Hydrogen Cooled Generator Upgrades .

There are many areas of hydrogen-cooled generators that could benefit from upgrades. Upgrades to the seals, inner oil deflectors, bearings, and oil systems can reduce hydrogen leaks, make assembly and testing easier, and reduce the chance of galling which can damage the generator rotor. Why use hydrogen instead of air when it's well known that hydrogen can be volatile? Hydrogen is the most abundant element. It is estimated to make up 75% of the mass in the universe. It's everywhere. Compared to air, hydrogen has low viscosity and it's lightweight. It takes much less power to cool the generator with hydrogen, and when hydrogen reaches a high level of purity, it's no longer combustible. Compared to other cooling medium, hydrogen is relatively inexpensive. Hydrogen is the first element on the periodic table. It has one proton and one electron. It has no neutrons. In the molecular form H2, it is very small, meaning it can leak through very small gaps. Generator upgrades can reduce these leaks, thus improving safety and reducing hydrogen losses into the seal oil conditioning system. A typical hydrogen-cooled generator has at each end a bearing, a hydrogen seal, and an inner oil deflector. Each end also has two hydrogen lines. One of these lines is a sensor line that has no flow. The pressure in this line provides feedback to the oil conditioning system. The hydrogen pressure in the generator must be greater than the ambient air pressure to keep the oxygen from penetrating the generator shell. The other line has a flow of seal oil which replaces the seal oil that passes through the seal rings. Years ago, the two-ring assembly was developed and installed in many generators. The seal ring assembly is located between the bearing and the inner oil deflector. It is the division between the air side and the hydrogen side. The rings are held tight to the housing with a garter spring. The upgraded seal housing is designed for easy installation and maintenance. This exploded view shows the housing upper half being disassembled and the seal rings removed. The housing surface that mounts to the frame is insulated with G-10. The screws that hold the seal ring housing to the frame are insulated with washers and sleeves which are also made from G-10. G-10 is a glass epoxy laminate that has a high dielectric strength and can withstand at least 250 degrees Fahrenheit. The inner oil deflector is on the hydrogen side of the seal. It has thin aluminum strips that block the oil that travels along the rotor. A small clearance between the aluminum strips and the rotor are necessary for the deflector to work properly. This exploded view of the inner deflector assembly shows the inner labyrinth and outer shell. The outer shell bolts to the frame while the inner labyrinth bolts to the outer shell. Alignment screws in the outer shell are used at assembly to simplify centering the inner labyrinth to the rotor. The inner deflectors on both the turbine and collector ends are insulated. Bearings are located on the air side of each hydrogen seal. Rotor vibrations can cause problems with hydrogen seals. An upgraded bearing can limit the motion of the rotor, preventing rubs or galling of the hydrogen seal bores. This bearing was part of an upgrade package. It is a double insulated five-pad bearing with lift oil pockets. It has saddle blocks for easy shaft alignments. The saddle blocks can be turned for either a cylindrical or spherical seat. The generator is filled with pressurized hydrogen. The tightest fitting seal rings alone would not be enough to stop the hydrogen from leaking from the generator into the air, where it would become explosive. To achieve a good seal, oil is pumped into the cavity behind the two seal rings. This oil is pressurized 5 to 10 psi above the hydrogen pressure. This difference in pressure is what keeps the hydrogen inside the generator. A small portion of oil leaks under the hydrogen side seal ring. This oil that leaks toward the generator absorbs some of the hydrogen and drains into the enlargement tank, where the hydrogen partially separates from the oil and returns to the generator. A larger portion of the oil goes to the unpressurized air side. The oil is collected and drains into an expanded chamber. The expanded chamber feeds the oil in two directions. Some of the oil from the expanded chamber is makeup oil, which compensates for the oil lost under the air side seal. The remainder of the oil in the expanded chamber returns to the main oil reservoir. There are improvements that can be made to many seal oil conditioning systems. The use of two pressure regulators assures that there is proper pressure at each end of the generator. Duplex filters reduce the particulate matter in the oil, reducing wear and extending the life of the seals. An oil-to-air heat exchanger, coupled with a thermostatic mixing valve, controls the temperature and viscosity of the oil. An emergency lube oil pump based on AC technology reduces the dependency on DC equipment. The two seal ring design has been around for quite some time. Developed by GE in the 1930s, it uses bronze rings split into quadrants held in place with garter springs. This design requires the garter spring to push the quadrants together at the butt joints. Any gap at the butt joint will allow oil and hydrogen to leak out. The garter spring also applies an axial force, keeping the quadrants tight against the seal housing wall. An upgrade design uses steel back seal rings with babbitted bores. These seals have two halves that bolt together. The garter spring is not required to keep the ring in the proper circular shape. The garter springs are only required for axial forces when the seal oil pressure is low, for instance at start-up. The springs apply a force that keep the rings tight against the housing. With seal rings that are bolted together, the springs can be less stiff because they are not required to keep butt joints together. More flexible springs means the axial forces pushing the seal ring against the housing are reduced, and this allows the seal rings to move more freely in response to rotor vibrations. Because the upgraded seals and rotor are made from similar materials, they expand with heat at the same rate. The old GE design with bronze quadrants requires a very tight cold clearance. As the generator warms up, bronze seals grow into a desired diameter which satisfy the proper clearance. When cold, the seals are more prone to damage due to vibration and particulate matter. Another benefit of upgraded seals is the babbitt lining, which is less likely to gall, and any particulate matter in the oil will be embedded in the babbitt instead of damaging the rotor. Butt joints in the old seal design is a potential path for leakage. These can be eliminated with an upgrade. Bolted rings have no gaps that can open up during periods of high vibration. Bolted rings also have the advantage of being easier to bore to tight tolerance and assemble. Because the upgraded housing has a top with a removable section, verifying the assembly is possible. A mega test between the seal housing and frame can be performed before the rings are installed. There are benefits to insulating all the seal housings, inner labyrinth deflectors, and bearings. During an outage, the coupling can be separated and grounding brushes removed. Proper electrical isolation can be tested without having to remove the seal housing or labyrinth seals. It's not the diameter of the seal that's important. The difference between the seal bore and the journal diameter determines how the seal functions. As discussed earlier, bronze and steel have different coefficients of thermal expansion. To know the proper bore diameter when upgrading, you must first calculate the rotor-to-bronze seal clearance at the typical operating temperature. Our example here is for a 22-inch diameter journal. The normal operating temperature is 140 degrees Fahrenheit, and room temperature is 68 degrees Fahrenheit. The bronze seal is installed cold, with 8 mils clearance. But this clearance increases to 13.5 mils as the unit heats up. This number, 13.5, is the clearance that we use when we manufacture the steel-backed seal in the cold state. The upgrade uses floating seal rings designed to move and center themselves on the journal. Here we see the calculation for the force required to move the seal laterally. Because the delta P, from the seal oil to the ambient air, is much greater than the seal oil to the pressurized hydrogen, the sliding forces required to move the air-side seal is much greater than the hydrogen- side seal. There are improvements that can be made to emergency lube oil systems. A reliable system that uses A.C. technology can replace D.C. motors and switchgears. This system uses an A.C. motor to drive a positive displacement pump. A U.P.S. powers the motor. The variable frequency drive starts the motor in less than a second and prevents the inrush current from over-amping the motor or the U.P.S. There are many reasons to make upgrades to aging hydrogen cool generators. Contact an engineer at TRI Transmission and Bearing Corporation if you have any questions about these upgrades or if you would like pricing and lead time on an upgrade package.