Article written by Cliff Knight of KnightHawk Engineering.

Once again, a debottleneck project is finished, and the plant is complete and coming on line. As it turns out, Process Engineering identified ways to achieve more pounds per hour by upgrading a quench tower with new trays and by adding a couple of new nozzles in the tower.

The turnaround went smoothly, and testing went well. One of the key components in the system was a turbine compressor train pushing cracked gas through the system. Things are looking good and, as the plant rotating equipment engineer, you have made your rounds and all the equipment is within allowable ranges regarding temperatures and vibration. You go home with a good feeling and rest easy as you get ready for the next day when the plant is slated to reach full production and start making product.

While the quench tower modifications were easy, the major part of the project involved the compressor upgrades that consisted mainly of tilt pad bearings, impellers and controls. New recycle lines, knockout drums and so forth were installed to handle the added capacity. It was not anticipated that there would be any problems, as this unit had previously run well in all aspects.

Compressor drive train noise and vibration

However, things went south when the train went on recycle. The noise was louder than you’ve ever heard, and operations reported cracking in one of the compressor drums. To make matters worse, the pedestal bearing between the turbine and compressor had unacceptable vibration levels. Since the train is critical to plant operation and the plant was unable to run, your office became a good stop over point for management to vent.

The problems that occurred with this upgrade are not unusual. The turbine and compressor were outfitted with the latest hardware as a technology upgrade in addition to the capacity increase with the change out of the impellers. The plant was “bit” by two problems with this upgrade.

Acoustical Analysis

First, the reconfiguration of the recycle system along with the impeller upgrade excited the fourth acoustical mode in the compressor recycle piping, which coupled to the drum natural frequency and led to the failure. No acoustical analysis of the system was done. The drum diameter was unnecessarily large and could easily couple into any acoustic pulsation.

The second problem was that no sensitivity study was done on the bearing stiffness when considering the new tilt pad bearing in the turbine. The pedestal bearing was too weak, and there was an interference at the first mode that caused the pedestal to vibrate. Another part of the problem was that the bearing base plates were not rebuilt during the upgrade.

Some points to consider when upgrading a turbine compressor train such as this one are as follows:

1. Perform a complete process analysis of the new train.

2. Develop a new process specification sheet for the equipment.

3. Revisit the old rotor dynamics studies of the existing equipment and operation.

Determine that the old simulation properly identified the critical components in the system. It would also be a good idea to revisit all previous analysis with the latest rotor dynamics tools. Finally, develop a tuned or normalized model of the existing operating system.

4. Develop a finite element model of the pedestal bearing to determine actual stiffness.

5. Perform a complete rotor dynamics study of the system.

6. Perform an acoustical analysis of the recycle piping and discharge. Make sure there is no interference with blade or vane pass in the system.

7. Conduct a review of results with process, maintenance, mechanical and operations before signoff.

This is just a brief review of the ballgame. Turbine compressor trains are complex, and each has its own characteristics. All work should be reviewed and approved by a professional engineer that is competent in rotating equipment.

Source: http://www.knighthawk.com/Related%20Pages/062_BICSept08.pdf

Problem Definition

It is the middle of the night and once again the phone rings at your home. Before you answer it, you know what it is about.  You are the maintenance manager at a major ethylene plant and before you left work today vibration levels started to climb on the pedestal bearing on the turbine compressor train. It happens several times a year, and no one really knows why. You have brought in the original equipment manufacturer (OEM) and all sorts of consultants to study the problem because this has shut the plant down at least three times in your tenure as maintenance manager. The “phenomenon” theories are free flowing and you have listened to opinions how operations are setting up a transient affecting the balance of the plant.  So you head out to the plant and once again it is shut down due to high vibration in this bearing.

Having coffee at midnight with one of the 35-year operators who has lived with this equipment brought great insight.  He told you that the problem did not start until the train was sped up by 300 rpm to increase production. He also told you the problem occurs with weather changes. It just so happened a “Blue Northerner” came though the previous afternoon and the temperatures dropped 30 F.

Now it just so happens that the structure is elevated and the turbine compressor train is on the second floor. In the mean time you have to start the plant back up and you do what you always do to get it to run — you change the oil temperature to stabilize the bearing. As the night wears on you look at the rotordynamics reports from the past few years. All of the work indicates a stiffness in the pedestal bearing but none show where it comes from. The next day you investigate and discover that the stiffness does not include the elevated structure or consider the ambient thermal growth of the structure. At this point it has all come together for you.

Consider the structural stiffness of pedestal bearings

Sometimes a turbine train will consist of a turbine and compressors all in one unit. A pedestal bearing is installed in some cases. Rotordynamics studies are conducted to ensure the unit will operate as intended.

However, many boundary conditions are assumed for the studies that may not always be valid. For example, the structural stiffness of pedestal bearings. Two problems exist with pedestal bearings regardless of the specific bearing design itself — the stiffness can affect the critical speeds calculated and the pedestal itself can experience vibration problems as well.

