Radioactive Tracers to Mark Interfaces and Measure Intermixing in Pipelines
development
DONALD E. HULL AND JAMES W. KENT' California Research Corp., Richmond, Calif,
W
HEX different petroleum products are transported in a
common pipeline, it is important t o distinguish the boundary between them so t h a t upon arrival a t the terminal consecutive products may be segregated in their respective tanks with a minimum of contamination. On long pipelines it is of value t o know t o what extent the interface has spread into the two products in order to determine whether the contaminated volume can be absorbed within the specifications of either product or whether it must be treated t o separate the original components. Two methods have been commonly used for this purpose-the measurement of density and the observation of color. This paper reports a newly developed method, the injection of a radioactive tracer a t the boundary between two liquids, which has several definite advantages over the conventional methods. Xeither the gravitometer nor the colorimeter will work in all cases. Both methods entail either the withdrawal of samples or the bypassing of a portion of the stream, and hence a time delay. The radioactive tracer can be detected instantly through the wall of the pipe, and its measurement provides a notably more precise and sensitive indication of intermixing. Several tests have been made of the uae of radioactive tracers t o label interfaces in commercial pipelines in this work. A variety of radioactive tracers may be used for this purpose. The requirements which the tracer must meet are: (1) it must emit penetrating gamma rays; (2) it must have a half-life at least comparable with the duration of its trip through the pipeline; and (3) it must be present in a stable, oil-soluble compound.
Selection and Preparation of Radioactive Tracers Barium-140 was selected as one isotope which meets these requirements. This element, formed by the fission of uranium in the pile a t Oak Ridge, was allocated by the Isotopes Division of the Atomic Energy Commission and supplied by the Oak Ridge National Laboratory. It has a half-life of 12.8 days and emits beta rays along with 0.5 m.e.v. gamma rays. The product of this disintegration is lanthanum-140, which is also radioactive, with a half-life of 40 hours. It has a complex gamma ray spectrum, in whirh the most prominent line has an energy of 1.6 m.e.v. It is this lanthanum gamma ray which actually contributes most t o the detection of the tracer by a Geiger counter outside the pipe. The barium was received from Oak Ridge in a dilute hydrochloric acid solution. From it was synthesized an alkyl phenate, dissolved in a few milliliters of oil. About 4 millicuries (me.) of barium was treated in the first synthesis and about 250 me. in a 1 Present address, California Research and Development Corp., 200 Bush Street, San Francisco, Calif.
later experiment. Yields of 30% were obtained, based on the activity of the oil-soluble isotope. Another isotope used for pipeline tracing was antimony-124. This isotope is not formed in fission, but is prepared in the pile by neutron irradiation of ordinary antimony. The isotope of mass 123 captures a neutron to form antimony-124. This is beta radioactive with a half-life of 60 days, emitting, among other gamma rays, one with a n energy of 1.7 m.e.v. A suitable compound in which t o incorporate the antimony into petroleum products is triphenylstibine. This compound, obtained from the Eastman Kodak Co., is irradiated in the Oak Ridge pile. The compound is partially decomposed by the pile radiations, and some of the antimony activity is lost in the decomposition products, but as much as 60 t o 70% of the active antimony was found t o be extracted with the original compound upon treatment of irradiated samples with hydrocarbon solvents. The solution thus obtained is stable to air, water, and dilute acids and bases.
