By Thomas Ward, Senior Training Consultant, We Energies Power Generation
Hydrogen is a very unique element. Listed as #1 on the periodic table of elements and with an atomic weight slightly above 1.0, it has no color, odor, taste, or apparent flame when it’s burning. In other words, your senses will not tell you when you’re in trouble.
But pure hydrogen is used to cool many of our AC generators, and for a variety of good reasons. Let’s learn why.
Back in 1935, the new Unit 1 Allis Chalmers 80 MW generator at Port Washington, Wisconsin was air cooled. A few years later, the plant commissioned Unit 2, which was identical to Unit 1 except the size of generator, which was 30% smaller because it was cooled using pure hydrogen. Hydrogen was a new idea which really helped to improve the efficiency and cooling potential of large generators for several reasons:
- When the purity of the hydrogen is kept well above 75%, hydrogen is considered to be a safe and viable cooling medium.
- Since hydrogen cooling replaced air cooling as the industry standard, the physical size of our generators has decreased and the generator output has increased due to the increased cooling potential of the hydrogen gas.
- The density of hydrogen is only 7% of the density of normal air, which reduces torque losses due to windage. The atomic weight of hydrogen is about 1.008, making it the lightest element on our periodic table of elements. The atomic weight of nitrogen is about 14. Reduced windage increases generator and turbine efficiency; mechanical drag is proportional to fluid or gas density, and relative to the square of the velocity of the machine.
- Hydrogen is able to conduct heat about 50% more efficiently than air. For the same reason, the hydrogen readily gives up its heat to the coolers.
- Hydrogen is a plentiful and relatively inexpensive gas. In 2021, the price for delivered bulk hydrogen averaged about $8.40 per 100 cubic feet.
- Hydrogen is a very clean gas and does not promote corrosion in a sealed environment. Air cooled generators require internal cleaning on a regular basis.
Risks associated with hydrogen gas
When hydrogen purity is between 4% and 75%, it becomes extremely flammable. This becomes even more dangerous when we consider how easily hydrogen mixes with its surrounding atmosphere. This is unlike a gas such as propane, which is very heavy and can accumulate in isolated locations. It is very important to monitor the generator gas purity continuously to ensure it is at least 90% pure at all times during operation. Most utilities maintain purity well above 90% to allow for a reasonable margin of error.
When hydrogen is burning (oxidizing), it does not produce a visible flame. Therefore, it produces very little (if any) radiant heat. This is very dangerous because when radiant heat is created, personnel can feel the heat from far away. With hydrogen, a person could simply walk right into the fire, being subject to severe burns and hot gas inhalation.
The high energy content of hydrogen implies that the reaction (explosion) of hydrogen releases about 2.5 times the heat energy as the same amount (mass weight) of gasoline. The Hindenburg disaster in 1937 was a result of a minor hydrogen leak which was ignited by either lightning or static electricity as it was trying to moor. The airship was completely destroyed in about 30 seconds.
Because hydrogen molecules are tiny, it’s virtually impossible to avoid leakage. Because of the small molecule size, there is a constant flow of hydrogen into and from the seal oil system. This is unavoidable, so it is important to have a means of continuous liberation, or “detraining” the hydrogen from the oil. The generator shaft seals by design are not 100% “leak proof,” so gas must be added regularly to maintain the proper pressure and purity. Minor leakage from the shaft seals usually does not present any hazards because most turbine and generator areas are well-ventilated, so the gas escaping will never reach dangerous concentrations. On the other hand, generators which are installed partially below the turbine deck can actually have hydrogen accumulations below a concrete turbine deck. These areas require frequent (or continuous) monitoring for unsafe concentrations.
Hydrogen is stored dry, added dry, and kept dry in the generator at a dew point of about -140 F. The dryness of the gas makes it very easy to self-ignite in air. In severe and catastrophic instances when a turbine experiences destructive vibration or overspeed, the vibration is instantaneously transferred to the generator bearings (which are less robust than the turbine journal bearings), causing a failure of the generator hydrogen seals and producing catastrophic results.
Causes of hydrogen accidents
- Poor unloading practices- failure to adequately ground the delivery trailer before delivering bulk ammonia. The slightest static spark from clothing, tools, or a running vehicle can initiate an explosion.
- Poor generator purging practices- most reported generator hydrogen events are the result of personnel not following approved generator purging practices. These practices and procedures include purging hydrogen from the generator with CO2 at the proper rate. Purging of the generator is not complete until it has been proven that all of the hydrogen has been purged from the machine and testing the atmosphere with a proper gas analyzer that has been recently calibrated.
- Poor work practices- when working in or around an environment which could potentially contain unsafe concentrations of hydrogen in air it is important to test the atmosphere frequently or continuously before entry and during the maintenance tasks. Equally important is the requirement of using spark-free tools and work lights. Something seemingly harmless like a light switch can cause a deadly explosion.
Using hydrogen as a cooling medium for AC generators has proven to be a safe and effective means of keeping the generator cool while minimizing efficiency losses in the prime mover. Hydrogen allows us to attain the maximum generation while keeping the stator and field windings within the OEM temperature limits. Additionally, hydrogen accidents can always be avoided by following safe operating and maintenance practices.
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