GB1570039A

GB1570039A - Measurement of phase fractions in flowing fluid

Info

Publication number: GB1570039A
Application number: GB5277077A
Authority: GB
Original assignee: Auburn International Inc
Current assignee: Auburn International Inc
Priority date: 1977-02-22
Filing date: 1977-12-19
Publication date: 1980-06-25
Also published as: CA1101070A; DE2806153A1; FR2381307A1; DE2806153C2; JPS53108497A; JPS6128095B2

Description

(54) IMPROVEMENTS IN OR RELATING TO THE MEASUREMENT OF PHASE FRACTIONS IN FLOWING FLUID (71) We, AUBURN INTERNA TIONAL, INC., of 1 Southside Road, Danvers, Massachusetts 01923, United States of America, a corporation organized and existing under the laws of the Commonwealth of Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present application relates to the measurement of relative fractions of liquid (nonconductive) and vapor, or solids (nonconductive) and gases such as occur in fuel or oil pumping and in pneumatic conveyance of solid particles.

The present application is related to U.S.

Applications 719 196 and 722 168,which led to U.S. Patents 4063 153 and 4082 994 respectively.

The prior art includes mechanical and electrical approaches to this kind of measurement which are of limited value because of their failure to deal effectively with the non-homogeneous character of the vapor and liquid or solid and gas mixture across the cross section of a conduit in most practical applications. Reference may be made to the following: 1. LeTourneau, B.W., and Bergles, A.E., Co-Chairmen of a Symposium on "Two Phase Flow Instrumentation," 11th National ASME/ AIChE Heat Transfer Conference, Minneapolis, Minnesota, 1969.

2. Hewitt, C.F., "The Role of Experiments in Two-Phase Systems with Particular Reference to Measurement Techniques," Progress in Heat and Mass Transfer, Vol. 6, 1972, p.

295.

3. Subbotin. V.I., Pakhvalov, Yu, E., Mikhailov, L.E., Leonov, V.A., and Kronin, I.V.. "Resistance and Capacitance Methods of Measuring Steam Contents," Teploenergetika, Vol. 21, No. 6, 1974, p. 63.

4. Olsen, H.Q.m "Theoretical and Experimental Investigation of Impedance Void Meters, "Kjeller report KR-1 18, 1967.

5. rbeck, I., "Impedance Void Meter," Kjeller report KR-32, 1962.

6. Maxwell, J.C., "A Treatise on Electricity and Magnetisim," Clarendon Press, Oxford, 1881.

7. Bruggeman, D .A.G., "BerechnungVerschiedener Physikalischer Konstanten von Heterogenen Substanzen," Ann. Phys., Leipzig, Vol. 24, 1935, p. 636.

8. Hewitt, G.F., and Hall-Taylor, N.S., "Annular Two-Phase Flow," Pergamon Press, 1970, p. 153.

9. Jones, O.C., Jr., and Zuber, N., "The Interrelation Between Void Fraction Fluctuations and Flow Patterns in Two-Phase Flow," Int. J. Multiphase Flow, Vol, 2, 1975, p. 273.

The present invention provides a method of measuring phase fractions in a flowing substantially dielectric fluid, the method comprising the steps of cyclicly making capacitative measurements across the flow in a series of relatively angularly spaced positions, and summing the capacitance measurements so obtained to produce a signal related to the phase fractions of the fluid.

The invention further provides a method of measuring phase fractions in mixed flow media of dielectric material, the method comprising the steps of applying a cyclic series of relatively displaced voltage fields across the flow in distributed, spatially overlapping, time sequenced, fashion, each field of the series being established between input electrode means and receiving electrode means of greater angular extent so as to have a tapered form, and summing capacitive currents so produced across the flow by such fields and affected by the flow medium dielectric constant to produce a signal correlatable with phase fraction of the flow, variations in the currents summed being averaged out in the signal.

The invention also provides an apparatus for measuring phase fractions in a fluid flowing in a conduit, the apparatus comprising a sensor assembly having an electrode array and arranged to be disposed relative to the conduit so that the electrodes of the array are spaced therearound, electrical means for applying measuring signals to the array and for measuring capacitative currents sequentially in directions relatively displaced across the conduit, and means for summing the measured currents.

