Hydrogen: 9-2

In particular, please provide all safety plans and designs concerning possible problems that could arise with the hydrogen.

9-2       ­In particular, please provide all safety plans and designs concerning possible problems that could arise with the hydrogen.  For example, the attached article claims:

  1. A typical tube trailer has the equivalent of 5,585 pounds of TNT;
  2. Hydrogen is especially dangerous because the explosive range of hydrogen in the air is from 4% to 74%;
  3. Hydrogen has a wide flammability range;
  4. Ignition of the hydrogen takes little energy;
  5. All hydrogen cooled generators leak;
  6. There is no shortage of ways to cause a hydrogen fire; and
  7. It is estimated that perhaps five hydrogen fires a year occur at power plants with hydrogen cooled generators.

Response:

The following is a list of design features and safety measures that will be incorporated into the design of the CREC systems to mitigate the hazard potential of hydrogen:

  1. In accordance with NFPA requirements, the systems will be designed and installed to prevent sources of ignition such as sparks from electrical equipment, static electricity, open flames, or extremely hot objects.  This will include use of properly rated equipment in the hydrogen storage and handling systems to limit potential ignition sources.
  2. The hydrogen purity will be controlled to a level where an explosive mixture is not present, (i.e.99.99 percent pure) which is greater than the explosive mixture range of hydrogen and oxygen, thus providing a measure of safety. To prevent mixing of hydrogen with oxygen in air that would create explosive mixtures, a control system is provided that purges the generator with an inert gas such as carbon dioxide before filling the generator with hydrogen.  Similarly, when hydrogen is removed from the generator, it is once again purged with carbon dioxide.  This control system allows safe and efficient filling and purging of hydrogen to avoid an explosive mixture in the generator.
  3. The generator is equipped with end shields that are designed to withstand a hydrogen explosion in the unlikely event of such a mishap and direct the blast away from possible occupied spaces around the perimeter of the generator.
  4. Enclosed spaces such as the generator neutral terminal enclosure will be furnished with hydrogen sensors to monitor the enclosure for hydrogen leaks.
  5. To remove hydrogen that is absorbed or entrained in the generator seal oil, a hydrogen detraining tank is provided in the seal oil system to remove hydrogen.  The seal oil system control is automated.
  6. The generator hydrogen seal oil system is equipped with emergency pumps that are powered by the plant emergency power system to maintain the hydrogen seal in the generator in the event of a loss of the normal power supply.
  7. The hydrogen system is furnished with a dedicated control panel that monitors hydrogen purity to ensure maximum efficiency and safety.  To maintain hydrogen purity in the generator casing, a small quantity of hydrogen is continuously scavenged from the seal oil drain and discharged to atmosphere. The function of the hydrogen control panel is to control the rate of scavenging, analyze the purity of the hydrogen gas and to monitor the gas composition during a generator purge cycle.  The electrical feed to the hydrogen control system is backed up by the plant uninterruptible power supply so that operation is maintained in the event of a power loss.
  8. Hydrogen is supplied to the generator casing through a hydrogen gas manifold. The hydrogen gas manifold includes a gas control valve assembly and instrumentation that ensure safe operation and control of the hydrogen supply to the generator.  The system monitors generator hydrogen gas pressure for alarm, trip, and safety functions, as well as the hydrogen supply pressure.
  9. The following design features will be incorporated to prevent accumulation of hydrogen in buildings and enclosures:
  • For indoor installations, building design will prevent the accumulation of hydrogen either by natural or forced ventilation of high points. Building ventilation flow will be sufficient to ensure there is no hydrogen gas build-up within the structure at all times of the year and in all weather conditions. Where needed, a forced ventilation system using redundant fans will be used that will prevent the accumulation of hydrogen.

     

  • When hydrogen is purged from the generator, it will be piped and vented to an elevated point outside of the generator building.  The low density gas will rise and disperse quickly from the vents.

     

  • A hydrogen sensor will be installed in all battery rooms with an externally mounted alarm and control panel outside of each room (Sensidyne SensAlarm Plus or equal). High hydrogen levels or loss of ventilation will be alarmed on the local panel.