Injection and Measurement of Radioactive Tracer An apparatus previously described ( 1 ) was used for injecting the tracer into the pipelines on the suction side of the pumps. The injection usually required 2 t o 4 seconds. The passage of the tracer a t selected points along the line was observed with a Geiger counter fixed against an exposed portion of the pipe. An allmetal counter tube, 1 inch in diameter and 16 inches long, made by the Geophysical Instrument Co. for cosmic-ray work was used in these studies. The tube was connected t o a high voltage supply and decimal scaler made by the Berkeley Scientific Co. (Model 1000-B). Power was taken from a conventional 110volt supply with extension cords (up t o 300 feet long) whenever possible. I n remote places power has been supplied from a gasoline-driven motor generator or from 6-volt storage batteries through an alternating current converter. The counting equipment was found t o be reliable under widely variable conditions of operation. The procedure for recording the passage of the marked interface was as follows: Each group of 1000 counts was registered with an audible click by a mechanical counter on the decimal scaler. The time reading a t each click was recorded, together with the reading of the counter register. From these data, the average counting rate during successive 1000-count intervals was calculated. The observations were usually started well ahead of arrival of the radioactive injection in order to record several intervals a t background rate. After passage of the wave the observations were continued until the rate returned t o background,
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SECONDS Figure I. Spreading of Radioactive Tracer Wave Injection P at Rangley
Experimental Tests of Tracers in Pipelines Experimental injections of radioactive tracers have been made in three pipelines operated by the Standard Oil Co. of California or its subsidiary, the Salt Lake Pipe Line Co. These lines are described in Table I. A brief description of the first test in the San Pablo line has been published earlier ( 1 ) . Table I.
Description of Pipelines
Location San Pablo t o Richmond, Calif. El Segundo to Montebello, Calif. Rangely, Colo., t o Salt Lake, Utah
Length, Miles 5
Velocity Diain.. of Flow, Reynolds Inches Ft./Sec. Number 20 4 51 103,000
24
8
3 61
300,000
182
10
2 68
24,000
Single injections were made in the San Pablo and Montebello lines. The Rangely line, which traverses rough mountainous terrain, was used for two series of four injections each a t interfaces between crude oil and crude oil-gasoline mixtures. These tests afforded the best opportunity to observe the behavior of the interface under varied conditions. Radioantimony was used in the El Segundo test, radiobarium in the others. I n all cases the wave has been observed in passage a t locations ranging from near the injection point to the terminal. X h e n the tracer is first injected, it is concentrated in a narrow band, and the counter shows its passage as a n-ave of high intensity and very short duration. As it travels along the pipe the tracer spreads into the adjacent liquid on both sides, so that eubsequent observations shox a broader wave of smaller amplitude. This is illustrated by experimental results shown in Figure 1, where the same tracer injection is plotted as seen a t three points along the line. The extent t o which the interface spreads with distance along the line is the characteristic of greatest importance in operation of the line. I n order to have a common basis for comparison of the different interfaces, the width of the v a v e was measured at the level where the activity was 5oy0 of the peak value. Thus, the spread of tracer waves of varying sizes, as in the series of Rangely injections, can be compared quantitatively. The half-value q-idths of the waves formed by injections a t Rangely are listed in Table 11, measured in seconds a t a constant flow
rate of 2.68 feet per second. It is seen that all the interfaces at a given station have a constant a.idth within a reasonable accuracy. The variation of width with distance is shown in Figure 2 on logarithmic coordinates. The experimental points fall well on a straight line, showing t h a t the width increases as the 0.56 power of the distance. The deviation of the last point, a t the 182 milepost, appears to be real and to be the effect of rough terrain. The route of the Rangely pipeline runs over broad plateaus b e h e e n the White River near Rangely and the Green River near Ouray, Utah, then climbs up the Duchesne Valley through Hanna to the crest of Wolf Creek Pass, a t an elevation of 9560 feet. From thcre it runs on steep slopes across deep valleys and successively lower peaks of the Wasatch mountains until it reaches the terminal a t Salt Lake, 4220 feet above sea level. The line is full continuously to the summit a t n701f Creek Pass, but beyond that point the stock flows under gravity faster than the pumping rate on several downhill runs. Contrary to expectation, the mixing over this part of the route was not abnormally high. Actually, the 50y0widths shoa. definitely less mixing than would be indicated by extrapolation of the curve for Figure 2 a t the rate
Table
II.
Width at Half Value of Tracer Waves in Rangely Pipeline (In seconds) a Milepost
Station Rangely White River Bonanza Green River Hanna (in) Hanna (out) Summit Provo River Salt Lake T o convert
1
Injection Siimber 2 4 5
0,062 17.0 13.8 21.2 . .. 1.9 69 65 . ., . .. 13.8 210 198 224 ... 43.1 404 400 402 108.5 670 644 690 670 108.5 670 635 730 650 125 742 735 805 715 ... ... ,,. 130,4 182.5 860 850 865' 980 to length, multiply by 2.68 feet per second.
