DD256367A1 - METHOD OF DENSITY DETERMINATION OF A FLUID USING A SWIVEL COUNTER TO INCREASE THE MEASUREMENT SECURITY BY REDUNDANCY IN VOLUME REVALUATION - Google Patents

Abstract

The invention relates to a method of determining the density of a fluid using a Wirbelzaehlers on the principle of Karman vortex street for the purpose of subsequent volume conversion by means of Dichtemengenumwertung and parallel executed state quantity conversion with subsequent density determination. Field of application is the flow measurement technology. From both density values, a relative density difference is to be determined as a decision criterion. If a predetermined permissible deviation is exceeded, a quick error message should be ensured. According to the invention, in a first branch, the frequency-analog signal obtained from the piezoelectric scanning system of the Wirbelzaehlers attenuated depending on the frequency and on the one hand this preprocessed signal by means of a Auswerteelektronik a noise-free pulse train and calculates the frequency and on the other hand by means of an effective value generator determines a voltage signal in dependence on the density. In a second branch, the density is directly determined using the magnitude of pressure, temperature, etc. Fig. 1

Description

Field of application of the invention

The invention relates to a method of determining the density of a fluid using a vortex counter according to the principle of Karman vortex street for the purpose of subsequent volume conversion by density conversion to increase the measured value by redundant volume conversion by means of state quantity conversion. The method is particularly suitable for the balancing of gaseous and liquid raw materials and energy sources, since in legal transactions the knowledge of the quantities mass and mass flow or reference volume or volume flow under consideration of the operating volume or volume flow and pressure, temperature and / or operating density of the fluid high measured value security for the clearing is required.

Characteristic of the known technical solutions

From the literature it is known that when using vortex frequency flow meters not only the average flow rate or the volume flow in a pipe on the Ablösefrequenz on a bluff body on the relationship

f ~ c

determined, but with an appropriate arrangement and selection of sensors or transducers, an electrical signal can be obtained, which images the fluctuations of the kinetic energy of the fluid after detachment of vortices to a bluff body proportionally. In the electrical signal obtained in this way, in addition to the signal frequency, it is also possible to evaluate the signal amplitude Ü, which is proportional to the density of the fluid ρ and the square of the mean flow velocity c. It applies

Cl- ρ c ²

US Pat. No. 3,885,432 describes a solution in which both effects can be exploited and the mass flow rate of a fluid can be determined by suitable circuitry. The pressure signal proportional electrical signal is obtained in this solution using a pressure transducer.

In EP 0046965 a method for the dynamic and density independent determination of the mass flow of fluids is proposed. The method is characterized in that on a single perturbation body on the one hand, the Wirbelablösefrequenz.und second measured at the same of the back pressure. From both sizes is then about the relationship

rh = 2 (-ypv ² ) ~ -A

the mass flow m determined.

In DE-AS 1498271 a further arrangement for measuring the mass flow is shown without moving parts. The solutions of US 3885432 and DE-AS 1498271 have in common that the output of the density is not possible. The two solutions as well as those described in EP 0046965 are also common that a further economically relevant problem, the determination of a reference volume, with these arrangements or methods is not solved. Further arrangements for the independent measurement of the flow and / or. the density for the purpose of determining the mass and / or. the mass flow of a fluid can be found in the documents DE 2948961, DE 2935891, DE 3218940 and DD 222121. The last

Named solutions are representative of a variety of similar solutions that require two independent measurements (flow and density). Therefore, these solutions should not be discussed further.

Object of the invention

The aim of the invention is to achieve a redundancy in the volume conversion with the least possible additional effort for the flow measurement and subsequent volume conversion in order to increase the Meßwertsicherheit.

Explanation of the essence of the invention

The object of the invention is to find the solution of a method for density determination of fluids using a vortex meter according to the principle of Karman vortex street and parallel executed state quantity conversion with subsequent density determination. From the two density values thus obtained, a relative density difference is to be determined as a decision criterion with a flow computer, whereby a fault message is triggered when a predetermined permissible deviation is exceeded. The method is intended to ensure rapid fault detection in flow measuring devices. The essence of the invention is to be presented on the basis of the density determination of gases, which means no limitation of the method for liquids. According to the invention, the frequency-analogue signal obtained from the piezoelectric scanning system of the vortex meter is attenuated as a function of frequency, since small frequency changes Δί already lead to large changes in the effective value AU without attenuation, which can lead to overshoot of subsequent stages with increasing frequency values. From the thus preprocessed with a preamplifier signal is by means of a Auswerteelektronik a free of disturbances pulse train Nx and from this in the Durchflußrechner the frequency fx according to the equation

fx = Nx / T

(T - time base) calculated.

