Leak Location by Radioactive Gases in Buried PipesAndrew Gemant, Edward Hines, and E. L. Alexanderson Citation: Journal of Applied Physics 22, 460 (1951); doi: 10.1063/1.1699984 View online: http://dx.doi.org/10.1063/1.1699984 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/22/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in LEAK LOCATION IN SPACECRAFT SKIN WITH ULTRASONIC ARRAYS AIP Conf. Proc. 975, 1528 (2008); 10.1063/1.2902617 Acoustic emission leak source location J. Acoust. Soc. Am. 92, 3453 (1992); 10.1121/1.404160 ULTRASONIC SYSTEM FOR LOCATING LEAKS J. Acoust. Soc. Am. 39, 1196 (1966); 10.1121/1.1910019 Tracer Diffusion in the Ground in Radioactive Leak Location J. Appl. Phys. 24, 93 (1953); 10.1063/1.1721141 UK admits submarines leaked radioactive material Phys. Today

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460 GEMANT, HINES, AND ALEXANDERSON

CONCLUSIONS

The crystallography and interfacial free energy of noncoherent twin boundaries in copper have been in­vestigated. It is found that the noncoherent twin boundary is approximately parallel to a {113} plane of one crystal and to a {335} plane of the other. The ratio of the interfacial free energy of noncoherent twin boundaries to the average grain boundary free energy in copper is found to be 0.80±0.015. Two measurements

by a second method confirm the magnitude of this value.

ACKNOWLEDGMENT

The author is indebted to many members of the Metallurgy Division of the General Electric Research Laboratory for assistance, and in particular to J. C. Fisher and G. W. Sears. The investigation was sup­ported by the AEC.

JOURNAL OF APPLIED PHYSICS VOLUME 22, NUMBER 4 APRIL, 1951

Leak Location by Radioactive Gases in Buried Pipes*

ANDREW GEMANT, EDWARD HINES, AND E. L. ALEXANDERSON

Research Department, The Detroit Edison Company, Detroit, Michigan (Received September 29, 1950)

This paper describes basic considerations and experiments on leak location by means of radioactive tracer gases in underground pipes. C-14 labelled carbon monoxide and radon were used as tracers. The principles of the method are described. Suitable techniques for preparing and detecting the tracer gas have been worked out. Calculations on the spreading of a column of tracer gas in a pipe follow. Leak location on two small scale installations was carried out experimentally.

INTRODUCTION

THE principle of locating, by means of radioactive gases, leaks in closed containers can be applied

to any closed system and has thus a wide range of applicability. The chief field of application appears to be containers that are not easily accessible, such as buried pipes. The method deserves serious consideration for pipe-type high voltage cable lines.

There are several existing methods for locating leaks in pipes, such as that developed by J. D. Piper,! but none of the existing methods is applicable to all con­ditions. The radioactive method has the advantages of simplicity, operating on a purely qualitative principle, great sensitivity, and the need of little equipment.

One of the chief problems is the proper choice of the tracer element and the gas containing the tracer atom. The chief requirement the tracer must fulfill is its detectability at a concentration sufficiently low so that it does not constitute either a health hazard or a danger to the equipment to be tested. The ideal tracer should have a moderately long (few days to few weeks) half-life.

A fairly suitable tracer (except for its long half-life) was found in the form of C-14 which can be converted into both carbon dioxide and carbon monoxide. Tests showed that a microcurie of such a gas can be de­tected in concentrations of 10-4 J.l.c/cc. According to present knowledge2 the tolerance concentration of C-14 in air in case of permanent exposure (worst condition)

* A preliminary report of this study was presented at the 1950 Fall Conference on Electrical Insulation of the National Research Council at Pocono Manor, Pennsylvania.

1 J. D. Piper, Trans. Am. lnst. Elec. Engrs. 67, 10 (1948). 2 K. Z. Morgan, ]. Phys. Colloid Chern. 51, 984 (1947).

is 5XlO-5 J.l.c/cc. Concerning the total body intake, the amount that could possibly be retained by a person in our present scheme would be only microcuries which according to Skipper appears safe. Although the half­life of C-14 is 5360 years, its effective life in the body is only a matter of days. The gas chosen thus appears satisfactory from a health standpoint.

