Machinery Components Maintenance And Repair Episode 2 Part 9 pps

• Journal surface—Surfaces that have been scratched, pitted, or

scraped to depths of 0.001 in. or less are acceptable for use.

Deeper imperfections in the range of 0.001 to 0.005 in. must

be restored by strapping.

• Thrust collar—does it have good finish? Use same guidelines

as for journals. Is the locking nut and key tight? If the collar

is removed, is its fit proper? It should have 0.001 to 0.0005 in.

interference minimum.

3. The journals, coupling fits, overspeed trip, and other highly pol￾ished areas should be tightly wrapped and sealed with protective

cloth.

4. The rotor should be sandblasted using No. 5 grade, 80/120 mesh,

polishing compound, silica sand, or aluminum oxide.

5. When the rotor is clean, it should be again visually inspected.

6. Impellers and shaft sleeve rubs—rubs in excess of 5 mils deep in

labyrinth areas require reclaiming of that area.

7. Wheel location—have any wheels shifted out of position? Wheel

location should be measured from a thrust collar locating shoulder.

There should be a 4–5 mil gap between each component of the

rotor; i.e., each impeller, each sleeve, etc.

8. On areas suspected of having heat checking or cracks, a dye pene￾trant check should be made using standard techniques or “Zyglo”:

a. Preparation

Cracks in forgings probably have breathed; that is, they have

opened and closed during heat cycles, drawing in moist air that

has condensed in the cracks, forming oxides and filling cracks

with moisture. This prevents penetration by crack detection

solutions. To overcome this condition, all areas to be tested

should be heated by a gas torch to about 250°F and allowed to

cool before application of the penetrant.

These tests require a smooth surface as any irregularities will

trap penetrant and make it difficult to remove, thus giving a false

indication or obscuring a real defect.

b. Application

The penetrant is applied to the surface and allowed to seep into

cracks for 15 to 20 minutes. The surface is then cleaned and a

developer applied. The developer acts as a capillary agent (or

blotter) and draws the dyed penetrant from surface defects so it

is visible, thus indicating the presence of a discontinuity of the

surface. In “Zyglo” an ultraviolet light is used to view the

surface.

9. A more precise method of checking for a forging defect would

require magnetic particle check, “Magnaflux” or “Magnaglow.” As

Centrifugal Compressor Rotor Repair 515

these methods induce a magnetic field in the rotor, care must be

taken to ensure that the rotor is degaussed and all residual mag￾netism removed.

10. The rotor should be indicated with shaft supported at the journals:

a. Shaft run out (packing areas) 0.002 in. TIR max.

b. Impeller wobble—0.010 in. TIR—measured near O.D.

c. Shroud band wobble—0.020 in. TIR.

d. Thrust collar—0.0005 in. TIR measured on vertical face.

e. Vibration probe surfaces 0.0005 in. TIR—no chrome plating,

metallizing, etc., should be permitted in these areas.

f. Journal areas—0.0005 in. TIR, 20 micro in. rms or better.

g. Gaps between all adjacent shrink fit parts—should be 0.004 to

0.005 in.

11. If the shaft has a permanent bow in excess of the limit or if there

is evidence of impeller distress, i.e., heavy rubs or wobble, the rotor

must be disassembled. Similarly, if the journals or seal surfaces on

the shaft are badly scored, disassembly in most cases is indicated

as discussed below.

Disassembly of Rotor for Shaft Repair

If disassembly is required the following guidelines will be helpful.

1. The centrifugal rotor assembly is made with uniform shrink fit

engagement (3

/4 to 11

/2mil/in. of shaft diameter), and this requires an

impeller heating process or, in extreme cases, a combination process

of heating the impeller and cooling the shaft.

2. The shrinks are calculated to be released when the wheel is heated

to 600°F maximum. To exceed this figure could result in metallurgi￾cal changes in the wheel. Tempil® sticks should be used to ensure

this is not exceeded. The entire diameter of an impeller must be uni￾formly heated using “Rosebud” tips—two or more at the same time.

3. Generally a turbine wheel must be heated so that it expands

0.006–0.008 in. more than the shaft so that it is free to move on the

shaft.

4. The important thing to remember when removing impellers is that

the heat must be applied quickly to the rim section first. After the

rim section has been heated, heat is applied to the hub section, start￾ing at the outside. Never apply heat toward the bore with the remain￾der of the impeller cool.

5. To disassemble rotors, naturally the parts should be carefully marked

as taken apart so that identical parts can be replaced in the proper

516 Machinery Component Maintenance and Repair