cooling tower and gearbox overhaul

7/27/2019 Cooling Tower and Gearbox Overhaul

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Cooling Tower and its Gearbox Overhaul

Operations and Maintenance

Report on the cooling tower design, principles andapplication, and complete report on overhaulingof its gearbox.

Advisor Engr.Wahab Javed7/15/2008

Submitted by Sharoon Saleem (MMD intern)


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Cooling Tower and its Gearbox OverhaulJul. 15

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Cooling Tower

A cooling tower is a heat rejection device, which extracts waste heat to the

atmosphere though the cooling of a water stream to a lower temperature.The type of heat rejection in a cooling tower is termed "evaporative" in that i t

allows a small portion of the water being cooled to evaporate into a moving airstream to provide significant cooling to the rest of that water stream. The heat fromthe water stream transferred to the air stream raises the air 's temperature and itsrelative humidity to 100%, and this air is discharged to the atmosphere.

The towers vary in size from small roof-top units to very large hyperboloidstructures that can be up to 200 meters tall and 100 meters in diameter, orrectangular structures that can be over 40 meters tall and 80 meters long.

At Lalpir/PakGen Cooling Towers are mainly used to condense steam comingfrom the low pressure turbine to create vacuum and hence act as driving force forsteam.The type of cooling tower installed at AES Lalpir/P akgen in the induced draughtcounter flow cooling tower.

Induced Draught Counter Flow Cooling TowerThis type of cooling tower is perhaps most common. It can be identified by the

fan at the top of the tower. The fan pulls air up through the tower in the oppositedirection to which the water is falling. The air usually enters the tower through inletlouvers on the sides of the tower.


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The name counter flow is derived from the air flow which is directly opposite of thewater flow. The term induced draft is because it is a mechanical draft tower with a

fan at the discharge which pulls air through tower. The fan induces hot moist air outthe discharge. This produces low entering and high exiting air velocities, reducing thepossibility of rec i rcula t ion in which discharged air flows back into the air intake. This

fan/fill arrangement is also known as draw-through .The counter flow tower has a fil lconfiguration through which air flowsvertically upward, counter to the fallingwater. The figure shows a schematic of theLalpir/PakGen cooling tower. The fill isarranged over the entire tower plan arearather than just at the outer perimeter as inthe cross flow tower. The air enters thetower through the openings in the lowerportion of the tower, turns 90 degrees, andpasses upward through the fill section,where heat and mass transfer between theair and the water take place. The air thenpasses through the drift eliminators above

the fill , enters the tower plenum space, and passes out through the fan stack.The fill in the counter flow tower is a typical film type fill . The fill consists of denselypacked, vertically oriented sheets of material [PVC], and functions by causing the hotwater to flow down the surfaces of the fill in a thin continous film. As air passes overthe water film, heat and mass transfer occur at the surface of the water film.This form of cooling tower has the advantage of maximum exposure of water toairflow. However, the thermal performance of this type of fil l is extremely sensitiveto poor water distribution, as well as to the air blockage and turbulence that a poorlydesigned fill support system can perpetuate. The overall tower design must assureuniform air and water flow throughout the entire fil l area.

Counter flow Advantages:

-Maximum thermal efficiency,-Smallest tower,-Lowest capital cost,-creates lower tower pumping head than cross flow tower.

The important points deduced are:


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1) The interaction of the air and water flow allows a partial equalization and evaporation of water.2) The air, now saturated with water vapor, is discharged from the cooling tower.

3) A collection or cold water basin is used to contain the water after its interaction with the air flow.

Components:

Blades:The fan model used at the plant is Hudson Tuflite series t-30 having a radial

blade of 16 in radius.The stability of blades is a crucial factor under rotation as a slight

malfunctioning can result in severe failure of the gearbox.

The claimed advantages of the manufacturer include:

High Toughness, Light-weight construction, Safety,High efficiency, High static pressure capabilit ies, Lownoise, Individual blades balanced to a master standard,Ultra-violet resistant, Erosion and chemical resistant.

Fills:Inside the tower, fills are added to increase contact surface as well as contact time between air and

water. Thus they provide better heat transfer. The efficiency of the tower also depends on them. Thereare two types of fills that may be used:

Film type fill (causes water to spread into a thin film) Splash type fill (breaks up water and interrupts its vertical progress)

Pressurized Nozzles:Pressurized nozzles are employed in the stated cooling tower type to sprinkle hotwater from the source, this water is cooled as it converts to streams by the air suckedin by the cooling tower fan.