Table 111.
6
... . .. ... 71'J 660 730 770 940
Average 18.0 67 211 402 678 669 746 770 895
Stocks Pumped in Rangely Test
Crude oil Crude oil plus gasoline
Spec. Gravity a t 70' F.
Viscosity a t 70' F., CS.
Reynolds Number
0.8461 0.8391
9.73 7.76
21,500 27,000
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1952
followed up t o the summit. The explanation for this unexpected result may be that the greater linear velocity of the downhill run has the same effect as a smaller pipe diameter in reducing mixing. At a n interface between two liquids of different viscosities it might be expected that the rate of mixing would not be the same in the two liquids. If mixing were more rapid on one side than on the other, a dissymmetrical wave form would develop. I n the Rangely tests, a n effort was made t o observe such dissymmetry by injecting the tracer a t interfaces made between crude oil and a mixture of crude oil with gasoline. The maximum available concentration of gasoline did not provide a very great difference in fluid properties, as shown in Table 111.
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Another illustration of the effect of viscous flow is seen in the results obtained a t El Segundo, where the line passes through an excelsior-packed filter just outside the pump station. The whole tracer injection was thus subjected t o laminar flow through a porous substance before the first observation was made on it. The effect of the filter was t o form an asymmetric wave which persisted as an abnormally large tail even after 24 miles of normal mixing. This effect is shown in Figure 4. The effect of passage through a pumping station on the dispersion of a n interface is found from the Rangely data to be small. The tracer waves issuing from the Rangely pump station had a width a t 50% of peak height of only about 48 feet. Passage through the intermediate pump station at Hanna did not broaden the waves appreciably. At least in this line, the effect of pumps was small compared with the normal mixing in pipeline flow.
Comparison with Other Methods of Marking Interfaces In the Montebello pipeline, the arrival a t Montebello of the interface between regular and premium grade gasolines is observed routinely with both a colorimeter and a recording gravitometer. The test on this line, therefore, provided a n opportunity to compare the sensitivity of the radioactive tracer with both these older methods. Care was taken in this test to inject the
I 20
I 2
I
I O
20
E4
100
rm
DISTANCE FROM RANGELY -MILES
Figure 2.
Half-Value
Width of Waves
Interfaces with the more viscous stock first leading and then following were labeled and compared with each other and with injections in the middle of a crude run. No definite differences in shape of the wave were detected in these experiments. Another factor that would introduce an asymmetric component into the wave form is viscous flow. This would have the effect of elongating the trailing side of the wave into a tail. Examination of the experimental curves from the Rangely data do not give any evidence, except in a few cases discussed below, of any marked dissymmetry. It may be concluded that, in normal operation, viscous flow makes only a small contribution to the mixing. I n the second series of Rangely tests a shutdown was carried out by slowing down the pumps gradually over a period of half a n hour. After 10 hours a t rest the line was started up again, gradually over a half-hour as before. During part of this time the line operated at laminar flow velocities. Under these conditions the viscous drag pulled the trailing side of the interface out into a pronounced tail. This was especially marked in an injection (number 8), which had been made at the Hanna pump station, and was still relatively sharp at the time of the shutdown, I t s appearance on its subsequent arrival at Provo River is shown in Figure 3. I n contrast, the shape of injection 7 in the same figure is seen to be quite normal after the interruption. This wave had already crossed the summit when the shutdown was started. It continued t o flow under gravity a t turbulent velocities until i t came to rest in the first trough. Thus, the extended period of flow under laminar conditions was avoided in this case and only the normal dispersion was exhibited. This experiment shows clearly the importance of maintaining pipeline flow in the turbulent region t o avoid undue mixing of different products and indicates that shutdowns and start-ups should be carried out quickly when interfaces are in transit.
w
eL
IO
ge W
6 4
L 800
4cQ
LOO
0
LOO
400
BW
COO
SECONDS Figure 3.
