United States Patent 119]
`Miller et a1.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,679,947
`Jul. 14, 1987
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`[22]
`[51]
`[52]
`
`[58]
`
`[56]
`
`METHOD AND APPARATUS FOR
`MEASURING STEAM QUALITY
`Inventors: Charles E. Miller; Gerald L.
`Schlatter, both of Boulder; Louis T.
`Yoshida, Longmont, all of Colo.
`Assignee: Engineering Measurements C0.,
`Longmont, Colo.
`Appl. No.: 755,493
`Filed:
`Jul. 16, 1985
`
`Int. Cl.‘1 ...................... .. G01N 9/36; GOlK 17/06
`US. Cl. ................................... .. 374/42; 73/32 A;
`73/29
`Field of Search ................. .. 73/32 A, 29, 30, 155;
`374/40, 42, 118
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,100,395 8/1963 Morley . . . . .
`3,516,283 6/1970 Abbotts ...... ..
`4,034,597 7/1977 Fredriksson ..
`4,096,745 6/1978 Rivkin et al
`4,149,403 4/1979 Muldary et a1. . . . . .
`
`. . . .. 374/42
`.. 73/32 A
`374/42
`.. 73/32 A
`. . . . . .. 73/29
`
`4,193,290 3/1980 Sustek, Jr. et al. . . . . . .
`
`. . . . . .. 73/29
`
`73/155
`4,409,825 10/1983 Martin et al.
`73/32 A
`4,466,272 8/1984 Stanfeld
`73/ 32 A
`4,480,461 11/1984 Ponzi
`73/32 A
`4,491,009 1/1985 Ruesch ........... ..
`4,524,610 6/1985 Fitzgerald et a1. ............... .. 73/32 A
`
`4,526,480 '7/1985 Ward ................................ .. 73/32 A
`4,542,993 9/1985 Mims et a1.
`374/42
`4,547,078 10/1985 Long 6181.
`374/42
`4,576,036 3/1986 Huang 6161.
`.. 73/29
`4,581,926 4/1986 Moore 6181. ......................... .. 73/29
`Primary Examiner-Charles Frankfort
`Assistant Examiner-Thomas B. Will
`Attorney, Agent, or Firm-James R. Young
`[57]
`ABSTRACT
`A method and apparatus for measuring steam quality of
`wet or two-phase flowing steam includes a ?ow
`through densitometer comprised of two parallel tubes
`connected to two common nodes, a vibrator for causing
`the tubes to vibrate, and a transducer for detecting the
`frequency and amplitude of the vibrations. A tempera
`ture probe detects the temperature of the steam, and a
`computer is used to monitor the vibrations and tempera
`ture and to calculate steam quality. The computer can
`also be used to control the vibrator. An intake sampler
`is positioned in a ?owing steam line to divert a represen
`tative sample of the wet steam into the densitometer.
`The bulk density of the steam is determined as a func
`tion of the fundamental frequency of the densitometer
`with the steam ?owing therethrough, and the steam
`quality is determined as a function of the bulk density
`and vapor density, which is a function of temperature.
`
`46 Claims, 8 Drawing Figures
`
`(‘<30
`
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`
`134
`136---/’ 4
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`
`1
`
`

`

`U. S. Patent JuL 14,1987
`
`Sheetl of4
`
`4,679,947
`
`2
`
`

`

`U. S. Patent
`
`Jul. 14,1987
`
`Sheet 2 of 4
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`U. S. Patent
`
`Jul. 14,1987
`
`Sheet 3 of 4
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`94
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`679,947
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`

`U. S. Patent Jul. 14,1987
`
`Sheet4 of4
`
`4,679,947
`
`g ! glad,
`
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`
`F|G.8
`
`5
`
`

