For large generators in power plants hydrogen cooling is provided for efficient heat transfer and electrical properties are superior to non explosive gases. High purity hydrogen will not support combustion. However when hydrogen mixed with air to the level of 95% purity, it is explosive in nature. Therefore a sealing system must be provided to prevent the hydrogen gas from escaping from the generator casing. This is done by maintaining pressure on an oil seal around the generator shaft. As long as the oil pressure is greater than the hydrogen gas pressure, a seal will be formed confining the gas inside the generator and preventing leakage to the areas outside the casing.
For a generating unit the seal oil motors would consists of the following:
- AC main seal oil pump (MSOP)
- DC emergency seal oil pump (ESOP)
- AC motor driven vacuum pump
The main seal oil pump is used normally and is backed up by the dc emergency seal oil pump, which is fed from the station battery. These pumps circulate oil through the seals while the vacuum pump removes the hydrogen from the oil in the hydrogen drain tanks. Some vendors will furnish two seal oil pumps instead of one. These pumps, one for each side of the seal, are called the air side and hydrogen side seal oil pumps