THE LIQUEFACTION AND STORAGE OF NATURAL GAS EXPLAINED*
East Ohio Liquid Gas Plant First of Its Kind in America
IN its early days the natural gas industry was divided into three parts, production, transmission and distribution. Later, a fourth part, closely integrated with all other three, was added due to increasing industrial commercial and consumer demands. That was storage. It was found necessary to store up the natural gas against the peak demands which could not at times be met by the existing transmission systems.
Natural gas can be stored by four general methods: (1) as gas at low pressure in the conventional cylindrical gas holders; (2) as gas in gas holders under medium pressures (5 to 25 pounds); (3) as a liquid at extremely low temperatures in well insulated alloy steel tanks and (4) underground, as gas at high pressures.
The third method, employed by the East Ohio Gas Company, was said by gas experts as late as February, 1944, to be “still so new as almost to be unique.” Certainly the system was revolutionary.
Stored in liquid form, natural gas occupies only about l/600th of the space required in the conventional low pressure holder. Contrary to the general belief; gas is liquefied by a combination of refrigeration and compression, rather than principally by compression. In this process, gas is first compressed to about 600 pounds, cooled to about 130 deg. below zero (F.) by ethylene and ammonia, and then allowed to expand suddenly to a pressure of about three pounds. The sudden expansion in heavily insulated vessels drops the temperature to minus 257 deg. (F.).
Storage in well-insulated containers can be effected over long periods of time without building up high pressures; the small quantities of gas which form within the containers during storage are piped off to the distributing system.
First Plant of Its Kind
The first plant to handle gas on this system was that of the East Ohio Gas Company, Cleveland and was put into operation in 1941. According to reports* it had a liquefying capacity of 4,000,000 cu. ft. of natural gas per day and a regasifying capacity of 70,000,000 cu. ft. per day. The plant consisted primarily of (1) apparatus to remove all the moisture and carbon dioxide from the natural gas prior to its liquefaction; (2) six gas engines having a total of 3,400 rated h.p. which supplied the power for compressing the natural gas and the ammonia and ethylene that were used as refrigerants; (3) four storage tanks previously described, and (4) three steam heated special heat exchangers which were used to regasify the liquid gas as needed. Considerable heat (20,000,000 B.t.u. per 1,000,000 cu. ft. of gas) was required to return the extremely cold liquid natural gas to its natural and gaseous state. When the plant was regasifying at maximum rate, 2,200 boiler h.p. were required to supply sufficient steam for the process.
*We are indebted to Gas Axe. Robbins Pub. Co., for much of this technical information.
From the outset, one of the most difficult problems to be overcome was the steel for the storage tanks. Ordinary pipe and tank steel becomes brittle at only minus 50 deg. (F.) so special alloys of steel had to be designed. It was found after much experimenting that cork was the best insulator but reports disclose that the cylindrical tank, last to be erected and considered by some to be the chief source of the trouble, was insulated by mineral wool because of the plant’s inability to secure cork due to wartime restrictions.
The Liquefaction Process
For students who desire to know, here are details of the liquefaction process.*
There is a simple rule of physics: when you have a gas or a liquid under pressure, and that pressure is released the temperature of the gas or liquid automatically drops.
Ammonia is cooled under pressure. Then the ammonia is brought into contact, in heat-exchanging equipment, with ethylene gas, which has previouslybeen compressed, and cooled by contact with water.
The ammonia pressure is then released, and as the temperature of the ammonia drops, it absorbs heat from the ethydene, which then becomes a liquid.
The cold ethylene is then passed into a heat exchanger with natural gas, and the same thing happens again; that is, the natural gas, which is under pressure of approximately 600 pounds, being cooled to about minus 130 deg. (F.) by the contact with the ethylene, becomes a liquid at that temperature and pressure. Special valves then expand the liquefied gas by reducing its pressure from 600 pounds to atmospheric, and this causes the temperature of the liquid to drop to minus 250 deg. (F.) thus obtaining a liquefied gas at the atmospheric pressure.
A portion of the liquid in the final step returns to the gaseous state, but since it is extremely cold by that time, around minus 200 deg. (F.). it can be put to useful work as a cooling agent, acting on ethylene ammonia, water and natural gas coming in from the field.
The Storage Process
The next step is the storage of the liquid gas. As said, ordinary steel becomes brittle at low temperatures. At minus 250 deg. (F.) it can be shattered by a slight blow, and ruptures even under the stresses of expansion and contraction. The special alloy developed for the storage tanks was said to have a .09 per cent carbon and 3 1/2 per cent nickel content, strong light and ductile; but even with this superior material it “as found to be good practice to support the weight of the spherical containers with an outer shell of ordinary storage tank metal. Thus, while the inner sphere was 57 feet in diameter, the other sphere had a diameter of 63 feet, The inner tank was all-weld construetion.
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Liquefaction and Storage of Natural Gas
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Between the two spheres cork blocks three feet thick were placed up to about the “equator” of the steel sphere, where the weight rested when the tank was full. According to reports, at that point powdered cork was used to insulate the Space between the inner and outer shells, since there was no weight on the spherical tank in the upper area, and powdered cork is usually effective, while less expensive than cork blocks. For an added protection, dry gases were circulated through the insulation, to prevent the accumulation of any moisture which might freeze and rupture the cork.
Lack of room prevents description of the cylindrical tank in detail. It was of the old low pressure design, but contained alloy steel, with rock wool insulation, substituted for cork because of wartime building restrictions. This reservoir was completely destroyed in the fire and explosion, along with one of the three spherical tanks.
Leakage Trouble Experienced
It is known that leakage trouble developed when this cylindrical tank was first tested in May, 1943. According to reported evidence, when the sub-zero liquid was introduced into the tank there was uneven contraction of the special steel plates which caused a rupture in the bottom of the tank. The rupture was said not to be in the welded seams but in the metal itself. Here may be a clue to the seat of the trouble if, as some believe, it originated in the cylindrical container.
Aftermath of the Holocaust
Unquestionably, the disaster will have far-reaching repercussions. Already Cleveland municipal authorities have drafted legislation prohibiting the installation of such a plant within 2,000 feet, or more, of inhabitated areas and it is said that reconstruction of the original liquid gas plant will not be permitted at its old location.
In St. Paul, Minn., the National Board of Fire Underwriters was expected to decide whether or not the Skelgas division of the Skeliy Oil & Gas Co. would be permitted to erect a gas plant in the Midway district of that city. The company had already been given a permit to erect a plant, but property owners in the vicinity petitioned for a rehearing of the case, and the National Board was asked to rule upon it. The Skelgas Company declares there is no similarity between its proposed plant and the one in Cleveland.
According to the National Board of Fire Underwriters the tragedy will result in additions to, or changes in, fire prevention and building codes and zoning laws.