Patent Application
20180038811
The present invention relates to a method for determining fluid parameters of an unknown fluid, in particular a calorific value of the fluid or a parameter related thereto, the fluid being preferably a burnable gas. It further relates to a measurement device and a computer program product configured to carry out said method.
Today's natural gas pipelines are organized in vast networks fed by numerous feed-in stations. As the gas in the pipeline system originates from different gas sources, the gas is naturally a mixture that varies over time.
A calorific value of the natural gas is an important technical and economic fluid parameter that is commonly determined by analysis of the gas composition. The gas composition may be measured by means of a gas chromatograph. Such devices are, however, expensive in acquisition as well as in maintenance. Moreover, measurement times are long and consequently, the measurement data are not available in real time but has to be gathered intermittently.
A further device for determining the calorific value of a natural gas is known from DE 101 22 039 A1. This document suggests using a gas flow meter with two temperature sensors and a heater arranged therebetween. From the two measured temperature values the calorific value of the natural gas is determined by linear interpolation. This method is, however, not as precise as desired. Moreover, a gas flow has to be kept constant during the measurement.
It is an object of the present invention to provide an improved method for determining a parameter of an unknown fluid. In particular, the method may be for measurement of a burnable gas or a mixture of such gases, in particular a natural gas of a mixture thereof, wherein the fluid parameter may be the calorific value of the fluid or a parameter indicative thereof.
This object is achieved by the method according to claim 1. Accordingly, a method for determining a characteristic parameter, in particular a calorific value Hρ or a parameter indicative thereof, of an unknown fluid g, the unknown fluid g preferably being a burnable gas, preferably a natural gas, said fluid g flowing in a fluid flow through a sensor device, the sensor device comprising:
(i) establishing the fluid flow with an unknown value through said sensor device with at least part of the fluid g overflowing said thermal flow sensor;
(ii) activating the heating element of said thermal flow sensor and measuring at least one first temperature T1 with said first temperature sensor and at least one second temperature T2 with said second temperature sensor;
(iii) measuring first and second absolute pressures p1, p2 in the fluid g at the first and second positions, respectively, or
measuring a differential pressure Δp in the fluid g between the first and second positions with said pressure sensor device; and
(iv) determining from the measured first and second temperatures T1, T2 and the measured pressure value(s) Δp or p1, p2, a heat capacity cPρ, a calorific value Hρ or a parameter indicative thereof, a methane number (MN) or a parameter indicative thereof, and/or a Wobbe index WI or a parameter indicative thereof, of the fluid g with correlation functions.
The term “fluid” relates in particular to gases, preferably burnable gases such as, e.g. natural gases, biogases, shell gases, town gases, or mixtures thereof or the like.
The methane number MN and the Wobbe index WI linearly correlate with one another.
Accordingly, the invention is based on the insight, that with two sensors, in particular a thermal flow sensor and a pressure sensor, parameters of an unknown fluid related to an energy content, in particular related to a calorific value, may be determined simply and effectively. Moreover, the present invention allows providing online (i.e. real-time) data. Accordingly, a fluid feed into a target device may be analyzed (and controlled) during the feed-in process.
In some embodiments, the method further comprises the step of:
(v) determining from the measured first and second temperatures T1, T2 a first characteristic parameter of the fluid g;
(vi) determining from the measured first and second temperatures T1, T2 and the measured pressure value(s) Δp or p1, p2 a second characteristic parameter of the fluid g, wherein the second characteristic parameter contains additional information of the fluid g with respect to said first characteristic parameter;
(vii) determining from said first and second characteristic parameters the heat capacity cPρ, the calorific value Hρ or a parameter indicative thereof, and/or the Wobbe index WI or a parameter indicative thereof, of the fluid g with the correlation functions.
Preferably, the first characteristic parameter of the fluid g is a heat conductivity λ of said fluid g and the second characteristic parameter of said fluid g is a microscopic force ξ of said fluid g.
