CA1115556A

CA1115556A - Flow meter

Info

Publication number: CA1115556A
Application number: CA350,692A
Authority: CA
Inventors: Vincent V. Tivy
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-05-14
Filing date: 1980-04-25
Publication date: 1982-01-05
Also published as: US4249164A

Abstract

FLOW METER
ABSTRACT OF THE DISCLOSURE

A delta-P transmitter generating a signal corresponding to the differential pressure produced by a primary element installed in a conduit through which a fluid flows, com-prising the primary element and a transducer implanted in the primary element to generate a signal corresponding to the differential pressure with which may be combined the signals generated by a temperature transducer and/or an absolute pressure transducer to generate an output signal corresponding to the weight or mass rate of fluid flow.

Description

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FLOW METER

This invention relates to a flow meter and more particularly to a meter for measuring the rate of flow of a fluid through a pipe or conduit.
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Presently such a flow is commonly measured by means of a primary element, such as a flat-plate orifice, flow noz~le or venturi tube installed in the conduit and producing a differential pressure proportional to the s~uare of the rate of flow. To measure the differential the conduit is pene-trated upstream and downstream of the primary element at selected distances by pressure taps connected to a delta-P
transmitter generating an output signal corresponding to the differential pressure.

A wide variety of delta-P transmitters are available such as, but not limited to, bellows operated, diaphragm operated and mercury manometer operated. All of such transmitters suffer at least loss of calibration and possibly catastrophic fail-ure, such as bellows or diaphragm distortion or rupture, when subjected to an over-range differential pressure as fre~uently occurs in placing a transmitter in or out of serv-ice or by rupture of a connecting tube.

If the fluid is a vapor, such as steam, it is frequently necessary to install radiators at the pres~ure taps in order

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that the delta-P transmitter and connectiny tubes be filled with condensate. When measuring the rate o~ flow of a deleterious liquid, or a li~uid carrying solids in sllspension, ~l it is necessary to provide`separating chambers in the con-: 5 necting tubes to maintain a benevolent liquid in the trans-I mitter, or to provide 'an elaborate back-flush arrangement to periodically or continuously flush the connecting tubes with a benevolent liquid.
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Additionally, the 'installation of available delta-P trans-mitters is expensive, req'uiring penetrations into the fluid carrying conduit, the'removal of burrs protruding into the conduit as a result of the penetrations, the running of con-necting tubes to the transmitter, and the installation of root valves at the penebrations and shut-off valves and on equalizing valve 'at the transmitter.

In accordance with this invention the foregoing deficiencies are eliminated by incorporating or, as it may be said, im-planting in the pr;mary element a delta-P transducer, there--by eliminating the necessity for pipe penetrations, valves, connecting tub~ing 'and the like and the probability that the transducer will be subject to a differential pressure other than t}:~t produced by the primary element.

Further in accordance with this invention pressure and tem-perature transducers may also be incorporated or implanted . in the primary element whi.ch may be utilized to compensate the output.signal of the delta-P transducer for variations in temperature 'and/or pressure of the fluid.

These and'other objectives.of this invention will be appar- :
~0 ent from the following description and from the drawings in which: .

IN THE DRAWINGS

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~igs. 1 and 2 are cross~section views of typical installa-tions of primary elements in which are incorporated trans-ducers according to this invention.
.i , Fig. 3 illustrates a primary element as viewed from the in-¦ 5 let or upstream side in which are incorporated transducers in accordance with this invention.

Fig. 4 is a cross-section view of one type of differential pressure transducer taken along the line 4 - 4 of Fig. 3.

, Figs. 5, 6 and 7 illustrate alternate forms of the differ-¦ 10 ential pressure transducer shown in Fig. ~.
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Fig. 8 is a cross-section view of an absolute pressure trans-ducer taken along the line 8 - 8 of Fig. 3.

Fig. 9 is a cross-section view of a-wireway shown in Fig. 3 taken along the line 9 - 9.

Fig. 10 is a cross~section view of plenum chambers for intro-ducing the upstream and downstream pressures into the trans-ducers shown in Figs. ~, 5, 6 and 7.
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¦ Fig. 11 is a one-line logic diagram o~ a computing circuit generating an output signal corresponding to the weight or mass rate of fluid flow.

