Fluid Drive Brake SystemsNovember 22, 2010
The Purpose For The Brake System On A Fluid Drive
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.
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:
– 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.
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.
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.
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.
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