TRI has been asked by a number of parties about the correlation between changes in LP Turbine Rotor Vibrations and changes in condenser back-pressure and between changes in LP turbine rotor vibrations and changes in condensate temperature, as well as what to do to reduce the sensitivity of the LP Turbine rotor vibrations to these changes in back-pressure and temperature.
The purpose of this case study is to address both the correlations and to provide recommendations to reduce the rotor vibration sensitivity.
Yes, there can be a correlation in each case. The way it works is as follows:
There can be, and usually is, a correlation between LP Turbine back-pressure and elevation of the cone support structures for the bearings in the LP turbine hoods. Also, there is usually a correlation between the LP Turbine condensate temperature and the elevation of the cone support structures for the bearings in the LP Turbine hoods.
Dropping the elevation of one bearing shifts the loading on several bearings in both directions along the shaft line. In particular, the two adjacent bearings will pick up loading, but others are affected, carrying slightly less or more load. The load shift for the various pairs of adjacent bearings of a unit, for example, Bearings (4,5), (6,7), and, if they exist, Bearings (8,9), may be on the order of 1000 to 4000 lbs per mil (0.001 inches) of elevation change between adjacent bearings.
A change of loading on a bearing changes the film stiffness of that bearing: Increasing the loading increases the stiffness, and vice versa. Changing the stiffness of the bearing film of a turbine shaft can change the effective critical speed, often not much, but in the extreme, the critical speed can actually pass through the operating speed. Furthermore, almost all rotors in service are unbalanced to some degree. If a critical speed is near the operating speed and that critical speed changes, the vibratory response (amplitude and angle) at the operating speed due to the existing unbalance usually changes.
In order to understand the actual degree of change of 1X, or synchronous, vibration in a given situation, it is helpful to plot the vibratory response on a polar plot, including the slow roll data point and the roll up data for each vibration probe of the machine. The most reliable data for understanding what is going on at each bearing in a GE machine is the “left side” data, even though the “right side” is usually the side with the larger amplitude. For a counter-clockwise rotating machine (GE), the left side is what is called the “hard side” because the thinnest film location is approximately directly in line with this probe on the lower right of the journal, and 1 mil of motion in the direction parallel to the left side probe typically represents greater vibratory force on the bearing than 1 mil of motion measured on the right side probe. The right side is the “soft side” because the journal can slide along the thin film zone, which is why the vibrations on this side are usually larger, but less important than the vibrations measured on the left (hard) side.
Typically, if the overall vibration is substantially more than the 1X vibration, then the remainder is generally sub-synchronous. If there is subsynchronous vibration, then this is important to note. A small amount of sub-synchronous is usually not a problem, but if it appears that the amplitude of the sub-synchronous component could jump to large amplitude motion, the situation should be evaluated for further action. If the unit has experienced large amplitude sub-synchronous vibratory motion, then it should be evaluated for remedial action, which TRI can offer in various forms.
There are typically two ways to support condensers. For reference, older condensers were fixed to, that is, hung from, the turbine deck and they had spring supports underneath the condenser. In this case, no flexible seal was necessary at the LP Turbine on the turbine deck. In the older units of this design, it was absolutely critical to have the condenser filled with the proper amount of water when cold alignments of the turbine-generator were made. Too much water in the condenser, and the turbine deck would be pulled down too far, and the alignment would not be right. Not enough water, and the turbine deck would not be pulled down enough, and the alignment would not be right.
The newer design, which began in the 1950s, is to have the condensers supported rigidly underneath from the basement, and to have a flexible sealing connection (dog bone) around the top of the condenser to the LP turbine at the turbine deck. The larger units, which are generally the subject of this Case Study, are made with the flexible sealing connection to the LP Turbine hood. When vacuum is drawn in the condenser, the entire LP Turbine hood is pulled down, as if a weight equivalent to (14.7 psia – actual condenser pressure in psia) times the projected area of the condenser connection to the LP Turbine (sq inches) is loaded evenly on top of the LP Turbine hood. The turbine hood and surrounding foundation drop accordingly. For an LP Turbine, the reinforced concrete (or fabricated steel) support structure must carry the LP Turbine hood, the LP Turbine rotor, and the entire vacuum loading. In addition, the ends of the turbine hood are pushed inward, and this helps to rotate the cone structures that support the LP turbine rotors. The internal support structure (struts, gussets, and the like) within each hood affects how the bearing elevation and bearing rotation are changed as a result of the forces due to the changes in vacuum loading.
