Method and apparatus for measuring a component in a flow stream

A method and apparatus for measuring the concentration of a particular component, e.g. water, in a two-component mixture, e.g. water-oil. A probe having three sets of sensor electrodes is positioned into the mixture. A first elastic sac is secured over the exposed ends of a first set of electrodes and is filled with water. A second elastic sac is secured over the exposed ends of a second set of electrodes and is filled with oil. The ends of the third set of electrodes are left exposed. The electrodes are energized and each generate a signal representative of a measured electrical property, e.g. resistivity, conductivity, or capacitance, of the liquid in which they are immersed. By properly combining these three signals, the concentration of the particular component is determined. Since any changes in temperature and pressure in the mixture being measured with affect the readings from all three sensors equally, the probe is considered self-adjusting and the accuracy of the final measurement is relatively unaffected by these changes.

BACKGROUND OF THE INVENTION 
The present invention relates to measuring fluid composition and more 
particularly relates to a method and apparatus for measuring the 
concentration of a particular component in a two-component liquid flow 
stream. 
There are many situations where it is necessary to determine the amount of 
a contaminant in a liquid stream. For example, in the oil industry, crude 
oil that is delivered through a gathering system to a pipeline or other 
terminal point normally is monitored with regard to its water content. 
This is routinely accomplished by means of a device commonly termed a 
"BS&W" or "water-cut" monitor which measures the concentration of water in 
the crude oil as the crude oil flows through a conduit. The water-cut 
monitors in most common use today are of the capacitance-probe type. The 
probe is inserted into the conduit and detects the presence of water by 
means of a change in the dielectric constant of the oil stream as it flows 
through the conduit and contacts the electrodes of the probe. This type 
monitor produces a read-out signal which is indicative of the percent 
water-cut, i.e. the concentration of water (and therefor of oil) in the 
crude oil stream at the instance that the measurement is made. Since the 
dielectric constants of most petroleum oils are in the nature of about 40 
times as great as the dielectric constant of water, relatively small 
amounts of water may be detected by this type of monitor. 
Available water-cut monitors of the type described above normally provide 
measurements of high accuracy and reliability when properly calibrated and 
operated under stable conditions. However, they are relatively expensive 
to use due to the sophisticated electronics involved. Also, they normally 
require a high caliber of preventive maintenance which may not always be 
convenient. Still more importantly, the readings of capacitance generated 
by presently known capacitance-type probes are directly related to the 
temperature and pressure of the liquid stream being monitored. This 
requires a particular capacitance-type probe to be recalibrated before 
almost every use to establish proper reference values, at the particular 
temperature and pressure expected to be encountered during the monitoring 
operation. Once calibrated for a particular temperature and pressure, a 
probe of this type will normally provide acceptable readings when minor 
temperature and pressure changes occur in the flow stream during a 
monitoring operation but is unable to maintain acceptable accuracy when 
major changes in temperature and pressure occur. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for measuring the 
concentration, i.e. percentage of a particular component (e.g. water) in a 
two-component (e.g. oil-water) liquid flow stream wherein the accuracy of 
the measurement is substantially unaffected by changes in the temperature 
and/or pressure of the monitored flow stream. A probe which automatically 
compensates for any changes in temperature and/or pressure of the flow 
stream during monitoring is used to measure an electrical property, (e.g. 
resistivity, conductivity, or capacitance) of the flow stream which is 
then processed to provide the concentrations of the components of the flow 
stream. 
More specifically, in carrying out the method of the present invention, a 
probe is positioned into the flow stream to be monitored. The probe is 
comprised of a housing having three individual sets of electrodes thereon. 
A ballon-like, elastic sac is positioned over the exposed, lower ends of a 
first set of electrodes to enclose the electrodes in a fluid-tight 
relationship. This sac is filled with 100% pure component to be measured, 
e.g. water. A second ballon-like, elastic sac is positioned over the 
exposed, lower ends of a second set of the electrodes to enclose the 
electrodes in a fluid-tight relationship. The second sac is filled with 
100% pure other component, e.g. oil. The lower ends of the third set of 
electrodes are left exposed so that they will be immersed in the flow 
stream when the probe is in an operable position within the flow stream. 
