Chemical and Petroleum Engineering, Vol. 33. No 6. 1997
METHOD OF DIAGNOSTICS AND CALIBRATION OF FLOWMETERS
A. V. Kisilevskii, A. B. Mirgazov, and V. V. Plotnikov
UDC 681.121.8:53.089.6
In the methods widely used to calibrate and verify liquid and gas flowmeters the metrological characteristics of the verified flowmeter (VFM) are evaluated after removing instrument from piping in service. This is done by taking the readings of the instrument under verification or, e.g., when constrictors (CS) are being verified, by measuring the real linear dimensions of the constrictor (the sharpness of the working edge, the ellipticity of the diameter of the constrictor aperture, and so forth). It is not feasible in practice, however, to employ those methods of calibrating and verifying a flowmeter in the case of a continuous technological process or cryogenic piping with shielded-vacuum heat insulation, from which it is difficult or impossible to remove the flowmeter. The "Kriogenmash" Firm (Cryogenic Machines) has developed a procedure for diagnostics and verification of flowmeters with a constrictor (or other types of flowmeters with a linear calibration characteristic and a fixed zero) under operating conditions, without removing them from the piping. This is accomplished by using a custom-made portable device which, without interrupting the operation of the cryogenic equipment, directs part of the flow through a bypass pipe with a turbine flow sensor (TFS) mounted on it as a reference flowmeter. A diagram illustrating the operation of the portable device is shown in Fig. 1. With the pipe functioning with a steady-state flow, the pressure drop in the CS being calibrated and the temperature of the flow of cryogenic liquid to the CS are measured. The measured pressure drop is converted into an electric current I~, which corresponds to the instantaneous flow rate, and is recorded. The valve is then opened and part of the working flow is diverted into the bypass pipe with a control TFS. The temperature in the bypass pipe is made equal to the temperature of the working flow in the main pipe and then another measurement is made of the pressure drop in the CS, is converted into an electric current 12, which corresponds to the new instantaneous flow rate, and its value is also recorded. At the same time the flow rate in the control TFS corresponding to the difference of the measured instantaneous flow rate in the main pipe is measured and recorded (Qrvs) and the difference of the output current signals 11 - 12 is determined. Then, making allowance for the fact that the scaling factors for the flow rate and the output current signal (e.g., 1 mA = 20 mm, 10 liter/sec = 15 mm) had been chosen arbitrarily from the conditions for constructing the calibration characteristics, we determine the slope tangent c~ of the flowmeter calibration characteristic (Fig. 2) (It-I2)Ki tg a =
QTFSK 2
Since this method extends to a flowmeter with a fixed zero and a linear characteristic, by determining o~ we in fact obtain a new calibration characteristic for the flowmeter. The accuracy of the determination of ~ (and the calibration characteristic as a whole) depends only on the accuracy with which the reference flowmeter measures the output current signals and the flow rate. Under operating conditions the accuracy of measurement of the current signal (in the range 0-5 and 4-20 mA) is usually no less than 0.001 mA and the error of the certified (water) characteristic of the TFS varies from 0.5 to 1%, depending on the diameter of the nominal opening of the TFS. Accordingly, the total error of flowmeter calibration (in the steady-state mode) should be less than + 1%. The certified characteristic of the TFS was obtained for water and when working with other liquids the TFS must be calibrated with the working medium or appropriate corrections must be made to the certified calibration characteristic [1, 2].
Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 6, pp. 24-25, November-December, 1997. 0009-2355/97/3306-0633518.00 9
Plenum Publishing Corporation
633
3#-
5
5
Fig. I. Using a portable device (PD) to calibrate a flowmeter: 1) valve; 2) bypass pipe; 3) indicator of the temperature T,, p in the main pipe; 4) resistance thermometer; 5) connector; 6) constrictor (CS); 7) transducer measuring and converting the pressure drop in the CS; 8) indicator of the output tone signal of the flowmeter in the main pipe; 9) indicator of the temperature Tpu of the temperature in the bypass pipe; 10) flowroeter of the TFS; 11) TFS. I, mA
t/
11
/
A
3
-
/
I
/ Iz
I
2
to
f I
I
441 50 r liter/sec
Fig. 2. Calibration characteristics of the flowmeter being checked: 1) from specifications; 2) experimental. Analysis of the flowmeter error for air separation units (existing and in the design stage) shows that for those units to function normally it is sufficient that the flow rate be measured with 3-4% accuracy. When a flowmeter is used under commercial conditions (e.g., in the sale of liquid oxygen or nitrogen) the accuracy of measurement of the outgoing product should be higher, this lowering the unjustified overdelivery of product. This method and equipment have been patented by the Kriogenmash firm [3].
REFERENCES 1.
2.
3. 634
A. V. Kisilevskii, "Calculation of the correction factors for the static characteristic of turbine flowmeters," in: Processes in Cryogenic Equipment [in Russian], "Kriogenmash" NPO ("Kriogenmash" Scientific-Industrial Firm (Cryogenic Machines)), Balashikha (1982). A. V. Kisilevskii, L. A. Kamyshev, and I. V. Morokhev, "Taking account of the influence of the structure of the liquid flow on the operation of turbine flowmeters," lzv. Vyssh. Uchebn. Zaved., Fiz. (1982). Favorable decision on Application No. 9304i807, December 3, 1995.