`

`1
`
`METHOD AND APPARATUS FOR MEASURING
`STEAM QUALITY
`
`FIELD OF THE INVENTION
`The present invention is related generally to steam
`analysis methods and apparatus, and more particularly
`to a method and apparatus for determining and monitor
`ing the quality, thus heat or energy content, of steam in
`steam systems.
`
`10
`
`4,679,947
`2
`water droplets combine to form large slugs ?owing
`through the pipe. These slugs of liquid water can also be
`stretched out in churning or turbulent ?ow to elon
`gated, amorphous masses twisting through the pipe.
`Also, in some conditions, a thin film of liquid water
`?ows along the interior surfaces of the pipe in combina
`tion with the other ?ow forms described above. These
`varying and unpredictable ?ow phenomena present a
`formidable, and heretofore unsolved, measurement
`problem. The literature abounds with theoretical disser
`tations which attempt to treat rather idealistic models of
`two-phase ?ow, but unfortunately they are of little
`consequence or utility to understanding the real world
`conditions encountered by industry.
`The total energy contained in wet steam is equal to
`the combined energy of the water particle and water
`vapor phases. Steam quality, Q, de?nes the ratio by
`weight of each phase. Thus, the energy or heat content
`of wet steam systems is a function of the steam quality,
`where steam quality Q is de?ned as the ratio of the mass
`of vapor, My, contained within a speci?ed volume, V0,
`to the total mass, M,, of ?uids in the steam system.
`Since the heat energy in the liquid water particles and
`the heat energy in the water vapor are both well estab
`lished functions of temperature, it is theoretically possi
`ble to determine the energy contained in a sample of
`wet steam by acquiring an accurate measurement of
`steam quality Q. However, prior to this invention, there
`was no instrument or technique available for determin
`ing or measuring steam quality in commercial or indus
`trial steam systems on a reliable and continuous basis.
`Unable to measure steam quality, many industrial steam
`systems operate inef?ciently, and it is dif?cult, if not
`impossible to accurately account for energy usage
`among a number of steam users on a steam system or to
`monitor the actual heat energy delivered to a point of
`use by a steam system.
`
`DESCRIPTION OF THE PRIOR ART
`Steam systems are used primarily in industry to gen
`erate and deliver heat to points where the heat energy is
`put to useful work. The amount of heat energy deliv
`ered is, of course, directly related to the amount of
`work that can be accomplished. Therefore, it is the
`amount of heat energy delivered by the steam system
`for which the steam generator must account and for
`which the steam user must pay. Without an accurate
`accounting of heat energy delivered, ef?cient and accu
`rate management of a steam system is really not possi
`ble.
`Prior to this invention, such steam system manage
`ment involved many assumptions that really were not
`much better than guesses. Also, some equipment, such
`as modern steam turbines, are very complex machines
`that require sophisticated controls. The condition of
`steam delivered to such equipment is critical in achiev
`ing optimum ef?ciency and reduced maintenance.
`Therefore, there is a great need for a practical method
`or device for measuring and monitoring the heat energy
