In using the Bunsen effusion method, specific gravities are determined by noting the rate at which a certain volume of the gas passes through a small orifice. The rate at which a like volume of air passes through the orifice is also determined. The specific gravities of the natural gas and air are then in inverse ratio to the squares of the rates of effusion.

α

·f

e

EFFUSION METHOD.

d

Some natural gas contains a large percentage of carbon dioxide. This is a heavy gas, having a specific gravity of 1.53 (air-1). If it happened to be present in a natural gas mixture and a test were not made for it, an experimenter might be misled into believing that the gas was heavy, because of paraffin hydrocarbons present.

Many samples of natural gas contain large percentages of nitrogen. In an extreme instance the bureau found that a natural gas issuing from the earth in the State of Washington contained 98.5 per cent of nitrogen. Proportions of nitrogen as high as 10 per cent in natural gas are not uncommon. The specific gravity of nitrogen is 0.97. If much nitrogen were present an investigator might be misled by the specific-gravity test in that the test would not show the specific gravity of the paraffin hydrocarbons but the specific gravity of the entire mixture, which depends in part upon the content of nitrogen or carbon dioxide, or both.

CONSTRUCTION AND USE OF SCHILLING TYPE OF
APPARATUS.

The authors have used the particular type of apparatus known as the Schilling (fig. 3) for the specific-gravity determination. It consists of a glass jar, b, with a metal top

FIGURE 3.—Apparatus for determin- into which fits a brass column having sus

ing specific gravity of gas.

pended from its base a long graduated tube,

a, and at its top a cock, c, and a ground-joint socket, d, into which sets a socket holding a small glass tip, e, closed at the top with a thin piece of platinum, f. In this platinum is a minute hole to permit the pasgas or air at a very slow rate. All metal parts are nickeled.

sage

of

The mode of operation is as follows: The glass jar is filled with water to the top graduation of the tube or to a point a little above it. The tube is then withdrawn so that it may be filled with air. The cock on the standard is then closed and the tube replaced in the jar. The cock is then opened and with a stop watch the time is taken that elapses while the water passes from the lowest graduation to the graduation above. The tube is then withdrawn and filled with gas and the procedure repeated. The specific gravity, air being 1, is obtained by dividing the gas time squared by the air time squared. Thus, if A represents the time gas requires to pass through the orifice, and B represents the time air requires to pass through the orifice, the specific gravity of the gas will be

2

represented by (1)

USE OF THE PITOT TUBE
FOR MEASURING THE
OPEN FLOW OF GAS
WELLS.

a

3

2

2

The quantity of natural gas that is discharged from a well is usually measured by means of a Pitot tube. (Fig. 4.) This instrument directly measures the velocity of the gas flow. In its most accurate form it consists essentially of two parts, first a tube pointing upstream for measuring the dynamic pressure and second a means of determining the static pressure. Two pressures are thus obtained. Their difference as read on a U gage gives the velocity or impact pressure of the flowing gas.

FIGURE 4. U tube for measuring flow of gas.

3

As ordinarily used for field work the static pressure of the gas flow is not obtained, the instrument consisting simply of a small tube, which is inserted in the flowing gas (a, fig. 4), just inside the pipe or tubing, a distance of one-fourth to one-third of the pipe's diameter

from the outer edge. The plane of the opening in the tube is held at right angles to the flowing gas. At a convenient distance, varying from 1 to 2 feet, a U-shaped gage (fig. 4) is attached to the other end, which is usually half filled with water. If the gas pressure is high enough to force the water out of the tube, mercury is used, and for pressures that are so great that mercury can not be used, a spring gage is attached. A scale graduated from the center in tenths of 1 inch is placed between the two limbs of the U gage. The distance above and below this center line at which the liquid stands in the gage should be added, the object being to determine the exact distance between the high and the low side of the fluid in inches and tenths.

The top joint of tubing or casing should be free from fittings for a distance of 10 feet below the mouth of the well where the test is made. The test should not be made in a collar or gate or at the mouth of any fitting. The well should be blown off at least three hours prior to making the test, and in some cases as much as 24 hours should be allowed. After the velocity pressure of the gas flowing from the well tubing has been determined in inches of water, inches of mercury, or pounds per square inch as outlined above, the corresponding rate of flow may be ascertained from Table 3, a table prepared by F. H. Oliphant" and presented below. The quantities of gas stated in the table are based on a pressure of 14.65 pounds per square inch absolute, and a flowing temperature of 60° F., for a gas having a specific gravity of 0.60 (air = 1). If the specific gravity of is other than 0.6 the flow should be multiplied by

the

gas

0.6

V specific gravity of gas.

For flowing temperatures above and below 60° F., 1 per cent should be deducted or added for each 10 degrees.

a See Wescott, H. P., Handbook of natural gas, 1913, pp. 105-106.

TABLE 3.-Table to be used in testing gas wells with Pitot tube.

Multipliers for pipe diameters other than given in Table 3.
[Apply the multiplier to the figures given in Table 3 for 1-inch tubing.]

O

Multiplier.

68

81

100

144

PITOT TUBE WITH ATTACHMENT FOR MEASURING STATIC

PRESSURES.

For the most accurate measurements by means of the Pitot tube a device is attached for recording static pressures (fig. 5). Accord

HE

FIGURE 5.-Pitot tube with attachment for measuring static pressures.

ing to Rowse the piezometer or its equivalent is the most reliable means of obtaining the static pressure. This device embraces a pipe with a few very small circular holes in it, so combined with a larger pipe as to leave an annular space between them (fig. 5). This annular space is closed at the outer end, the other being connected to one end of the gage glass by means of a rubber tube. The other end of the gage glass is so connected to the Pitot tube as to give the dynamic pressure. The difference between the two pressures (the velocity head) is read on the gage glass.

a Rowse, W. C., Pitot tubes for gas measurement: Jour. Am. Soc. Mech. Eng., vol. 39, September, 1913, p. 1341.