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Gearbox:The most complex and driving part of the cooling tower is the gearbox of the fan.

The Sumitomo gearbox at AES Lalpir/Pakgen comprises of a simple gear train withhelical gear, spiral bevel pinion shaft and a spiral bevel gear set. The purpose of thegearbox is to reduce the driving force by a set of two gears,

The shaft at inlet of the gearbox has a pinion gear on the other end connected to anassembly of bevel gears (labeled 100 ) which act as the first speed reducer.

Spiral-bevel gears have teeth that are curved and oblique to their axes. The contact

Inlet

To Fan


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begins at one end of the tooth and progresses to the other.

An important point to consider in this design is the backlash 3 factor, the backlash iscrucial as it can make the gear movement jerky which in turn can affect the l ife of thebearings attached (labeled 600,6oi). From bevel gear duo the rotating shaft holdingthe bevel gear further drives the shaft that runs the fan blades.The bearings in this assembly are chosen to be tapered roller bearings because of their ability to store both radial and axial loads.


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Rolling-contact bearings use balls and rollers to exploit the small coefficients of when hard bodies roll

on each other. The balls and rollers are kept separated and equally spaced by a separator (cage, orretainer). This device, which is essential for proper bearing functioning, is responsible for additional friction.

End plates mark the end of the inside of the gearbox assembly and the exposed partof the shaft connected to the fan.

Labyrinth Cover accounts for the dire need of a water tight environment inside thegearbox.


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Overhauling Report:

Cooling tower, (for common terms consult glossary), gear reducers historically havebeen one of the least reliable pieces of equipment on the cooling tower. The gearreducers are typically mounted in the moist exit airstream directly below the fans,and routine scheduled maintenance is iften neglected. Inadequate lubrication,inappropriate lubricant viscosities, infrequent oil changes, and general neglect willeventually lead to bearing failure.

Other problems can arise from inadequate service factors and bearing liferating 3 . CTI Standard 111 recommends a minimum service factor for cooling towergear reducers of 2.0 and a minimum output shaft bearing L-10 expectancy of 100,000h*.

In large cooling towera, fans have large diameters to handle the large volumesof air more efficiently and to reduce fan horsepower requirements. For many years,

this has necessitated a two piece drive shaft with an intermediate bearing couplingbetween the two pieces. This textbook model is the one in operation atLalpir/PakGen. The intermediate bearing has historically been a high maintenancerequiring item*(replacement).

Malfunction was reported on 5 t h July 2008, the gear box was disassembled andbrought to mechanical maintenance workshop for repair. The end plate was badlydamaged, the labyrinth cover showed signs of damage as well. The tapered rollerbearing at the mouth of the gearbox at the exit shaft was damaged and rendereduseless.

The end plate was to be made by casting. Critical analysis was carried out tofind the root cause of failure.

The following theories of failure were developed at MMD on inspection of thegearbox:

1) Initially there was a finding of copper tube blockage due to incompatible oil causing insufficientgear lubrication leading to gear damage. As a result, the copper tube was replaced by SS tube.This problem was solved but the damage of gears continued indicating some other factors also.

2) Initially the fans were running with the blade pitch angle of 16 17 over loading the gear boxand causing to increase the temperature of oil which in turn became the cause of damage of gear boxes. Later on, Sumitomo was consulted. The pitch angle provided by them was 14 to15. This helped to reduce further the damage of gear boxes.


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3) Initially the fans were running with the blade pitch angle of 16 17 over loading the gear box

and causing to increase the temperature of oil which in turn became the cause of damage of gear boxes. Later on, Sumitomo was consulted. The pitch angle provided by them was 14 to15. This helped to reduce further the damage of gear boxes.

4) Shell Omala 220 oil is being used in CT gear boxes. The operating range of which is 30 Cambient temperatures but in actual, we have got 45 C to 48 C in summer season which isanother cause of CT gear boxes damage. To overcome the problem we have recently replacedShell omalla 220 with synthetic oil (Mobil HSC 630) in three gear boxes namely 1E, 1F & 1G. Theresult of this change is encouraging as there is a reduction in temperature of 2 C to 3 C as

compared to other gear boxes having Shell Omala 220, for details see section below.

5) Blade thrust imbalance could cause the shaft to be under large forces that if unbalanced by the bearing could damage it and once the bearing was damagedthe cycle mentioned in theory 1 could have continued.

6) A particle of the gears assembly could have been torn off due to pitting,causing bearing failure.

