Info

shutdown for

Immediate

Smooth

Acceptable

Marginal

repairs

shutdown

0.1 in/s (p) and less

0.1-0.2 in/s (p)

0.2-0.3 in/s (p)

0.3-0.5 in/s (p)

0.5 in/s (p)

Note: For gearing, add 0.1 in/s to all values. p = peak mm/s = 25.4 X in/s

Note: For gearing, add 0.1 in/s to all values. p = peak mm/s = 25.4 X in/s

2. Bent rotors can sometimes be straightened by the "hot-spot" procedure, but this should be regarded as a temporary solution because bow will come back in time. Several rotor failures have resulted from this practice. If blades or disks have failed, check for corrosion fatigue, stress corrosion, resonance, off-design operation.

3. Straighten bow slowly, running on turning gear or at low speed. If rubbing occurs, trip unit immediately and keep the rotor turning 90° using a shaft wrench every 5 minutes until the rub clears; resume slow run. This may take 12 to 24 h.

4. Often requires complete rework or new case, but sometimes a mild distortion corrects itself with time (requires periodic internal and external realignment). Usually caused by excessive piping forces or thermal shock.

5. Usually caused by poor mat under the foundation or thermal stress (hot spots) or unequal shrinkage. May require extensive and costly repairs.

6. Slight rub may clear, but trip the unit immediately if a high-speed rub gets worse. Turn by hand until clear.

7. Unless thrust bearing has failed, this is caused by rapid changes of load and temperature. Machine should be opened and inspected.

8. Usually caused by excessive pipe strain or inadequate mounting and foundation, but is sometimes caused by local heat from pipes or the sun's heating the base and foundation.

9. Most trouble is caused by poor pipe supports (should use spring hangers), improperly used expansion joints, and poor pipe line up at casing connections. Foundation setting can also cause severe strain.

10. Bearings may become distorted from heat. Make a hot check, if possible, observing contact.

11. Watch for brown discoloration, which often precedes recurring failures. This indicates very high local oil film temperatures. Check rotor for vibration. Check bearing design and hot clearances. Check condition of oil, especially viscosity.

12. Check clearances and roundness of journal, as well as contact and tight bearing fit in the case. Watch out for vibration transmission from other sources and check the frequency. May require antiwhirl bearings or tilting-shoe bearings. Check especially for resonances at whirl frequency (or multiples) in foundation and piping.

13. This can excite resonances and criticals and combinations thereof at two times running frequency. Usually difficult to field balance because, when horizontal vibration improves, vertical vibration gets worse and vice versa. It may be necessary to increase horizontal bearing support stiffness (or mass) if the problem is severe.

14. Usually the result of slugging the machine with fluid, solids built up on rotor, or offdesign operation (especially surging).

15. The frequency at rotor support critical is characteristic. Disks and sleeves may have lost their interference fit by rapid temperature changes. Parts usually are not loose at standstill.

It is often confused with oil whirl because the characteristics are essentially the same. Before suspecting any whirl, make sure everything in the bearing assembly is absolutely tight with an interference fit. This should always be checked.

It usually involves shading pedestals and casing feet. Check for friction, proper clearance, and piping strains.

To obtain frequencies, tape a microphone to the gear case and record noise on magnetic tape.

Loose coupling sleeves are notorious troublemakers, especially in conjunction with long, heavy spacers. Check tooth fit by placing indicators on top, then lifting by hand or a jack and noting looseness (should not be more than 1-2 mils [0.025 to 0.05 mm] at standstill, at most). Use hollow coupling spacers. Make sure coupling hubs have at least 1 mil/in (1 mm/m) interference fit on shaft. Loose hubs have caused many shaft failures and serious vibration problems.

Try field balancing; more viscous oil (colder); larger, longer bearings with minimum clearance and tight fit; stiffen bearing supports and other structures between bearing and ground. This is basically a design problem. It may require additional stabilizing bearings or a solid coupling. It is difficult to correct in the field. With high-speed machines, adding mass at the bearing case helps considerably.

These are criticals of the spacer-teeth-overhang subsystem. Often encountered with long spacers. Make sure of tight-fitting teeth with a slight interference at standstill and make the spacer as light and stiff as possible (tubular). Consider using a solid or membrane coupling if the problem is severe. Check coupling balance.

Overhang criticals can be exceedingly troublesome. Long overhangs shift the nodal point of the rotor deflection line (free-free mode) toward the bearing, robbing the bearing of its damping capability. This can make critical speeds so rough it is impossible to pass through these speeds. Shorten the overhang or put in an outboard bearing for stabilization.

Casing resonance is also called case drumming. It can be very persistent but is sometimes harmless. The danger is that parts may come loose and fall into the machine. Also, rotor/casing interaction may be involved. Diaphragm drumming is serious because it can cause catastrophic failure of the diaphragm.

Local drumming is usually harmless, but major resonances, resulting in vibration of the entire case as a unit, are potentially dangerous because of possible rubs and component failures, as well as possible excitation of other vibrations.