`content of ?owing steam at various points in steam
`systems. However, prior to this invention, there were
`no such practical methods or devices available.
`When saturated steam gives up energy, condensation
`occurs and the basic nature of the steam changes.
`Rather than being a consistent, homogeneous vapor, as
`is the case with superheated or saturated steam, wet
`steam is a mixture of liquid water particles with water
`vapor. The liquid water particles and the water vapor
`coexist in the steam system at the same temperature.
`This condition is known as two-phase ?ow. As more
`and more energy is extracted from the wet steam, as
`occurs in heat exchangers, more and more vapor con
`denses to water particles until eventually only liquid
`water remains. At this point, much of the useful energy
`is extracted from the steam.
`The energy or heat content of saturated or supersatu
`rated steam can be determined easily from temperature
`and/or pressure measurements of the steam system.
`Unfortunately, the task is not so easy in wet steam sys
`tems wherein two-phase ?ow is encountered. The phys
`ics which controls the ?ow of two-phase ?uids, such as
`wet steam, is not well understood. Simpli?ed mathemat
`ical models derived from laws of thermodynamics and
`fluid mechanics, so useful in predicting ?ow of single
`phase liquids and gases no longer applies when two
`phases coexist.
`The distribution of the water liquid particles in a pipe
`containing wet steam under ?owing conditions is very
`unpredictable and changes with variations in pipe ge
`ometry, direction of ?ow, temperature, and the like.
`The water particle and water vapor phases flow at
`different ?ow velocities with the water particles usually
`lagging the water vapor. Sometimes there can be a
`fairly even distribution of small droplets of liquid water
`flowing in the steam vapor, but at other times the liquid
`
`SUMMARY OF THE INVENTION
`Accordingly, it is a general object of this invention to
`provide a method and apparatus for determining wet
`steam quality, thus energy in the steam.
`Another object of the present invention is to provide
`a method and apparatus capable of determining steam
`quality in ?owing wet steam systems.
`A further object of the present invention is to provide
`a method and apparatus that can be used in a wet indus
`trial steam distribution system to determine and monitor
`steam quality at selected points in the system.
`A still further object of the present invention is to
`provide a method and apparatus for determining and
`monitoring steam quality in an industrial steam distribu
`tion system that is accurate, reliable, inexpensive, and
`easy to use.
`Additional objects, advantages, and novel features of
`the present invention shall be set forth in part in the
`description that follows, and in part will become appar
`ent to those skilled in the art upon examination of the
`following or may be learned by the practice of the in
`vention. The objects and advantages of the invention
`may be realized and attained by means of the instrumen
`talities and in combinations particularly pointed out in
`the appended claims.
`To achieve the foregoing and other objects and in
`accordance with the purpose of the present invention as
`embodied and broadly described herein, the method of
`this invention may comprise the steps of collecting the
`
`45
`
`65
`
`6
`
`

