I
D.
E. HULL
California Research Corp., Richmond, Calif.
Total-Count Technique in the Refinery A New Principle in Flow Measurements, Applicable to a Wide Variety of Problems b Measuring flow rates is fundamental in many fields of science and industry but industrial flowmeters often prove inaccurate in plant service. The totalcount method offers a new principle of promising versatility. Its direct simplicity, both in theory and in use, may earn it an important place in flow measurements.
A principle in measuring flow rates, called the totaJ-count method is NEW
based on measurementof thetotalnumber of counts from a radioactive tracer as it flows past a detector. As the total count bears a simple inverse relation to the flow rate, the method is also useful for measuring other quantities related to flowing streams. A measured quantity of radiotracer is introduced into a flowing stream above a counter, which is fixed in or near the stream and accumulates counts while the tracer passes. Counts recorded are independent of tracer distribution along the stream. If mixed homogeneously across the pipe, radioactive atoms passing the counter station have the same chance of discharging the counter, whether or not neighboring radioactive atoms are present. But if the rate of flow of the tracer atom past the counter increases, its chance of tripping the counter decreases in proportion. So the total count N , is inversely proportional to the flow rate, Q. Thus, Nz/b*i
(1 1
Qi/Q%
for a constant quantity of tracer. More
A
TRACER INJECTOR MILLICURIES
COUNTS MILLICURIES M m / GALLON
Nzlh’i = Az/Ai X Q i / Q z (2) More simply, N = AF/Q (3) where constant F is characteristic of the isotope, counter, and the geometric relation between the counter and stream. If F could be determined, we would have not a relative but an absolute method of measuring flow rates. A piece of pipe similar to the one used in the flow measurements, several inches longer at both ends than the counter tube, is filled with a solution of the tracer a t a known concentration in the same geometric configuration as in the field test, and the counting rate is measured. The rate is proportional to the concentration, and the ratio has the units: counts per minute divided by microcuries per gallon, which can be rearranged as counts per microcurie times gallons per minute. These are the dimensional units required of F in Equation 3.
N counts
=
A pc. Q gal./min. X
counts ~
PC.
gal.
x min.(4)
The numerical value of F in Equation 4 is that found in the static calibration. This techique answers a variety of problems in the refinery. I t measures flow rates in pipes where no flowmeters exist or where they have defaulted, and leak rates between cross streams and liquid entrainment in distillation units. I t has natural advantages in the special cases of confluent and divergent streams.
Flow rate by total count
----c
‘
generally, the amount of tracer, A , used in different tests varies; and the number of counts is proportional to the number of tracer atoms used. To allow for this,
=& A
It is an inherent property of the total count that it may be measured in a sample stream of indeterminate size. This is exploited to measure gas flow rates. The principle applies ‘ t o open streams; wherever a fluid flows in a definite course, total count can measure flow.
Flow Rate by Total-Count Method For measuring condenser rates in the refinery, cesium-134 chloride was used as a water-soluble tracer, diluted in 1 pint of 1% sodium chloride solution. I t was measured by counting its gamma rays with a Geiger counter in a standard position (7). The tracer solution was injected into the line with a Sparklet bottle, designed for preparing “carbonated water” with carbon dioxide cartridges. A special cap, equipped with a valved exit tube and a pressure gage, provided for attaching the bottle to the pipe. When carbon dioxide was admitted into the bottom of the inverted bottle, the tracer was forced into the pipeline. The tracer was counted in the pipeline with counter tubes attached to the pipe downstream. The precision of the measurement was increased by connecting four I X 12 inch counter tubes in parallel. A portable, battery-operated scaler was connected to the tubes. After a few minutes the time and the scaler reading were noted to compute the background counting rate. Then the tracer was injected, and when the scaler returned to background, readings were again noted. Then a final background count was taken. Ideally, this should check the initial background. For determining the calibration factor for the 8-inch pipe, a section of pipe about 2 feet long was closed with a plate welded across one end. I t was filled with a solution containing 42.3 microcuries per gallon of cesium-1 34 in 1% sodium chloride. A single Geiger tube was taped to the outside of the pipe midway between the two ends. The counting rate was 222 counts per second, to 0.5% standard deviation. The corresponding F value is
315 counts/minute microcurie/gallon VOL. 50, NO. 2
FEBRUARY 1958
199
I n a typical measurement of condenser flow rate the tracer injection was 4.24 mc., and the net count on four tubes during passing of the tracer wave, after subtraction of background, was 4856 counts. Thus,
Q
=
4240
m6X
4 X 315 = 1100 gal./min.
The meter on the feed line to this condenser indicated a water flow of 510 gallons per minute, but it was believed unreliable because of the corrosive effect of hydrogen fluoride in the line. Calculations based on heat supplied to the feed preheaters had painted to a flow rate of about 1100 gallons. The radioactive test corroborated the heat balance figure.
