lubrication
- Greatly reduced bearing failures
- A trouble-free system with no moving parts
- Alarm systems that monitor oil level and flow rate
In pure-mist systems, pure oil is reclassified into a light film that's continuously deposited on the bearings. Any excess liquid goes to the collection container. The sole function of purge mist is to protect against contaminants by maintaining a slight positive pressure in the headspace above an oil bath. The oil bath does the lubricating. Sometimes, two different oils are used. That's fine as long as they're compatible.
Lubricants used for oil-mist on pumps and motors are the same. In warmer climates, a low-wax ISO 68 or 100 typically is used. In colder climates, to prevent waxing of the reclassifiers, an ISO 68 PAO or diester is recommended.
Troubleshooting
Be proactive in resolving any lubrication-related pump problems. Pay close attention to the following issues:
Contamination…
Contamination can contribute to cavitation (see pg. 13), a major cause of failed bearings. Contamination results from:
- Poor seals
- Improper venting
- Loose fill plug
- Poor storage and handling resulting in dirty oil
- Delivery of dirty oil
Steps for ensuring clean lubricants include:
- Following proper storage, handling and transfer procedures (i.e., indoor bulk storage, oil safe transfer containers, filter carts and clean, dedicated hoses)
- Testing incoming oil deliveries for particle counts and water content
- Installing desiccant breathers to help stop particle and water ingression from pump aspiration. (Some new models incorporate sealed fill-ports for top offs.)
- Replacing worn lip seals or upgrading to bearing isolators
- Training and certifying oil technicians in best practices
Mechanical issues…
While there are countless mechanical issues with pumps, two that are typically associated with lubrication are vibration and cavitation (see previous page).
Pump vibration can start for a variety of reasons, but when it is caused by structural stresses on the bearing housing—which affects alignment of the pump's bearings—the lubricant may not be able to do its job. Pipe strain on suction and discharge flanges, hollow bedplates or soft-foot on anchor bolts and misalignment of pump and driver couplings are the most common culprits. Each of these conditions twists, tweaks or pulls on the pump frame and bearing housing, causing changes in the small clearances of the bearing races. These clearance changes will have an adverse effect on lubrication film thickness and the ball's track in its groove. Such situations lead to ball sliding or skidding and rapid overheating.
Cavitation—implosion of gas bubbles on the interior surfaces of a pump or piping—is an amazingly destructive force. It can easily erode and break impellers, volutes, pump cases and piping.
Cavitation can be caused by leaking gaskets and O-rings that lead to air entrainment. A more common cause may be the effect of rising temperature and/or pressure on the vapor point of the liquid being pumped. Once vapor pressure is reached, some of the liquid flashes to a gas and the remaining pressurized liquid compresses and implodes this bubble of gas on the nearest surface. Pumps that are cavitating may sound like they are pumping rocks or marbles.
Best practices for operators in early lubrication-related mechanical-problem resolution include:
- Monitoring pumps and drivers closely to recognize subtle changes in oil levels, color, foaming and cleanliness
- Using infrared thermometers to check bearing and oil temperatures and inlet and outlet temps on oil coolers to determine efficiency
- Frequently draining small amounts of oil from bearing housings to inspect for particle ingression, wear debris and water content
- Noting changes in vibration, unusual sounds or oil leaks
- Being vigilant in contamination-control practices. (It can cost 10 times as much to remove particle contamination from oil as it does to prevent contamination in the first place.)
- On oil-misted bearings, draining manifolds and collection bottles and regularly checking mist flow at lube points
- Learning the proper use of desiccant breathers, filter carts and vacuum dehydrators
Conclusion
Pumps are an integral part of almost every plant. The key to their reliability is the early detection of potential problems. Operators and lubricant technicians are vital players in any reliability program. Utilizing some of the troubleshooting techniques discussed in this article will go a long ways in preventing premature pump failures at your site.
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