The vertical component of the change in vacuum loading in the present case is calculated as follows: When the backpressure in the condenser is on the order of 1.4 in Hg, which corresponds to 0.7 psia, the total vacuum loading on the foundation is (14.7-0.7 = 14 psi) acting over the entire area of the LP Turbine shell. If each LP Turbine shell is 20 ft x 20 ft, this is 400 sq ft or 57,600 sq inches, the total force downwards is 806,000 lbf. This force will drop the elevation of the entire LP Turbine structure. If the overall stiffness of the LP hood support structure is approximately 100 million lbf/inch, then the 14 psi pressure will cause the turbine deck to drop by 0.008 inches (8 mils).
Dropping the backpressure from 1.4 inches Hg (0.7 psia) to 0.8 inches Hg (0.4 psia) increases the downward pressure on the Hood by 0.3 psi, which corresponds to another 17,280 lbf downward loading. This 0.3 psi change of loading would drop the turbine deck by an additional 0.17 mils.
Each of the bearings of an LP Turbine is in a structure called a cone. It is, in effect, a tunnel that extends out into the LP Hood. These cone/tunnel structures are designed to move the bearings toward the center of the LP Turbine rotors, reducing the bearing spans of the LP Turbine rotors. When vacuum is drawn, the hood changes shape and each cone/tunnel structure rotates.
A change of temperature can cause a significant change in the shape of a large steel structure, such as an LP hood. For instance, a 10 degree F change in a steel bar that is 16 inches long will change the length by 0.001 inches. If a steel structure that is 48 inches in length is subjected to a temperature change of 30 degrees (105 – 75) deg F, it will change length by 0.009 inches.
Experience over many years indicates that it is very difficult to know how the bearings and the shaft line are going to respond to a change in vacuum (back-pressure) and/or to a change in temperature of the LP hood before a machine and its mounting structure are built. Similar units react in different ways. How the LP hood is constructed including what gussets exist, how the hood is supported by the foundation, and how the foundation is built (concrete or steel) affect how the bearing elevations will change with a change in vacuum and/or condensate temperature.
Usually the foundation is a reinforced concrete structure down to bedrock or to substantial pilings, but some foundations are steel structures. The long vertical piers under the bearings are cooled and change length when the ambient temperature changes, in an amount as described above. Some columns change length more or less than others because some are more exposed to radiant heat from steam pipes than are others, and some are more exposed to changes in ambient temperature than others. Some units are so sensitive to ambient air temperature that opening a big door in cold weather will cause the vibrations to change almost immediately, and in some extreme cases, the sound of the machine vibrations will change, and will do so in a repeatable manner.
The alignment of the LP turbine rotors relative to each other and to the adjacent turbine or generator rotors is often intentionally adjusted from time to time by turbine engineers, so that at normal operating temperatures, the bearings are fairly evenly loaded as represented by the bearing metal temperatures. It is imperative to retain plant records of what alignment conditions provide the best operating conditions, and to use the latest records of what works best for each unit in the process of aligning the rotors for that unit during each subsequent outage.
In 1973, I took a considerable amount of test data on a 500 MW tandem compound GE LST-G which showed the following trends. When cold and when no vacuum was drawn, the LP Turbine Bearing 5 was substantially unloaded. As the temperature in the condenser increased, the LP hood expanded, raising Bearing 5. When the unit was at normal operating temperatures, the bearing metal temperatures evened out showing that the loading on these two bearings evened out.
On the other hand, other GE units of a slightly larger size have Bearing 4 unloaded and Bearing 5 loaded when cold. Yet, at operating conditions, the bearing metal temperatures are approximately the same.
In any case, it is typical that the largest mismatch between couplings in cold conditions along a GE tandem-compound train occurs between the last IP Bearing and the first LP Bearing. In other words, the first LP Bearing sees more variation in loading than any other LP turbine Bearing. The last Turbine bearing, adjacent to the generator, also sees variation in loading, but usually not so much variation as the first LP Bearing experiences.