A second gas, used in this study, was radon. After its decay the long-lived Pb-210 remains, and if the original activity used was not more than about 100 J.l.C, the residual activity will be less than 0.05 J.l.C. Actually less than this will stay in the pipe, if the pipe is flushed after each test. Radon might be replaced by another gas if such should become available. Xenon-127, for instance, with half-life of 34 days and 0.9-Mev gammas, might be suitable for the purpose.

The present study must be considered as a first step along the lines mentioned. Experimentation was carried out in the Laboratory and in two short pipes in the field, but not in long buried pipes so far. The material to be presented in the following pages is somewhat heterogeneous, involving physical, chemical, and engi­neering aspects. The results of this study are presented under the following headings: principles of leak loca­tion; preparation of the tracer gas; detection of the tracer gas; introduction and subsequent spreading of the gas in the pipe; location of leak.

I. PRINCIPLES OF LEAK LOCATION BY RADIOACTIVE TRACERS

Among the various possible principles, two were studied in this investigation.

3 Skipper, White, and Bryan, Science 110, 306 (1949).

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LEA K L 0 CAT ION B Y R A D lOA C T I V EGA S E SIN BUR lED. PIP E S 461

FIG. 1. Diagram of apparatus for generating and detecting labelled carbon monoxide.

3

1. A radioactive gas having soft radiation (CO) is introduced at a suitable point into a buried pipe. If the pipe has a leak, the tracer will move in that direction, and, reaching the leak, will escape into the ground where it will diffuse out in all directions, but prefer­entially along the pipe where the soil may be looser. At certain spacings bore-holes in the ground over the line are prepared and the air from the soil is pumped into the detecting apparatus. The bore-hole with posi­tive indication then identifies the location of the leak.

2. A radioactive gas emitting hard radiation (Rn) is introduced into the pipe. As it spreads out in the direction of the leak, its progress may be detected by surveying the line with a counter. At the leak the gas escapes and from that point on no further radiation along the line is detected. An alternative of this prin­ciple consists in measuring the activity merely in the immediate vicinity at both sides from the point of introduction. The higher activity indicates the direction of flow and of the leak from the point in question. In this manner a pipe, accessible by manholes, may be investigated and the leaky man-hole section sought. Having ascertained the latter, Principle 1 may be used for narrower location of the leak.

II. PREPARATION OF THE TRACER GAS

1. Carbon monoxide

Flask G (Fig. 1) (volume 20 cc) served for generating CO2 from radioactive barium carbonate and concen­trated sulfuric acid. Usually a charge of about 0.01 g BaCOa, containing from 0.1 to 0.2 mc of C-14 was employed. A small heater served to expel the absorbed gas from the liquid phase. The gas was flushed by non­radioactive CO into the tungsten-filament light bulb B (volume 1000 cc), the method suggested by Kummer4

being used for conversion of CO2 into CO. In order to remove traces of O2, the gas, prior to its entering B, was passed through a tube containing fine copper wire, heated to about 500°C. The bulb was operated at 60 volts and 3 amperes for 20 hours. About 95 percent conversion efficiency is obtainable.

• J. T. Kummer, J. Am. Chern. Soc. 69, 2239 (1947).

FIG. 2. Diagram of quartz-platinum cell for liberating radon from radium.

2. Radon

The generator (Fig. 2) is a quartz container with a platinum crucible inside; the top of the quartz cell is closed after the crucible is placed inside. The crucible contains a small amount (0.1 mc) of radium salt with four times its weight of a mixture of sodium and potassium carbonate. Placed upon a Meker burner, the mix melts and the occluded Rn is liberated. One end of the cell is connected to the tank in which the gas is to be collected and which is previously evacuated. By opening the stopcock, the gas is swept into the con­tainer. The other stopcock serves for flushing the cell and transferring the maximum possible Rn into the container.

III. DETECTION OF THE TRACER GAS

Simple immersion of a thin-window counter into a gas space containing the tracer produces relatively low counts. Accordingly, a technique was devised involving enrichment of the CO in a small and light filter. Such a filter, brought close to the mica window of a Geiger­Mueller counter, radiates into the counter under very favorable geometric conditions. Since it is not easy to absorb carbon monoxide by a filter, the latter was first converted into carbon dioxide which is easily absorbed by a suitable filter.

