Part V: Lubricating Process Fan Systems
EP Editorial Staff | December 5, 2012
Protection of what goes around will come around, in terms of reliability and productivity.
(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.)
This final part of a year-long series explores fans from the perspective of lubricant selection and application, and techniques for enhancing equipment reliability. The focus will be on these three fan-system types:
1. Heater fans that provide air in the heating of process streams (i.e., heating crude oil in refining processes).
2. Fin fans that cool processes by removing heat from process fluids.
3. Cooling-tower “fan” systems that transfer process-waste heat to ambient air.
Air must be introduced into high-temperature heaters for the combustion of natural gas that provides heat to the fluid in heat-exchanger tubes. The fan supplies that air in one of two ways: forced draft (where the air is introduced directly into the heater); and induced, where air is pulled into the heater through a venturi effect. In some cases, both a forced and induced fan is used. Referred to as “balancing,” this method requires less energy from the fan. That’s because bearings on the induced blower will run hotter than those of the forced blower.
A heater fan resembles a large paddle wheel—and functions like the impeller in a large centrifugal compressor. It’s connected to an electric motor on one side and a steam turbine on the other side. The electric motor is the primary driver; the turbine is the backup. The speed controller on the turbine is set at 200 RPM lower than the running speed of the motor. In the event of a power failure or if the motor slows down by 200 RPM, the turbine becomes the primary driver and there is no loss of air flow to the heater. Large turbines are stepped down from 3600 RPM, with a gearbox to run fans at up to 1800 RPM.
The lubricated components in a heater-fan system include the fan itself and the motor and turbine bearings.
Large, slow-speed fans (which have large pillow-block journal bearings) are lubricated with ISO 150 mineral or synthetic PAO oil. Smaller, higher-speed fans (which have pillow-block cylindrical roller or journal bearings) are lubricated with ISO 68 mineral or synthetic PAO oil. The electric motors in these fans usually operate in the 1800 RPM range. Smaller motors are sealed for life. Larger motors (> 75 hp) are lubricated with ISO 68 mineral oil by way of a bath with oil rings. Steam turbines with step-down gears are typically lubricated via circulating systems with ISO 68 mineral oil. When turbine-bearing temperatures are high, PAO synthetic oil is preferable. The bearings and gears are lubricated with the same oil.
Troubleshooting heater fans…
Heater-fan and electric-motor bearings with circulating systems or oil-bath lubrication are excellent candidates for oil-mist lubrication. The positive bearing housing pressures of these mist systems greatly reduce the ingression of particle and water contamination. All bearing temperatures, no matter how they are lubricated, should be monitored daily: Increased temperature is usually a first sign of trouble. On circulating systems, reservoir water should be drained and the oil-cooler inlet and outlet temperatures should be checked daily. Oil and grease levels should be checked at least weekly—if not daily.
Ultrasonic monitoring is the preferred method when it comes to adding the right amount of grease to a bearing. Oil-bath-lubricated pillow-block fan and electric-motor bearings with constant-level oilers usually contain only a quart or two of oil in their housings. It’s wise to drain approximately one-tenth of this amount once a month. This does three important things: 1) Water, debris or wear metal collected in the bottom of the housing will be removed and can be inspected. 2) The constant-level oiler should activate, proving that it is working and its pathway to the bearing is open. 3) Once the oiler activates, the oil level is returned to its preset height with fresh oil.
Increased fan vibration can be traced to a number of causes: The two most common are loose fasteners (i.e., foundation bolts, bearing housing capscrews, etc.); and buildup of debris on the impeller from unfiltered air that leads to an imbalance. The fan may have to be water-washed to remove this debris and the balance rechecked.
Fin fans are used in a large number of fluid-cooling applications. The main advantage over cooling with water is that they can be used in plants that aren’t near a supply of cooling water. These fan systems can be very large (i.e., those that cool the turbines in a power-gen facility) or very small (i.e., units that cool car radiators). A bank of fin-fan heat exchangers is shown in Fig. 1 (pg. 10). Representative of the most common type of fin fan, axial-flow, propeller-driven units like these can range in size from three to 60 feet in diameter and incorporate two to 20 blades.
Fig. 1. Axial-flow, propeller-driven heat exchangers like these represent the most common type of fin fans.
A fin fan cools by way of a tube bundle—an assembly of tubes, headers, side frames and tube supports. The fins on the tube surface exposed to the air essentially create an extended surface that provides better heat transfer. There are two basic arrangements for air distribution based on the fan location: forced draft when the fan is located below the tube bundle and air is forced over the tubes; and induced flow when the fan is located above the bundle and air is pulled over the tubes. There are advantages and disadvantages to both types.