As the flow stream flows past the probe, the temperature of both the water 
and oil in their respective sacs will quickly reach the same temperature 
as the flow stream due to conduction through the thin-walled sacs. Also, 
due to the elasticity of the sacs, the pressure on both the water and oil 
inside the sacs will be substantially equal to that of the flow stream. 
Therefore, when the electrodes are energized, the set immersed in the 
water will generate a signal representative of an electrical property 
(e.g. resistivity) of 100% water; the set of electrodes immersed in the 
oil generates a signal representative of the resistivity measurement of 
100% oil; and the exposed set of electrodes generates a signal 
representative of the resistivity of the actual flow stream being 
monitored. 
These three signals are fed to a processing unit, e.g. a properly 
programmed general purpose digital computer, wherein a scale is 
established having the 100% water signal as one reference limit and the 
100% oil signal as the other reference limit. The signal from the flow 
stream is then related to this scale to determine the concentrations of 
water and oil in the flow stream at that specific instant in time. 
During a monitoring operation, the measurement step described above is 
continuously repeated at extremely rapid intervals and the results of each 
step is cumulated and averaged within the process unit so that upon 
completion of the operation, a read out is provided of the average 
concentration of each of the two components present in the flow stream 
during the entire monitoring operation. It can be seen that, since any 
change in temperature and/or pressure of the flow stream affects each of 
the signals equally, the resulting relationship between the three signals 
remains the same and, hence, the accuracy of the final measurements are 
unaffected by any such changes in the flow stream during a monitoring 
operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring more particularly to the drawings, FIG. 1 discloses a 
multi-component flow stream monitoring system utilizing the present 
invention. A two-component flow stream, e.g. a mixture of basic sediment 
and water (BS&W) and oil from a production well, is flowed through conduit 
10 and through flow meter 11 which measures the total volume of flow. The 
stream then flows past probe 12 which is used to measure the concentration 
of a particular component, e.g. water, in the flow stream. 
Referring now to FIGS. 2 and 3, probe 12, as illustrated, is comprised of a 
housing 15 having a threaded portion 16 adapted to mate with a threaded 
opening 17 (FIG. 1) in conduit 10. Three substantially identical sensor 
supports A, B, and C are affixed to housing 15 and extend downward 
therefrom. Positioned through each support A, B, and C are a pair of 
electrodes, 18, 19, and 20, respectively. Each pair of electrodes extend 
completely and by a precise amount through its respective support and are 
exposed at their lower ends below said support. The upper ends of each 
electrode terminate within protective shell 15a of housing 15. 
Housing 15 and supports A, B, and C are all constructed of a nonconductive 
material, e.g. polyvinyl chloride, and are preferably molded as a unit 
with electrodes 18, 19, and 20 in place. However, it should be recognized 
that other assembly techniques may be used without departing from the 
present invention. Open passages 21, 22, 14 (FIG. 3) are provided through 
supports A, B, C respectively, and terminate within shell 15a for a 
purpose to be described below. 
A circumferential groove 25 is preferably formed on at least supports A and 
B near their lower ends. Individual ballon-like sacs 26, 27 made of a 
thin-walled, elastic material, e.g. rubber, are positioned over the lower 
ends of supports A and B, respectively, and are held in place by snap ring 
28 or the like. If needed, a skeletal frame-like structure (not shown) 
made of non-conductive material, e.g. polyvinyl chloride, which may 
resemble a slotted thimble or the like may be positioned within sacs 26, 
27 to prevent collapse, tearing, or major deformation of the respective 
sac. For the sake of clarity, only support A and sac 26 have been shown in 
its entirety in FIG. 2; support B being shown as broken away. When sacs 
26, 27 are in place, they completely enclose electrodes 18 and 19, 
respectively, in a fluid-tight relationship. The upper ends of each set of 
electrodes 18, 19, 20 are adapted to be connected to a set of electrical 
lead lines 18a, 19a, 20a, respectively, (FIG. 1) which electrically couple 
probe 12 to processing unit 30, which will be described in more detail 
below. With the structure of probe 12 now having been fully described, the 
operation of probe 12 and processing unit 30 will be set forth. 
