Selection of secondary instrument input signal
The commonly used output signal of a vortex flow sensor (i.e. The input signal of the secondary instrument) is in the form of a pulse signal or 4~20 mA DC current signal. Now vortex flow meters on the market have products that output pulse signal or 4~20 mA DC current signal at the same time, and each of the two signals has its advantages and disadvantages. The frequency signal is the original signal generated by the sensor, but it is easy to be interfering in the transmission process, especially when there are vibration sources, high-power electrical equipment, and other interference sources on site. It should be noted that some vortex flow meters still have flow indication under the condition of no flow, or the flow indication suddenly increases when the working condition is stable, which is caused by the interference source. The current signal is not easy to interfere in the transmission process, but because the current signal needs to be converted through the F/I frequency current conversion unit, at least 0.02% accuracy will be lost in this process. Therefore, the environmental conditions on site shall be fully considered when selecting the type, and reasonable instruments shall be selected.
Pressure measurement calibration
On the premise that the pressure transmitter meets the accuracy level, the reliability of pressure measurement is mainly affected by the setting of the impulse pipe. In general, when laying pipelines and installing instruments, in order to verify and maintain instruments more conveniently, the installation position of the pressure transmitter and pipeline pressure tap is often not at the same height. Due to the gravity condensation in the impulse pipe, there will be a deviation between the pressure value measured by the pressure transmitter and the actual pressure value of steam. Taking the pressure transmitter 2 m below the pressure tap as an example, if the height of condensate in the impulse pipe is 1 m, the indication of the pressure transmitter will be 9.8 kPa higher.
Taking the superheated steam with the actual pressure of 0.5 MPa and a temperature of 170 ℃ as an example, the density value calculated by looking up the table of the secondary instrument due to the high-pressure indication is 2.04% higher than the true density value, resulting in the corresponding high mass flow. The most convenient way to celebrate the influence of condensate in the impulse pipe is to set the pressure range to - 9.8~1590.2 kPa (the range of the pressure transmitter is 0~1.6 MPa) in the intelligent secondary instrument, which is labor-saving, time-saving, accurate and convenient. Of course, in order to accurately calibrate the pressure measurement, it is necessary to fully insulate the pipeline, especially the impulse pipeline, and then use the same pressure transmitter to measure installation position A and calibration position A ' respectively under stable working conditions. The pressure deviation caused by the gravity of condensate can be obtained by subtracting the pressure value of A' from the pressure value at position A.
Effect of high-temperature steam on instrument coefficient
The instrument coefficient (pulse/m3) of the vortex flow meter is generally celebrated in the normal temperature air of the laboratory. When the vortex sensor is used on site, it is in the high-temperature steam medium. The inner diameter of the measuring pipe and the width of the generator will change due to thermal expansion, which will affect the instrument coefficient of the vortex flowmeter. Therefore, the instrument coefficient should be corrected according to the different materials of the sensor and the measuring pipe, Refer to the relationship between the instrument coefficient and fluid temperature provided by the manufacturer, and put it into the secondary integrating instrument.
For example, the relationship given by a domestic manufacturer is Kt=[1-4.8 × 10-5 (t-t0)] Km, Kt: instrument coefficient when the steam temperature is t; T0: medium temperature during instrument calibration, refer to corresponding test report; Km: Instrument coefficient obtained through calibration laboratory conditions. If the field temperature working condition is stable and close to a constant temperature, the new instrument coefficient Kt calculated can also be directly put into the secondary integrating instrument, which is more convenient for the secondary integrating instrument that cannot be put into the above formula through programming. If the instrument coefficient is not corrected, taking the superheated steam at 170 ℃ as an example, assuming that the vortex street flow sensor is calibrated in an air environment at 20 ℃, the instrument coefficient will shift by -0.72% due to thermal expansion according to the above relationship. If the instrument coefficient at the time of calibration (greater than the instrument coefficient at the site of use) is still used for integration, the accumulated value will be lower, and the higher the steam temperature, the lower the amount will be. It can be seen that the deviation caused by not calibrating the instrument coefficient is considerable.
Instrument coefficient nonlinear calibration
In general, the instrument coefficient of the vortex flowmeter is a single value obtained through calibration in the laboratory environment. This value is obtained after the average of the maximum and minimum values of the instrument coefficients at the three flow points of Qmax, 0.4 Qmax, and 0.2 Qmax (Qmax: maximum flow value). For a vortex flowmeter with poor linearity, when the flow fluctuation deviates from the flow point corresponding to the middle value of the instrument coefficient, errors will inevitably occur.
Especially for large flow trade settlements, a 1% error may cause a huge trade balance. Therefore, in order to maintain the fair interests of both the supplier and the demander, a higher accuracy measurement level is required. It is recommended to configure intelligent secondary instruments with non-linear calibration of instrument coefficients. At present, domestic manufacturers are using the six points and five-segment non-linear correction method for instrument coefficients. The main principle is to put the instrument coefficients of multiple flow points calibrated under laboratory conditions (the number is determined by the linearity of the instrument, generally speaking, the number of instruments with poor linearity is more, otherwise it can be reduced accordingly) and the corresponding frequency values of each point into the intelligent flow Integrator so that the measurement accuracy is higher than that of a single instrument coefficient, and the accuracy of each point can be improved accordingly, It is a good way to improve the measurement accuracy.