It is to be understood that instead of two temperature sensors the heater element may also be designed to provide alternative parameters, such as one temperature and one heater current strength or the like such that at least two parameters that are indicative of first and second temperatures T1, T2 are available. In some embodiments, first calibration data and second calibration data are used that comprise first and second temperatures T1, T2 and the pressure value(s) Δp or p1, p2 for at least one, preferably for two of more calibration fluids g1, g2. Accordingly, the sensor device according to invention may be calibrated with at least one, preferably two or more fluids. Preferably, said fluids do have characteristic parameters which are not too different from the characteristic parameters of the unknown fluid. The characteristic parameters of the calibration fluid may be within a range i.e. if λ is in the range with a lower bound of 50% to 300%, preferably 80% to 150%, more preferably 80% to 120% of the characteristic parameters of the calibration fluid .
The first calibration data may be data related to the first characteristic parameter, the second calibration data may be data related to the second characteristic parameter.
Preferably, the first and second calibration data is stored in at least one lookup table.
In some embodiments, said first characteristic parameter and said second characteristic parameter are determined by interpolation, preferably by linear interpolation. Accordingly, the calibration fluid(s) may be chosen such that an interpolation, preferably a linear interpolation, is possible.
Preferably, the second characteristic parameter is determined by resealing at least part of the relevant calibration data, particularly the second calibration data, prior to interpolation.
In some embodiments, the first calibration data further the heat conductivity λg1, λg2 of the calibration fluid g1, g2 and the second calibration data comprise the microscopic force τg1, ξg2 of the calibration fluid g1, g2.
In some embodiments, the correlation functions are based on a quadratic Ansatz. Any coefficient of the correlation functions may be determined empirically, i.e. one uses available data of similar fluids, i.e. fluid with similar parameters (e.g. natural gases or mixtures thereof), and uses fitting techniques such as the least square method to determine the coefficients. Data sets for fluids are readily available in literature and publicly accessible data bases.
In some embodiments, the method further comprises:
(viii) determining a volume flow Q of the fluid flow of the fluid g from the heat capacity cPρ and the heat conductivity λ. This may be done by the following equation:
Here, the function Θ1=Fλ(φQ) denotes the flow signal of the thermal flow sensor as function of
If λ is close enough to λg1 and λg2, the function Fλ(φQ) may be obtained by linearly interpolating the calibration data, as shown in Eqs. (9) and (10). The term “close enough” is explained below. Since, with the method according to invention, the parameters may be determined more precisely, also the heat capacity cPρ may be determined more precisely. As the volume flow Q strongly depend on the heat capacity cPρ, the method according to invention also allows for determining the volume flow Q more precisely and without knowing the fluid, e.g. the gas sort, i.e. whether its propane or the like.
In some embodiments, the method further comprises:
(ix) determining an energy transfer per time unit from said volume flow Q and the calorific value Hρ of the fluid g. Accordingly, the sensor device according to invention may be operated as an energy meter, which may help to optimize a combustion process in a combustor.
Preferably, the method also comprises:
(x) outputting the heat capacity cPρ, the calorific value H, and/or the Wobbe index WI or the methane number MN. Any other parameter determined by means of the sensor device may be outputted on a display or the like or made available to further circuitry using said parameter, e.g. a combustion engine may use the calorific value or methane number determined in online measurements to control the volume flow Q of the fluid into the combustion zone or to control the combustion process itself.
Preferably, said method is carried out with the fluid g being a natural gas or a mixture thereof. This is advantageous as natural gases are widely known, i.e. one can find details data sets about their properties, and it is widely used, e.g. in combustion processes. Accordingly, there is a significant demand for small, inexpensive, and reliable devices for online measurements of natural gas flows.
It is another object of the present invention to provide a sensor device that is configured to carry out a determination of the calorific value of an unknown fluid or of a fluid parameter indicative thereof.