A sample of the flow stream to be monitored is taken and the components are 
separated. For example, a quantity of a particular lease production stream 
of oil and BS&W is separated into substantially "pure" lease oil and 
"pure" lease BS&W components by settling, centrifuging, or the like. Sac 
26 on support A is completely filled at atmospheric pressure through 
passage 21 with the pure lease oil component so that the lower exposed 
ends of electrodes 18 are completely immersed in the pure lease oil 
component. Likewise, sac 27 on support A is completely filled through 
passage 22 with the pure lease BS&W (i.e. water) component so that the 
lower exposed ends of electrodes 19 are completely submerged in the pure 
lease water component. The upper ends of passages 21, 22 are then sealed 
with threaded plugs, friction stoppers, or the like. Passage 14 in support 
C is also closed off but preferably with a valve (not shown) which, during 
operations, can be intermittently opened for drawing a sample of the 
mixture being measured at a specific time for analysis purposes. Probe 12 
is assembled into conduit 10 and leads 18a, 19a, and 20a are attached to 
multi-plexer 31 in processing unit 30. 
The oil-BS&W stream flows through flow meter 11 (e.g. 2"-3" positive 
displacement flowmeter with magnetic pulse pickup, such as Barton Model 
380, ITT Barton, City of Industry, Calif.) which measures the total flow 
of the stream. The physical location of this flowmeter upstream from the 
probe assists turbulence and thorough fluid mix at the probe. The output 
of meter 11 is transmitted as pulses through line 35 to counter 33 which 
in turn is connected into computer 34 of processing unit 30. The oil-BS&W 
stream flows from meter 11 past probe 12 in conduit 10. Electrodes 18, 19, 
and 20, are energized and all measure an electrical property (e.g. 
resistance) of the liquid in which the respective electrodes are immersed 
and generate respective signals representative of said measurements. That 
is, electrodes 18, being immersed in pure lease oil, will provide a signal 
representative of the resistivity measurement of pure lease BS&W; and 
electrodes 20, being immersed in the flow stream, will provide a signal 
representative of the resistivity measurement of the actual flow stream 
which is in contact with probe 12 at that time. 
Each of these resistivity signals is transmitted to a terminal (unnumbered) 
in multiplexer 31 through its respective lead, 18a, 19a, 20a. As 
understood in the art, multiplexer 31 feeds each signal, in its proper 
sequence, to an analog to digital converter (ADC) 32 which, in turn, feeds 
the respective converted digital signal into computer 34. Computer 34, of 
course, can be a hard-wired apparatus or preferably can be any properly 
programmed general purpose digital computer. Due to the simplicity of the 
present invention, a relatively inexpensive "home" computer, e.g. TRS 80 
manufactured by Tandy Corp., Ft. Worth, Tex. or "PET" microcomputer, 
manufactured by Commodore Business Machines, Inc., Santa Clara, Calif., is 
preferably used to substantially hold down costs in most applications 
unless a larger unit used elsewhere in a process can "time-share" this 
function. 
Computer 34 is programmed to accept the signal from electrodes 18 and set 
the value of this signal as being representative of a liquid comprised of 
100% lease BS&W. The value of the signal from electrodes 19 is set within 
computer 34 as being representative of a liquid comprised of 100% lease 
oil. Computer 34 then subtracts the two signals to calculate a difference 
therebetween and establishes a scale wherein the resulting difference is 
spread over a 0-100% range. The signal from electrodes 20 (i.e. 
resistivity measurement of actual two-component flow stream) is then 
related by computer 34 to this scale to determine the actual percentage of 
BS&W (and hence oil) in the monitored flow stream. 
To further clarify the above description, an example of a typical 
measurement operation will be set forth. For a particular lease oil and 
lease BS&W at a specific temperature and pressure, the actual resistivity 
readings from electrodes 18 might be 2000 ohms (i.e. 100% oil) and from 
electrodes 19 might be 20 ohms (i.e. 100% BS&W). These readings are 
supplied to computer 34 which subtracts the two and takes the resulting 
difference (i.e. 1800 ohms) and establishes a scale wherein the 1800 ohms 
is linearly spread over a 100% spectrum. On this scale, a change of 18 
ohms in a particular resistivity reading from electrodes 20 would equal a 
1% change in the components' concentrations in the monitored flow stream. 