In most cases, induced-draft fan advantages outweigh the disadvantages, including better distribution of air across the bundle and greater process control. Among their disadvantages, induced-draft units are typically less accessible for maintenance; fan blades and bearings are exposed to high effluent air temperatures; and the finned tubes are exposed to sun and rain.
Fin fans can be driven by a number of sources, with electric motors being the most common. The most popular speed reducer is the high-torque positive belt-drive used with motors up to 60 hp and fans up to 18 feet in diameter. Gear drives are used for very large electric motors and fan diameters. Two types of bearings are normally used: deep-groove ball bearings with small systems; and spherical roller bearings in pillow-block housings with larger systems.
Motor and shaft bearings in fin fans are normally grease-lubricated. Plants that use pure oil-mist lubrication with their pumps and motors now have the option of using it in the lubrication of their fin-fan and motor bearings. In the past, attempts to use oil mist for fan bearings led to housekeeping issues due to escaping mist. A recently introduced system, however, is capable of capturing the excess/stray mist, thus resolving the housekeeping problem. In addition, a new mist-lubricated motor design has been developed for fin-fan applications. Pure-mist lubrication of this new motor eliminates concerns associated with greasing in a difficult environment and—very importantly—assures that the lubricant will be applied properly. The result is extended bearing life. The bearings can be lubricated with either an ISO 220 R&O or PAO synthetic oil.
There are a number of options for the type of grease used on fin-fan bearings. The most effective are synthetic-based greases—with the most common incorporating an NLGI 2 lithium complex thickener with an ISO 220 PAO synthetic lubricant. In some cases, because of the high torque experienced with belt drives, an NLGI 2 thickener with an ISO 460 PAO synthetic lubricant is recommended. Mineral-oil-based greases with both lithium and polyurea thickeners are also used. The most common grease for electric motors is a polyurea thickener with an ISO ~ 100 mineral oil.
Remember: The application of a lubricant is as important as the selection of the right grease type. The following are options for greasing bearings:
- Manual delivery: Accomplished with a grease gun at each bearing (or with the aid of a divider block at multiple points). Because of the difficulty in manually greasing each point, the preferred method is use of a divider-block system from a remote location. Typically, one divider block can lubricate 30-40 fans—or more, depending on the system. Note that there can be problems with separation of oil and thickener from infrequent greasing. Therefore, it’s better to grease more frequently with smaller amounts of lubricant.
- Automated delivery: Accomplished with single-point lubricators or single-line-parallel centralized grease systems. Single-point electromechanical lubricators have also been used with success. (Some can pump up to a pressure of 350 PSI and lubricate two to six points.)
Troubleshooting fin fans…
Automated grease systems, proper sheave alignment and correct belt tension are the keys to long fin-fan life. Constant regreasing via an automated system prevents water and particle intrusion into the bearings. These systems should be inspected quarterly to ensure that they’re functioning as designed, lubricating at the correct rates and not leaking.
- Belts should be inspected semi-annually for wear, tension and sheave misalignment. If a belt is noisy (i.e., squeaking, roaring, etc.), it needs attention.
- Induced-draft fin-fans with intermittent duty service should be monitored closely: Their hot fin tubes can allow grease temperatures to spike when these fans are off and, thus, not pulling cool air across the motor and fan bearings.
Cooling-tower fan systems
Industrial cooling towers remove heat absorbed by water in circulating systems and transfer it to the atmosphere. Power plants, oil refineries, petrochemical facilities and natural-gas operations are large users of cooling water, as are many food processors. The two major cooling-tower manufacturers are Marley and Amarillo.
Fig. 2. Gearboxes in cooling towers are typically single- or double-reduction units, with right-angle spiral-bevel or right-angle helical gears. (Courtesy: Colfax Corp.)
Cooling-tower fan-system lubrication…
The main lubricated components in a cooling-tower system are the gearbox and electric-motor driver. The gearboxes are typically single- or double-reduction units (as shown in Fig. 2), with are spiral-bevel or helical right-angle gears. The spiral-bevel design features intersecting shafts; the shafts in the helical design are non-intersecting.
The bearings in these gearboxes are roller types. Tapered roller bearings are used to handle both radial and thrust loads. Cylindrical bearings set an an angle to handle radial and thrust loads can also be used, as can bearing types based on the gearbox OEM.
Gears and bearings in cooling-tower systems operate in extremely difficult environments—marked by high moisture content, high-temperatures and high risk of particulate contamination. This makes changing oil and monitoring oil condition quite challenging.