Injections M a d e at Hanna
Observed at Provo River after ahutdown
tracer exactly in the interface. The error of timing was less than 5 seconds. The gravitometer record of the arrival at Montebello is shown on the same time scale with the tracer wave in Figure 5 . It is seen t h a t the peak of the wave comes three minutes earlier than the midpoint of the gravitometer trace. The first indication of the approach of the interface was seen in the counting rate 4.5 minutes before the gravitometer began t o respond. Forty seconds of this can be attributed t o the volume of pipe and filter tank between the two instruments. The rest of the lag must be the time required t o fill the gravitometer. The color change was seen simultaneously with the upturn in the gravitometer
Table IV.
Total Counts in Wave at Various Stations (El Segundo-Montebello pipeline)
Milepost 0.25 1.5 14.5 24.5 AV.
Counts 4646 i.140 5020 f 180 4920 f 180 4790 =t180
Pipe Size, In. 6 8 8 8
4910 f 100
8
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trace. This demonstrates how both the high sensitivity and the immediate response to the radioactive tracer contribute t o its advantage over the other instruments. The time required for transit between different stations can be measured with high accuracy with the radioactive tracer. Inspection of the tracer waves shows that even the most widely dispersed has a peak which can be located to the nearest minute. During the Rangely tests, care was taken to maintain the pumping speed as constant as possible. A comparison of the measured transit times of five injections made a t Rangely as observed a t eight stations along the line shows at each station a variation of only =t1 minute. Measurements made in this way, combined with pump displacement volume, provide a method of determining pipeline capacity more accurately than any other may of measuring transit times, even more so than calculation from pipeline schedules.
MINUTES
Figure 4.
Effect of Filter on Interface Injection in El Segundo line
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injection in the moving liquid, in terms of the peak activity, A L , of t h e tracer wave after being transported a diitance, L , along the pipeline, and the width parameter
w
d2E
=
t being the time after injection. square root of t.
AL
(3)
A L decreases in proportion to t h e
(4
= At-1/2
The validity of this relation for distribution of a tracer TT-ave can be tested most thoroughly with the data from the Rangely pipeline. In transport a t constant velocity, so that L = ut, the nidth of a tracer n-ave should increase as the one-half power of L , according to Equation 3. This is in fairly good agreement with the empirical function L (Figure 2). A check of Equation 4, which predicts an inverse one-half power in the variation of peak height with distance, is seen in Figure 6. Appropriate corrections have been made in the observed peaks to take account of radioactive decay and of reduced counting efficiencies a t 6ome stations. Satisfactory agreement is found, the empirical exponent from the slope being -0.54. An over-all test of the theory is involved in the calculation of D, the mixing coefficient. In a given pipeline a t a steady flow rate, this should be constant. From 23 values of wo.5 and L listed in Table I1 from the Rangely tests, the values of D have been calculated. The average of all values from this pipeline is 1.28 5 0.22 square feet per second. A similar calculation from the results of the San Pablo test gives a D of 2.04 square feet per second. The accuracy with n-hich Equation 2 represents the experimental curves is illustrated by Figure 7 , in which are plotted the counting rates observed during the passage of one of the injections from Rangely a t a station 43 miles downstream. The solid curve is calculated from the equation, with the parameter A determined from the size of the injection and the parameter D taken as the average of all values calculated from the waves observed in the Rangely tests. The experimental points fall on the calculated curve within the accuracy of counting.
Under standardized conditions of flow and measurement and with an appropriate correction for radioactive decay, the total number of counts recorded from a passing wave is constant,, no matter what shape the wave has. This constancy of total counts is best illustrated by results obtained in the El SegundoMontebello line (Table I V ) where three widely spaced measurements were made under identical conditions, with the counter tube strapped to a clean exposed pipe of constant dimensions. The integral count on the %inch pipe is seen t o be constant within the error of counting. The first count, in the 6-inch pipe, was somewhat smaller.
Mathematical Analysis of Tracer Waves The shape of the tracer waves has been found to fit a normal probability curve almost within the error of measurement. This function for the activity of the tracer in terms of displacement and time measured from the point of injection may be derived from the differential equation for mixing by diffusion
I
! 626
6 26
1 6 30
I
6 32
I
I
I
6 34
38
TIME
Figure 5.
