It has been nearly two years since our last Tech Note, but today we have released the latest write up. This month’s topic is about bearings for turbine-generators found in nuclear electric generation plants.
Many nuclear plants run at 1800 RPM where many fossil fueled steam plants run at 3600 RPM. Considering the generator, half the speed means twice the poles dramatically increasing the weight of the rotor.
The new Tech Note focuses on heavily loaded bearings.
There are various ways to rebabbitt a bearing that has been in service previously, among them are centrifugal casting, static casting, and hand welding. Each one has advantages and disadvantages. The choice used depends, in part, on the material composition of the bearing backing material.
Some of the most difficult applications to rebabbitt are cast iron fan bearings that have water jackets. Essentially, these bearings have two walls to consider, an outside wall and an inside wall.
There are various concerns with these bearings.
TRI has just completed the installation of a fluid drive brake. This system is similar to previous system that we have installed with the exception of larger calipers.
See our web site for more information about brakes and TRI Transmission and Bearing Corp.
Some of the first parameters needed when selecting the fluid drive are the power and speed requirements of the working fluid. These parameters will determine the size of the circuit diameter. TRI has created an online interactive application for this purpose. The Interactive Sizing Chart is free to use at the TRI Web Site
Here is a photo I snapped in the shop this morning. It is a TRI Transmission & Bearing pad that has been sent back for refurbishing. The surface is rough because the babbitt has been puddled. The next process will be machining the surface to the proper diameter.
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.
“Sliding Pressure” simply mans reducing boiler pressure in proportion to a reduction of MW generation. When “Power Wheeling” is fully effective, clearly those electrical generating units with the lowest cost will operate the most. Overall operating costs, efficiency, and/or “”eat rate”" as well as “turn down ratio” for each turbine-generator unit will be critical.
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.