7)

Use of an inappropriate lubricant, whose viscosity level was higher than thatrequired by the bearings and gears.

8) Initially, backlash 2 /end floats of all gears were unknown and were adjusted by hit & trialmethod. During the course of time, Sumitomo was consulted and they provided backlash /endfloats data. After using this data although, the problem was reduced but could not beeliminated.

9) The fan installed on the gear boxes for self cooling is not providing sufficient cooling air to the gearboxes.

Besides all above factors the gear boxes are being operated above there capacity due to which any slightchange in any of the parameters affect adversely.


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Estimated cost of gear boxes overhauled

From March, 2002 to March, 2008Total gear boxes overhauled

38 Nos.

Material cost per gear box Rs.450000 approx.

Labor cost per gear box for removal,Overhauling and installation.

Rs.40000 approx.

Total amount per gear box Rs.485000 approx.

Heat Rate loss per day per cooling tower $ 4658

Table 1 . Based on data provided by mechanical maintenance depar tment .

Information acquired from condition:

1) Bearing failure, the bearing at the mouth of the output shaft to the fan bladewas found to be damaged.

2) End plate at the opening was also completely damaged.3) The labrynth cover showed slight indications of damage due to shaft movement

as well.

Lubricant being used in the Gearbox

The gearbox uses splash type lubrication employin g the oil pump at base of gearbox.Need of lubricant:

Extreme temperatures, Excessive shut downs from

mechanical problems, Equipment failure with mineral

lubricants, High energy consumption, Excessive parts consumption,

such as bearings, Inaccessible lubrication points.


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Bearing failure as per inspection was due to improper lubrication, the datasheet for the bearing is:

d D T C/N Co /N P u /N Speed ra t ing,lubr icant(oi l )r /min

Speedrat ing, lubr icant(grease)r /min

m/kg Number

150 270 49 429000 56000 57000 1300 1800 11 30230

The tripping temperature was reported to be 960

C, implying the lubricant to beused should have a high viscosity index.

The lubricant being used inside the gearbox was Shell Omala 220, a mineral oil .Its properties are:

COOLING TOWER GEAR BOX OIL ANALYSIS (Shell Omala 220)As per 19-Mar-08

SampleKinematic

viscosity (cSt)@ 40 C

Kinematicviscosity (cSt)

@ 100 C

ViscosityIndex 2

EmulsificationCharacteristics

Total acidnumber

(mg KOH/gm)

Foaming(ml)

Water

220(198 ~ 242)

19.4(17.46 ~21.34)

Min. 100EmulsionMax. 3 ml

Max. 0.5 Seq IIBy

CrackleMethod

LP Omala-220 Batch#10016876-062006(New unopened Top)

223 19.79 102 3 0.24 50/0 NilLP Omala-220 Batch#10017665-072007(Open drum LP Top used) 218 19.86 105 3 0.21 0/0 NilPG Omala-220 Batch#10019314-072007(Open drum PG Down used) 221 21.00 112 3 0.29 0/0 Nil


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Performance Features and Benefits

Outstanding oxidation and thermal stability

Withstands high thermal loading and resists the formation of sludge. Provides extended oil life, evenwith bulk oil temperatures of up to 100C in certain applications.

Effective corrosion inhibition

Protects both steel and bronze components, even in the presence of contamination by water andsolids.

Lead-free

Operator acceptability. Reduced health and product removal risks.

Wide range of viscosities

Caters for the most varied and arduous industrial applications.

Water shedding properties

Omala also have excellent water separation properties, such that excess water can be drained easilyfrom lubrication systems. Water can greatly accelerate surface fatigue with gears and bearings as wellas promoting ferrous corrosion on internal surfaces. Water contamination should therefore beavoided or removed as quickly as possible after the occurrence.

However for high temperature and efficiency requiring assemblies, a switchover has been made to synthetic oils, the graph below provides the necessarycomparison:


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At increasing temperatures, the lubricant shows a higher viscosity than mineraloil . The pink region marking the increased protection at higher temperatures andsuperior flow at lower temperatures.

The viscosity index of mineral oil only bare ly met the requiremts of the gearboxas temperatures of 96 were often reported at which the cooling tower trips.

An improvement that can be made and was actually made was to use asynthetic oil as replacement, as clearly shown by the deductions f rom the graph.

Synthetic oil lubricants cater to the lubricating needs of the gearbox moreeffectively.


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Recommendations:

1) The lubricant change is likely to enhance performance and decrease the failure ratesubstantially; results should be monitored of the number of times the tower tripsdue to temperature issues in the gearbox.