I DETAILED DESCRIPTION

Referring to the drawings, wherein like reerence characters designate like or corresponding elements throughout the sev-eral views, there is shown in cross-section a fluid conduct-ing conduit, generally indicated at l, having a section 2and a section 3 joined together, in Fig. l by a pair of flat faced flanges 4 held together by means of a plurality ~f bolts 6. Clamped between-the flanges is a primary element having an orifice 10 through which the fluid flows. Suitable , -4~ 5~

gaskets, such as shown at 12, are installed on either side of the primary element to maintain a pressure tight assembly.
In Fig. 1 the primar~ element is illustrated as an orifice plate 8A, whereas in Fic~. 2 the primary element is illus-trated as a flow nozzle 8B clamped between a pair of ~langes14, each having a raised face 16 providing an annular plenum or piezometer ring 37 on the inlet and outlet sides of the primary element 8B. As evident, the orifice plate 8A can be installed in the arrangement shown in Fig. 2 and, conversely, the flow nozzle 8B can be installed in the arrangement shown in Fig. 1. The primary element 8, Fig. 3, shows either the orifice plate 8A or the flow nozzle 8B as viewed from the upstream or inlet side. Hereafter, where in the description reference is made to the primary element 8, it shall be i 15 taken to mean an orifice plate 8A, a flow nozzle 8B or other type of differential pressure producing device.

Implanted in the primary element 8 is a transducer 18 for generating a-signal proportional to the differential pressure produced by the flow of fluid through the orifice 10 in the primary element, which in the case of an orifice plate 8A
can be any one of the known types such as concentric, ec-centric or-segmental. There may also be implanted in the primary element-8 a temperature transducer 20 such as a ther-mocouple or resistance thermometer and a duplex absolute pressure transducer 22. The transducers 18 and 22 may gener-ate the signal by a change in resistance, capacitance or inductance.- By way-of illustration, this invention will be described by specific reference to transducers producing a change in resistance and capacitance.

Referring to Fig. 4 there is shown in cross-section the transducer-18 as comprising a capsule having a supporting rim ll, of circular or other clesired configuration, to which are secured in pressure tight relationship flexure diaphragms ~;
7 and 13 to form a sealed chamber 15 filled with a non-compressible, low temperature coeffi~ient, high boiling :

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point liquid, such as silicone. The outer face of one dia-phragm, such as the'diaphragm 7., ;s subject to the fluid pressure (Pu) on the'inlet or upstream side of the primary element, wher'eas the'outer face of the other diaphragm 13 is subject to the 'fluid pressure:(PD~ on the outlet or down-stream side of the primary element. The differential pres-sure acting on the 'd;aphragm is therefore the differential pressure produced by the flow of fluid through the orifice 10 and each diaphragm will flex from a null or neutral posi-tion in proportion to.'this.differential pressure. To pro-' 10 duce a signal proporti~nal to this differential pressure thereis bonded, as shown-in Fig. 4, to the flexure diaphragm 7 a bi-ax;al strain guage 5, having, as shown in Fig. 11, one, winding predominantly sensitive to diaphragm flexing and a temperature'compensating.winding 52 predominantly sensitive to ambient temperature.:
., j The transducer 18 is implanted in a conforming hole drilled l or otherwise'formed'`:in the primary element 8 and bonded ' thereto by any suitable means such as welding, brazing or ., the like. I~ stres'ses-on the primary element are likely to '~ 20 produce significant strains, the transmission thereof to .I the transducer can be'inhibited.by bonding the rim 11 to the primary el'ement through flexures or an equivalent means.

In fig. 5 there is shown a modified form of the transducer 18 wherein the strain gauge 5 is secured to a flexure spring 19 having one'end anchored.to the-rim-ll and the free end provided wi*h'a cusp engaging the diaphragm 7.

A further modification of the transducer 18 is shown in Fig.
6 therein there-is provided''a two-ply diaphragm 23 to which the strain.gauge:5 is.~onded ~etween the plys~ The plys . may be bonded'.. together by epox~:brazing or welding, or, in the altenative,' the'space~between-the'diaphragm plys can .. be.evacuated' to ther'é~y.'cause the~.. diaphragms to collapse - 35 toward ea'ch other' and flex as a unit~

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: The strain gauge 5, as illustrated in Figs. 4, 5 and 6 can be either the wire wound or thin-film type, the latter being particularly suitable for the transducer shown in Fig. 6 with the thin-film configuration bonded to the interior surfaces of the plys.

In Fig. 7 the diaphragm 7 forms a capacitance plate movable toward and away ~rom ~n insulated non-flexing capacitance plate ~1 to thereby generate a signal proportional to differential pressure.

