but a mechanical mixture of several combustible and diluent gases and vapors thoroughly diffused through each other, the number and exact proportion of the various crude natural constituents varying for the different localities and somewhat during the working lives of individual wells. The term "casing-head gas" is applied to a natural gas that flows from oil wells, coming out between the casing and tubing. It is collected by means of a metal head-called "braden-head "1-connecting the casing with the tubing, as shown by the dotted lines at the top of fig. 1. The term "braden-head gas" is sometimes used synonymously for casing-head gas. This is a mixture where two or more substances are brought together in a thoroughly commingled state, without, however, any of the constituent substances losing their individual identity. The various vapors and gases going to make up natural gas are merely intermingled as mechanical mixtures. Another very common illustration is atmospheric air, where water vapor and the gases oxygen and nitrogen are merely mixed in the form of a mechanical mixture; that is, the water vapor has undergone no chemical change and the oxygen and nitrogen have undergone no chemical change by the mixture. This word literally means a warm exhalation. A vapor is the gaseous state of a substance which at ordinary temperature exists as a solid or liquid; that is, the vapor is the result of the action of heat on a solid or liquid. On removal of the heat the vapor will return to its former solid or liquid state. When a liquid, by the action of heat, goes into a vapor or gaseous form it is said to vaporize or evaporate, the meaning of these two terms being the same. The most common form of vapor is the moisture always present in greater or less degree in the atmospheric air. GASES AND VAPORS DISTINGUISHED. A vapor is an aeriform substance in the gaseous state at any temperature below the critical point, the critical point being the line of demarcation between a vapor and a gas. The temperature of fluid at the critical point is the critical temperature, and the pressure which at this critical temperature just suffices to condense the gas to the liquid form is called the critical pressure. A vapor can be reduced to a liquid by pressure alone, and may exist as a saturated 1 Named after its designer Mr. Glenn T. Braden. vapor in the presence of its own liquid. A gas is the form which any liquid assumes above its critical temperature, and it can not be liquefied by pressure alone, but only by its combined pressure and cooling. All vapors are gases, but not all gases are vapors. The difference between vapors and gases may be summarized as follows: Aeriform fluids. Vapor. Below its critical temperature and pressure. Can be condensed by pressure alone. Gas. Above its critical temperature. Can be condensed only by both pressure and cold. Gasoline found in natural gas always exists there in the form of a vapor, while methane, for instance, in natural gas exists only as a gas. NATURAL GAS' MAY BE WET OR DRY. Natural gases coming from the ground may be classed-according to their gasoline vapor content-into two main groups, namely: 1. Wet gas. This is gas intimately associated with oil, usually produced with oil, and is ordinarily known as casing head natural gas. 2. Dry gas.-This is gas not intimately associated with oil, but may nevertheless contain gasoline vapors. The term "dry" does not refer to water vapor that may be carried by the gas, but rather to the gasoline vapor, and, furthermore, this is a relative term since a strictly dry gas would be one containing no gasoline vapors.1 66 While natural gas is a natural product made by nature, it is no more natural than other minerals, like coal, oil, or iron ore. The word "natural "" came into common use probably as contrasted with manufactured gas, and the use of the word appears to have given a fallacious impression that natural gas was a free and unlimited resource. Merely being made by nature does not mean that a substance is cheap and of low value. Natural gas is a natural resource, which men have learned to use for the satisfaction of their wants. The misconception regarding the position of natural gas has arisen from failing to appreciate that man creates no new matter and can merely get the materials of nature ready for consumption. Food, clothing, wealth in all its forms, are derived 'originally from nature. The forces of nature, working through the ages, have created things which mankind needs. Human effort expended on these products of nature, converts them into forms which are usable.2 1 For further discussion, see Bureau of Mines Bulletins No. 88 and No. 120. 2 Suggested by Ely's Outlines of Economics. 1 NATURAL GAS A MINERAL. Broadly, the word mineral means the inorganic materials of which the earth consists. "The word minerals in the popular sense means those inorganic constituents of the earth's crust which are commonly obtained by mining or other process for bringing them to the surface for profit." That is, the term "mineral" is not, per se, a term of art or trade, but of general language, and in addition to its broad scientific meaning is also used in a commercial sense where it may include any inorganic substances found in nature, having sufficient value, separated from its condition as a part of the earth, to be mined. Natural gas is now universally classed as a mineral. However, on account of its adventitious origin, migratory habits, and fugitive tendencies, it is regarded as a mineral with special attributes. Since it is a mineral, it is, therefore, a crude product. As so aptly stated by the United States Supreme Court, " Natural gas is a crude mineral, not advanced in value or condition by refining or grinding or by any other process of manufacture." 