Maintenance

Introduction

The care of a fan unit should be carried out by routine inspec­tion performed according to a definite schedule. The timing in­tervals required are dependant upon the working conditions and environment of the fan and the demands of operational reli­ability. The latter should have been a major requirement in the fan specification and in the criteria for evaluation of competing products.

Good maintenance practices cannot reverse deficiencies of fan selection. Some fans may require attention every day; topping up oil reservoirs for example. Periods exceeding one week are inadvisable, especially in the case of fans in distant fanhouses and lacking alarm systems. Observations such as listening and feeling for vibrations, and checks on pressure, flow and power consumption should be performed with every inspection in ad­dition to the checking of gaskets and shaft seals. More detailed investigation should, of course, be undertaken if deviations from normal operation are noted.

If several fans are included in the system, the starting sequence should be adjusted with the lag-lead switch so that the running time is divided between all the units. Automatic and alarm de­vices should also be regularly tested.

In the absence of any specific instructions, the recommenda­tions given in Section 18.5.3 should be followed.

Routine inspection

In the selection of fans and the planning of their systems, it is necessary to take into account all aspects of maintenance. The assessment of a particular fan should be based on technical grounds connected with maintenance such as ease of disman­tling, availability of spare parts, trade skills required and above all, a record of reliability.

The difference between routine inspection on the one hand and routine maintenance on the other is often difficult to decide. In general, it is recommended that inspection, as well as mainte­nance, should be carried out on a regular basis. Some form of database of faults noted and measures taken should be initi­ated and up-dated using PCs to allow the storage and retrieval of considerable quantities of information. Software is available which enables machinery data to be stored and individual fan service histories to be retrieved.

Under normal circumstances handling clean air, the system will need cleaning about once a year. However, with the service his­tory of the fan and with regular routine inspections it should be possible to detect any unusual accumulation and modify these instructions accordingly.

The fan impeller should be specially checked for build up of ma­terial or dirt which may cause an imbalance, with resulting un­due wear on bearings and vee belt drives. A regular mainte­nance programme must be established to prevent this build-up.

The rotating assembly should be inspected regularly to detect any weakening of the impeller shaft and bearings resulting from corrosion, erosion, or metal fatigue.

Do not attempt any detailed inspection of a fan unless the elec­trical supply has been completely disconnected. If a disconnect switch has not been provided, remove all fuses from the circuit and lock the fuse panel so that they cannot be accidentally replaced.

Routine maintenance

Maintenance in this context refers to preventive maintenance intended to reduce the number of breakdowns and the resulting unscheduled shut-downs. Attempts have been made to calcu­late optimum maintenance statistics for fans, but the results are uncertain and the systems are difficult to handle.

Generalisation of fan maintenance is a waste of time. Fans op­erate in widely differing circumstances with a vast array of ma­terials. The best policy is to initiate a strict, very regular, routine inspection of the equipment and continue this until a pattern of equipment behaviour is apparent. At this stage, it may be possible to relax the inspection routine in some areas.

Running and maintenance instructions should be issued with every fan, if the requirements of the Machinery Directive are to be met. It is surprising how often this instruction is ignored. In the absence of any specific instructions, the following are sug­gested:

Every shift

When taking over plant at beginning of shift, operators should check that all bearings are cool.

Every week

Check for undue vibration. If present, stop fan at earliest oppor­tunity, check impeller for any dirt build-up on the blades, and clean as necessary.

Every 6 months

(a) Consult motor manufacturer’s manual and carry out in­structions.

(b) Examine vee belt pulleys for any chipping, tension of ropes, or

C) Check coupling alignment and condition.

Every 12 months

A) Examine impellers, fan bearings, inlet spigots/venturi. Check vee belts and pulleys or coupling element(s). If any wear, replace as necessary.

B) Check clearance at impeller, level of shaft, and general alignment. Adjust as necessary.

C) Check all H. D. bolts for tightness.

D) Refer to motor and control gear manufacturer’s mainte­nance instructions and act accordingly.

E) Grease-lubricated bearings should be cleaned out and grease renewed.

Refer also to all proprietary item literature and act as instructed. Lubrication should be carried out regularly according to operat­ing conditions.

Bearing lubrication

The whole question of bearing lubrication is addressed in Sec­tion 10.6. It will have been noted that the greasing intervals for ball bearings are invariably higher than for roller bearings. Nev­ertheless, spherical roller bearings are used particularly in many indirectly driven fans (e. g. vee belt drives) as they can re­sist the high radial loadings due to belt pull.

The instructions which follow do not apply to arrangement 4 and

5 fans (see Chapter 9) which should follow the motor manufac­turer’s recommendations.