Check with Two-Point Method
In a later test, the total-count method was checked more directly against a radioactive method in which the time of transit between detectors a t two points is measured with a pulsed injection of radioactive liquid. The linear flow rate is multiplied by the cross section of the pipe to get the volume flow rate, This test was made on a line supplying cooling water to a section of the refinery. Two Geiger tubes were mounted on the 12-inch pipe, 132 feet apart, and connected in parallel into a ratemeter with a recorder attached. Several hundred feet farther downstream, four tubes were taped to a 6-inch pipe and connected to a scaler. The tracer was injected as quickly as possible, to produce sharp peaks on the ratemeter. The same tracer pulse, passing through the totalcount setup, wa? used to get the flow rate by this method also. The volume of water between the two points was
The net count has a 6% standard deviation, based on counting statistics. The agreement between the two methods is within this range.
-300 =
Accuracy of Technique Accuracy of the total-count method depends on several factors. Sources of error can be: Statistical error of counting. Differences in mounting the tubes on the pipe from test to test and from the laboratory calibration test. Retention of tracer in the I-pint bottle, injector, and connections to the line. The cumulative resultant of these sources of error is a standard deviation of 2 to 5% in the average plant test. There are also sources of absolute error in several assumptions implicit in using factors measured in the laboratory for field tests. Sensitivity of each counter tube equals that of the calibration tube. The contribution to the counting rate from solution more than 6 inches beyond the ends of the counter tubes is negligible. No corrosion or deposits have affected the wall thickness of the pipe in the field. The last factor is probably the largest source of error in this method; it could amount to as much as 10% in old pipes, and it is difficult to estimate.
l e a k Rates The total-count principle has been applied to measure quantitatively leaks from one liquid stream into another through heat exchangers and condensers. The flow rate is known; to determine quantity of tracer diverted to the cross stream from the stream in which it is injected, Equation 3 is rewritten :
X 132.2 X 12 699 gallons 231
The two peaks on the ratemeter were 0.626 minute apart. Thus, the twopoint method showed a flow rate of 699/0.626 = 1120 gallons per minute The factor for a 6-inch pipe was 264 counts per second per microcurie per gallon. The injection of 2.66 mc. gave 2380 counts of the four Geiger tubes. Thus, flow rate was 2660
264 2380
= 1180gal./min.
factor of 480 was estimated. Thus, activity leaking into the product stream was
A = NQ/F
(5)
To measure leakage of cooling water from a condenser into a gasoline condensate line, a solution of 2.3 mc. of cesium-1 34 chloride was injected into the cooling water line, flowing at 75 gallons per minute. Water leaking into the gasoline stream from this and other lines was collected in a settler and drawn off through a 12-inch pipe a t a rate of 1.4 gallons per minute. A net total count of 300 i 85 was observed from a Geiger tube on the pipe. The 12-inch pipe had not been calibrated, but a
480
0.9
pc.
only 0.047, of the 2300 bc. in the cooling water line. Thus, only 0.03 gallon per minute of the water in the gasoline line came from this source. Other sources were responsible for the major leakage. Alternatively, counters may be exposed to both streams to measure the total count from a charge of tracer. The ratio between the tracer in the cross stream and that in the primary stream is
In this case the quantity of tracer injected does not have to be accurately known. If the two pipes are the same size, = Fz, and the equation becomes
I n heat exchangers, this primary stream sometimes returns as the secondary stream after passing through a process vessel, and even the flow rates cancel out: A2/Ai = N 2 / N 1
(8)
Entrainment in Distillation Another application is entrainment in distillation processes. Tar must be excluded from refined petroleum products to a very high degree, because it contains nonvolatile metals and coke which would leave deposits in engines and burners. These can enter the distillate only by mechanical entrainment which a nonvolatile tracer measures with high sensitivity. Data obtained from the injection of 10 mc. of cobalt40 naphthenate into a lubricating oil still illustrate this type of problem (Table I). The distillate was removed a t several different points on the column, The condensate lines from two take-off points were logged with Geiger counters several hundred feet downstream. A counter was also attached to the asphalt line to count the main charge of tracer. As the pipes were all the same size, Equation 7 could be used for the numerical comparisons.
literature Cited
(1) Hull, D. E., Keirns, G. H., Nucleoizics 14,No. 8, 95 (1956). Table I. Product Line Asphalt bottoms Lube G overhead Lube F overhead
200
Entrainment in Lubricating Oil Distillation Flow Rate, %. Bbl./Hr. Tubes Counts Entrained 311 33 64
1 4
4
INDUSTRIAL AND ENGINEERING CHEMISTRY
62,300 490 =!= 78 158 i 45
...
0.021 i 0.003 0.013 z t 0.004
RECEIVED for review May 17, 1957 ACCEPTED August 22, 1957 Division of Petroleum Chemistry, Symposium on Nuclear Technology in the Petroleum and Chemical Industries, Joint with Division of Petroleum Chemistry, 13191 Meeting, ACS, Miami, Fla., April 1957. Patents applied for by the California Research Corp.