This object is achieved by a sensor device according to claim 11. Accordingly, a sensor device for measuring a heat capacity cPρ, a calorific value Hρ, methane number MN and/or a Wobbe index WI of a fluid g, or a parameter indicative of any one thereof, is suggested, the sensor device comprising:
a flow channel for the fluid g, the flow channel having an inlet and an outlet for the fluid g,
a thermal flow sensor device with a heater element arranged between first and second temperature sensors,
a flow restrictor arranged between first and second positions in said flow channel,
at least one pressure sensor device for determining a pressure difference Δp in the fluid g between said first and second positions,
and a digital control circuit comprising a memory,
wherein the digital control circuit is configured to carry out a method according to invention.
In some embodiments, a memory stores the first and second calibration data, wherein the digital control circuit is configured to retrieve the first and second calibration data from said memory and to determine the first and second characteristic parameter, wherein, preferably, the correlations functions are stored in said memory and the digital control circuit is preferably configured to determine the heat capacity cPρ, the calorific value Hρ or a parameter indicative thereof, and/or a Wobbe index WI, a methane number MN or a parameter indicative thereof, of the fluid g with said correlation functions.
In some embodiments, the thermal flow sensor device is a CMOS flow sensor. This allows a particularly compact and cost efficient design of the device.
It is another object of the present invention to provide a computer program that, when carried out in a processor of a sensor device, carries out a determination of a calorific value of an unknown fluid or of a fluid parameter indicative thereof.
This object is achieved by a computer program according to claim 15. Accordingly, a computer program is provided that comprises computer program code that, when carried out in a digital control circuit of a sensor device according to invention, causes the digital control circuit to carry out a method according to invention.
Accordingly, in yet another aspect, the present invention provides a computer program product comprising computer program code that, when carried out in a processor of a digital control circuit of a sensor device, the sensor device comprising a thermal flow sensor device with a heater element arranged between first and second temperature sensors, a flow restrictor arranged between first and second positions in said fluid flow, and at least one pressure sensor device for determining a pressure difference Δp in the fluid g between said first and second positions, causes the digital control circuit to carry out any of the methods as described above, in particular, a method comprising:
(i) establishing an unknown flow of the fluid g through the sensor device with at least part of the fluid g overflowing said thermal flow sensor;
(ii) activating the heating element of the thermal flow sensor and measuring at least one first temperature T1 with said first temperature sensor and at least one second temperature T2 with said second temperature sensor;
(iii) measuring first and second absolute pressures p1, p2 in the fluid g at the first and second positions, respectively, or a differential pressure Δp in the fluid g between the first and second positions with said pressure sensor device; and
(iv) determining from the measured first and second temperatures T1, T2 and the measured pressure value(s) Δp or p1, p2 a heat capacity cPρ, a calorific value Hρ or a parameter indicative thereof, a methane number MN and/or a Wobbe index WI or a parameter indicative thereof, of the fluid g with correlation functions.
The computer program may be provided in a source code, in a machine-executable code, or in any intermediate form of code-like object code. It can be provided as a computer program product on a computer-readable medium in tangible form, e.g. on a CD-ROM or on a Flash ROM memory element, or it can be made available in the form of a network-accessible medium for download from one or more remote servers through a network.
Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,
The first temperature sensor 2 measures a first temperature T1, the second temperature sensor 4 a second temperature T2. In the present embodiment, the temperature sensors 2, 4 are thermopiles; in other embodiments, the temperature sensors 2, 4 may be of a different type, e.g. resistive temperature sensors. Typically, the heater element 3's working principle is based on the Joule heating effect, i.e. the heater element 3 is a resistive heater.
The temperature of the heater element 3 is generally stabilized above a temperature of the substrate 6. As a fluid g overflows the heater 3, heat energy is transported from the heater element 3 to the second temperature sensor 4. Accordingly, the second T2 is higher than the first T1. The temperature difference T2−T1 (or, equivalently, the ratio of these temperatures) depends, inter alia, on the fluid flow 5 and on the heat transfer properties of the fluid g, in particular, on its heat conductivity and heat capacity.
This type of flow sensor 1 is described, e.g., in WO 01/98736 A1 and U.S. Pat. No. 7,188,519 B2; the disclosure of these documents is incorporated herein by reference in its entirety for teaching the setup of the flow sensor device 1.