`

`4,679,947
`4
`FIG. 6 is a simple mechanical oscillating system used
`in describing the operating theory of this invention;
`FIG. 7 is an elongated tubular container used in de
`scribing the operating theory of this invention; and
`FIG. 8 is a conceptual representation of an elongated
`tubular container divided into individual sections used
`to describe the operating theory of this invention.
`
`3
`wet steam or a representative sample thereof and pass
`ing it through a densitometer to determine the bulk
`density or mass per unit volume of the wet steam, mea
`suring the temperature of the steam and determining
`vapor density or mass of the vapor phase, and determin
`ing steam quality therefrom. The method includes pass
`ing the steam through a continuous tube or chamber,
`causing the chamber to vibrate, determining the funda
`mental frequency of the vibrating tube or chamber, and
`determining the bulk density as a function of the funda=
`mental frequency. The method also utilizes a computer
`for monitoring the temperature and fundamental fre
`quency, calculating and outputing steam quality deter
`minations, and controlling the, vibration frequencies. An
`enhancement of the method includes swaging or reduc
`ing the steam pipe to a smaller diameter and stripping
`liquid ?lm from the interior surface of the steam pipe at
`the swage to obtain a uniform mixture of vapor and
`liquid phases at the sampling location.
`To further achieve the foregoing and other objects
`and in accordance with the purpose of the present in
`vention as embodied and broadly described herein and
`to implement the method of this invention, the appara
`tus of this invention may comprise a continuous ?ow
`through densitometer and temperature sensing device
`25
`to obtain bulk density and vapor density values neces
`sary to determine steam quality. The densitometer com
`prises two tubes mounted at common mass end nodes, a
`magnetic coil for inducing the tubes to vibrate, and
`strain guages on one of the tubes to measure the vibra
`tions.
`A computer is connected to the temperature sensor
`and to the strain guages to monitor those measurements
`and to a coil driver control to vary the frequency of
`vibration through a band that includes the fundamental
`35
`or resonant frequency of the tubes and connecting
`structure as the wet steam ?ows therethrough. The
`computer also rapidly calculates and outputs steam
`quality and related data.
`The apparatus also includes a sampling structure for
`diverting a representative sample of wet steam from a
`main steam system pipe for flowing the sample through
`the densitometer chambers and reinjecting it into the
`steam pipe. A pipe reducer or swage coupling swage
`and ?lm stripper are positioned in the main steam pipe
`immediately upstream from the sampler for enhancing a
`thorough mixture of the liquid and vapor phases at the
`sampling location.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The accompanying drawings, which are incorpo
`rated in and form a part of the speci?cations, illustrate a
`preferred embodiment of the present invention, and
`together with the description serve to explain the prin
`ciples of the invention. In the drawings:
`FIG. 1 is a perspective view of the steam quality
`meter of the present invention with portions of the body
`thereof cut away to reveal the components inside;
`FIG. 2 is a side elevation view of the steam quality
`meter of the present invention;
`FIG. 3 is an enlarged cross-section view of the steam
`quality meter of the present invention taken along lines
`3-3 of FIG. 2;
`FIG. 4 is a logic diagram of the control circuit of the
`steam quality meter of the present invention; and
`FIG. 5 is a logic diagram of an alternative control
`circuit for the steam quality meter of the present inven
`tiOn;
`
`50
`
`55
`
`65
`
`20
`
`30
`
`40
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`A steam quality meter 10 according to the present
`invention is shown in FIG. 1 attached to a steam flow
`line S in a typical steam distribution system. In order to
`appreciate the method and apparatus of this invention
`for determining and monitoring steam quality, thus heat
`or energy content, in a wet steam system where two
`phase fluid flow is encountered, it is helpful to review
`the theory underlaying this invention.
`As discussed in the background section above, the
`heat energy in the liquid water phase and the heat en
`ergy in the water vapor phase are well-established func
`tions of temperature and of the masses of those phases.
`Steam quality Q is an expression of the relation of the
`masses of the liquid and vapor phases, thus the total heat
`energy of the wet steam is a function of the steam qual
`ity Q. Therefore, if steam quality Q for wet, flowing
`steam can be measured accurately, then the heat energy
`in the wet ?owing steam can be determined accurately
`by utilizing known enthalpy relationships.
`Steam quality Q for a volume V0 of wet steam is
`de?ned as the ratio of the mass of vapor My contained
`within the volume V0 to the total mass M, of the ?uids
`in that volume. Thus:
`
`M
`
`(l)
`
`The method and apparatus of this invention makes
`use of the density relationship between the liquid and
`vapor phases and the bulk density of all the fluid in the
`volume to determine steam quality Q. The bulk density
`Fof the wet steam is de?ned as follows:
`
`T=M:/Va
`
`(2)
`
`Since the total mass Mris equal to the sum of the mass
`of the vapor Mv plus the mass of the liquid ML then:
`
`_ _ Mv + ML
`P _
`V.
`
`Mv
`V0
`
`+ Va
`
`(3)
`
`The volume occupied by each phase is also impor
`tant, and the relationship between the volume of the
`vapor Vv and the total volume V0 can be expressed as
`the “void fraction” (1, as follows:
`
`°a= WV.
`
`(4)
`
`Since the total volume V0 is the sum of the liquid
`volume V1, and the vapor volume VV, then
`
`Va= VL + VV
`
`and
`
`VI. =_ Vo— VV
`
`From equation (4),
`
`(5)
`
`(6)
`
`7
`
`