As shown in Fig. 8, the duplex absolute pressure transducer 22 comprises a bi-axial strain gauge 24, which may be either the wire:wound or thin-film:type bonded to a dia-phragm 25, responsive to the pressure on the upstream side of the primary element 8, and a similar strain gauge 26 bonded to a diaphragm 27, responsive to the pressure on the downstream side of primary element 8, separated by an evacuated chamber 28.

The leads to and from the transducers 18, 20 and 22 are carried through;suitable seals, a wireway 30 implanted in the primary element 8 and ear or tab 32, to a connector 34, which because of the low potentials, may, if required, be of the intrinsically safe t~pe. Fig. 9 shows a typical cross-section of the wireway 30, as taken along the line 9 -9 of Fig. 3, which comprises a pressure tight metal sheath29 housing the required number.of leads packed in a ceramic insulating material 31.
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While a.single set of transducers.18, 20 and 22 are suffici-ent to satisfy the requirements of the great majority of applications, where the service is critical and redundancy is required, a duplicate set of transducers as shown at `18', 20' and 22' and a duplicate wireway 30'. may be im-planted in the.primary element 8.

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As shown in Fig. 10, the transducer 18, and as evident transducers 20 and 22, may be housed in plenum chambers 33, 36 on the upstream and downstream sides respectively of the primary element 8. Such chambers serve to protect the transducers from injury caused by careless handling prior to or during installation, or by solids which may be carried in the flowing fluid. Static pressure inlet ports 34, 35 may be located relative to the primary element to con~oxm to the distances prescribed for the so-called corner taps or by the one inch upstream and downstream taps. The plenum chambers 33, 36 may-be packed with a filtering material such as ceramic beads, wire screening or the like, serving to dampen pressure pulsations usually present and commonly referred to as noise.

The transducers 18, 20 and 22 may be located in primary element 8 as required by the specific type of primary 20~ element and flanges. Thus with a flat orifice plate 8A
installed between flat faced flanges, as shown in Fig. 1, the transducers can be located in the primary element between the inside diameter of conduit 1 and the orifice 10.
- If the primary element is installed between flanges 14, such as shown in Fig. 2, the transducers can be located in the annular space 37 begween the conduit inside diameter and raised faces 16.

Referring to Fig. 11, which is a one line schematic diagram of a circuit for computing weight or mass rate of flow from the signals generated by the transducers 18, 20 and 22, it will be noted that conventional logic symbols have been used,-as it will be recognized that the components, or hardwarej` as-it is-sometimes-called, which such symbols represent, are commercially available and their operation well understood. Further, conventional lo~ic symbols have been us-ed to avoid specific indentification with any particular t~pe o components such as analog or digital, or as may be incorporated in a digital computer.

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As known, the rate of fluid flow through the orifice in a primary element varies as the s~uire root of the differen-tial pressure~produced by the primary element divided by the specific volume of the fluid. Practical considerations dictate that a delta-P transmitter generate a predeter-mined maximum output-signal at a selected maximum differen-tial pressure and that the primary element be designed to produce that differential pressure at a selected maximum rate of fluid flow and specific volume.
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In Fig. 11 there is shown a function generator 40 respon-sive to the signal generated by the transmitter 18, which at design conditions-produces an output signal E~ varying in straight line relationship with the rate of fluid flow.
If re~uired, the programmed relationship between the input signal to and the output signal from the function generator 40 can also be such as to compensate for nonlinearity be-20- t~een the signal generated by the transmitter 18 and differential pressure.

The pressure and temperature of the flowing fluid are used to determine the corrections necessary for deviations in the specific volume or density of the flowing fluid from the design values. If the flowing fluid is a liquid ordinarily the speci-fic volume or density is solely a function of temperature, however if the flowing fluid is a vapor or gas the specific volume or density is a function of both temperature and pressure.

Incorporated in the computing circuit of Fig. 11 is a - function generator 42 responsive to the signal generated by transducer 20 which generates an output signal correspond-ing to the correction required for deviations in specific volume caused-by deviations in actual fluid temperature fro~
the fluid temperature at design conditions. Also incorporated in the computing circuit is a unction generator 44 respon-sive to the signal generated by the transducer 24 which - ,:, . , ,. : .

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generates an output signal corresponding to th~ correction required for deviations in specific volume caused by devia-~ 5 tions in actual fluid pressure from the fluid pressure at i design conditions. A signal corresponding to a composite correction factor for both temperature and pressure varia-l tions is generate~ in a multiplier 46 and applied to the I output signal Eo in a multiplier 48, generating an output ! lo signal E~o Gorresponding to the weight or mass, as it is sometimes called, rate of fluid flow.

