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The puddling process is a manual operation that takes some time. You can see the babbitt is laid down in rows.  After machining the bore, porosity will be revealed.  More babbitt will be laid down to fill the voids and the machining will be repeated.

by Turbo Research Inc.

There is only one purpose for having a brake on the output shaft of a fluid drive; to aid in the operation of the boiler feed pump by stopping the boiler feed pump shaft and to keep it from rotating when the pump is out of service.

There are certain boiler feed pumps which are designed and built in ways that will cause them to gall and consequently to seize if they rotate for more than a few seconds or a few minutes at low speed with no or low flow through them. Typically, these pumps have stainless steel components that can rub together. They are often the higher performance, higher pressure pumps, but this is not always the case.

There usually is sufficient experience with each boiler feed pump over the years to know if that pump has or has not seized when it was at low speed during those times when a) the pump was out of service and simultaneously, b) the brake is imperative.

For those boiler feed pumps which have not seized when they were operated at low speed for several hours, or for which the manufacturer indicates that no problem will arise by operating at low speed with low/no flow, the brake is not required, and there is no value in using it.

Brake Capacity

Usually the hydraulic and mechanical brake system can stop and keep the output shaft/coupling/boiler feed pump from rotating, if the following conditions are met:

Conditions:

-    The scoop tube tip clearances are properly set.
-    The scoop tube is intact, i.e., it does no break off.
-    The scoop tube linkage internal and external to the fluid drive is intact.
-    The scoop tube is kept at the minimum power position.
-    The circuit oil flow is at minimum.
-    No vane is broken in either the impeller or runner.
-    The gap between the impeller and runner is properly set.
-     The journal and thrust bearings are not worn excessively so that the impeller and runner are not touching, either axially or radially. There are very close clearances between the input and output rotating elements.
-    The brake disk is intact.
-    The calipers are not excessively worn.
-    The hydraulic brake system is not leaking oil.
-    The pin has not been replaced with a lower strength piece of steel.

Etc, etc. Then:
The hydraulic brake is designed to stop the shaft, and to keep the shaft from rotating assuming that the control power remains on and – at least until the mechanical pin can be inserted through the disk.
The mechanical pin, usually made of high strength alloy steel, is designed to be strong enough to keep the disk from rotating.

Brake Failures

Historically, for those fluid drives that have brakes, the brake is one of the highest maintenance items, and often the item which causes most of the forced outages.
They fail for a variety of conditions, usually one or more of the above conditions is violated, with the result that the brake is not able to keep the shaft from rotating.

Keep in mind that the output shaft is attached via a fluid coupling to what is essentially an infinite power source. For a 330 MW unit, a full load, the available power is 440,000 HP. For a 750 MW unit, the available power is 1,000,000 HP. This does not count the
available power when the generator is considered a motor taking power from the grid.

Another consideration is that the fluid drive has first priority on the steam power – ahead of the generator, regardless of whether the fluid drive is on the turbine end or on the generator end of the machine.

There are innumerable failure reports on brakes. Pins have been sheared, calipers worn out in a few seconds, brake disks glowed red hot, brake disks scattered, etc.

The net conclusions which are drawn from this long recital of failures are these:

- If the brake does not need to be applied in order to prevent the pump from galling and seizing, then the brake should be remove.

- If the brake does need to be applied in order to prevent the pump from galling and seizing, then use the brake, but keep it in excellent condition, and use it sparingly.

Maintenance

A subject which continues to arise, and about which TRI is continuously questioned, is the use of the brake to keep the output shaft, coupling, and the boiler feed pump from rotating while maintenance is performed on these components.

It must be clearly and unequivocally stated that the brake system – either the hydraulic brake and/or the mechanical brake – is/are not capable in preventing the output shaft/coupling/boiler feed pump from rotating with assurance to permit maintenance.

In fact, there is nothing that is going to be able to prevent the output shaft from rotating if one or more of the above conditions is/are violated, or possibly a new condition arises.