`

`4,679,947
`
`therefore, combining equations (6) and (7),
`
`Since density is de?ned as mass per unit volume, then
`
`PL=
`
`10
`
`(8.1)
`
`Therefore, the liquid mass ML and the vapor mass M Vin
`terms of density are
`
`ML=pL VL and M,=p,V,
`
`and, in terms of total volume V,,,
`
`ML =PL(l — <1) V0
`
`and
`
`Mv=pvaVa
`
`(8.2)
`
`20
`
`(9)
`
`25
`
`(10)
`
`Equations (9) and (10) show the relationships
`ML/Vo=PL(1'—0-)
`
`(11)
`
`and
`
`Combining equations (11) and (12) with equation (3)
`provides a de?nition of bulk density'?in terms of the
`void fraction (1, as follows:
`
`35
`
`T=pva+(1—a)PL=Pv<1+PL—aPL
`
`Therefore,
`
`‘F'—PL=a(Pv—PL)
`
`(13)
`
`(14)
`
`and the void fraction (1 in terms of density is
`
`45
`
`_ F — PL
`
`_ Pv — PL
`
`(15)
`
`pv .i.
`
`0-H. )
`
`(13)
`
`The result is that steam quality Q is effectively de
`?ned as the ratio of the vapor density pv to the bulk or
`average densityi)’.
`Fortunately, for purposes of this invention, the vapor
`density pv and liquid density P1, are well-established
`functions of the temperature T. While the precise func
`tional relationships are mathematically complex, they
`are easily handled by microcomputers providing real
`time measurement of steam vapor density pv and liquid
`density PL.
`The principle variable in the steam quality Q determi
`nation is the bulk density 7T Theoretically, the best
`method of determining bulk density Fis to weigh a ?uid
`of known volume. Unfortunately, conventional weigh
`ing techniques have not been successfully applied to
`two-phase ?owing ?uids, such as wet steam, except
`under rather ideal laboratory conditions. The problem
`is further compounded where such weight measure
`ments are attempted on ?owing two phase ?uids where
`the phase distributions are constantly changing and
`where the individual phases are moving at different
`velocities in the system.
`The method and apparatus of this invention solve
`these problems and provide ef?cient, accurate, and
`relatively easy effective measurement of the bulk den
`sity 'pTof the ?owing two-phase wet steam by use of
`physical principles of vibrating beams. This invention,
`therefore, includes a vibration densitometer arrange
`ment for obtaining a bulk density value for use along
`with the temperature T, vapor density pv, and liquid
`density P], relationships to derive a value for steam
`quality Q.
`In a simple model such as that illustrated in FIG. 6,
`comprised of a container 110 of mass MC containing a
`?uid 112 having a mass Mf suspended on a spring 114
`having a spring constant K, the container 110 will oscil
`late in a simple harmonic motion. A direct application
`of Newton’s law to such a simple harmonic system
`shows that the frequency of vibration f is inversely
`proportional to the square root of the mass Me of the
`container 110 and the mass Mfof its ?uid contents 112.
`Thus:
`
`Then, combining the relationships in equations (9)
`and (10) with the steam quality Q of equation (1),
`
`K
`Mc-l-Mf
`
`19
`(
`)
`
`pvaVn
`Q
`= PM. + pro — on =
`
`Pta + PL(1 —- :1)
`
`(16)
`
`55
`
`From equation (2), the bulk density, E is de?ned as:
`
`Finally, substituting equation (15) into equation (16)
`to eliminate the void fraction yields an expression for
`steam quality Q solely in terms of density, as follows:
`
`PL
`
`(
`
`1
`
`(17)
`
`If the liquid density pL is always very large compared
`to the vapor density pv, i.e., pL>>pv, a very useful
`simpli?cation of equation (17) results, as follows:
`
`Thus, where the mass of the total ?uid M, in equation
`(3) is the equivalent of the mass of the ?uid 112 in the
`container 110 Mfin equation (19), and substituting:
`
`60
`
`f:
`
`K
`M: + 5V0
`
`l
`
`65
`
`(20)
`
`Expanding equation (20) and solving for bulk density
`77 yields:
`
`8
`
`