1 ORIGIN OR FORMATION OF NATURAL GAS. How, when, and where the constituents of natural gas were formed is not definitely known. For our purpose we need not bother about the various theories that have been propounded regarding the origin or formation of petroleum generally or natural gas constituents in particular. That is, whether these constituents originated from cosmic, organic, inorganic, animal, vegetable, volcanic, animal bacterial, plant bacterial, diatomic, or fatty algal sources is not germane. Neither is the matter of adventitious and migratory or indigenous and accumulative relationship with regard to any geological formation of vital importance. The incontrovertible facts are that we have in natural gas a crude natural substance made up of mixtures of widely varying constituents-even though we may not know how these mixtures were thrown together for different natural gas fields in the United States. Some of these natural gases are wet, while others are dry; some are high in heating value, while others are low, and some are heavy, while others are light in weight. STARTING POINT OF GAS ACTIVITY. As far as temperature is concerned, gas activity begins at a point 460° below zero on the Fahrenheit scale, and as far as pressure is concerned, it begins at the point of absolute vacuum, or 14.7 pounds below atmospheric pressure at sea level. Neither point has ever 1 United States versus Buffalo Natural Gas Fuel Co., 172 U. S., p. 339. been reached by man's physical senses, but both form the bases from which all gas volume calculations must be made as shown on pages 15 and 16. BAROMETRIC PRESSURE. Atmospheric pressure is measured by a barometer-usually in inches of mercury, 1 inch of mercury equaling 0.49 pound to the square inch pressure-and is synonymous with barometric pressure. Sea level is the basis from which atmospheric pressures are reckoned. At that point dry air at 32° Fahrenheit exerts a pressure of 14.7 pounds to the square inch. This pressure varies with altitude and temperature, the pressure decreasing with an increase in altitude or temperature. 14.4 pounds represents a fair average barometric pressure for most natural gas using communities. GAGE PRESSURE. This is simply the pressure indicated by a pressure gage. Two general classes of gages are used for measuring gas pressure: 1. Spring gages.-Where the effect of the pressure exerted against some form of spring is made to move a pointer over a graduated dial or scale. 2. Fluid gages.—Where the effect of the pressure is indicated by the height of the column of fluid in a U-shaped tube. One side of the U-shaped tube is open to the atmosphere and the other is attached to the pipe where the pressure is to be measured. The gas pressure in this pipe then lowers the fluid in one side of the tube and raises it in the other. The total difference in the heights of the fluid on the two sides represents the total fluid pressures. When no pressure is applied to such a U tube gage other than the prevailing atmospheric pressure, the liquid will stand at the same level in both tubes. The pressures in natural gas distributing plants are almost universally measured in ounces to the square inch, while the pressures in manufactured gas distributing plants are measured in inches of water, 1 ounce equaling 1.73 inches of water. Where the word pressure occurs in ordinances or rules it invariably means gage pressure. ABSOLUTE PRESSURE. This is the sum of the gage pressure and the barometric pressure. Thus, if the gage pressure is 4 ounces-equaling 0.25 pound—and the atmospheric pressure 14.4 pounds to the square inch, the absolute pressure will be 14.65 pounds to the square inch, as shown on p. 15. This must be used in all gas calculations dealing with change of volume due to effect of pressure. Failure to appreciate that the absolute pressure, rather than merely the gage pressure, must be used when computing the effect of pressure on gas volume, or heating value content, has been responsible for most of the misunderstanding regarding the effect of variation in gage pressure on gas quality and gas service. DIFFERENTIAL PRESSURE. This is simply the difference between the pressure at the inlet and outlet of a gas line. Thus, if the inlet gage pressure of a gas line were 50 pounds and the outlet gage pressure 10 pounds the differential pressure would be 40 pounds. In gas transmission it is necessary to have a differential pressure in order to secure driving power to force the gas through the line. That is, the differential pressure is the pressure that is lost in overcoming the friction of the gas moving through the line. EFFECT OF PRESSURE ON GAS VOLUME, KNOWN AS BOYLE'S LAW. There is a definite relationship existing between the volume and pressure of natural gas. That is, when the gas is compressed or allowed to expand, it approximately follows Boyle's law. This law may be stated as follows: "The volume of a gas at constant temperature varies inversely as the absolute pressure to which the gas is subjected; or, what is the same thing, the product of the absolute pressure and the volume of a given quantity of gas remains constant." Thus, if the volume is doubled, or one-half of the gas is removed from a fixed reservoir, the absolute pressure will be reduced one-half. Conversely, if the absolute rock pressure in a fixed reservoir is reduced to one-half, the volume of gas remaining compressed in that reservoir will be reduced to one-half. GAGE PRESSURE 402LB 14 FIG. 2. DIAGRAM It has been the universal custom of the natural gas industry to disregard the small deviation of natural gas from Boyle's law and in measuring computations to assume that the gas follows the law exactly. Tests made on the West Virginia gas indicate that the per cent of deviation increases with the pressure. while there is no perceptible deviation at pressures under 15 pounds, at 150 pounds the deviation would be about 6 per cent. That is, the That is, |