The capacitance measurements are preferably made so as not to overlap in time. The capacitance measurements are algebraically summed and the sequence steps controlled by logically counting the capacitance measurement excitation high frequency at a rate which is high in respect to the flow rate through the sensor so that the solids or liquid is essentially standing still for purposes of the measurements to be summed. The frequency of the excitation applied for capacitance measurement may be from 10-100 KHZ, and is preferably 30 KHZ, thus being greater than common power frequencies but lower than radio frequencies. The summed varying capacitive response currents from the sensor can be converted to a voltage signal, amplified, and displayed on a meter as % solids or liquid.The electrodes are preferably a peripheral array of plates oriented around the flowing fluid material of a conduit being monitored, preferably without breaking through the conduit wall or entering or otherwise disturbing or touching the fluid flow.

By utilizing an electrically transparent (non-conductive) section of the flow tube, the measurement may be accomplished from outside the flow tube. The sensor housing preferably comprises a pair of semi-circular tube sections which clamp around outer periphery of the flow tube. Each semicircular section may comprise a laminated assembly containing electrodes or sensor plates on the inner surface, an intervening insulator layer and an external ground plane surface. The ground plane surface contains coaxial connectors through which electrical connection to each sensor plate is accomplished.

The sensor is thus readily moveable for measurements at several locations in the system whilst being totally external to the site piping. Portability and repair without interrupting site operation can be ensured.

Further, this ability to monitor flowing materials from outside the flow system make the apparatus a highly economic and reliable device having regard to materials and operation.

The invention will be further explained below, by way of illustration, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a preferred measurement apparatus embodying the invention; Figures 2A to 2F are diagrams showing sequential rotation of a measuring field within the sensor; Figure 3 is a block circuit diagram which shows the "Switching Logic" of the apparatus of Figures 1 and 2 in greater detail; Figure 4 is an expanded circuit diagram of a plate switch circuit element contained in Figure 3; Figures 5 and 6 are respectively longitudinal front and cross sectional views of a sensor instrument portion usable in the apparatus of Figures 1 to 4; Figures 7 and 8 are respectively longitudinally and cross sectional views of a flow conduit with two of the instrument portions of Figures 5 and 6 applied thereto; and Figures 9A to 9C are electrical schematic diagrams of major circuit assemblies incorporated in the apparatus.

The apparatus embodying the invention shown in Figure 1 comprises a sensor assembly S with six electrodes or plates P1 to P6 therein, a SPAN control comprising a voltage divider VD, a comparator COMP, two logic units, an oscillating voltage source 0, a summing junction, and metering elements, some being analogous to those described in the applications cited above. The oscillator source 0, which is preferably a Wien Bridge oscillator, produces a stable 30 KHZ sine wave output. This output is applied to the SPAN control and to the comparator COMP. The comparator COMP, an operational amplifier, produces a square wave output from 0-5 volts with transitions at each crossing of the axis of the 30 KHZ output.

The output of the comparator is divided by 16 and then converted to a 1 to 6 decimal sequence in a 1 to 6 LOGIC unit which in turn controls the switching sequence of the sensor plates P1 to P6 via a switching LOGIC unit.

The output of the SPAN control is connected to the appropriate sensor plate through the switching logic unit and also feeds a ZERO control with a low impedance drive. The output of the ZERO control is inverted in a inverter I, reduced in level, and applied through a capacitor CAP as a zeroing current to the summing junction l;. The ZERO control is connected after the SPAN control to minimize interaction between zero and span adjustments. The appropriate sensor plates are also connected to the summing junction ,. The algebraic sum of currents at the summing junction is converted to a voltage by an I to E converter I/E.

A field effect transistor FET is employed to cut off the signal during sequence transi tions to prevent the pulses which occur during transition from saturating a downstream ac amplifier ACA. This provides signal amplification and an inverter I inverts the signal so that synchronous rectification in a circuit element SYN RECT can be employed to derive a DC signal. The output of the comparator COMP provides switching input and clocking input for the synchronous rectification. A DC amplifier DCA provides a 0-10V output which is proportional to solids or liquid in the line and drives a front panel meter M. For operational convenience, one or two and one-half second time constant damping is provided for the meter. A DC zero control is used to offset any DC components present in the signal.