`

`k
`
`M,
`
`(21)
`
`By de?ning a variable A=K/Vo and a variable
`B=M¢/Vo and substituting into equation (21), the result
`yields an expression for bulk density, E as a function of
`frequency, f, as follows:
`
`Fem/2+8
`
`(22)
`
`4,679,947
`8
`mination of bulk density 'p‘can be determined by mea
`suring the resonant frequency f of the vibrating tube
`120.
`Since the densities of the liquid and vapor phases (pL,
`pv) can be determined from temperature T of the steam,
`as discussed above, and bulk density of the ?uid can
`now be determined from the resonant frequency of
`vibration f of the tube 120. Then, from equation (18), the
`quality Q of the steam can be determined. Therefore,
`the quality Q of the wet steam can be determined by
`monitoring steam temperature T and frequency of reso
`nant vibration f of the tube 120.
`It is signi?cant to mention that steam vapor tempera
`ture is a function of its pressure for saturated steam.
`Therefore, while this description of the invention fo
`cuses primarily on temperature measurements to deter
`mine vapor density p,., it should be understood that such
`determination can be made from pressure measurements
`as well. Thus, the use of pressure to determine vapor
`density p, is considered to be the equivalent of the uses
`of temperature for this purpose in this invention.
`One of the most important advantages to the use of
`frequency of vibration f of a ?ow-through tube 120 in
`determing steam quality Q is speed of response. For
`example, with the tube 120 vibrating at approximately
`4,000 Hz, rapid changes in mass of the two-phase wet
`steam ?uid ?owing in the tube 120 are immediately
`translated into changes of resonant frequency f that can
`be detected and measured in milliseconds with great
`precision.
`,
`Another important consideration is that since the
`total mass of a two-phase ?uid in a wet steam system is
`relatively small, extraordinary sensitivity is required to
`detect small changes in ?uid phase concentrations.
`Large acceleration ?elds present in the vibrating tube
`120 means that relatively large force changes are gener
`ated for small changes in mass. The greater the acceler
`ation, the greater the force. Thus, small changes in the
`two-phase ?uid mass are detectable with extreme accu
`racy by measuring the resulting more dramatic changes
`in frequency of the vibrating tube 120.
`With the above theoretical discussion of the princi
`ples utilized by this invention in mind, it is appropriate
`to now describe the speci?c method and apparatus of
`this invention used to determine steam quality Q thus
`heat or energy content, of a two-phase ?owing wet
`steam system. Essentially, this method and apparatus
`use the fundamental or harmonic frequency of a tube
`having two-phase ?uid ?owing therethrough along
`with the temperature of the ?uid, to determine the
`steam quality Q of the ?uid.
`The steam quality meter apparatus 10 according to
`the present invention is shown in FIGS. 1 and 2
`mounted on a conventional steam system ?ow line S. It
`is comprised of a vibrating tube type densitometer 60
`enclosed in a cylindrical housing 12. Two-phase ?uid
`from the steam ?ow line S is conducted to the densitom
`eter 60 by the stand pipe 20, which extends into the
`interior of the steam ?ow line S and is mounted thereon
`by a swage loc coupling 30. An electrical component
`compartment 14 is mounted by a neck tube 16 to the
`housing 12.