The reason that CO and not CO2 was used as the tracer gas is that the latter is readily absorbed by soil, as special experiments have shown; thus, it would not spread out sufficiently in the ground after it had escaped from a leak.

The conversion of CO into CO2 prior to filtering is accomplished by passing the gas through a tube H (Fig. 1) containing Hopcalite,* a mixture of chiefly copper and manganese oxides; this material converts CO into CO2• The filter F (fiberglass mat, diameter 1.3 cm, thickness 0.05 cm) is cemented to the end of a glass tube. The filter contains alcoholic potassium hydroxide solution. The filter is kept wet while the gas is passing through it; this is accomplished by means of the alcohol dropper shown. The gas to be analyzed is drawn through the filter for about 10 minutes, a pump being connected to opening 3. Toward the end of this process the filter is permitted to dry, formation of K2COa being noticeable by its white color.

In order to determine the sensitivity of the filter detector, a known amount of tracer was drawn from

* Mine Safety Appliances Company, Pittsburgh, Pennsylvania.

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462 GEMANT, HINES, AND ALEXANDERSON

o FIG. 3. Diagram of a tracer gas column in a pipe.

the bulb B (Fig. 1) into the sampling tube S, and diluted with air in the cylinder C so that the mixture corresponded to a definite concentration of tracer in air. Two p.c of CO in a concentration of 10-3 p.c/cc produced after filtering about 10,000 counts per minute. Removal by means of a suitable absorbent all extraneous CO2

from the tracer-air mixture prior to its passage through Hopcalite is expedient.

The efficiency of CO collection by the alkaline filter was obtained from counts taken at a known distance of the filter from the counter window. The expected counts for the case that the entire 2 p.c was fixed by the filter, can be calculated6 from the weight of the filter (18 mg); the actual counts indicated an efficiency of about 24 percent. This low efficiency was expected in view of the thinness of the filter, permitting a large fraction of the CO2 to pass unabsorbed.

The effect of the soil in using this technique was shown by permitting a sample to stand in contact with soil in cylinder C for 16 hours: a count of 800 per min was obtained in a particular case; a control test without soil gave the same counts. Hence, CO can be used in underground installations as was to be expected from the low solubility data for CO in water (Ostwald ab­sorption coefficient 0.02 as compared with 0.9 for CO2).

IV. INTRODUCTION AND SPREADING OF TRACER GAS IN PIPE

The question arises how much tracer gas is required if a leak in a given length of pipe is to be located. The travel of a column of tracer gas down the pipe is due to the drop of pressure at the leak, while the original

I~--.-----r---r---,__--_,

C

40

25

FIG. 4. Spread of tracer gas column for various values of ktla'.

5 Edward Hines and Andrew Gemant, Science 110, 19 (1949).

I.or---r-,__----r---.__--,------,

c ---- --INITIAL

AFTER

_---1-- 5 _------ ,10

..- /' '/20 DAYS

A~-I"I-+-7"_-tt'-----I_---I_--_1

I

j-t-" 2

DD 40 60 80 It 100

FIG. 5. Spread of originally sharply bounded column (a= 10 feet) after 1, 5, 10, and 20 days for carbon monoxide (k=0.2 -cm2/sec).

pressure is maintained by pressure tanks connected to the pipe. In addition to this linear displacement of the column, the latter will spread because of diffusion. In the radioactive technique this spreading of the column by diffusion is desirable, since in this manner the tracer gas reaches the leak faster and the time required until the tracer can be detected is reduced.

Consider a gas column of length 2a in a pipe, any point along which is designated by x; the center of the column coincides with x=o (Fig. 3). This column contains a tracer gas at a concentration designated by unity. The boundary condition is: for the time t=O, the concentration c= 1 for -a<x<a, and c=O outside this range. The concentration for any time and any location can be calculated by starting out from a general equation, as given, for instance, by Carslaw and Jaegar. 6 The result of this calculation is:

[ (x/a) + 1

c=0.5 erf---2 (kt)l/a

(x/a)-1] erf .

