Mercury Turbine Now a Success (Mar, 1931)
Thankfully Mercury Vapor Turbines are now obsolete.
Mercury Turbine Now a Success
New Power Idea Proves Its Value in Practical Use As Quicksilver Vapor Spins Electric Generator
By EARL CHAPIN MAY
WHEN, in 1914, William LeRoy Emmet, General Electric Company research engineer, first proposed that mercury vapor instead of steam could be used to drive a turbine wheel, scientists and engineers scoffed at him.
They said that while the physical characteristics of mercury, its boiling point, specific heat, and so on made the idea theoretically possible, insurmountable obstacles lay before anyone who tried to work it out in a practical way.
Pointing to the tiny leaks that often occur in steam piping, they said that similar leaks would certainly occur with mercury vapor, which is even harder to confine than steam. Such leaks, they claimed, would waste valuable mercury and kill all the engineers, because mercury vapor is exceedingly poisonous.
However, Emmet refused to listen and built an experimental machine. These early tests were described in Popular Science Monthly.
A few days ago, I saw in operation a mercury vapor power plant of full commercial size that, during the last year, has set a low figure for the use of coal such as the world has never before seen!
This startling result was made possible by the foresight of T. H. Soren. vice president of the Hartford Electric Com- pany, of Hartford, Conn. He was so impressed with the results that were obtained from Emmet’s first tests that a deal was made with the General Electric Company to continue the work on a larger scale. So in 1923 a 5,000-horsepower experimental mercury vapor unit was constructed and installed at the Hartford Electric Company’s Dutch Point power generating station.
During the years that followed, test runs bore out Emmet’s theories, but some of the troubles predicted by the scoffers also were encountered.
A boiler head blew out and thousands of dollars worth of the valuable mercury escaped. Only part of it was recovered. A turbine wheel went to pieces under the strain and caused a shutdown for several months. Workers were overcome by the fumes of the poisonous mercury. There were, however, no fatalities, nor was anyone permanently injured, because of the unusual precautions taken by the company to protect the men who had charge of the work.
After Soren, James Orr, the company’s superintendent of power, and their assistants had learned all they could from the first experimental plant, a second was built with such changes as experience dictated. This unit gave improved results but also developed some defects.
Then, utilizing the engineering knowledge gained from the years of experimenting with the first two units, the company decided to build a real, commercial size, mercury boiler unit at its South Meadow generating plant.
It was this plant I recently inspected. I had gone prepared to see only experimental apparatus that possibly produced results when it ran during the intervals between costly and dangerous breakdowns. I found instead a power house unit that was solidly engineered, permanently installed, and running without any sign of trouble. In fact it is now planned to follow the general design of this plant in building a unit twice as large, 20,000 kilowatts in capacity, in the new power plant of the Schenectady works of the General Electric Company.
THE South Meadow mercury boiler unit is producing as high as 143 kilowatt hours of electrical energy for each 100 pounds of coal burned. The finest steam power generating plant in operation today produces only 112 kilowatt hours of energy from that much coal. According to United States Government reports, the average efficiency of all the public utility power plants in the country is fifty-nine kilowatt hours from 100 pounds of coal.
This means that, while the average power plant is getting out of a pound of coal as much useful work as could be done by a strong horse in a little over three quarters of an hour, the mercury boiler installation is getting from the same amount of coal as much work as could be done by the same horse working for nearly two hours!
There is, therefore, no longer any doubt about the practical value of mercury boiler installations. However, talk of mercury entirely replacing water in the boilers of the world’s power plants is a bit premature.
In the first place mercury vapor does not, even in the South Meadow plant of the Hartford Electric Company, entirely replace steam. The production of steam from water still is a vital part of the process.
IN THE second place, much as we may want to profit by the efficiency of the mercury vapor process, it remains an open question as to whether the world’s supply of mercury can be increased sufficiently to provide for any large number of mercury vapor plants. Of course it must be remembered that, except for accidental losses, the mercury, once installed, can be used forever. It is not used up in generating power. A study of the process will show why this is so.
At the South Meadow plant, the mercury boiler consists of seven forged steel drums from each of which project 440 dead end tubes of low carbon steel. These projecting tube ends form the studded wall of the furnace chamber in which powdered coal is burned.