`The cross-sectional view in FIG. 3 shows the struc
`ture and functional features of the steam quality meter
`10 of the present invention in more detail. The steam
`quality meter 10 utilizes a densitometer 60 comprised of
`twin hollow tubes 62, 72, rigidly ?xed at each end. The
`
`35
`
`Thus, equation (22) is a basic law which describes a
`vibrating type densitometer. Since frequency f can be
`measured with extreme precision, there is signi?cant
`appeal in using this relationship to determine density by
`measuring frequency. However, to be of practical value
`for measuring density of a ?owing fluid, a ?ow-through
`container is almost essential.
`A hollow tube such as the tube 120 shown in FIG. 7,
`can be used as a simpli?ed ?ow-through container for
`purposes of this theoretical analysis. In a simpli?ed
`sense, for comparison with the mechanical oscillating
`systems shown in FIG. 6, the length of a hollow tube
`120 can be analyzed as being comprised of a series of
`individual containers 120 coupled together with ?exible
`25
`sections or bellows 124, as illustrated in FIG. 8. Thus,
`each tube section 122 in FIG. 8 is analogous to the
`container 110 in FIG. 6, and each ?exible section or
`bellows 124 in FIG. 8 is analogous to the spring 114 in
`FIG. 6. Essentially, each tube container 122 in FIG. 8
`being suspended by a bellows 124 is capable of simple
`harmonic motion similar to the container 110 suspended
`by spring 114 in FIG. 6. However, unlike the single
`container model of FIG. 6, the entire structure in FIG.
`v8 can be made to vibrate at a number of discreet fre
`quencies (harmonics) and modes.
`According to a mathematical model based on calcu
`lus and differential equations, the speci?cs of which are
`not necessary for the purposes of this explanation, the
`' sizes of the tube section containers 122 and bellows 124
`vcan be decreased and their numbers increased to in?
`nite, where they are mathematically equivalent to the
`solid length of tube 120 in FIG. 7. In the in?nite limit,
`therefore, the model in FIG. 8 is mathematically identi
`cal to the model in FIG. 7. Thus, the tube 120 in FIG.
`7 functions like both the bellows 124 in FIG. 8 and the
`spring in FIG. 6.
`While appearing to be rigid, the tube 120 is really
`elastic and will de?ect under a load in a very predict
`able manner, as does the more familiar helical spring
`114. The primary difference is that the de?ection of the
`tube 120 is very small and imperceptible to the human
`eye. This difference in magnitude of de?ections, how
`ever, does not affect the validity of the mathematical
`and physical equations that describe the oscillation or
`vibration of the tube 120. In fact, the fundamental physi
`cal laws that govern the dynamic behavior of the single
`container model in FIG. 6 are equally valid with respect
`to the tube model of FIG. 7. Therefore, the bulk density
`p of the ?uid in the tube has a direct effect on the fre
`quency at which the tube 120 vibrates under a load.
`Speci?cally, as shown in equation (22), the bulk density
`T is inversely proportional to the square of the fre
`quency, i.e., f2. Thus, by measuring the reasonant fre
`quency f of the vibrating tube 120, a value for the bulk
`65
`density p‘of the ?uid in the tube can be determined. As
`mentioned above, the frequency f of vibration can be
`measured very accurately, thus a very accurate deter
`
`55
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`60
`
`9
`
`