2 (kt)!ja

200~----,_------r------.__----_r----_,

II

1501----+----+----1_----[---_1

°0~--~5---~IO--~1~5~~~20~~da-Ys~2·5

(1)

FIG. 6. Rate of spread of tracer column for 2a= 10, 20, and 40 feet, for carbon monoxide and tritiated hydrogen gas.

6H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids (Clarendon Press, Oxford, England, 1947), page 34.

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LEA K L 0 CAT ION B Y R A D lOA C T I V EGA S E SIN BUR lED PIP E S 463

In this equation k is the diffusion constant of the tracer gas.

A graphical presentation of Eq. (1) is given in Fig. 4, showing c as a function of xl a; the parameter of the set of curves being ktl a2• By using this graph, any other desired graph may be drawn. Such a graph is shown in Fig. 5 for carbon monoxide as a tracer, k of which is about 0.2 cm2/sec. It shows the spread of an originally sharply bounded column of 20 ft length (2a) after 1, 5, 10, and 20 days.

Another graph is given in Fig. 6. Suppose the original concentration of the radioactive gas is such that 1 or 0.1 percent of it, leaving the leak, could be detected by the method described. It is then of interest to know at what distance from the origin these small concentra­tions prevail after a number of days. That distance must be added to the displacement of the column due to convection as mentioned earlier. This information is given in Fig. 6 for an original length of 10, 20, and 40 ft; for both carbon monoxide and tritiated hydrogen (H3H). This latter gas whose diffusion coefficient is

co

J 11ntntntl'Hr \ /'

FIG. 7. Diagram of underground pipe installation for leak location by labelled carbon monoxide.

about 0.5 might be used with advantage; its diffusion, also in the soil, is 2! times faster than that of CO.

From the results obtained it is possible to give a design of a leak test on a longer pipe line. If an installa­tion covers several miles, it might be desirable as mentioned first to ascertain which manhole section is the leaky one. When the leaky section of, say, !-mile length has been located in that manner, the carbon monoxide technique can be used for a closer location.

The laboratory test showed that 2 JJ.C of CO in a concentration of 10-3 JJ.c per cc produced counts of up to 10,000 per min. From this it can be estimated that in a pipe of, say, 2000 ft, perhaps five mc of CO should be used. If the original column is spread over 20 ft, the original concentration (assume, for example, an 8-in. pipe containing three 138-kv cables and having an open cross-sectional area of 0.23 sq. ft) is 40X 10-3 p.cl cc. The rate of travel of this column is about 60 ft per day if the leak is such that the loss of gas at 200 lbl sq. in. pipe pressure is one standard 200 cu. ft tank per day. If the leak is, say, midway, i.e., 1000 ft away from the point of introduction, 15 days are required

TABLE 1. Field test with CO, in dry weather.

Net counts per min with G-M Gas introduced counter i bore-holes:

Time schedule into pipe 2 3 4 5 6

First day, A.M. First tank 6 0 31 First day, P.M. Second tank 0 19 60 1880 24 Second day, A.M. 11 11 7 90 Second day, P.M. Third tank 36 20 Third day, A.M. 0 12 11 265 9

for the column to reach the leak. Figure 5 shows that in 15 days the column has flattened out to six times its original length, and its average concentration is one­sixth of the original. The column has then a tracer concentration of 7X 10-3 p.c/cc, and it takes about 2 days while all the gas escapes from the leak into the ground. The concentration of the escaping gas is about 0.5X 10-3 JJ.c/cc.

By applying an artificial leak of, say, two tanks per day at the other manhole (not the one at which the tracer was introduced), the rate of progress may be trebled. Instead of 15 days, it would need only 5 days to find the leak. In this fashion only i of the tracer would escape, but the sensitivity of the method might still be sufficient for finding the leak.

V. LEAK LOCATION IN PIPES

1. Carbon monoxide tracer

Field tests showing the possibilities of leak detection on a pipe buried underground were carried out in the following manner. A pipe, 1 in. in diameter was in­stalled (Fig. 7) in the ground, the horizontal portion being 20 ft long and 5 ft under the ground surface. This portion was jacked from a trench into position in order not to disturb the soil around the pipe; the end of the horizontal part contained a hole, /6 in. in diam­eter, serving as artificial leak. The soil immediately above the pipe was pure clay, thus presenting unfavor­able conditions for the spreading of the gas. In a sandy soil detection should be easier.