Bearing size (mm)

Fan speed rev/min

125

250

500

1000

2000

4000

20

8000

8000

6300

3150

1600

710

30

8000

8000

5000

2500

1250

560

40

8000

8000

4500

2240

1000

400

50

8000

8000

4000

1800

800

315

55

8000

7500

3550

1700

750

280

60

8000

7100

3150

1600

710

224

65

8000

6300

2800

1400

630

180

Table 18.1 Relubrication interval (operating hours) for spherical roller bearings 18.5.4.1 Split roller bearings

Lubrication — Grease lubrication is usually satisfactory up to the likely fan maximum speeds, subject to consideration for temperature and axial loads. Greases are grouped according to maximum working temperature at the bearing. They are of­ten water-absorbent and many contain moisture and oxidation inhibitors. Extreme pressure additives can be advantageous especially for high axial loads.

For high temperatures and speeds it is always advisable to con­sult the fan manufacturer.

Procedure — Apply grease as follows:

For speeds up to dn* = 20 000 the roller cage should be coated with grease and the other parts lightly covered for protection, grease weights are as shown in Table 18.2. The remaining space in the lower half cartridge should be filled with grease or the whole cartridge may be completely filled to aid sealing in wet or dirty conditions.

(*d = bearing bore (mm), n = rev/min)

For speeds over dn = 20 000, the cage and parts should be coated as above plus 25% of listed grease weight in the car­tridge. Cartridges fitted with thrust bearings, which are used only up to dn = 20 000, should be completely filled with grease on assembly, including the bore of the thrust bearing.

All cartridge end bore seals should be well lubricated on assem­bly including the bores of the revolving triple labyrinth seals. Blanking plates should be sealed with grease or jointing com­pound. Swivel seatings should be lubricated: anti-scuffing com­pounds such as Molycote are useful.

Note: Never assemble the bearings dry or inject the grease after closing the cartridge during assembly.

Bearing bore (mm)

Series

01

02

03

50

0.03

0.05

60

0.05

0.07

65

0.05

0.07

70

0.06

0.10

80

0.10

0.15

90

0.12

0.20

0.40

100

0.17

0.30

0.46

125

0.20

0.37

0.46

Table 18.2 Recommended grease weights (kilograms) for split roller bearings Courtesy of Cooper Roller Bearings Ltd

Routine greasing

Expansion EX bearings: One or two shots from a grease gun two or three times a year (say every 1000 hours) is usually suffi­cient.

Fixed GR bearings for thrust: One or two shots from a grease gun every two weeks (say every 100 hours) or longer according to duty and experience.

Recommended lubricants

All the major oil companies have greases suitable for anti-fric­tion (ball and roller) bearings. Competition is fierce and contin­ued improvements are being made. The bearing manufacturer should always be consulted, but those shown in Table 18.3 have been found to be satisfactory in many types of fan.

Manufacturer

Normal temp (up to 80 0 C at brg)

High temp (up to 120 °C at brg)

Shell

Alvania RA

Darina R2

Mobil

Mobilplex 47 or 48 Mobilux 2 or 3

Mobiltemp 1

Beacon 2 or 3

Spheerol AP2 or 3

LS 2 or 3

Esso

Regal AFB 2

Ultra Temp

Admax L2 or L3

Admax B3

Multi-purpose No 2

Hi temp

Lupus A2

Bellatric 2

Rocol

BRB 1200

BRB 1200

Table 18.3 Recommended lubricants for many types of fan

Fixed GR bearings used for location only: Treat as expansion bearings.

Clean out and replace the grease yearly or as determined by the conditions.

Lubrication points — Cartridges are tapped ys or yA pipe ac­cording to size and series. Lubricating nipples or temporary plugs are fitted as standard. The lubricant is injected through the outer race directly to the rolling surfaces.

Grease weights — The weights given are sufficient to coat the roller bearing as described. A similar amount fills the remaining space in the lower half of the cartridge and thus three times the values given will completely fill the bearing and cartridge. All weights are approximate.

Extreme pressure greases are usually normal range and suffixed EP. High temperature greases should be checked for speeds over dn = 100 000, and replenishment intervals may be reduced.

Deviation from these standard recommendations is notified separately when required. It is essential not to overgrease as this will raise the running temperature of the bearing and may shorten its life.

Excessive vibration

Check for build-up of material on the wheel. Generally this will show up as material flaking off the fan wheel and causing an im­balance which may lead to fatigue failure of the wheel. Never al­low a fan to operate if the amplitude of vibration is above the maximum safe limit. Contact the manufacturer for this informa­tion, if it is not included in maintenance instructions.

High motor temperature

Check that cooling air to the motor has not been diverted or blocked by dirty guards or similar. Check input power: an in­crease may indicate that some major change has been made in the system. For other motor problems refer to motor manufac­turers’ instructions.

High fan bearing temperature

Usually caused by improper lubrication (either “over" or “un­der”). In every case if the cause of the trouble is not easily seen, experienced personnel should examine the equipment before it is put back into operation. (See Section 18.5.4.)

Posted in Fans Ventilation A Practical Guide


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