Consequently, the brake system – hydraulic and/or mechanical – cannot be used to assure maintenance personnel that it is safe to work on the output shaft/coupling/boiler feed pump, etc.

For the record then, the position of Turbo Research Inc. on this matter is quite clear, and has been the same from the earliest time that TRI has been associated with fluid drives: The brake system – regardless of who made it or of its design – does not provide assurance of safety for maintenance of any form, no matter how simple and no matter how short in duration. The only purpose for which the brake is installed is to aid in the operation of the boiler feed pump.

Circuit Oil Flow and circuit cooling oil

The torque transmitted is related to three variables:
-    the scoop tube position;
-    the speed difference between the input and output shafts, and;
-    the circuit oil flow rate.
The circuit oil flow comes from two sources. One is from the circuit oil supply system which is in service during operation. The second is Circuit Cooling Oil, the source of which is usually
the bearing oil system, and this is used when the fluid drive is out of service.
When the output shaft is at low speed or stopped, there is considerable windage between the impeller(s) and runner(s). In order to keep them cool, it is essential to have a low flow of oil through the circuit. If there is no circuit cooling oil, then the impeller(s) and runner(s) will overheat with possible damage.
Please note that even if the cooling oil is shut off, there is a small amount of bearing lube oil which discharges from the bearings into the circuit elements. While this oil may be inadequate to keep the parts sufficiently cool, it is adequate to transmit sufficient torque to destroy the brake, should the scoop tube be operated or the tip break off, etc.
Again, there is no way to make the brake safe for maintenance. Safe Methods to Perform Maintenance
There are only three ways which to date have been shown to provide safety for maintenance on the boiler feed pump:

-    Remove the coupling between the turbine-generator and the fluid drive;
-    Remove the coupling between the fluid drive and the boiler feed pump; and/or
-    Install a disconnect coupling between the turbinegenerator and the fluid drive, and have it in the disengage position.

Disconnect Coupling

The disconnect coupling is the method which involves the least amount of outage time for maintenance on the boiler feed pump, and incidentally, on the fluid drive.
When the disconnect coupling is operated, and moves to the disengage position as measured by a limit switch and by a visual check, it is safe to work on the fluid drive and the boiler feed pump. The electrical power to the device which moves the disconnect coupling can be secured to provide even greater assurance of safety.
The great advantage for having the disconnect coupling is that no outage is required to disengage the coupling, and that an outage measured only in minutes is required to reconnect the coupling.
The coupling can be disconnected at 3600 rpm, and is reconnected on turning gear. Therefore, an outage of about one hour bus-to-bus is all that is required to pick-up the fluid drive and boiler feed pump. The operational staff – without any maintenance personnel involved – operates the controls for disengaging the coupling and reengaging the coupling. When fault conditions are detected, the disconnect coupling can be operated automatically to separate the fluid drive and boiler feed pump from the turbine, in a matter of seconds.
The only way which has been shown to date to provide safety for working on the boiler feed pump and the fluid drive is the disconnect coupling.

Summary :

In summary, there may be expectations that the brakes can be used as “clearance points” for working on the Boiler Feed Pumps.
The TRI position is that the brakes cannot be used as clearance points for maintenance, no matter who built the brakes.
The disconnect coupling arrangement should be strongly considered for implementation on fluid drives where a brake has been previously used.

TURBO RESEARCH INC.
Melbourne F. Giberson, Ph.D., P. E. President

The “turn down ratio” is the ratio of maximum to minimum load. Those with high ratios are preferred because they can take a big swing from maximum load in the day to minimum load at night. Without coming off line.

“Sliding Pressure” for most sub-critical boilers is usually required to obtain the lowest minimum load a unit can achieve continuously and the maximum efficiency at that minimum load.

For fluid drive applications, a reduction of boiler pressure corresponds to a reduction of the output shaft speed/BFP shaft speed, resulting in increased heat, usually more than the amount for which the unit was designed, and often, high amplitude vibration.