The six diagrams of Figures 2A to F show the steps 1-6 of the voltage field rotation sequence. In each step, a 30 KHZ voltage is applied to a transmitter plate T, and the three opposing plates are connected together to form a single electrode means or common receiving plate R from which a capacitive current proportional to the average dielectric constant within the sensor can flow to ground. The intervening plates are connected to earth to separate the transmitter plate T from the receive R plates. The sensor assembly S has an outer jacket also connected to ground to act as a shield or guard around the sensor assembly.Since one position would not provide adequate electric field distribution for averaging the dielectric constant of the entire cross section of the sensor, the switching logic advances the electrical position of each plate in a continuous six step sequence; thus rotating the field to achieve good averaging. The peripheral spread of the field at the R plates approaches 1800 and is in any event substantially above 90" which it should be to avoid fringe errors which would require considerable effort to correct.

The logic units are shown in Figures 3 and 4. Figure 4 shows a plate switch circuit, six of which are used in the circuit of Figure 3 to selectively interconnect the sensor plates to the oscillating voltage source 0, the summing junction X, of ground. The Plate Sequence Logic of Figure 3 generates Xmit, Gnd and Rec Logic levels for each plate switch circuit from the 1 to 6 sequence.

The Plate Switch Circuit of Figure 4 consists of five FET switches; two are connected in series with an intermediate load resistor to earth, for both the 30 KHZ input and the output to the summing junction S, to isolate the plate when not connected. The FET switch is used to connect the plate to ground (GND BUSS).

The applied oscillations are preferably single-phase and at generally higher fre qL -ncies than one employed in the apparatus described in the patent applications cited above. The effective plate area is maximized as shown in Figures 2A to 2F for greater signal strength appropriate for dealing effectively in the submicrofarad capacitance values involved. The spatial overlapping patterns produced as shown in Figures 2A to 2F should provide a "first capacitance plate" with a spread of at least 90 , preferably approaching 1800, of arc, while the opposing "second capacitance plate" is limited to less than 90" opposite the first plate to avoid predominantly annular concentration of high strength field and V channel field centrally.

District sequence commutation steps, with intervening isolation of field rotation, establish a scan of the whole flow, and an individual cross section of the flow is fully scanned several times as it flows axially down the conduit. This rotational scanning can be contrasted with known capacitance spiral sensor meters for measuring void fraction in fuel lines and the like in that the former provides effective coverage of the central core of the flow channel and effectively counteracts adverse effects of annular flow.

The apparatus of Figure 1 can be regarded as comprising Driver Board, Switching Board and Receiver Board circuits. Figure 9A is the schematic of the Driver Assembly Operational amplifier Z1A and its associated circuitry form a stablized Wien Bridge (frequency and amplitude stable capacitance-resistance bridge) oscillator with automatic gain control. The output of the oscillator is fed to the comparator Z2A (shown as COMP in Figure 1) which produces a square wave output from 0 to 5 volts with transitions at the axis cross points of the sine wave generated by the oscillator. Also, the output of the oscillator is coupled to the SPAN control (Figure 1).The output of the SPAN control is connected to the noninverting input of the operational amplifier Z4A (Figure 9A) which is connected as a non-inverting follower to provide isolation for the SPAN control and a low impedance drive to the ZERO control (Figure 1) and sensor drive via the Switching Assembly (Figure 9C below). The output of the ZERO control is inverted and reduced in level by an operational amplifier Z3A and is fed to the Receiver Assembly as a zeroing input. The ZERO control is connected after the SPAN control to minimize interaction between the zero and span adjustments.

The output of the comparator is fed to the Receiver Assmebly as a switching input for synchronous rectification. Also, the output is connected as a clock input to two four-stage shift register counters Z6A and Z9A. The output of the counter Z6A at terminal SYNC out is 30 KHZ divided by 16 or 1.875 KHZ and is also fed to the input of the counter Z9A. The parallel BDC outputs of the four stages of Z9A are connected to a binary to decimal converter Z10A which generates a one to six sequence which is inverted by the Hex Inverter Z11A and is then fed to the Switching Assembly via the ELECTRODE SELECT OUT connections. A seventh count of the converter Z10A is connected back to the "reset" input of Z9A thus bringing the decimal count back to position one.