The first by-pass channel 11 carries a by-pass flow 5. In the first by-pass channel 11, said thermal flow sensor 1 is arranged for measuring the fluid g conveyed through the sensor device 10. Flow meter 1 is preferably a wall-mounted sensor device. The thermal flow sensor 1 is integrated to the sensor device 10, e.g., with conventional circuitry 209 (e.g. analog amplifiers, an A/D-converter, and/or digital signal processors).
A second by-pass channel 13 is provided between the first and second positions 18, 19. In said second by-pass channel 13, a differential pressure sensor device 15 is arranged. No fluid flow is established in said second by-pass channel 13, wherein, in the first by-pass channel 11, fluid g may flow without being noticeably disturbed by the thermal flow sensor 1.
In the main-pass channel 12 is arranged, between the first and second positions 18, 19, a laminar flow restrictor 14. The flow restrictor 14 established a pressure drop Δp in the fluid g between the first and the second positions 18, 19.
Accordingly, with the differential pressure sensor device 15, the pressure difference Δp in the fluid g between positions 18, 19 and the two fluid temperatures T1 and T2 at positions 18 and 19, respectively, may be measured.
An alternative embodiment of the sensor device 10 is depicted in
It is to be understood that other designs of the sensor device 10 may be used. For example, the flow meter device 1 may be arranged directly in the main flow 55.
In the following, a preferred method of determining a calorific value of the unknown fluid g, in particular a natural gas (or a mixture of such gases), based on the data set Δp, T1 and T2 and calibration data LTU1-LUT4 is described.
The heat conductivity λ and the microscopic force ξ of the fluid g may be determined experimentally. Generally, the microscopic force ξ of the fluid g is given by
ξ=η2/(ρ Pr2α).
Here, the parameters of the fluid g are the following: η is the dynamic viscosity, ρ denotes the density, and Pr is the so-called Prandtl number Pr=cPη/λ, where cP is the specific heat and λ heat conductivity of the fluid g. Parameter α is a real number in the range fluid 0≦α≦1. Typically, the sensor device 10 is operated at temperature of −40° C. to 150° C. and at pressures of 0.01 MPa to 1 MPa.
Now, in a first step, the heat conductivity λ of the fluid g may be determined from the two independent temperature T1 and T2 signals obtained by means of the thermal flow sensor 1. The T1 and T2 signals depend on the flow velocity ν of the fluid g and on physical parameters of the fluid g as follows:
T
i
=F
i(cPρν/λ, λ, Pr)with i=1, 2. (1)
The functions Fi(cPρξ/λ, λ, Pr) in Eq. (1) depend on the details of the measurement setup (e.g. the geometry of the main-pass/by-pass channel system, flow restrictors . . . ) as well as on the design and location of thermal flow sensor 1 in the by-pass channel 11. The functional dependence of Eq. (1) follows from the heat equation.
Generally, the heat conductivity λ of the fluid g is given as:
λ=f(T1, T2). (2)
The function f(T1, T2) in Eq. (2) can be determined, for example, through calibration measurements.
Let Θ1=T2−T1 and Θ2=T1+T2. It is noted that, in other embodiments, other linear combinations of T1 and T2 signals may be used.
Two calibration gases g1 and g2 with known heat conductivities λg1 and λg2, respectively, may be measured in calibration measurements. One sends both fluids g1 and g2, one at a time, through the sensor device 10 and varies the volume flow Q in the relevant flow range to determine the curves or sets of value pairs Θ2g1(Θ1g1) for fluid g1 and Θ2g2(Θ1g2) for fluid g2.
