Summary . At the time the bearings of the Chumacera were lowered, a continuous sparking was observed in the internal track of the same, which led to the conclusion that the effect of static currents, product of the passage of hot air and with ferrous particles by the blades of the K216 fan and it's because of the bearings, they produced it. A collecting system was devised, through a carbon brush, of the bearing current to be drained to the ground, this being the same that the motor casing carries.
Keywords. Electric current, fan bearing, grounding system, bearing .
INTRODUCTION
In 2003, the Management of the Corporación de Cemento Andino CA through its projects department establishes the acquisition and assembly of a Cross Bar cooler, designed by the FL Smith group, for November of this year the complete system consisting of the hydraulic grill system, pneumatic cannons, transport system, air-air exchanger and filter were operational for a production of 2300 Ton / h.
For the beginning of 2009, damage to the bearings of the fan bearings, mainly on the free-side side, occurred with an increase in frequency, becoming monthly, and it began to be related to the higher passage of fine particles towards the chimney product of failures in the air cooling system, producing a static effect between the fan and the bearing. In previous years the effect also existed but the frequency of damage was less, close to three months, and after the scheduled furnace stops it was increased to 5 months, which was not so noticeable.
Fig 1. Clinkerization process
In the clinkerization process (Fig 1), the adjustment of the negative pressure air flow in the furnace is allowed to maintain a countercurrent to the material flow depending on the operating temperatures to produce its precalcination, and subsequent chemical reaction that transforms it into a clinker. After the clinker leaves the oven, it is necessary to pass it through the Cross Bar cooler, to lower the temperature from 900 C to 100 C, sending the material to the storage silos to be used in the cement grinding process. In the cooling stage, there is a draft of air caused by the K216 fan driving towards the filter and then being sent to the environment (Fig. 2).
Fig 2. K216 fan draft chimney
The K216 Cooler Filter fan system has a 315 KW 440 V motor (Fig. 3), a variable speed drive and a gate for the passage of gases through a duct. The operation of this equipment is based on having a controlled draft of air to maintain a pressure very close to zero in the furnace head to avoid distorting the burner flame. The variator through the speed adjustments allows a controlled draft to also establish a pressure differential in the filter that ranges from 12 to 15 mbar.
Fig 3. K216 fan motor
MATERIALS AND METHODS
By the K216 fan draft the particles pass at a speed that is a function of the speed of the variator, this occurs when the air exchanger does not have breaks in its heat exchange tubes; but due to the cooling process, the particles break the tubes and it is necessary to increase the draft to the maximum value to maintain a production of 140 Ton / h which is lower than the design one. To reestablish conditions that do not affect the environment, broken tubes are condemned in its head, which leads to the speed of the particles increasing and passing through the system causing the appearance of static currents that reached the bearing of the fan.
A study of bearing failures was carried out to determine how the current in the bearing can affect and the location in the causes of damage thereof (Fig. 4), and the solution depending on the type of failure that was related according to the Features found in the lowered bearing of the K216.
Fig. 4 Cause of bearing failure.
In failure in the bearing it was observed that the electric discharges have given rise to the formation of fusion craters in the zones of contact of the rolling elements with the raceway; sometimes multiple consecutive craters or complementary chains on the perimeter. The crater surface is sometimes in the form of weld beads (Fig. 5). The cause of this has been related to the continuous welding work carried out in the area where the fan bearings are in the furnace stops, since the current path to ground is part of the structure where the bearings are attached..
Fig. 5 Crater formation in the raceway.
Marks parallel to the axis that take on a brown color and occupy large parts of the raceway and even the entire perimeter of it (Fig. 6). The causes are due to the continuous flow of alternating or direct current, even in reduced current flow.
Fig. 6 Streak formation in the outer ring raceway.
Possible solutions to the two problems presented by the bearing were studied, determining that it was necessary not to let current flow through the bearing or to use bearings with insulation against the passage of current. It was chosen to apply the first proposal with a method of creating a path to earth from the currents that affected the bearing
Installation of the grounding system
Fig 7. Installation of the free-side grounding.
The grounding points of both the free-side and the drive-side pillow block were brought to the engine grounding system (Fig. 8).
Fig 8. Installation of the drive side grounding
Special bearing proposal
The K216 fan motor nameplate data (Fig. 8) and bearing type C2217C3-SKF were sent to a company in the United States to submit a quote for a special galvanic insulated bearing which was received and the amount of the It was five times greater than that normally used by the drive. The process for changing it was similar with both types of bearings.
Fig 8. K216 Fan motor nameplate data.
Bearing change process
With a retainer, a retaining ring is removed. With the extractor the bearings are removed, the new bearings are introduced, it must be greased, covering internally until the grease is horizontal to the bearing. Approximately one hour adjusts the lubrication to the sound that the motor presents and to the immediate results that are obtained with the vibrations. You put more fat if necessary, you must also be observing the temperature of the Pillow.
Summary of Costs - Benefits
Investment bearing C2217C3-SKF plus brush: $ 500.
Investment bearing with insulation: $ 2,500
Estimated annual benefit for savings on unscheduled stops $ 500,000.
III. RESULTS AND DISCUSSIONS
It was preferred to mount a manufacturing brush system with a medium voltage motor replacement to use a special bearing with galvanic isolation due to the delay time in the acquisition of the same and the higher cost of this compared to the first system.
Although there is no experience with the bearing with galvanic isolation, the good results that may be obtained with it for future application in this or other similar systems are not ruled out.
Since the brush system was installed, the bearing of the K216 fan was not damaged again due to the cause.
It is recommended to keep the air-air exchanger system in optimal conditions and if necessary, make a redesign so that the K216 draft fan works at its nominal conditions depending on the power supply of the oven.
IV. CONCLUSIONS
Based on the study carried out on bearings that have failures due to partial or continuous circulation of direct, alternating or parasitic currents through their tracks, a practical method was determined to carry them through a grounding path so that they do not affect the bearing. The economic saving for the Cemento Andino SA factory is very significant given the little investment made.
The solution to this problem guarantees technological independence, by developing an own alternative adapted to the failure that affected the cement plant.
REFERENCES
Siemens “Startup Manual and Advanced Topics for Adjustable Speed Drivers for Perfect Harmony Series AC Air Cooled Motors with Next Generation Control.” Manual number 902232 Version 3.0 July 2004, pp 1-1 a 8-33.
Fag bH Sales Europe GmbF "Bearing Breakdown Manual". Publ. No. WL 82 102/2 SB
Calle, OW and Lapo, A. F, Diagnosis of the state of three-phase induction motors through the analysis of vibration and current consumption with the use of the Labwiev software, Ecuador, 2009
Hidalgo, CJ, Analysis of electric motor failure zones, Grupo Termogram, Costa Rica, 2009.
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