Comparison of Radiotracer with Gravitometer
Injection From El Segundo observed at Montebello
where D is a mixing coefficient similar in physical dimensions t o a molecular diffusion coefficient, but larger by a factor of approximately 109, since the movements within vortices in turbulent flow are of much greater magnitude than molecular free paths. The integrated equation may be written in the form a = A L exp
where a
iF
II-I'):(
the artivity a t any dipplacement, z,from the point of
No appreciable deviation from the probability curve is apparent in the normal wave plotted on rectangular coordinates. However, if the integral of the counts (corrected for background) is plotted against time on probability coordinate paper, the normal curve gives a straight line and deviations from the theoretical distribution become more readily apparent. Treatment of the data in this way (Figure 8) reveals a definite small tail on all the waves. This is to be expected as a result of laminar flow in a thin layer next to the pipe m-all.
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Intermixing across Interface
It can be shown t h a t when a tracer is injected in a n interface between two consecutive liquids, A and B , at the moment i t is formed, the concentration, c, of one of the liquids across the interface a t any subsequent time is given by the normalized integral of the tracer activity.
Table V.
1 to 99% Widths of Interfaces (Feet)
Milepost
Exptl.
Theoretical
1.9 13.8 43.1 108.5 125.0 183.5
...
351 1110 2100 3580 3960 4750
1330 3030 4320
4400 5790
Ratio, Exptl./
Tbeor.
(9)
Ratio Smith and Schulae/ Theor.
216 738 1500 2660 2900 3670
0.62 0.66 0.71 0.72 0.73 0.77
Calcd. from
...
1.20 1.44 1.21 1.11 1.22
(5)
L
-m
Further, the integral of the concentration gives the quantity of the product which has passed a given point. [Lefevre ( 2 ) has arrived a t expressions which are similar in form t o these equations through mathematical induction from the arithmetic averages of
2,
2
5
0
20
50
IW
Comparison with Other Measurements of Interface Widths Smith and Schulze ( 3 ) have reported a n extensive series of mixing measurements in an experimental 2-inch line, using a gravitometer with products of different densities. They have attempted to correlate their laboratory results with some field measurements on commercial lines with partial success. They give an empirical equation for the 1 t o 99% width as a function of length of run and Reynolds number. I n order t o compare results of this work with those of Smith and Schulze the widths of waves a t the 6.7740 level, which marks t h e 1 t o 99% concentration range, have been measured. I n Table V is listed the average width a t each milepost as measured on curves through the experimental points. Also listed is the width measured on a theoretical curve ( a straight line on probability coordinates) fitted t o the experimental data. For comparison the corresponding widths are calculated from the equation of Smith and Schulze. It is seen that their values are consistently smaller than the theoretical, but t h a t the real widths (on account of the viscous flow effect) are larger by 10 to 45%.
Po0
concentrations in a series of adjacent sections.] I n Figure 9 are plotted, in terms of the characteristic width parameter, w, the activity of the tracer wave, the concentration of the product B which follows the interface, and the volume of B which has flowed past. This graph together with the value of w at a given station can be used t o calculate the extent of intermixing from measurements of the tracer activity. As a n illustration of such a calculation, consider the wave arriving a t Salt Lake from Rangely, having a width at the 50% level of 2240 feet. This is related to the parameter w by the proportionality derived from Equation 2
0
200
400
BW
800
IWO
1200
1400
SECONDS
Figure 7.
Tracer Wave of Injection 1
Observed at Green River
Thus w = 1340 feet a t Salt Lake. Multiplying by the cross section of the pipe gives the equivalent volume, 134 barrels. From the graph, the 10 t o 90% concentration range is seen t o extend between x/w = k0.907 or 2430 feet. Similarly the 1 t o 99% concentration range extends between x/w = 11.645 or 4410 feet. Observed values of the 10 t o 90% range on probability coordinate plots agree well with the calculated values. Because of the tail on the interface, observed 1 to 99% values vary from 10 t o 50% higher than the theoretical. At the 10% concentration level, only five barrels of B have flowed past the counter, but the counting rate is already 45% of its peak value. Even the 1% concentration level is marked by 6.7% of the count rate; a t this point only 0.1 barrel of B has contaminated A . These figures illustrate emphatically the large factor of sensitivity gained by measurement of a radioactive injection instead of differential measurement of a property depending on concentration.