By measurement of the fluid g, one determines its heat conductivity λ by means of linear interpolation of the Θ2 signal while keeping the Θ1 signal constant:
λ=x λg1+(1−x)λg2, (3)
with
x=[Θ
2−Θ2g2(Θ1)]/[Θ2g1(Θ1)−Θ2g2(Θ1)]. (4)
Instead of using two calibration fluids g1 and y2 one may also use master data and calibration data from only one calibration fluid. In this case, the thermal conductivity is determined from the relation λ=λg1+m(Θ1)[Θ2−Θ2g1(Θ1)], where the slope m(Θ1) was determined beforehand by averaging calibration data (with two calibration gases) of many sensors. Said slope may be the master data.
Preferably, the data is averaged over a plurality of single measurements. The single measurements may be performed at different values for the fluid flow Q.
ξ=g(Θ1, Θ2, Δp). (5)
By means of the pressure sensor device 15 or 16, 17 the differential pressure Δp is measured. The differential pressure Δp depends on the flow velocity ξ as well as on the physical parameter η and ρ of the flowing fluid g as follows:
ξ=η2/ρ*H(ρν/η), (6)
wherein the function H(ρξ/η) depends on the details of the measurement setup (e.g. the geometry of the main-pass/by-pass channel system, flow restrictors . . . ). The functional dependence of Eq. (6) on the fluid parameters follows from the similarity transformation of the Navier-Stokes equations for incompressible fluids.
The function g(Θ1, Θ2, Δp) may be determined by means of calibration measurements. Let Θ1=T2−T1 and Θ2=T1+T2. Again, it is possible to use other linear combinations of the T1 and T2 signals.
For the calibration measurements, the two calibration gases g1 and g2 are used again, wherein the microscopic force parameters ξg1 and ξg2 of both gases g1 and g2 are known. During the calibration measurements, the fluid flow Q is varied in the relevant flow range, i.e. in the range the sensor device 10 shall operate with fluid g, and the two independent T1 and T2 signals obtained by means of the thermal flow sensor 1 and the differential pressure Δp is obtained by means of the pressure sensor device 15 or 16, 17. From these data, the two curves or sets of value pairs Δpg1(Θ1g1) for the first calibration gas g1 and Δpg2(Θ1g2) for the second calibration gas g2 are determined.
During use, the gas g is measured by means of the thermal flow sensor 1 and the pressure sensor device 15 or 16, 17. From the thermal flow sensor Θ1 and Θ2 are obtained. On the basis of the Θ1 and Θ2 data, resealed parameters Θ1′ and Θ1″ are calculated.
Θ1′ and Θ1″ are the resealed Θ1 parameters which are chosen such that the heat conductivity value λ corresponds to the heat conductivity values λg1 and λg2 of the calibration gases g1 and g2, respectively. Accordingly, Θ1=F1(cPρξ/λ, λPr)→Θ1′=F1(cPρξ/λ, λPr) and Θ1=F1(cPρξ/λ, λ, Pr)→Θ1″=F1(cPρξ/λ, λg2, Pr). In this way, the explicit dependency of the Θ1 and Θ2 data on λ is removed.
Then, the microscopic force ξ is given by the linear interpolation formula:
ξ=w ξg1+(1−w)ξg2 (7)
with
w=[Δp−Δp
g2(Θ1″)]/[Δpg1(Θ1′)−Δpg2(Θ1″)]. (8)
The resealed sensor signals Θ1′ and Θ1″ in Eq. (8) may be determined as follows. By means of Eqs. (3, 4), one first determines the heat conductivity λ of the unknown gas g as outlined above. If said heat conductivity λ is close enough to λg1 and λg2 the Θ1 sensor signal may be linearized in parameter λ as follows:
Θ1=uΘ1g1(φ)+(1−u)Θ1g2(φ) (9)
with
u=(λ−λg2)/(λg1−λg2). (10)
Here, φ=cPρξ/λ. Said heat conductivity λ being “close enough” to λg1 and λg2 means that λ is in the range with a lower bound of 50% to 125%, preferably 80% to 100% of the smaller of λg1 and with an upper bound of 100% to 150%, preferably 125% of the larger of λg1 and λg2, preferably it means that λg1≦λ≦λg2, wherein λg1 and λg2 differ by not more than 20% of the larger one of both.