The liquid mercury is boiled into vapor in these tubes and passes by way of a pipe to a turbine that runs an electric generator. After turning the wheel of the turbine, the mercury vapor passes into what is actually a combination mercury condenser and steam boiler. The mercury vapor gives up its remaining heat to the water in the boiler and so produces steam.
This steam operates another turbo-generator and the mercury, again in liquid form, runs back into a boiler by way of a preheater located in the furnace chamber. The high boiling point of mercury, 675° Fahrenheit, thus makes possible the absorption of the maximum amount of heat from the fire and the subsequent economical use of this heat to produce power by the method outlined.
After the mercury vapor has developed 10,000 kilowatts by operating its own turbine, the heat remaining in it is sufficient to generate 128,000 pounds of steam an hour at a pressure of 280 pounds.
Note that the mercury simply circulates around in the system and is not lost or destroyed. Note, also, that it is the combination of the mercury vapor generation and the steam generation that makes possible the high efficiency.
NINETY tons is the weight of the liquid mercury in the boiler of the South Meadow plant, representing a value of $300,000—or at least that is what it would cost to replace at the current price of $125 a flask of seventy-five pounds. Of course when you consider that the total annual consumption of mercury in this country is only about 1,000 tons, it is obvious that it would be difficult, if not entirely impossible, either to buy or sell ninety tons of mercury in a single lot. These figures also show why a great increase in the production of mercury is necessary before it can be used widely in power generation.
Mercury long has been one of our most neglected elements. It has been used in thermometers, barometers, percussion caps, mirror backing, cosmetics, insect destroying, medicinal disinfecting, roughening the hairs of felt products, anti-fouling ships’ hulls and, lately, in giving blue color to neon lights.
You will find it in dentists’ amalgam, as “roller bearings” for the Mount Wilson Observatory telescope, and you may have swallowed some of it in calomel.
In its relatively small use in arts and sciences, mercury has kept company with other now almost indispensible elements. For example, a widely read textbook of 1876 carries a picture of two little pieces of aluminum exhibited as rarities in the British Museum! Tungsten, through which we get much of our electric illumination; chromium, now recognized as a valuable rust reducer; vanadium, employed with chromium in making armor plate, were almost strangers to art and industry ten, twenty, or forty years ago.
YET mercury has been used in gold and silver mining since the early fifteenth century by Spaniards, Italians, and those who came later to be known as Americans. The Phoenicians were mining mercury in Spain long before the Christian era.
Because mercury has an affinity for gold and silver it played an important part in developing placer mining in North and South America. Most placer miners were spendthrift fellows. Having used their mercury they abandoned it. Only last month an enterprising westerner told me that he was making good money by shoveling mercury out of California and Nevada sands or retrieving it from thousands of iron “flasks” in which it had been shipped from European or American miners to westerners who “washed” for gold.
The ninety tons of pure mercury from which the Hartford mercury boiler unit gets its unprecedented power arrived at the South Meadow plant in the same type of cast-iron “flask” in which it reached the miners after the great Sutter’s Mill strike in California. Each cylindrical flask, about eleven inches high by four-and-a-half inches in diameter, weighs about ten pounds and contains seventy-five pounds of liquid mercury.
As a strong man could carry such a “flask” over mountain trails or sling two of them across a horse’s back, they were ideal for mining under pioneer conditions. So little has the demand increased that mercury still is shipped in these archaic “flasks” and an order for ten tons of this element would have a serious effect on the market. Yet there is plenty of it available.
Mercury is found almost entirely in red-dotted sandstone, called cinnabar. Ninety percent of it comes from Spain and Italy. The Almaden Mine, in southern Spain, first worked by the Phoenicians, still produces from one third to one half of the world’s mercury. Yet, even in Spain, production has not reached its limit. In 1927 Spain produced 52,599 flasks. During the next year it turned out 74,000 flasks. That same year, Italy mined 64,000 flasks of mercury.
DURING 1928 American mines produced about 17,000 flasks, mostly from California and Nevada. But during the peak war year of 1918, this country mined 32,883 flasks. So there is no scarcity of mercury.
Normally there is danger in working close to or with mercury. Mercurial poisoning manifests itself in decaying teeth or serious intestinal disturbances. At ordinary atmospheric pressure, mercury condenses at about 675 degrees F.