Independently of the frequency, a signal U _{p is} formed from the preprocessed frequency-analog signal Ü with an RMS-value generator and subsequent smoothing, for which applies

Up ~ p-fx ^{2 + n} , (2)

where f ¹ (where η S = 0) is an expression of the attenuation realized in the preamplifier. A proportionality factor K ₂ , determined by calibration of the measuring device in a normal and stored in the memory of the flow computer, allows the calculation of the stored in the program memory of the flow computer equation, after entering the values of U _p via a voltage input of the analog input of the computer.

(3)

In another branch of processing, the density is determined using the terms pressure and temperature, compressibility numbers and standard density. For this purpose, the analogue input of the flow computer obtained from pressure and temperature sensors and read in via corresponding signal inputs and with the standard density stored in the memory and the field of the compressibility numbers according to the equation

pu = To pn (4)

7 ₌ _P_ _Jn 1 ₍₅₎

^u p θ _n + K T _n (p, T, gas type)

V _B = Z _U V, V ₈ = J, Z _u Vdt (6), (7)

the thermal (5) necessarily also for the volume evaluation (6) and (7) must be calculated and so this calculation does not have to be made again for the density determination. The standard density is sufficiently determined as a constant with high stability of the composition of the fluid. If the composition fluctuates more, the constant should be replaced by values that are entered into the computer via a standard density transmitter. After the density determination in two independent branches and according to two different principles of action, a relative density difference according to equation

_{ρ (8)}

Pu

calculated as the decision criterion for fault messages z. B. disturbances of the sensors, wire breaks, drifting of sensors u. a. can be used.

-3- 256 387

embodiment

FIG. 1 shows the principle measuring arrangement for carrying out the method. FIG. 2 shows the principle of the vortex meter

According to FIG. 2, in the presence of a flow in the sender tube 12 on the baffle 1 of the swirl counter, vortices 13 are released which lead to fluctuations in the kinetic energy of the fluid behind the baffle 1 and cause the deflection of the spring plate 10 with a piezo scanning system 11. The output from the Piezoabtastsystem electrical signal Ü is an image of the pressure fluctuations behind the bluff body.

According to FIG. 1, the signal Ü (frequency-analogue signal) is fed to a preamplifier 5 with low-pass behavior and subsequently further processed separately. The signal Cl 'is connected to a transmitter 7 with frequency-dependent controlled low-pass and high-pass filters and a subsequent trigger stage. As an output signal, the pulse train Nx (fx) for further program processing in Durchflußrechner 8 is available. In addition, the signal Ü 'is performed on an effective value generator 6 mit.Kleichrichtung and low-pass filter and processed the resulting output voltage U _p in Durchflußrechner 8. The quantities absolute pressure p _abs and temperature θ are detected via sensors 3 and 4 and are fed into the flow computer 8 as unit flows l (p _abs ) and Ι (θ). If necessary, the standard density Pn can be made available to the flow computer 8 via the standard density transmitter 2 as the frequency f _p . All influencing variables are linked in the flow computer 8 according to the program parts stored in the program memory for the equations (1) and (3) to (8). As well as the quantities reference volume V _B and reference volume flow V ₈ , the relative density difference is available as a decision criterion in the event of faults in the measuring device. The method is particularly suitable for triggering fault messages in the case of faulty or defective sensors, in the case of breaks in signal lines to the sensors and in the flow computer, in the event of excessive drift of individual sensors or signal inputs on the computer, etc. With the aid of additional program plausibility tests of the individual influencing variables and calculated variables including the relative density difference and the application of two different principles of action can increase the reliability and reliability of a flow measurement on the principle of Karman vortex street.

Claims (2)

Method for determining the density of a fluid using a vortex meter to increase the measured value by redundancy in volume conversion by means of sensors for the determination of pressure and temperature in the sensor tube, characterized in that / in a first way, the frequency-analogue signal Ü in from the Piezoabtastsystem the vortex counter Depending on the frequency is attenuated and subsequently on the one hand by means of a transmitter in a no-interference pulse train N _x and on the other hand by means of an RMS converter with subsequent smoothing in a density-dependent voltage signal U _{p is} converted and that the thus determined magnitudes of the pulse train N _x and of. Voltage signal U _{p are used} to calculate the operating density, that on a second way by the sensors obtained operating variables pressure and temperature and compressibility numbers associated with these variables and reference variables, the state number is calculated and using the standard density den.Betriebsdichte is determined and that in another processing section the operating density determined with the aid of the vortex frequency measurement and subsequent calculation is compared with the operating density determined via the state number for deriving a decision criterion for fault detection. For this 2 pages drawings