Due Diligence
Q: What causes Thermal Imbalance?
A: 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.
Q: What is a proper clearance for a tilt pad bearing?
A: Proper tilt pad and bearing radial clearance is .0015" per inch journal diameter up to a 12" journal.
Q: What is a proper clearance for a tilt pad bearing?
A: Proper tilt pad and bearing radial clearance is .0015" per inch journal diameter up to a 12" journal.
Q: What is a proper clearance for sleeve type?
A: Sleeve type bearing radial clearance is .002" per inch journal diameter up to a 12" journal.
Q: What is a good clearance for steam seal clearance?
A: Proper gland steam seal clearance is .002" per inch shaft diameter.
Q: In a nut shell how do you calibrate a vibration monitoring system?
A: 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.
Q: My rotating equipment seems to be getting louder over the years?
A: Large rotating equipment that appears to be excessively loud may be due to the grout failing. ST CTS, Inc 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, this lowers the operating vibration level.
Q: What is the correct distance for vibration probe separation?
A: 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.
Q: What is the proper depth for setting bearing temperature detectors?
A: 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.
Q: How do tilting thrust pads work?
A: 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.
Q: What is oil whirl?
A: 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 than 1/2 speed.
Q: My seal oil and lubricating oil are 10 degrees different, it this OK?
A: 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.
Q: What should our alarms be set to go off at for bearing lube oil?
A: 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.
Q: What is the proper way to check crush?
A: 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 in line 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 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.
Q: Spherical bearing self align to the shaft and seat right?
A: 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.
Q: What are the parameters for normal bearing operation and lube oil?
A: 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.
Q: When and how is the best way to perform online step inspections?
A: 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.
Q: What is one way to stop oil whirl?
A: When rotating equipment exhibits oil whirl under normal operating parameters, the solution is usually to install tilt pad radial bearings
Q: When should field breakers and trip systems be inspected?
A: 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 back fed 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.