When the header of the Hartford Electric Company’s boiler gave way at Dutch Point, the flue and stack temperature was rather high and much of the escaping element found its way into the Dutch Point yard.
Because of the relatively low specific heat of mercury and the low boiler pressure, a mercury boiler does not explode as a steam boiler does. Not until they noticed mercury falling in the level or gage glass did the Dutch Point workmen discover the boiler failure. At that time little thought had been given to the hazard of mercurial poisoning; hence, no precautions were taken during construction, operation, or reconstruction. Several workmen, therefore, were slightly poisoned by breathing mercury fumes. Light exercise in open air soon cured them. The most stubborn case lasted only four days.
WHEN one of the South Meadow boiler tubes gave way and a good deal of mercury escaped into the furnace, the coal supply was shut off and as the temperature at the base of the smokestack was only about 300 degrees, much of the mercury was recovered.
Better knowledge about welding has eliminated mercury leakage or loss at the South Meadow plant. But in order to provide for emergencies, the General Electric Company has developed a simple detector—a paper ribbon coated with selenium sulphide and unwound from a spool across an aperture through which gas or plain air is forced by a fan. The sulphide turns dark when in contact with mercury vapor or mercury.
In addition to this selenium paper, a photoelectric cell gives instant notice of the presence of mercury in flue gases by ringing a bell. All ventilation, where any leaks might occur, is delivered to the smokestack below the point of draw-off of the gases to the test paper. Hence, if a single tablespoonful of mercury were thrown into the boiler furnace a bell instantly would ring a warning.
When there is any possibility of workmen coming in contact with mercury vapor they wear face masks. If they are handling mercury or any parts that have been in contact with mercury, their hands are protected by rubber and leather gloves. If they are close to any mercury equipment, they wear overalls that are frequently washed. So far, none of the experimenters or their assistants has had a serious case of mercurial poisoning. And the loss of mercury has been negligible.
Though the South Meadow plant represents a large financial investment, it is not as great as the original estimate. Each hour, 1,100,000 pounds of mercury descends from the top of the unit to the boiler tubes. While ninety tons of mercury thus circulates six times an hour through the boiler, its descent is accomplished by gravity. Expensive boiler feed-pumps and valves and regulators thus are eliminated.
THE operation of the South Meadow unit is simple. There are no valves in the mercury cycle except a throttle valve, an emergency valve, and two safety valves on the turbine floor. In the event of excess pressure on the mercury boiler the latter are used to pass vapor into the condenser-boiler.
When heat from the coal fire has built up enough vapor pressure, the throttle is slowly opened until temperatures are uniform within the unit. The turbine is then started. By throttling the turbine the generator is cut in and synchronizing accomplished.
There are several reasons for believing that cost of electric production by means of a mercury unit can be reduced materially and that maintenance costs will be lower than for a straight steam station. As internal cleaning is not required a mercury boiler need not be opened, after being sealed. There are no scale deposits and no corrosion or pitting in the mercury boiler, and no tube failures due to scale formation from the water, as in ordinary steam systems.
As the mercury tubes are short and practically vertical, they easily can be cleaned externally. There is small probability that erosion, corrosion, or electrolysis will cause a failure in the condenser tubes. The condenser, though it has a “dual personality” because it acts both as a condenser and a steam boiler, will not suffer from burned or distorted tubes, because there is no fire underneath it. Hard scale will not form in its tubes because mercury vapor will be at a comparatively low temperature when it reaches the condenser from the turbine, and there is a vigorous circulation in the tubes.
BECAUSE simplicity of equipment affects dependability, a mercury plant will be more reliable than a modern all-steam plant, where refinements have introduced complications. As mercury does not attack steel, special construction materials are not necessary. Lightweight containers are practicable because mercury temperatures are high but pressures low.
Finally the initial cost of a mercury boiler system, plus steam capacity, is not greatly in excess of the cost of an equivalent all-steam system. But the overhead running cost, measured in kilowatts, is much lower than the best, most efficient steam plants now operated, and there is every reason for believing that the unit installed at South Meadow will run satisfactorily and efficiently without material repairs and replacements for a quarter century. It has run that way since February 4, 1930.
The results obtained since then clearly indicate that we may be passing from the age of steam to the age of mercury, at least so far as the generation of electric energy is concerned.