Similar problems exist as in 24 and 25 with the added complications of settling, cracking, warping, and misalignment. This cause may also produce piping troubles and possible case warpage. Foundation resonance is serious and greatly reduces unit reliability.

Pressure pulsations can excite other vibrations with possible serious consequences. Eliminate such vibrations using restraints, flexible pipe supports, sway braces, shock absorbers, and so on, plus isolation of the foundation from piping, building, basement, and operating floor.

It occurs mostly at two times line frequency (7200 cpm), coming from motor and generator fields. Turn the fields off to verify the source. It is usually harmless, but if the foundation or other components (rotor critical or torsional) are resonant, the vibrations may be severe. There is a risk of catastrophic failure if there is a short circuit or other upsets.

This can excite serious vibrations or cause bearing failures. Isolate the piping and foundation and use shock absorbers and sway braces.

Valve vibration is rare but sometimes very violent. Such vibrations are aerodynami-cally excited. Change the valve shape to reduce turbulence and increase rigidity in the valve gear. Make sure the valve cannot spin.

The vibration is exactly one-half, one-quarter, one-eighth of the exciting frequency. It can be excited only in nonlinear systems; therefore, look for such things as looseness and aerodynamic or hydrodynamic excitations. It may involve rotor "shuttling." If so, check the seal system, thrust clearances, couplings, and rotor-stator clearance effects.

32. The vibrations are at two, three, and four times exciting frequency. The treatment is the same as for direct resonance: change the frequency and add damping.

33. If the cause is intermittent, look into temperature variations. Usually the rotor must be rebuilt, but first try to increase stator damping, add larger bearings (tilting-shoe), increase stator mass and stiffness, and improve the foundation. This problem is usually caused by maloperation, such as quick temperature changes and fluid slugging. Use membrane-type coupling.

34. This is basically a design problem, but is often aggravated by poor balancing and a poor foundation. Try to field-balance the rotor at operating speed, lower oil temperature, and use larger and tighter bearings.

35. Add mass or change stiffness to shift the resonant frequency. Add damping. Reduce excitation and improve system isolation. Reducing mass or stiffness can leave the amplitude the same even if resonant frequency shifts because of stronger amplification. Check "mobility."

36. Stiffen the foundation or bearing structure. Add mass at the bearing, increase critical speed, or use tilting-shoe bearings (which is the best solution). First, check for loose fit of bearings in bearing case.

37. Same comments as 36 with additional resonance of rotor, stator, foundation, piping, or external excitation; find the resonant members and the sources of excitation. Tilting-shoe bearings are the best. Check for loose bearings.

38. Sometimes you can bear the squeal of a bearing or seal, but frequency is usually ultrasonic—very destructive. Check for rotor vanes hitting the stator, especially if clearances are smaller than the oil film thickness plus rotor deflection while passing through the critical speed.

39. Usually accompanied by rocking motions and beating within clearances. It is serious especially in the bearing assembly. Frequencies are often below running frequency. Make sure everything is absolutely tight, with some interference. Line-in-line fits are usually not sufficient to positively prevent this type of problem.

40. This problem is very destructive and difficult to find. The symptoms are gear noise, wear on the hack side of teeth, strong electrical noise or vibration, loose coupling bolts, and fretting corrosion under the coupling bolts. There is wear on both sides of coupling teeth and possibly torsional-fatigue cracks in keyway ends. The best solution is to install properly tuned torsional vibration dampers.

41. It is similar to 40, but encountered only during startup and shutdown because of very strong torsional pulsations. It occurs in reciprocating machinery and synchronous motors. Check for torsional cracks.

Impeller Unbalance Impeller unbalance appears as a 1X running speed frequency vibration approximately 90% of the time and may be mechanical or hydraulic in origin. Impeller mechanical unbalance is a frequent cause of mechanical seal and bearing failures. Many mechanics will never think of checking impeller balance until heavily pitted areas appear. Because of the nonhomogeneous nature of most castings, corrosion is usually more aggressive in one area of the impeller. The degree of etching or surface pitting is a judgmental indicator of balance change. Impeller balancing should be part of the shop repair procedure for impellers over 10 in (25 cm) at 3600 rpm. It is good practice, when balancing an impeller, to keep the impeller bore to balance mandrel fit no greater than 0.001 in (0.0254 mm) loose. Installing the impeller with the keyway up on the balance mandrel and pump shaft will help eliminate some of the unbalance due to shaft centerline shift.

example A 38-lb (17.2 kg), 15 ji-in (39.4-cm) diameter impeller operating at 3600 rpm is balanced on a machine good to 25 X 10~6 in (635 X 10~6 mm) using an expanding man drel. The impeller is then installed on its shaft, which has a loose fit of 0.0035 in (0.0889 mm). The forces created by this shift in the center of mass is calculated as follows:

in USCS units

Unbalance = eccentricity of impeller (in) X impeller wt. force (oz) 0.0035

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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