2) The number of instances reported of gearbox overhauls exceed 35 implying there is aneed for an improvement in design, and since the gearboxes have cost more thanthere pric e in maintenance, its a very considerable option to replace them with themodern enhanced design gearboxes.

3) Maintenance procedure requires a proper code of instructions to be provided, whichclearly mentions the heating temperature to fix bearings etc. So maintenanceoperations are well documented and are free of errors of all sorts.

4) The market leaders design for the cooling tower gearbox with its claimed benefits isattached; it provides an edge because of its ability to supersede competitors on thetemperature sustainability, low maintenance and high performance grounds.


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Glossary

Some commonly used terms in the cooling tower industry

Drift - Water droplets that are carried out of the cooling tower with the exhaust air.

Drift droplets have the same concentration of impurities as the water entering the

tower. The drift rate is typically reduced by employing baffle-like devices, called drift

eliminators, through which the air must travel after leaving the fill and spray zones of

the tower.

Blow-out - Water droplets blown out of the cooling tower by wind, generally at the air

inlet openings. Water may also be lost, in the absence of wind, through splashing or

misting. Devices such as wind screens, louvers, splash deflectors and water diverters

are used to limit these losses.

Plume - The stream of saturated exhaust air leaving the cooling tower. The plume is

visible when water vapor it contains condenses in contact with cooler ambient air,

l ike the saturated air in one's breath fogs on a cold day. Under certain conditions, a

cooling tower plume may present fogging or icing hazards to its surroundings. Note

that the water evaporated in the cooling process is "pure" water, in contrast to the

very small percentage of drift droplets or water blown out of the air inle ts.


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Blow-down - The portion of the circulating water flow that is removed in order to

maintain the amount of dissolved solids and other impurities at an acceptable level.

It may be noted that higher TDS (total dissolved solids) concentration in solution

results in greater potential cooling tower efficiency. However the higher the TDS

concentration, the greater the risk of scale, biological growth and corrosion.

Noise - Sound energy emitted by a cooling tower and heard (recorded) at a given

distance and direction. The sound is generated by the impact of falling water, by the

movement of air by fans, the fan blades moving in the structure, and the motors,

gearboxes or drive belts.

Approach - The approach is the difference in temperature between the cooled-water

temperature and the entering-air wet bulb temperature (twb). Since the cooling

towers are based on the principles of evaporative cooling, the maximum cooling

tower efficiency depends on the wet bulb temperature of t he air.

Range - The range is the temperature difference between the water inlet and water

exit.

Fill - Inside the tower, fil ls are added to increase contact surface as well as contact

time between air and water. Thus they provide better heat transfer. The efficiency of

the tower also depends on them. There are two types of fil ls that may be used:

Film type fill (causes water to spread into a thin fil m)

Splash type fill (breaks up water and interrupts its vertical progress)

1 VI (Viscosity Index)

An arbitrary scale used to show the magnitude of viscosity changes in lubricating oils with changes intemperature. Oils with low VI number such as VI=0 ("zero") have high dependence of viscosity changeon temperature. They thicken quickly with decreasing temperature, and thin out quickly withincreasing temperature. Oils with high VI number such as VI=200, will still thicken with decreasingtemperature but not as rapidly, and also will thin out with increasing temperature, but again not asmuch as low VI oil.


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VI number can also be "negative"

Tables found in ASTM Method D 2270 are widely used to determine VI number.

However, VI does not tell the whole story -- it only reflects the viscosity/temperature relationshipbetween temperatures of 40C and 100C. Two lubricants or base oils with the same VI number mayperform dramatically different at low temperatures in the -5C to - 50C range.

2. Backlash is the amount by which the width of a tooth space exceeds the thickness of the engagingtooth measured on the pitch circle

3. Bearing life is defined as the length of time, or the number of revolutions, until afatigue spall of a specific size develops. This spall size, regardless of the size of thebearing, is defined by an area of 0.01 inch 2 (6 mm 2 ). This life depends on manydifferent factors such as loading, speed, lubrication, fit t ing, setting, operatingtemperature, contamination, maintenance, plus many other environmental factors.Due to all these factors, the life of an individual bearing is impossible to predictprecisely. Also, bearings that may appear to be identical can exhibit considerable lifescatter when tested under identical conditions.

4. Detailed cross sectional picture of gearbox from maintenance books attached withindicated failure points.

cooling tower and gearbox overhaul

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