JPH0560142B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a computing unit used for flow measurement and the like. Computing units used for flow rate measurement are required to have various computing functions in response to various fluids such as natural gas, general gas, steam, and liquids, so recently they have been constructed using microcomputers. By the way, some calculations in flow rate measurement include very complicated calculation formulas, such as the calculation of specific weight based on the temperature and pressure of steam. To accurately calculate complex arithmetic expressions using a microcomputer,
Calculations require a large amount of memory and take a long time. For this reason, a table of specific weight with respect to temperature and pressure is provided, and the specific weight is determined by table interpolation calculation. However, in this case as well, the specific weight is largely curved with respect to temperature and pressure, so in order to obtain it with high accuracy, the table must be divided carefully, and a large amount of memory capacity is required. An object of the present invention is to realize an arithmetic unit that can perform complex operations at high speed and with high precision using a small amount of memory. In the present invention, the temperature of the steam is converted into a temperature signal by a temperature sensor, and the pressure of the steam is converted into a pressure signal by a pressure detector, which are inputted respectively, and based on these signals, the practical international state of the steam is determined. an approximate formula calculating means for calculating an approximate specific weight that approximates the specific weight based on the formula; a table storing the specific weight difference between the previous specific weight and the previous approximate specific weight with respect to the previous temperature and pressure; a correction value calculation means for calculating a correction calculation formula using the temperature signal, the pressure signal, and the specific weight difference, and calculating the specific weight correction value therefrom; an addition calculation means for calculating the addition ratio weight by algebraically adding the
The above object is achieved by providing a weight flow rate calculation means for calculating a weight flow rate by multiplying a flow rate signal obtained by converting the steam flow rate by a flow rate detector by an addition specific weight. FIG. 1 is a block diagram functionally illustrating an embodiment of the computing device of the present invention, illustrating the case where the weight flow rate of steam is determined. In the figure, 1 is the steam flow rate signal Q (m ³ ) from the flow rate detector (not shown).
2 is the terminal to which the steam temperature signal T(〓) from the temperature detector (not shown) is applied, 3
is the pressure signal P (Kg) from the pressure detector (not shown)
f/m ² ) is added, 4 is an approximate expression calculation means,
Based on the temperature signal T and pressure signal P, an approximation calculation of the following equation is performed to calculate an approximate value γ' (Kgf/m ³ ) of specific weight. 1/γ'=47.06T/P-0.668/(T/100
) ³ −437 + 6.31x10 ^-3 P/(T/100) ⁸ (1) 5 is the difference (γ − γ′) between the specific weight γ and the approximate value γ′ based on the practical international equation of state as a function of temperature and pressure. On a table, calculate in advance on a computer, etc., the specific weight γ based on the practical international equation of state based on the international equation of state for steam and the approximate value γ′ of the specific weight based on the approximation formula of equation (1), and calculate the difference (γ −γ′). Here, the specific weight γ calculated based on the practical international equation of state regarding steam will be explained. For details, see the 1980 Japan Society of Mechanical Engineers Steam Table (Publisher: Japan Society of Mechanical Engineers, Publication date: February 1980)
The following is an overview of the publication. This specific weight γ is given as the reciprocal of the specific volume v, and the specific volume v is the converted specific volume X=v/v _C1 (however, v _C1 =
0.00317m ³ /Kg), it can be calculated using the following formula (1A). However, θ is the converted temperature T/T _C1 (T _C1 = 647.3K),
β is the converted pressure P/P _C1 (P _C1 = 22120000N/m ² ), I ₁
is the reduced ideal gas constant and I ₁ = 4.260321148×10 ⁰
It is. Here, X=exp{b(1-θ)} ...(1B) β _L =β _L (θ) = L ₀ +L ₁ θ+L ₂ θ ² ...(1C). However, the induction constants L ₀ , L ₁ , L ₂ are L ₀ = 1.574373327×
10 ¹ , L ₁ = −3.417061978×10 ¹ , L ₂ = 1.931380707
It is given by ×10 ¹ . Also, the number of n(μ) and 1(μ) for μ,
Alternatively, the values of the indexes z (μ, ν) and x (μ, λ) are as shown in Table 1.

【table】

[Table] Furthermore, the constant B in which μ and ν are parameters in equation (1A) is given in Table 2.