One important parameter in the rotordynamics study is the stiffness of the pedestal in all-principal directions. Of course the bearing damping coefficients and bearing type are important as well. The stiffness of the pedestal bearing is not always easy to obtain. Sometimes the support structure should be figured into the ball game as well. The structural stiffness can be calculated using a finite element tool. A structural dynamics model can be developed to determine the dynamic response of the support structure. The boundary conditions can be derived from the finite element model and incorporated into a rotordynamics model. If problems exist, the pedestal model can be revised as required to obtain the desired response.

How to tackle pedestal bearing problems

Pedestal bearings are a major player in many turbine compressor and turbine generator trains. A suggested procedure for tackling the issue is as follows:

1. If the unit is existing, perform a field study to determine natural frequencies of the structure.

2. Create a base rotordynamics model of the train.

3. Perform a sensitivity study on the stiffness of the pedestal bearing.

4. Develop a finite element model of the equipment’s support structure.

5. Incorporate the stiffness into the rotordynamics model. This should include any temperature effects.

6. Develop a detailed rotordynamics evaluation including the support structure.

7. Look at changes in bearing design and structure that can detune the system away from detrimental criticals.

Every situation is unique to itself and each application should be reviewed by a professional engineer who is competent in rotordynamics.

Source: http://www.knighthawk.com/Related%20Pages/BICFeb09.pdf

It is your first week on the job in a plant that is old but new to you. You find out that, once again, the plant is faced with another failure of an integrally geared compressor.  To make matters worse it has failed five times in the past two years. This last failure occurred within just a few weeks of the previous failure. Now the plant manager has had enough and profits of the plant are suffering. One of the main functions of your new job is to perform a failure analysis of the latest failure and to assist with getting the compressor train operating reliably.

Bolt Failure

The failure has always been in a tie bolt that fastens the open face impeller to the shaft. This failure consisted of the third stage impeller separating from the shaft while running full speed during normal steady state conditions. The steady state failure is new for you because you have been told by the operators that the other failures have occurred during a start-up or shutdown.

To start the project you read all the previous work on the other five failures. You also see where the compressor manufacturer has been called in on every failure. Every failure has been attributed to corrosion fatigue. The words “corrosion fatigue” catch your eye and you recall that in your 20 years of experience, rotating equipment OEMs (original equipment manufacturer) have almost always attributed failures to corrosion fatigue, slugs or surge. You rarely if ever found an admission of a design error as a probable cause from an OEM. What really caught your interest on this project was that the corrosion seemed to always occur on the same impeller bolt. 

Root Cause Failure Analysis

Realizing that there must be something else going on, you have a contractor perform a root cause analysis of the failure and the findings are interesting. The contractor concluded that the cause of the failure was due to an inadequate design of the stage impeller bolt and fastener system. Finite element analysis (FEA) indicates high stress in the failed area.

This provides no allowance for any normal dynamic stress that impeller components would experience. During the investigation a few other items came to the table. Contributing factors were excitation of the impeller from a process instability while running coincident with blade pass frequency. Also maintenance procedures for bolt-up regarding mount and dismount cycle life of the tie bolt were an issue.

In most cases the problem was easily fixed with a change of material for the tie bolt, illumination of the process instability and scalloping the impeller to change the natural frequency. The problem was addressed from all fronts to ensure that the unit would not fail again.  It just happens that on initial tightening most of the load of the tie bolt is carried with only the first few threads of the tie bolt. If tightening continues such that the first three threads yield, the next three threads start to pick up more load. If the bolt is over-torqued, it is possible to compromise the bolt preload if a significant number of threads have yielded. With a loss of preload the full dynamic load is transferred to the tie bolt and a fatigue failure of the tie bolt could occur.

Open Face Impeller Issues

In approaching open face impeller issues to avoid failures, consider the following:

1. Perform a CFD (computational fluid dynamics) analysis of the gas path to insure a sound aerodynamic design.  Consider any secondary wake interaction or acoustic effects.
2. Perform a detailed structural dynamics analysis of the impeller and tie bolt assembly. FEA is a good tool to evaluate the design.
3. Look at natural frequency interferences with the Campbell and nodal diameter mode shape interference diagrams.
4. If necessary create a Goodman Diagram for any anticipated dynamic loads.
5. After the impellers are built, perform “ring checks” to evaluate the natural frequencies. The dynamic stresses can be checked with strain gauges mounted on the impeller. Mode shapes can also be determined and evaluated.
6. Compare the measured frequencies to the FEA results for the impeller.
7. When the impellers are installed, the impellers can be instrumented to look at vibration during the first runs.

Not all of these steps are required but the return on investment is quite high for most large machines. All work should be performed under the direction of a professional rotating equipment engineer competent to do this work.