For given sensor signals Θ1 and Θ2, Eq. (9) implicitly defines the parameter φ=φ(Θ1, Θ2). From said parameter φ one can calculate the resealed sensor signals Θ1′=Θ1g1(φ(Θ1, Θ2)) and Θ1″=Θ1g2(φ(Θ1, Θ2)).
Preferably, the data is averaged over a plurality of single measurements. The single measurements may be performed at different values for the fluid flow Q.
If one uses master data, φ(Θ1, Θ2) may be determined by measurement of one calibration gas only. In this case, Eq. (9) may be replaced by Θ1=[1+q(φ)(λ−λg1)]Θ1g1(φ) where the slope q(φ) has been determined beforehand by averaging over the calibration data (with at least two calibration gases) of many sensors. Said slope may be used as master data.
This method may be used for providing, e.g., an energy meter, e.g., for burnable gases such as natural gases. Generally, an energy meter determines a volume flow and the energy specific parameters such as calorific value Hρ or Wobbe index WI (in energy per volume) of an unknown burnable gas.
Accordingly, the sensor device 10 may be configured for measuring T1, T2 and Δp of the fluid g. Moreover, fluid measurement device may comprise a digital control circuit 20 configured for carrying out the method as outlined above.
Via a data interface 207, the processor 201 communicates with various peripherals, including the thermal flow sensor 1, the differential pressure sensor device 15 (or, in the embodiment according to
Moreover, by means of the heat capacity cPρ and heat conductivity λ the volume flow Q of the unknown gas g may be determined using the equation above. In combination with the calorific value Hρ one can obtain the energy transport per time unit.
The correlation functions between the fluid parameters may be found by regression analysis. One may analyze e.g. a large data set containing physical properties of burnable gases or mixtures thereof. Such data sets are publicly available, e.g. in the report “Report on gas composition range in Europe” (available under Intp://www.ingas-eu.org/docs/DB0.1.pdf).
Moreover, if the composition of the burnable gas, in particular the natural gas is known, physical parameters relevant here may be calculated by means of commercially available programs such as Refprop NIST (available under http://www.nist.gov/srd/nist23.cfm) or PPDS (available under http://www.tuv-sud.co.uk/uk-en/about-tuev-sued/tuev-sued-in-the-uk/nel/ppds-thermodynamic-properties-suite).
The correlations functions for a quadratic Ansatz may be written as:
c
P
ρ=A
1
+B
1
λ+C
1λ2+D1ξ+E1ξ2+F1λξ
Hρ=A
2
+B
2
λ+C
2λ2+D2ξ+E2ξ2+F2λξ
WI=A3+B3λ+C3λ2+D3ξ+E3ξ2+F3λξ (11)
Using the least squares method, one may obtain the coefficients Ai to Fi, i=1 to 3. The following set of coefficients was found (Tab. 1):
Here, the coefficients Ai to Fi, i=1 to 3 are normalized such that the heat conductivity λ an the microscopic force ξ are given with reference to the heat conductivity λ an the microscopic force ξ of methane. For the calculation of ξ the value α=0.7 was used.
The standard deviation of the relative error of the fit using the coefficients Ai to Fi, i=1 to 3 of Tab. 1 was found to be 0.28% for cPρ, 0.15% for Hρ, and 0.43% for WI.
An illustrative set of exemplary values is given in Tabs. 2, 3, 4, wherein all values are calculated at temperature T=25° C. and pressured p=0.1 MPa.
Theoretical values as calculated by said commercially available programs (Tab. 2):
wherein gas 1 and gas 2 have the following compositions (Tab. 3):
Values calculated by means of correlation function, i.e. Eqs. (11) (Tab. 4):
From the Wobbe index one may determine the methane number MN.
Accordingly, the present invention provides a small, inexpensive sensor element 10 and an inexpensive method which may be used for determining fluid parameters, wherein a pressure drop over the flow restrictor of maximum 2 millibars, as required by the relevant norm, is complied with.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/054648 | 3/5/2015 | WO | 00 |