TRI Transmission & Bearing Corp. has proven solutions in hand for fully evaluating and resolving these issues for all sizes and types of fluid drives, in BFP or other applications.

For certain units, during sliding pressure operation, new control valve opening patterns occur, leading to different “nozzle block” forces on the turbine rotors. TRI evaluates the changing rotor vibration conditions which result and makes recommendations, including the benefits provided by TRI Align-A-Pad ® bearings and other TRI products for reducing rotor vibration and bearing maintenance.

My first direct exposure to the resultant phenomenon of the formation of these compounds came about 10 years ago.  During the dis-assembly of a high speed gas compressor, the thrust pads were removed from the unit for inspection.  In this particular bearing, the pads were designed with ASTM-B23 Grade 2 Babbitt bonded to a copper alloy containing approximately 2% chrome for increased mechanical strength.  In this application, the high sliding velocity present in the oil lubricated thrust bearing would have yielded unacceptably high bearing temperatures if conventional steel backing material had been used.  The copper alloy backing material was used due to its high thermal conductivity to provide improved bearing performance.  In this instance, following successful dimensional checks, and ultrasonic inspection of the Babbitt bond, the pads were returned to the compressor deck to be re-installed in the machine.  During the installation process, one of the pads was inadvertently dropped from a height of about three inches on to a steel workbench.  As a result of this minor impact, the Babbitt completely separated from the copper alloy backing material.  This was indeed somewhat disturbing that the Babbitt could fall off of an otherwise acceptable part that was ready for installation in a very expensive machine that operates in excess of 10,000 RPM.

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There are many combinations of constituents that are used in manufacturing Babbitt.  The first distinction is whether the Babbitt is “tin-based” or “lead-based”.  When tin was difficult to obtain during WW 2, some equipment manufacturers added portions of lead to the Babbitt used in bearings in order to stretch the supply of tin. Many of these bearings failed because lead made the tin-lead Babbitt brittle. This experience was a good justification for using only tin-based Babbitt for rotating equipment where ductility and endurance are always important.

Two widely used Babbitt compositions are known as ASTM B23 Grades 2 and 3.  There are other grades, however, for rotating machinery, Grades 2 and 3 are very common.  Grade 3 has higher strength than Grade 2, but Grade 2 is easier for Mechanical Technicians to use in the refurbishment of existing bearings, and for this reason, it is more commonly used. Other compositions may be used.  TRI typically uses a proprietary Babbitt material that has a unique combination of constituents and special methods of manufacture, with the result of significantly higher strength at higher temperatures.

One of the most important issues that affects the success of the performance of a bearing is the attachment of the Babbitt layer to the backing of the bearing. Carbon Steel is an excellent backing material.  The surface of the steel can be machined and then tinned, avoiding the use of mechanical Babbitt anchors, or dovetails.  Suitable tinning compounds are readily available in the commercial market.

Babbitt thickness is also an important factor in the ability of oil-lubricated babbitted bearings to take abusive pounding.  Thinner layers can survive higher levels of pounding forces, yet thin layers cannot permit large particulate matter to embed without damage to journal or runner surfaces.  Consequently, a compromise is required, usually in the range between 0.030 inches to 0.125 inches, depending upon the application and cleanliness of the lube oil.

Copper based backing materials that are not properly coated before tinning can be expected to develop a brittle coating at the boundary between the tin-based Babbitt and the copper-based backing material.  This phenomenon is called “copper-tin embrittlement and debonding”, and was discovered several years ago by another well-known Babbitt bearing manufacturer. This phenomenon can cause Babbitt layers of bearings with copper-based backing materials to fall off in the storeroom even if they have never been used.  They can also fall off in service, which obviously can damage a machine.  With proper surface preparation and coating, a copper-based backing can have a tin-based Babbitt layer attached, and this bearing can be expected to have excellent performance and service life.

For more information about Babbitt, Babbitted Bearings and Babbitted Bearing Repair, please contact an engineer at TRI Transmission & Bearing Corp.