! Transducer 18, incorporated in the primary element 8, is, as previously mentioned, subject solely to the differential ` pressure produced by the fluid in flowing through the orifice 10 in the primary element 8, accordingly, no protective over range device is ordinarily re~uired. In extremely critical applications where false differential pressures may be encountered due, for example, to partial or complete blocking of the orifice 10, this invention further com-1 20 prehends the generation of an alarm and/or scram signal.¦ As shown in Fig. 11 in a difference unit 50 the signal SPD
generated by strain gauge 26 is subtracted from the signal 1 SPU generated by strain gauge 24 and when the output signal ¦ Eo therefrom exceeds a predetermined value as established by a manually adjustable unit 56, a signal EA, generated in a comparison unit 58, is available for alarm, control I or scxam. A flow reversal is detected by comparing the relative magnitudes of signals SPD and SPU in a logic unit 38, which, when the first named signal is greater than the last named signal, generates an alarm, control or scram signal EA.

While the`output signal of the transducer 18 may be com-pensated for changes in temperature by the inclusion of a resistance element 52, oriented to be responsive to changes in temperature and coupled with the differential pressure responsive strain gauge element in a conventional bridge circuit ~not shown~, in the alternative the function - ., ;

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generator 42 may be programmed to produce a composite output ' signal incorporating strain gauge temperature compensation ¦ 5 for the transducer 18 as well as-compensation required for ¦ deviations in specific volume of the fluid due to deviations in actual fluid temperature from the fluid temperature at . design conditio s.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A delta-P transmitter for producing an output signal varying in functional relationship with the rate of fluid flow through a conduit, comprising, a primary element consisting of a rigid disk for installation in the conduit and provided with an orifice for the flow of fluid there-through producing a differential pressure between the inlet and outlet sides of said disk varying in functional rela-tion to the rate of fluid flow, a first aperature between the inlet and outlet sides of said disk, a first flexure diaphragm sealing said aperture flexing in proportion to said differential pressure and a transducer operatively connect-ed to said diaphragm generating an output signal proportion-al to the flexing of said diaphragm.

2. A delta-P transmitter as set forth in claim 1 further including a hermetically sealed liquid filled chamber com-prising said first and a second flexure diaphragm secured to and separated by a supporting rim implanted in said first aperture and said transducer is mounted within said chamber responsive to-the flexing of said diaphragms in accordance with changes in said differential pressure.

3. A delta-P transmitter as set forth in claim 1 wherein said transducer is a strain guage.

4. A delta-P transmitter as set forth in claim 1 further including a temperature transducer implanted in said primary element for producing an output signal proportional to the temperature of the fluid.

5. A delta-P transmitter as set forth in claim 1 further including an absolute pressure transducer implanted in said primary element producing an output signal proportional to the absolute pressure of the fluid.

6. A delta-P transmitter as set forth in claim 1 wherein said primary element is provided with a second aperature further including a duplex absolute pressure transducer comprising an evacuated chamber implanted in said second aperture having a first transducer responsive to the pressure on the inlet side of said primary element for producing an output signal proportional to the absolute pressure on the 'inlet side of said primary element and a second transducer responsive to the pressure on the outlet side of said primary element for producing an output signal proportional to the absolute pressure on the outlet side of said primary element.

7. A delta-P transmitter as set forth in claim 1 wherein said transducer comprises variable capacitor having a stationary plate and a cooperating flexure diaphragm movable toward and away from said stationary plate in accordance with changes in said differential pressure.

8. A delta-P transmitter as set forth in claim 1 further including a wireway for transmitting said signal to a receiving device comprising a plurality of conductors packed in an insulating material hermetically sealed in a metal sheath implanted in said primary element.

9. In combination with a delta-P transmitter as set forth in claim 1 wherein said output signal varies in non-linear relationship to the weight rate of fluid flow at a pre-determined density of the fluid, further including means generating a second signal corresponding to changes in the density of the fluid and computer means responsive to said signals generating a second output signal linearly proportional to the weight rate of fluid flow through said conduit.

10. In combination with a delta-P transmitter as set forth in claim 6 further including means responsive to the output signals of said duplex absolute pressure transducer generating an output signal proportional to the difference between said signals and means generating an alarm signal when said output signal exceeds a pre-determined value.