`

`0
`
`20
`
`25
`
`30
`
`9
`steam samples from the steam ?ow line S ?ows continu
`ously through these tubes 62 72. The tubes are forced to
`resonate, in a similar fashion to that of a tuning fork,
`creating an acceleration force on all matter within the
`tubes 62, 72, regardless of whether it is liquid, gas, or
`solid. Consequently, each particle of mass within the
`tubes 62, 72, exerts, in turn, a force which is propor
`tional to the product of its mass and acceleration, i.e.,
`Force=Mass X Acceleration.
`As the total mass or bulk density within the tubes 62,
`72 increases, the resonant frequency decreases by the
`relationship shown in equation (22) above. The con
`stants A and B in equation (22) are determined by cali
`bration of the device. Thus, a measure of resonant fre
`quency of the tubes results in a measure of the total mass
`or bulk density. Then, with a precision measurement of
`the steam temperature by temperature probe 94, the
`device determines the vapor density or the mass that
`would exist if tubes were full of saturated steam. That
`theoretical vapor density is the same for any vapor that
`exists in the system under that temperature and pres
`sure. Thus, the vapor density pv of the vapor in the two
`phase ?ow is also determined by the temperature or
`pressure measurement. The ratio of the vapor density to
`the bulk density of the wet steam is the steam quality Q.
`Referring primarily to FIG. 3 therefore, the steam
`quality meter 10 is comprised of a cylindrical housing 12
`enclosed on the top by a cover plate 18 and on the
`bottom by a base block 50. The densitometer 60 of the
`present invention is positioned inside this cylindrical
`housing 12. A neck 16 extending through the top cover
`plate 18 supports a cylindrical container 14 that is uti
`lized primarily to house the electronic components of
`the steam quality meter 10. The outlet plug 15 in con
`tainer 14 accommodates the passage therethrough of
`35
`the necessary electrical wires (not shown) for connect
`ing the electronics of the steam quality meter to a power
`source, as well as to desired peripheral control and
`monitoring equipment, as will be described in more
`detail below.
`The densitometer assembly 60 is essentially com
`prised of two vertical parallel tubes 62, 72 positioned in
`the housing 12 a spaced distance apart from each other.
`These tubes 62, 72 are rigidly mounted in separate con
`duit 52, 56, respectively, in base block 50. They are also
`rigidly mounted at their tops in a crossover head 66
`having a conduit 68 therethrough that connects the tube
`62 with the tube 72. This arrangement is designed to
`conduct a ?ow of two-phase wet steam from the steam
`?ow pipe S upwardly through tube 72, through the
`crossover head 66,.and downwardly through tube 62
`and back to the steam ?ow line S, all as indicated by the
`?ow arrows in FIG. 3.
`This densitometer assembly 60 also includes a mag
`netic coil 80 mounted on a pedestal 88 between the
`tubes 62, 72. This magnetic coil is comprised of a metal
`lic core 82 and a set of wire windings 84 positioned in a
`spool 86 around the core 82. A pair of strain guages 90,
`92 preferably coil-type, are mounted on the upper end
`of tube 62, and a temperature probe 94 is positioned in
`the crossover head 66 to measure the temperture of the
`two-phase fluid ?owing through the conduit 68 therein.
`The densitometer tubes 62, 72, are connected to the
`steam ?ow line S by the stand pipe 20. The stand pipe 20
`is uniquely designed to continuously collect a represen
`tative sample of the two-phase ?uid flowing through
`the steam pipe S that accurately re?ects average steam
`quality. It also creates extreme turbulence, hence mix
`
`4,679,947
`10
`ing, of the steam liquid and vapor phases in the sample.
`The steam sample ?ows continuously through the den
`sitometer tubes 72, 62, and then returns to the steam
`?ow pipe S.
`As best seen in FIG. 3, the structure of the stand pipe
`20 is in the form of a double tube arrangement having an
`outer tube 22 with a smaller diameter inner tube 26
`positioned concentrically therein. The inner tube is
`small enough in diameter to leave an annulus 24 be
`tween the outer tube 22 and the inner tube 26. A top
`plug 44 closes and seals the top of the annulus 24 from
`the interior 28 of the inner tube 26 at the top. A bottom
`plug 46 is also positioned in the bottom of inner tube 26
`to close the bottom end thereof. The inside ?ow conduit
`28 of the inner pipe 26 is aligned in fluid ?ow relation
`with the conduit 56 adjacent the bottom of tube 72 The
`annulus 24 is connected in ?uid ?ow relation to conduit
`52 through an appropriately positioned hole 48 near the
`upper end of the outer tube 22.
`A plurality of inlet holes 38 through the lower end of
`the outer tube 22 are connected by small transverse
`tubes 39 to the interior 28 of inner tube 26. These inlet
`holes 38 are distributed across the cross-section of the
`steam ?ow pipe S in order to admit into the inner tube
`26 a representative sample of the two-phase ?uid ?ow
`in the steam ?ow pipe S. The admitted two-phase ?uid
`then ?ows upwardly through the inner tube 26 into the
`?rst densitometer tube 72. From the ?rst densitometer
`tube 72 the ?uid ?ows through the conduit 68 in cross
`over head 66 and downwardly through the second
`densitometer tube 62, all as illustrated by the ?ow ar
`rows in FIG. 3. The two-phase ?uid then continues to
`?ow out of the bottom 64 of the second densitometer
`tube 62, through the conduit 52, and into the annulus 24.
`The two-phase ?uid then exits from the annulus 24
`through the opening 40 at the bottom thereof to return
`into the main stream ?uid ?ow in the steam pipe S.
`It is appropriate to mention at this point that the ?ow
`direction through the densitometer could