TGM’s Steam Technical Tips

1. Some generator fields are subject to what is called a thermal mass imbalance condition. The thermal mass imbalance condition occurs when the windings grow during warm up at uneven rates. This causes changes in the balance of the rotating field. One way to counter this is changing the vars (up and down) as the unit is brought up. This changes the current across the field warming it up evenly.



2. A good rule of thumb in determining proper tilt pad and bearing radial clearance is .0015" per inch journal diameter up to a 12" journal.



3. A good rule of thumb in determining proper sleeve type bearing radial clearance is .002" per inch journal diameter up to a 12" urnal.



4. A good rule of thumb in determining proper gland steam seal clearance is .002" per inch shaft diameter.



5. In order to properly calibrate vibration monitoring equipment, you should chart the output of the matched set of components. These items include the proximeter, extension cable and probe. Each set will be marginally different. The monitor should then be calibrated to match. Once the system is calibrated, the components should never be interchanged.



6. Large rotating equipment that appears to be excessively loud may be due to the grout failing. TGM has experienced a reduction of up to 30db after re-grouting old units with new epoxy grout. Not to mention the substantial improved dampening effect of the pedestal, which lowers the operating vibration level.



7. Non contacting vibration probes must have a minimum clearance of 0.23" around the probe tip because the radio frequency (RF) field extends outward from the side of the probe tip. Inadequate probe clearance will result in false information.



8. Vibration probe seperation - To ensure no cross-coupling of the electromagnetic field of probe tips, make sure that a minimum clearance of 1.5" between probes in any direction is maintained.



9. Bearing temperature detectors should be embedded close to the running surface. (0.030" back from the babbitt to backing metal bond interface) at the position of minimum film. It is critical that the metal backing is not machined through. This will provide an excellent indicator of bearing running status.



10. Tilting thrust pads that are supported on articulated leveling links which will distribute thrust load uniformly from thrust pad to thrust pad should be the bearing of choice as a retrofit.



11. Oil whirl is a purely oil film excited vibration, which can be visualized as a wave traveling around the shaft within the bearing clearance. The wave travels with the average velocity of the oil film, which is about half the shaft surface speed. You can diagnose it by its characteristic "1/2 frequency" which is actually slightly less that 1/2 speed.



12. Oil supplied to Hydrogen Cooled Generator bearings and hydrogen seals should be maintained within five degrees. By diverting temperature controlled bearing lubrication oil through the seal oil cooler, rather than water, the two oil supplies will always be close to equal in temperature.



13. Bearing temperature monitor alarms should be set at fifteen degrees above normal operating temperature. It is important to be notified of a deviation from normal rather than a deviation from a generic number.



14. An easy solution to verify bearing "Crush" can be accomplished with the use of a dial indicator. Simply bolt down one side of the pedestal cover and then set a dial indicator on that side of the joint inline with the center of the radial bearing. Now bolt down the one on the other side of the pedestal cover. This side must remain tight. Loosen the side of the cover with the dial indicator and the pedestal cover should lift if there is an interference between the bearing and the cover. Divide the reading on the dial indicator by two and this is the actual Crush/Pinch on that bearing.



15. Many people believe that a Spherical (Ball) seated bearings automatically align themselves when the rotor is placed in the bearings or during operation. This is not the case. Spherical seated bearings cannot self adjust when installed with the correct "Crush" of 1 to 3 mils.



16. A typical bearing design today uses oil supplied at 15 to 18 psig @ 120°F. With these conditions and normal operating speed, the anticipated oil drain temperature is 140°F to 160°F and the babbitt metal temperature in the loaded area can be expected to be 185°F to 240°F.



17. With the advent of modern motoring instruments it is very realistic to complete online step inspections, provided the data is recorded at regular intervals. It is critical that the initial data be recorded directly after a major inspection or installation of a new unit.



18. When rotating equipment exhibits oil whirl under normal operating parameters, the solution is usually to install tilt pad radial bearings



19. The unit trip system and generator field breaker should be tested after each unit inspection or at least once every 2 years. A unit will overspeed when it is backfed and motorizes. When this occurs, even for a short period of time, it can cause overheating and burning of the generator field insulation. This issue can be compounded if the field has any oil contamination issues. Additional concerns for a unit overspeed included rotor blading stress, high vibration rubs, and potential bearing problems. Any or all of these issues can be very expensive and can be avoided by properly testing your unit protection systems.