Along the 20-ft pipe, !-in. tubes, 5 ft apart, were driven into the ground, their open lower ends being 1 ft above the pipe. These bore-holes are shown in the figure, and numbered as 1 to 6, No. 5 being over the leak.

For each test about 0.2 me of CO was produced. The gas was transferred into from two to four 17 cu. ft

TABLE II. Field test with CO, in wet weather.

Net counts per min with G-M Gas introduced counter; bore holes:

Time schedule into pipe 2 3 4 5 6

First day, P.M. First tank Second day, A.M. Second tank 7 8 7 51 3 Second day, P.M. Third tank 6 4 7 58 670 Third day, A.M. Fourth tank 5 5 9 15 Third day, P.M. 7 22 51 13

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464 GEMANT, HINES, AND ALEXANDERSON

.v

G K

A B

G

FIG. 8. Diagram of pipe installation to determine direction of leak from a sleeve, using radon.

cylinders, the total pressure in each being brought to 1QOO lb/sq. in. These tanks were emptied into the pipe which contained the leak; by means of a reducing valve the pressure in the pipe was kept at 200 lb and the rate of flow was such that each tank emptied in about 3 hours.

From time to time the gas-collecting equipment, con­sisting of a Hopcalite tube and a filter, was connected to the various bore-holes, and air was pumped out of the soil. The pumping process lasted about 15 min each. Since the Hopcalite retains part of the CO2, the material must be renewed after each measurement with positive indication.

The results of one particular test are summarized in Table I, giving indications of the time, the radioactive charge emptied into the pipe, and the net counts per min obtained by a recording counter. This test was carried out after a few weeks of comparatively dry weather; the gas permeability of the soil was relatively high.

The results of a second test are shown in Table II. This test was carried out in rainy weather; the moisture content of the soil was higher and the gas permeability of the soil lower than in the first test. The counts ob­tained were, therefore, generally lower.

It may be seen that the relatively small amount of 0.2 mc dissipating from a leak into the soil is easily detectable by means of the filter technique. There is a maximum of counts at the leak; the counts diminish with increasing distance from the leak along the pipe. Small counts can be found as far as 20 ft away; longer distances along the pipe could not be tested with this installation. A further bore-hole 5 ft away beyond hole 6 in undisturbed ground did not give positive results. The gas obviously travels along the pipe easier than into undisturbed ground. It follows that bore-holes must be spaced not more than 40 ft apart.

TABLE III. Results of first Rn test for directional detection .

Time in hours after introduction of gas

6 24

Counts per minute Toward leak Away from leak

180 110

110 60

TABLE IV. Results of second Rn test for directional detection.

Time in hours after introduction of gas

2 3 5 6

Counts per minute Toward leak Away from leak

140 140 80 70

2. Radon as tracer

90 60 40 40

As mentioned in Sec. I, radon was used in order to find the direction of a leak from an accessible point of a pipe.

The test was carried out in an installation shown in Fig. 8. Two 30-ft lengths of 8-in. pipe are joined by a 14-in. sleeve, 9 ft long. The sleeve has a valve K which served for introduction of the radon from a container C. The pressure in the container was brought to 800 lb/ sq in., and a needle valve V permi tted the slow passage (in about 30 min) of the gas into the pipe the pressure in which was 180 lb.

One end of the pipe was connected to a needle valve V' and a flowmeter F. The needle valve served as an artificial leak permitting a drop of pressure in the pipe of SO lb/sq. in. in the course of 6 hours.

The counter tube G, of a portable instrument was placed at points A and B at certain times. The readings (background counts deducted) are shown in Table III for one of the tests. In this test about 100 p.c of Rn was passed into cylinder C of 3.5 lit.

In a second test, summarized in Table IV, SO fJ,C of Rn were used in a small cylinder of 400 cc.

In introducing the gas into the sleeve, its spreading toward both sides could not be prevented. After a few hours, however, the counts toward the leak are higher than in the opposite direction. This shows that determination of the leaky section appears feasible by the method indicated.

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