In the early years, a majority of the turbines that had entered our shop were repaired by competitors, that included a range of expertise that ranged from OEM shops to local "mom and pops". Recently, we have spent a lot of time and effort to examine the type of problems that are common to small turbines. Examining, five years of incoming inspections and photographs of turbine split lines, we can conclude that the number one problem appears to be leakage at the joints. This can be followed closely by leakage along the shaft. With regards to the repair of small steam turbines, little has changed in the process for the last fifty years. We are always on a quest to find ways of doing things better, and improving equipment reliability for our customers. In order to better correct part of the problem, we have developed a couple of unique turbine shaft sealing systems – our Brush Seal and our "Disrupter" bearing isolator seal. Both of these products are unique. Both are aimed at improving the efficiency and reliability of small turbines.
Leakage along the split line has been an ongoing problem that we have been examining for quite some time. There seems to be no single solution. Careful examination of a single area of concern only reveals additional areas that need to be better looked at.
Steam can somewhat be compared to mice. It's amazing just how small of an opening is needed for mice to move about freely both in and out of a structure. Steam is kind of like a "one way" mouse. Steam seeks out every avenue of escape, moving from high pressure to low pressure.
The forces of escaped steam can be extremely destructive. The velocity and pressure of steam can cause cuts through the toughest of materials. Steam cuts grow at an always accelerated rate as the escape path gets larger.
This results in the destruction of the turbine itself. Escaped steam lowers the efficiency and run life of the turbine and, adds a safety hazard to those working in close proximity of the machine. The escaped vapor then migrates towards the bearing housings, thereby contaminating the lubrication system, further reducing the turbines reliability.
Our deeper investigation into this problem brought up the following areas of concern
(Or, what we call - The Quest For The Better Mouse Trap):
Are the split lines being adequately prepared?
Does sealant need to be better looked at?
Is the proper grade of bolting being used?
What are the proper torquing sequences and techniques?
How does a "run test" affect proper sealing of a turbine?
Is there adequate follow-up once the turbine has been installed at the plant? What about start up procedures?
Steam quality? (this item is beyond our control)
1. ARE THE SPLIT LINES BEING PROPERLY PREPARED?
Common practice at CTS is to completely disassemble the turbine, breaking apart both the horizontal and vertical split lines. Split lines are then ground to remove steam cuts and any other damage. Split lines are to be cleaned up to have at least an 80% contact. 100% is desirable but not always attainable if the damage to the case was indeed too severe. The horizontal split lines on the bearing housings are also cleaned up. The housings have to be lined up with the case and the ID's need to be re-bored to make up for metal that was removed from the split line of the housing.
Several types of sealant have been used over the years. These include Copaltite, SilverSeal and Linseed Oil. Each type of sealant has its merits, but at CTS we primarily use Copaltite. There are cases where the split lines have been too badly pitted to clean up. In this case, Temptite String is added to the split lines (inside of the bolt holes) in order to better fill-in imperfections and improve sealing. Copaltite, as with other sealants, has one inherent problem. In order to be effective, the sealant has to be properly cured. In many cases where there was initial leakage at the split lines, it was found that the sealant had been "blown out". This is a good indication that the sealant was never properly cured in the first place. There is the assumption (and we know what that word means) that the sealant will be properly cured in the field as part of the start-up procedure. This was not always the case.
Grade "8" is used for bolting
Grade "B-7" is used for studs.
4. Torquing Sequences and Techniques and other Considerations?
Bolt Torquing and sequencing is a critical part of turbine assembly. It is often either overlooked or performed improperly. Most manufacturers specify Torquing sequences and procedures in their turbine manuals. Beyond the manufacturer's instructions, a few other steps must be taken in order for the procedure to be completed.
a. First hand - tighten ALL bolting until they are snug.
b. Following the "crisscross" pattern as illustrated below.
Torque each bolt (or nut) to 50% of the desired torque.
c. Following the same pattern, begin again, this time
Torquing to 100% of desired amount of torque. As each bolt is tightened, mark each bolt with a paint pen or Sharpie to indicate that the bolt is tight.
5. The "Run Test" (Air Test)
Completed turbines are "run test" on air to check for leakage, vibration, misalignment, and over-speed trip.
Considerations for "Run Test"
• Do not over pressurize – you can blow out sealant
• Since this test is run on air, you may introduce moisture
into the interior of the case
6. Start – Up Procedures
In MANY cases proper start-up procedures are not followed once the turbine is ready to run in the plant. Has the turbine been pre-heated properly to allow for a uniform distribution of heat throughout the case?! This is important in that improper start – up can cause distortion leading to steam leakage or
leakage due to not having cured the sealant properly. As a turbine re-builder, we have to assume that the turbine will not be properly pre-heated.
So – HERE IS WHAT WE HAVE IMPLEMENTED!
7. CTS - Our Better Mouse Trap
There is a Better Mouse Trap – a better way of sealing turbines. It doesn't involve any single item listed above, but a combination of ENHANCED repair procedures and techniques.
The Better Mouse Trap – The Oven
Compressor and Turbine Services has carefully reviewed leakage problems on small turbine split lines and has ultimately changed our method of small turbine repair. Our thoughts are that turbine repair procedures must be changed in order to facilitate the proper sealing of small turbines.
Based on the points listed above, we have instituted the following procedures (as our customers allow):
1. Horizontal and vertical joints will be ground to improve finish.
2. Sealant will be properly cured PRIOR to leaving our facility. Turbines are to be heated to a temperature as specified by the
sealant manufacturer. The desired temperature will be held for a specified duration.
3. Bolting will be as specified:
Grade "8" is used for bolting
Grade "B-7" is used for studs
Nuts, washers, and studs will be used on the main body of the turbine. This will eliminate the chances of a "bottoming out" in cases where studs are used. Bolting with washers will be used on the packing boxes and bearing housings.
4. Proper Torquing procedures will be used as per Item number 4 (as described above).
5. Turbines will be pressurized to 100 psi or less during testing to reduce the risk of "blowing out" the sealant.
6. Turbines will be "tagged" with suggested procedures for start-up and installation upon shipment.
7. The Better Mouse Trap – As noted above, turbines are heated in our oven (Mouse Trap) prior to shipment. This will help to insure that the sealant has cured, the paint has dried, and that moisture is removed from the interior of the turbine. Bolting torque will again be checked.
Once again, we feel that these enhanced repair techniques will provide our customers with a more efficient, and reliable turbine. Our goal is to be the BEST turbine repair facility in the business!