Electric-motor bearings are typically greased manually every six to 12 months. While the grease of choice is a polyurea with an ISO 100 mineral oil, a lithium complex thickener with PAO base oil can also be used. Be advised that oil selection differs between Marley and Amarillo cooling towers. Both require non-EP oils. Marley, though, recommends an ISO 150, while Amarillo requires an ISO 220. Some users consolidate for both OEM’s gearboxes and use ISO 220 oil. Because of the high-temperature/high-moisture conditions in cooling towers, synthetics are recommended (for the important degree of oil/water separation they provide). PAOs also provide good lubricity for protection of gears without using EP. (The oxidative life of a PAO is far superior to a mineral oil.) Cooling-tower OEMs recommend oil changes every six months—and also recommend, with the use of synthetics, that changeouts be based on oil condition. Synthetic oils have been known to last in excess of three years.
Splash lubrication is used for cooling-tower gearbox systems, along with an oversized slinger on the input shaft that provides oil to the bearings by way of channels and baffles. Some double-reduction systems can be supplied with a pump for better oil distribution.
Facilities with oil-mist systems may want to consider using purge-mist in their cooling-tower gearboxes to keep out contaminants (especially water). To prevent corrosion, cooling towers that sit idle over winter months can be protected with mist lubrication to coat thier internal surfaces. Desiccant breathers are another way to minimize water-vapor intrusion in this equipment.
It goes without saying that it’s tough to add oil and sample from a gearbox surrounded by a shroud. Some operations, therefore, choose to run cooling-tower gearboxes to failure without an oil-condition-monitoring system. At a minimum, a sight glass should be installed outside the shroud to monitor oil levels.
Some plants use filter carts on a semi-annual basis to sample and clean cooling-tower oil (an approach that calls for the help of a crane). Use of the drain line outside the shroud to collect samples requires draining at least two gallons of oil, depending on the size of the line. Oil is drained of water until warm oil flows. After proper purging, a sample is collected and the purged oil is returned to the gearbox.
Another method involves installing an off-line pipe circulation system from the gearbox to outside the cooling tower shroud to connect the fill and drain lines. A pump can then be used to sample the gearboxes.
The ability to clean and monitor the oil will contribute to longer gearbox life. For condition of the oil and the gearbox, oil analysis should be performed—preferably on a quarterly basis or, at a minimum, semiannually. The recommended tests include:
- Atomic Emission Spectroscopy
- Viscosity at 40 C
- Acid Number
- Water by Karl Fischer
- Particle Quantifier or Direct Read Ferrogram
- Particle Count (for filtered gear boxes)
- Analytical Ferrography (as required)
An effective oil-analysis program will optimize change intervals and identify potential equipment problems at an early stage, thus helping prevent unexpected failures. As with other equipment, clean oil can go a long way toward enhancing gearbox life. Consider the experience of one major steam-turbine power generation plant that installed an offline filter circulation system to remove particles and moisture from its cooling-tower gearboxes. The system also allowed the plant—for the first time—to collect oil-analysis samples. This analysis verified the effectiveness of the offline filtration system and led to longer equipment life.
The above options reflect just a few ways ways to lubricate and monitor cooling-tower fan systems. Many innovative end-users have developed their own effective approaches. Be on the lookout for such strategies.
Troubleshooting cooling-tower systems. . .
By design, cooling towers can have very long driveshafts between motor and gearbox. Precise alignment of driveshaft couplings is a must for long service life.
Cooling towers made of wood are not as structurally sound as their metal or plastic counterparts. All types of fasteners—nails, capscrews, mounting bolts, etc.—will loosen over time. If a fan picks up an increase in vibration, check its fasteners first.
Newer towers have solid composite plastic fan blades. Older towers may have hollow sheet metal blades with weep holes in the end of each blade to expel condensed water. If these holes become plugged, trapped water will cause severe imbalance and harsh vibration.
Gearbox vent lines should be run to the outside of the fan shroud and have desiccant breathers installed to reduce ingressed water.
As mentioned previously, cooling-tower oil changes, top-offs and routine analyses aren’t easy. Adding a circulating system with offline filtration and sample ports—or nothing more than a header system to assist with drains and top-offs—can dramatically increase gearbox life.
Process fans represent critical systems with specific lubrication and troubleshooting issues. Recommendations in this article are just that: recommendations. Always adhere to OEM guidelines regarding lubricant selection and to guidelines from your lubricant supplier regarding correct application. LMT
Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Telephone: (281) 250-0279. Email: firstname.lastname@example.org.
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