Smith and Schulze used a sine curve t o fit their interface concentration curves empirically. They point out t h a t a t the 1% concentration level the gravitometer charts are subject t o considerable error. Apparently their values of the 1 t o 99% concentration ranges were not read directly from the data but were extrapolated along the sine curve fitted t o the gravitometer record in the region where it was more accurate. Actually, sine curves drawn through the 1 to 99% points calculated from their formula fit most of the present data reasonably well, except a t the leading and trailing edges. It appears likely t h a t the discrepancy indicated in Table V arises not in the data themselves but in the different ways of reducing the data. The ratio of the 1 t o 99% width on a sine curve t o t h a t on a n integrated probability curve tangent t o it a t the 50% concentration level is 0.74, a factor of the same magnitude as the ratios in the last column of Table V.
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Vol. 44, No. 11
ioc
80
3
60
z x
s L L 40 0
x 20
9 0
-15
-io
-05
0
05
id
15
20
f Figure 8.
Tracer Waves on Probability Coordinates
Figure 9.
Injection 1 as obiorved at Green River
Radiological Safety in Use of Tracer The advantages of the radioactive tracer for studying flow in pipelines are so marked t h a t i t seems well t o include a brief mention of the disadvantage arising out of the precautions necessary t o avoid the hazards of radiation. The synthesis and other handling of the concentrated tracer preparatory to injection must be done with due care t o avoid exposure t o the hard gamma rays and contamination with the radiotracer. Once the tracer is injected into the pipeline, it undergoes such rapid dilution that no further hazard is associated with it. Passage of a tracer 17-ave exposes a person in contact with the pipe t o only a few microroentgens. I n the storage tank at the terminal the injected tracer is further diluted so t h a t even immersion in the oil would give a radiation level only a fraction of that produced by cosniic rays.
Relation of Activity to Concentration and contamination
Under normal conditions of turbulent flow the interface spreads symmetrically in both directions. Under conditions of viscous flow, whether due t o reduced pumping speed or passage through a filter, the interface is extended on the trailing side. The concentration of the stock following an interface marked by a radioactive tracer is represented by the integral of the tracer activity curve. The radioactive method of labeling interfaces offers several marked advantages over other methods. It is generally applicable to all kinds of liquids; it does not require any diversion of the flow or the removal of samples; and there is no time lag in noting its arrival. The interface tracer is inherently more sensitive than any measurement which is proportional to the concentration, such as color or specific gravity changes; it provides precise information on the extent of mixing. The disadvantage of having t o cope with radiation hazards has been met without difficulty
Conclusions By use of radioactive tracers injected at the interface between different liquids in a pipeline the passage of the interface a t any subsequent point can be accurately noted and the extent of intermixing determined. The spread of a tracer injected a t one point in a stream and pumped through a pipeline a t a uniform speed sufficient t o maintain turbulent flow can be represented with good accuracy by the probability function
Acknowledgment The authors wish t o acknowledge the assistance of R. C. Mithoff, who suggested the use of radioactive tracers t o mark interfaces in pipelines, and R. K. Bond, who took an indispensable part in conducting the tests on the Rangely pipeline. The developments described in this paper are the subject of patent applications by the California Research Gorp.
literature Cited (1) Hull, D. E., Kent, J. W., and Lee, R. D., World Oil, 129,No. 1, 188 (1949).
with two arbitrary parameters: A , determined by the size of the injection, and D,characteristic of the pipeline flow conditions. The width of the interface increases approximately as the square root of the distance pumped, all other conditions being held constant. Mixing is less than normal on steep downhill runs over mountainous terrain.
(2) Lefevre, P., Congr. mondial pe'trole, 3rd Congr., The Hague, 1951. Section IX.Preoriiit 7. (3) Smith, 'S.S., and Schulse, R. K., Petdeuin Engr., 19, No. 13, 94 (1948); 20, No. 1, 330 (1948). RECEIVED for review M a y 7, 1952. ACCEPTED August 4 , 1952. Presented as p a r t of a Symposium on Radiochemistry before the Division of Industrial a n d Engineering Chemistry a t the 121st Meeting of the AILIERICAN CHEMICAL SOCIETY,Milwaukee, Wis., April 2, 1962.