The parallel output of the counter Z6A is decoded to a 0-16 decimal count by a unit Z7A. Two NAND gates of a Quad NAND gate Z8A are connected as a latch. When the "fifteen" count from the unit Z7A is reached, the latch is set and when the "Z" count is reached the latch is reset. The output of the latch is connected to an operational amplifier Z5A which is connected as a Schmidt Trigger thus producing the GATE OUT output which is fed to the Receiver Assembly (Figure 9C) as a commutation blanking gate signal.

Z1A to Z5A amplifiers are preferably Harris HA2-2625-5 operational amplifiers except Z4A which is preferably a nominal 311 Comparitor. All the "Z" elements are standard logic chips.

Figure 9B schematically shows the Switching Assembly. The function of the Switching Assembly is to connect each electrode plate of the sensor to 30 KHZ (MIT IN), Receiver Assembly (REC OUT) or to ground (GND).

The switching functions are accomplished by Quad FET switches ZIB-Z7B, Z15B and Z16B. Two switches are cascaded for each transmit or receive function for adequate isolation. The selection logic consists of triplethree input NOR gates Z1 1B-Z14B and Hex Inverters Z8B-Z1OB. The logic is arranged so that when one Electrode Select Line is high, the associated electrode plate is connected to the 30 KHZ supply for transmit and the preceding and following plates are connected to ground. The remaining plates are connected to the Receive line since with- out excitation from an Electrode Select Line as transmit on ground, the plate is automatically connected to the Receive Line. Diodes are provided to prevent accidental overvoltaging from sensor connections. The resistance in Figure 9B are preferably 47K, the diodes IN914.The FETS are Harris H1-1201-5.

Figure 9C is a schematic diagram of the Receiver Assembly. The zeroing voltage (ZERO IN) from the Driver Assembly (Figure 9A) is applied to a 100 picofarad capacitor which produces a capacitive current into the Summing Junction of operational amplifier ZIC 1 80C out of phase with the capacitive current entering the summing junction from the sensor via the Switching Assembly (REC IN). The capacitor in the feed-back loop of ZIC converts the summed input currents into a voltage and shifts the voltage into phase with the oscillator and also the SYNC IN from the Driver Assembly.

The FET (E105) is shut off during plate sequence commutations by a negative GATE from the Driver Assembly. This is done to prevent saturation of the following operational amplifier Z2C by commutation spikes.

Z2C provides voltage amplification and operational amplifier Z3C acts as a unity gain inverter. The inverted and non-inverted signals are connected through two FET switches of the squad switch Z5C. The two switches are alternatively turned on by the SYNC signal to provide synchronous rectification of the signal. The third section of Z5C is connected to provide an inverted SYNC signal for the "inverted signal" switch.

Operational amplifier Z4C provides DC gain for the rectified signal and a low impedance output to both drive a meter and provide a 0-10 volt output proportional to the volume of the sensor occupied by the solid or liquid being measured. A ZERO control on the board is used to remove any DC offsets incurred in the circuitry and the METER CAL allows the full scale of the meter to be adjusted at exactly 10 volts.

Meter damping is provided by a switch (Figure 9) with positions of none 1 second, and 2.5 seconds time constants.

The mechanical configuration of the sensor assembly shown in Figures 5 to 8 comprises two laminates 1 of semicircular crosssection each consisting of a conductive ground plate 2, an insulating layer 3 onto or into which conductive sensor plates 4 are fastened and to which electrical connections are made by conductors 5 which extend through the insulating layer and connect to, or are part of, the isolated conductor of coaxial connectors 6 which are fastened to the ground plate.

Two of the laminates 1 constitute the sensor assembly which substantially surrounds the outside periphery of an electrically transparent (non-conductive) portion of a flow tube 7 inside which the material 8 being monitored flows.

The laminates 1 are fastened over the flow tube 8 by clamps 9, the shape and size of the laminates allowing intimate contact with the outer surface of the flow tube and providing minimal gaps 10 at the adjacent edges of the laminates. The sensor plates 4 are equiangularly spaced about the flow tube axis and their common axial length is such that their ends are sufficiently spaced, as by at least 1 inch, within the guarded insulated laminate to eliminate external electrical disturbances.