In the instant example, a reading of 38 ohms from electrodes 20 would 
indicate a liquid having 1% oil present in 99% BS&W. Again, it is pointed 
out that the actual resistivity values given above are by way of example 
only and actual readings in each practical operation will vary widely 
depending upon individual probe construction, e.g. electrode spacing, 
actual field conditions, etc. 
Presently known instruments which measure an electrical property of a flow 
stream to determine the concentration of a component therein are all 
sensitive to changes in the temperature and/or pressure of the flow stream 
during measurement. This requires recalibration of these instruments at 
frequent intervals to reflect the changes in temperature and pressure of 
the flow stream being monitored to thereby insure accurate measurement of 
the components when substantial changes occur in the flow stream. The 
present invention, as will be explained below, is self adjusting to 
automatically compensate for any changes in both the temperature and 
pressure of the flow stream substantially instantaneously as they occur. 
Although both electrodes 18 and 19 are isolated from direct liquid contact 
with flow stream by the liquid-filled sacs 26, 27, respectively, both are 
directly subject to the pressure and temperature of the flow stream. That 
is, both the oil and the water which fill sacs 26, 27, respectively, will 
quickly equal the temperature of the flow stream due to heat conduction 
through the thin-walled sac material. Further, due to the elasticity of 
sacs 26, 27, the actual pressure of the flow stream will be applied 
directly to the liquids in the sacs thereby equalizing the pressures 
inside and outside each sac. Therefore, any changes in temperature and/or 
pressure of the flow stream during a monitoring operation will result in 
substantially the same change on all three pairs of sensor electrodes. The 
simultaneous and equal pressure and temperature changes on each sensor 
effectively cancels any effects that such changes may have on the actual 
flow stream measurement. 
Again, to further clarify the above description, a change in pressure 
and/or temperature of the flow stream will change the actual resistivity 
measurements from each of the three sensors. However, since these changes 
will inherently be of the same magnitude on all three sensors, the actual 
relationship between all of the resistivity measurements remains 
unchanged. By programming computer 34 to (1) record the instantaneous 
measurement from electrodes 18 and set this value equal to 100% oil, (2) 
record the instantaneous measurement from electrodes 19 and set this value 
equal to 100% water, (3) substract the two measurements to get the 
instantaneous difference, (4) record the instantaneous measurement from 
electrodes 20 and relate it to the instantaneous difference, the 
concentration of water, and hence oil, can be calculated and recorded by 
computer for each series of resistivity readings. 
Individual sets of the three measurements (i.e. 100% water, 100% oil, and 
actual flow stream) are made at rapid intervals (e.g. at rates of from 1 
to 200 sets of measurements per second) and each set, which represents the 
concentration of the flow stream at the time said set of measurements is 
made, is calculated and stored in processing unit 30. These sets of 
measurements are then cumulated and averaged within computer 34 so upon 
completion of a monitoring operation, the average concentration of water, 
and hence average concentration of oil (i.e. 100%-concentration of water) 
in the monitored flow stream is available as a final readout from computer 
34. Alternatively, these final concentrations of water and oil can be 
combined within computer 34 with the volume readings from meter 11 (FIG. 
1) to calculate the actual amounts of oil and water that were flowed 
through conduit 10 during the monitoring period. 
Although resistivity has been used as the electrical property being 
measured in the above description, other electrical properties of liquids 
can be used as well. Conductivity which is the reciprocal of resistivity 
can be used by properly programming the processing unit and modifying the 
electrodes energizing circuit. Capacitance can also be used although it is 
the least preferable as the other two mentioned properties since 
additional electronics are required which must be mounted as close to the 
electrodes (e.g. on probe 15) as possible to avoid long lead lines with 
consequential extra capacitance. Where capacitance is the measured 
property, a bridge circuit 50 (FIG. 4) would need to be provided for each 
set of electrodes 18, 19, 20, respectively, with the electrodes, e.g. 18, 
and leads 18a being coupled into bridge 50 as shown. As understood in the 
art, additional circuitry for frequency generation would also be required.