It is difficult to try to modify the hood structures in such a manner as to reduce the sensitivity of bearing loading to changes of vacuum condition or to condensate temperature. If these changes become concern items on a continuing basis, then the least costly options are (a) to change the first LP Bearing from an elliptical bearing to a tilting pad bearing, such as a TRI Align-A-Pad ® Bearing, and/or (b) to use a variable speed pump to control the circulating water flow rate to maintain uniform condensate temperature and backpressure.
Summary of Recommendations:
- Because most of the GE LP turbine bearings in the original configurations are marginally stable, that is, they are not far from experiencing sub-synchronous vibrations such as “oil-whip”, it is very helpful to have the LP Bearings mounted as rigidly as possible in the cone structures, or standards, as the case may be.
- For those LP bearings that are adjacent to the IP turbine or to the generator and experience substantial elevation changes, and have demonstrated any tendency toward sub-synchronous vibrations, consideration of changing to a tilting pad bearing such as the TRI Align-A-Pad ® Bearing is appropriate.
- The standard GE elliptical bearings can be modified using a TRI proprietary design method in a way to improve the stability of the bearings while keeping the original Babbitted bearing length. In some cases, converting to shortened elliptical bearings is advantageous. In other cases, the bearings should not be shortened because the Babbitt becomes overloaded, and in this case, other design modifications can be made by TRI proprietary methods to reduce the amplitudes of synchronous rotor vibrations as well as to improve stability (resistance to sub-synchronous vibrations).
- It is very important that the turbine engineers maintain accurate records of what alignment works for each machine. Trying to follow the OEM recommendations in the original mechanical design data (instruction) book is not necessarily advisable because this data may have been written before the unit was actually built and does not take into consideration how the unit is being operated, peculiarities of the foundation design, whether it is in indoor or outdoor unit, the range of ambient temperature conditions to which the unit is exposed, or what has been learned from this unit about what makes it work best. Bearing metal temperatures, vibratory characteristics, orbit shapes, and wear patterns should be used to adjust the alignment from time to time to optimize the performance of the machine. In many cases, bearing designs have been changed to permit a wider tolerance on the alignment data that is used. This is a clear benefit of using the TRI Align-A-Pad ® tilting pad bearings in places that are susceptible to a range of alignment conditions during operation, such as result from high temperature conditions of the standard when at high load and cooler temperature conditions when at low load.
- Where over-cooling of the condensate has occurred with the normal operating conditions for the circulating water pumps, it is appropriate to consider variable speed motors. Under certain circumstances, two-speed motors can be used. In extreme circumstance, turning on or off the circ water pumps may be appropriate to control the cooling of the condensers. Exercising any one of these options may help to optimize the cycle efficiency under various plant conditions and/or ambient conditions.
Because most Large Steam Turbine-Generators were built to operate at or near full load, and because capital costs were (and are) always a critical factor in new equipment, most LST-G units were made with a certain number of constant speed circulating water pumps, and these were intended to be on all of the time, and not cycle on and off. Nevertheless, some units have been modified to have two speed condenser cooling water pumps. For some units, variable speed circulating water pumps are being considered. Incidentally, in some cases, variable speed pumps are being considered to minimize water removed from a river primarily for external reasons, but would provide the possibility of cycle efficiency optimization.
There is a compromise in establishing the preferred condenser back-pressure: In almost all cases, the colder the cooling water, the lower the back-pressure, and the more MW load that is generated, with all other conditions being the same. However, the colder the condensate, the more heat that is required to heat the condensate to make the steam for the turbine. The objective should be to cool the condensate no more than is necessary to optimize the efficiency of the cycle. For those units that are operating at maximum firing rate, over-cooling the condensate will actually reduce the MW generated.
An issue to be considered in selecting an option for varying the circulating water flow is this: The motors for circulation water pumps are huge, and these motors and the associated switchgear typically were not designed to be started often. Stopping one of these motors and restarting it in a cyclic manner is not wise, even if it might optimize the cycle efficiency.
It is not often that an FD Fan, ID Fan, BFP, or other large auxiliary is forced out of service. Consequently, when the MW load is dropped for a period of several days to repair one of these items, it might be worthwhile to take a circulation water pump off, so long as the condenser function remains reasonably uniform for all of the condenser and LP Turbine sections.