A sensor assembly with these features affords a precisely positioned sensor plate array required to accomplish the measurement described, whilst retaining ease of attachment and portability.

It is evident that the specific disclosure can be modified in various ways within the scope of the invention as claimed below.

\\ HAT WE CLAINI IS: 1. A method of measuring phase fractions in a flowing substantially dielectric fluid. the method comprising the steps of cyclicly making capacitative measurements across the fl,, in a series of relatively angu marly spaced positions. and summine the capacitance measurements so obtained to produce a signal related to the phase fractions of the fluid.

'. A method as claimed in claim l x-herein the capacitative measurements are made between electrode means of different angular extents.

3. A method as claimed in claim 2 wherein the electrode means comprise transmitter electrode means of lesser angular extent than receiver electrode means.

4. A method as claimed in claim 3 wherein the receiver electrode means comprises a plurality of electrodes electricallv connected together.

5. A method as claimed in claim 3 or 4 wherein the transmitter electrode means comprises a single electrode.

6. A method as claimed in any preceding claim wherein the measurement positions are equiangularly spaced.

7. A method as claimed in any preceding claim "'herein the measurement positions are rotated around the fluid flow.

S. A method of measuring phase fractions is mixed flow media of dielectric material. the method comprising the steps of applying a cyclic series of relatively displaced voltage fields across the flow in distributed. spatially overlapping. time sequenced. fashion. each field of the series being established between input electrode means and receiving electrode means of greater angular extent so as to have a tapered form.

and summing capacitative currents so produced across the flow bv such fields and affected by the flow medium dielectric constant to produce a signal correlatable with phase fraction of the flow. variations in the currents summed being averaged out in the signal.

9. A method as claimed in claim 8 wherein the capacitative measurements are applied with a peripheral spread of the voltage field of at least 90" at the one larger electrode.

10. An apparatus for measuring phase fractions in a fluid flowing in a conduit, the apparatus comprising a sensor assembly having an electrode array and arranged to be disposed relative to the conduit so that the electrodes of the array are spread therearound electrical means for applying measuring signals , to the array and for measuring capacitative currents sequentially in directions relatively displaced across the conduit, and means for summing the measured currents.

11. An apparatus as claimed in claim 10 wherein the electrical means is arranged sequentially to excite each electrode as a transmitter electrode. whilst grounding at least one adjacent electrode and exciting a plurality of non-adjacent electrodes as receiver electrodes.

12. An apparatus as claimed in claim 11 wherein three electrodes spanning substantiallv 1800 of arc are excited as receiver elec trodes.

13. An apparatus as claimed in claim 10.

11 or 12 wherein the electrode array comprises six electrodes of angular equal spacing and extent.

14. An apparatus as claimed in anv one of claims 10 to 13 wherein the electrical means are arranged to supply a high frequench oscillation to the array and the measuring and summing means comprises means for synchronous rectification of the summed currents to produce a d.c. measuring signal.

15. An apparatus as claimed in any one of claims 10 to 14 wherein the electrical means comprises means for shutting off the application measuring signals to the electrode array between changes in the measuring direction.

16. An apparatus as claimed in anv one of claims 10 to 15 wherein cascade Field Effect Transistors are provided for isolating electrodes of the electrode array.

17. An apparatus as claimed in anv one of claims 10 to 14 wherein the electrodes are mounted in a tubular shell arranged to be placed around an electricallv non-conductive tubular portion of the conduit.

18. An apparatus as claimed in any one of claims 10 to 17 wherein measuring signals are single phase.

19. A method of measuring phase fractions in a flowing fluid substantially as herein described with reference to the accompanying dranings.

20. An apparatus for measuring phase fractions in a flowing fluid substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. be modified in various ways within the scope of the invention as claimed below. \\ HAT WE CLAINI IS:

1. A method of measuring phase fractions in a flowing substantially dielectric fluid. the method comprising the steps of cyclicly making capacitative measurements across the fl,, in a series of relatively angu marly spaced positions. and summine the capacitance measurements so obtained to produce a signal related to the phase fractions of the fluid.

13. An apparatus as claimed in claim 10.