JPS61164117A - Instrument for measuring flow rate of gas - Google Patents

Abstract

PURPOSE:To perform approximate correction on a measured flow rate of a fluid without receiving any influences of the change in the temperature and pressure of the fluid, by multiplying the volume flow rate under a designed standard condition by a correction coefficient corresponding to the measured absolute temperature of the fluid together with the temperature and pressure correction terms and performing moisture correction. CONSTITUTION:An absolute moisture detector 5, such as zirconia type hygrometer, etc., and the 2nd arithmetic means 7 are installed to a measuring instrument composed of a differential pressure transmitter 2, pressure detector 3, temperature detector 4, and 1st arithmetic means 6. When the output DELTAP of the transmitter 2, output P of the pressure detector 4, and output T of the temperature detector 4 are sent to the 1st arithmetic means 6, the means 6 calculates a flow rate QNO against which the temperature and pressure are converted into the reference condition. Then the 2nd arithmetic means 7 finds a moisture correction coefficient KW as the function of the absolute moisture Hf and calculates a moisture-corrected flow rate QNf by multiplying the flow rate QND by the coefficient KW. Therefore, moisture correction of a wet gas can be performing very easily by applying prescribe approximate correc tion of a linear formula irrespectively of the temperature and pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a gas flow rate measuring device that can calculate the flow rate of wet gas by correcting the moisture content.

<Prior Art> When measuring the flow rate of moist gas using a differential pressure method, temperature correction, pressure correction, and moisture correction are also required. That is,
In reality, the influence of moisture in the measurement gas is so large that it cannot be ignored, and it is necessary from the point of view of energy conservation to accurately measure the flow rate while taking the influence of moisture into account.

Generally, the correction formula for the flow rate QNf of the humidifying gas is given by the following formula.

The radical of the eye is compression! The fifth radical sign represents the moisture correction term.

Here, γNbtr: The value obtained by converting the trivial weight of the gas under the partial pressure of the dry gas in the wet gas with the design standard composition to the standard state. Value obtained by converting the specific weight of gas to the standard state Tb: Absolute temperature in design reference state Tf: Absolute temperature in use state Pb: Absolute pressure in design reference state Pf: Absolute pressure in use state zNftr 'Standard for fluid with composition in use state Compression coefficient Zbtr in state: Compression coefficient of dry gas in design standard state “Nbj
r'Compression coefficient of fluid with design standard composition in standard state zftr'Compression coefficient of dry gas in use state QNO:
'The moisture correction term in the apparent volumetric flow rate (1), which converts the apparent volumetric flow rate in the design standard state to the standard state, includes the relative humidity ψf1. ψb
It is expressed as follows.

``ゝPfs: Absolute saturated water vapor pressure Pb8 in use condition:
Saturated water vapor absolute pressure in the design standard state ψf: Relative humidity in the operating state ψb: Relative humidity in the design reference state PN: Absolute pressure in the standard state γbs: Saturated water vapor specific weight in the design reference state γfs: Saturated water vapor specific weight in the operating state ( 2) As is clear from the equation, the moisture correction term is too complicated;
High-precision correction requires extremely complicated arithmetic processing, which is impractical, and direct measurement of relative humidity is difficult and requires conversion calculations, resulting in complicated and expensive equipment.

<Problems to be Solved by the Invention> An object of the present invention is to provide a flow rate measuring device that is not affected by changes in the temperature and pressure of the fluid and is capable of approximate correction using the following equation.

<Means for Solving the Problems> The structural feature of the present invention is that when the flow rate of humid gas is corrected for temperature and pressure and is measured using a differential pressure method including moisture correction, the moisture correction coefficient is absolutely fixed. It is calculated as a function of humidity, and when measuring gas flow rate, the volumetric flow rate in the design standard state is multiplied together with the temperature correction term and pressure correction term to calculate the moisture-corrected humid gas flow rate. That's what I did.

<Function> When flow rate is measured using the differential pressure method, absolute humidity is detected by an absolute humidity detector along with temperature and pressure, and a humidity correction coefficient is calculated based on a predetermined calculation formula or from a pre-stored table. The humidity is read out, temperature and pressure are corrected, and humidity is corrected and converted into a volumetric flow signal converted into humidity in a standard state.

<Embodiment> Fig. 1 is a configuration diagram showing an embodiment of the present invention. 1 is a pipe through which wet gas P flows, 2 is a differential pressure transmitter disposed in the pipe, and ΔP is its output, 3 is the pressure detector that detects the gas pressure in the piping, P is its output, 4 is the pressure detector that detects the temperature of the gas in the piping, T is its output, and 5 is the pressure of the gas in the piping. H (indicates the output of an absolute humidity detector that detects absolute humidity).

6 is a first calculating means which multiplies the differential pressure ΔPiCP/T and square root, and outputs the flow rate QNO obtained by converting the temperature and pressure to the standard state. 7 is the absolute humidity I detected in the main part of the present invention.
(Outputs the flow rate QNf converted to the standard state based on (). If the correction coefficient in this case is Kw, QNf = Kw - QNo
(3) becomes.

The process of expressing Kw by H() will be explained below.

First, we assume that the gas can be treated as an ideal gas.

First, a correction formula for calculating the true volume QNf in the standard state is derived from the indicated value (apparent volumetric flow rate) QNb of the meter whose scale reference is in the standard state.

Since the weight flow rate of dry air does not change with respect to state changes,
The following equations (4), (5), and (6) hold true.