[Table] In addition, grid points in Table 5 (points with data)
are normally selected at equal intervals, but in this example, since the steam is limited to the range defined by the saturated vapor pressure curve as shown in FIG. , pressure P, only one of temperature T and pressure P is selected at equal intervals. Figure 2A shows the case where the temperature T is selected at equal intervals, and Figure 2B shows the case where the pressure P is selected at equal intervals. In addition, in the figure, considering the practical range of steam, T is 100 to 400℃ and P is 1 to 100Kg.
It is shown in the range of f/cm ² obs. Reference numeral 6 denotes a calculation means for calculating the correction value Δγ, which calculates the correction value Δγ from the table 5 by table interpolation calculation based on the temperature signal T and the pressure signal P. The table interpolation calculation is
Since the value of Δγ is determined for the two variables T and P, we find the coefficients a, b, and c of the plane equation Δγ=a・T+b・P+c, and use these to calculate the value of Δγ. . The calculation begins with the four grid points A (T _Nt , P _Np , Δγ _Nt,Np ), B (T _Nt+1 , P _Np ,
Δγ _Nt+1,Np ), C(T _Nt+1 , P _Np+1 , Δγ _Nt+1,Np+1 ),
D
(T _Nt , P _Np+1 , Δγ _Nt,Np+1 ) are determined. Next, inside this quadrilateral ABCD, triangle ABC and triangle
Considering two triangles of ACD, measurement points (T, P)
Find out which triangle belongs to. As a result, if the measurement point (T, P) belongs to triangle ABC, the equations of the three planes corresponding to grid points A, B, and C are a・T _Nt +b・P _Np +c=Δγ _Nt,Np a・T _Nt+1 +b・P _Np +c=Δγ _Nt+1,Np a・T _Nt+1 +b・P _Np+1 +c=Δγ _Nt+1,Np+1 (3) and calculate Δγ using equation (2). Note that the measurement point (T, P) is a triangle.
If it belongs to ACD, the coefficients a, b, c are
Find the value of. 7 is an addition means which adds the correction value Δγ from the correction value Δγ calculation means 6 to the approximate value γ′ from the approximate formula calculation means 4, and calculates the value (γ′+Δγ) obtained by correcting the error of the approximate value as the specific weight. It is something to seek. Reference numeral 8 denotes a weight flow rate calculating means which multiplies the flow rate signal Q by the calculated specific weight value (γ'+Δγ) from the adding means 7 to calculate and output the weight flow rate W. The arithmetic means 4, 6, 7, and 8 described above are realized by a computer program. The operation of the present invention configured as described above will be explained below. First, based on the temperature signal T and the pressure signal P, the approximation calculation means 4 performs the approximation calculation of equation (1) to obtain an approximate value γ' of the specific weight. However, this approximate formula has 2
There is an error of about %. Next, using the table 5 of the difference between the specific weight γ and the approximate value γ' based on the practical international equation of state, the correction value calculation means 6 performs table interpolation calculation to calculate the correction value according to the temperature signal T and the pressure signal P. Δγ
seek. Adding this correction value Δγ to the addition means 7,
The approximation value γ' from the approximation formula calculation means 4 is added, and γ' is corrected (γ'+Δγ), which is output as the calculated value of the specific weight. This correction calculation results in an error of ±0.2
% or less. Thereafter, the weight flow rate calculating means 8 calculates the weight flow rate W by calculating Q+(γ'+Δγ) between the flow rate signal Q and the calculated value of specific weight (γ'+Δγ). The approximation calculation using equation (1) is simpler than the calculation of the specific weight γ based on the practical international equation of state from the international equation of state for steam, requires less memory capacity, and can be performed at high speed. . In addition, the table interpolation calculation for calculating the correction value shows that the difference between the specific weight γ based on the practical international equation of state and the approximate value γ′ is the temperature,
The bending in response to pressure becomes very small, and the table can be divided into sections that much more coarsely, so the memory capacity can be very small. According to the present invention, which combines approximate formula calculation for obtaining an approximate value and table interpolation calculation for obtaining a correction value, specific weight can be calculated accurately and at high speed with a small amount of memory. In the above description, the case where the approximation calculation circuit 4 performs the calculation of equation (1) has been exemplified, but it is also possible to further simplify equation (1), for example, to perform the approximate calculation of the following equation. 1/γ'=47.06T/P-0.668/(T/100) ³ (4) However, in this case, the error will be larger than in the approximate calculation of equation (1), and the curve of the correction value will also be larger, so It is necessary to divide the table into smaller sections. The required approximate expressions and tables may be combinations that can be calculated accurately and quickly with a small amount of memory.
Further, in the above description, the case where the specific weight of steam is calculated has been exemplified, but it goes without saying that the present invention is not limited to this and can be applied to calculations of other complicated calculation formulas. As explained above, in the present invention, an approximate value is obtained by first calculating an approximate value by simplifying the complex calculation formula for calculating specific weight based on the practical international equation of state, and then calculating an approximate value based on the practical international equation of state. Using a table of differences between specific weights and approximate values, the correction difference is calculated by table interpolation calculation, and the approximate value is corrected using this correction value, so complex calculation formulas can be written with less memory.
An arithmetic unit that can perform calculations with high precision and high speed can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram functionally showing an embodiment of the arithmetic unit of the present invention, and FIG. 2 is a diagram for explaining grid points of a table. DESCRIPTION OF SYMBOLS 1... Input terminal for flow rate signal, 2... Input terminal for temperature signal, 3... Input terminal for pressure signal, 4... Approximate formula calculation means, 5... Table, 6... Correction value calculation means, 7... ...addition circuit, 8...weight flow rate calculation means.

Claims (1)

[Claims] 1 The temperature of the steam is converted into a temperature signal by a temperature sensor, the pressure of the steam is converted into a pressure signal by a pressure detector, and these signals are used to calculate the ratio based on the practical international equation of state of the steam. an approximate formula calculating means for calculating an approximate specific weight that approximates the weight; a table storing the specific weight difference between the specific weight and the approximate specific weight with respect to the temperature and pressure; a correction value calculating means for determining a correction calculation formula using the specific weight difference and calculating a specific weight correction value therefrom; and an addition calculation means for calculating an added specific weight by algebraically adding the approximate specific weight and the specific weight correction value. and weight flow rate calculation means for calculating a weight flow rate by multiplying a flow rate signal obtained by converting the steam flow rate by a flow rate detector by an addition specific weight.