Qf・γfd wealth QNf emperor γNfd (true flow rate) (4)
Qb'rba=QNb'77Nha (apparent flow rate)
(5) By substituting equations (4) and (s) into (6) and rearranging the results, equation (7) is obtained.

Where, Qf: Volumetric flow rate in use condition Qb: Volumetric flow rate in design standard condition QNf: Conversion value of Qf to standard condition QNb: Conversion value of Qb to standard condition γfd: Specific weight of dry air in use condition γbd= Specific weight of dry air in design standard condition γNfd 'Conversion value of γfd to standard condition 7Ny:
Regarding the conversion value of ybd to the standard state and the specific weight of the used state, γf :rfd” rfω= rfb(1+XO(8)
Then, the following equation 6 [I, u holds true.

γb: γbd+γbω8γbd(”Xb)
(G) Here K.

γf : Specific weight of humid air in use condition rfoa : Specific weight of water vapor in use condition γb : Specific weight of humid air in design standard condition γbo : Specific weight of water vapor in design standard condition Tf : Absolute of humid υ air in use condition Temperature T6 ' Absolute temperature TN of humid air in design standard state
'Absolute temperature of humid air in standard condition P ('Total pressure of humid air in service condition (Pf = Pfd + Pf (, J) Pfd 'Partial pressure of dry air in service condition P (m) 'Partial pressure of water vapor in service condition pb'R meter reference state, total pressure of humid air (Pb"'
pba + Pbal) pbd' Partial pressure of dry air in design standard condition Pbat : Partial pressure of water vapor in design standard condition PN : Pressure of dry air in standard condition Xf : Absolute humidity in operating condition (Xf = γ field/γfd) Xb
:Absolute humidity in design standard state (Xb=γb, /γbd)e
f: Pressure ratio in use (ef=Pfa+/PI)eb:
Pressure ratio in design standard state (eb=Pb(,/Pb) Formula (8
), (9), (g) By substituting the α force into equation (7) and rearranging the results, the following equation (6) is obtained.

The positive coefficient KT, the (3) term is the pressure correction coefficient Kp, and the (4) term is the humidity tank 4 positive coefficient KW. Therefore, .

Next, consider the relationship between pressure ratio and absolute humidity. In general, absolute humidity X, partial pressure Pd of dry air, and partial pressure P of water vapor. It is known that there is the following relationship between:

Here K.

Md: Molecular weight of air (Md = 28.95) Mo: Molecule of water vapor ffi (Ma+ = 1111.0106)
The pressure ratio e is expressed as follows using 2〇〇.

Here, P = Pd + P.

Now, by substituting the formula α force into 03, the following result is obtained, and the humidity correction coefficient Kw can be expressed only by the absolute humidity X.

Furthermore, when absolute humidity is expressed as the weight of water vapor in dry air 1 (Nm"), the specific weight of dry air at 0°C and 76°rITrIHz is 1293 (g/Nm").
3), so X: □
(2) becomes. Therefore, the humidity correction coefficient Kw is calculated using the following formula.
, = 18.0106, the following formula % formula % Here, the range of absolute humidity H (-5 to 60 (g/Nm3
) An example of calculating the correction coefficient Kw for K is shown in the following table. The design standard in this case is Hb = 20 [27 m3], and therefore the numerator of formula (13C) is: how many i shoulder i stones 〒i=hang πi1 san 1 city i.

Relationship between humidity change and humidity correction coefficient (table) [Based on formula (13c)] As is clear here, KW is 1.0150 to j90.
However, it is noted that the difference ΔKW between each data is a substantially constant value, which indicates that the correction coefficient can be approximately linearly approximated to a change in absolute humidity.

FIG. 2 is a graph of this relationship, and it is clear that a linear relationship is obtained.

The correction calculation in the second calculation means 7 in the present invention is carried out by computer means (13c) at each correction timing.
) formula to find the correction coefficient -, or if the range of absolute humidity is predicted in advance, the above table can be stored in a table in the head etc. and the correction coefficient - can be calculated by calculating the range of absolute humidity Hf. K may also be used to read out the correction coefficient -. It is also possible to use analog polygonal function generating means.

The absolute humidity detector used in the present invention is as follows:
Zirconia-type hygrometer A high-precision absolute hygrometer with a quick response is available, and by using this, it is easy to implement the hygrometer. Alternatively, it is also possible to use a means of detecting the dew point temperature with a deacel dew point meter and converting it into absolute humidity.

<Effects> As explained above, according to the present invention, moisture correction of humid gas can be performed extremely easily by linear approximation correction by detecting absolute humidity, regardless of temperature or pressure, and a highly accurate flow rate measuring device can be obtained. can be realized at low cost.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a graph showing the relationship between absolute humidity and correction coefficient. F... Moist gas to be measured, 1... Piping, 2... Differential pressure transmitter, 3... Pressure detector, 4... Temperature detector, 5
... Absolute humidity detector, 6... First calculating means, 7...
-Second calculation means.

Claims (1)

[Claims] When measuring the flow rate of humid gas using the differential pressure method, which includes temperature and pressure correction as well as moisture correction, the moisture correction coefficient is calculated as a function of absolute humidity, and when measuring the gas flow rate, it is calculated according to the measured absolute humidity of the gas. A gas flow measuring device characterized in that a volumetric flow rate in a design reference state is multiplied by a correction coefficient together with a temperature correction term and a pressure correction term to calculate a moisture-corrected wet gas flow rate.