Research and application of cancelling the instrument difference adjuster of crude oil volumetric flowmeter

0 Introduction

Most of the external metering stations under Sinopec Pipeline Company are equipped with liquid volumetric flowmeters and volume tubes for calibrating flowmeters for crude oil handover measurement. According to relevant national regulations, the accuracy level of flow meters used for trade settlement should be 0.2, that is, the basic error should not exceed ±0.2%. Therefore, if the accuracy of the flowmeter at the external metering station where the pipeline company and the refinery hand over reaches 0.2%, both parties can directly use the indicated value of the flowmeter as the settlement value. This indicator is indeed a considerable data for both the units that transport oil or the units that receive oil at the external metering station all year round, and the impact is still very large. In order to improve the accuracy of handover measurement and achieve handover close to zero error as much as possible, according to the requirements of Sinopec Group and the experience of some metering stations in carrying out coefficient handover over the years, most metering stations of the pipeline company have adopted the flowmeter coefficient method for crude oil handover.

Flowmeter coefficient handover is to use the flowmeter coefficient to correct the flowmeter. According to JJG667-1997 "Liquid Volumetric Flowmeter", the flowmeter should be periodically calibrated within a period of no more than 6 months. If the basic error exceeds ±0.2% during the calibration, the flowmeter's instrument error adjuster needs to be adjusted. Within 6 months, the supply and demand parties shall calibrate the flowmeter coefficients handed over every 1 to 2 months, calculate the actual amount of oil handed over during this period based on the flowmeter coefficients of the two previous and subsequent calibrations, and determine the amount of compensation. This approach solves the "±0.2%" problem. Therefore, the implementation of the coefficient handover method not only puts forward requirements for the reliability of the operation of the on-site flowmeter, but more importantly, it puts forward higher requirements for the accuracy and long-term stability of the measurement standard (volume tube flow standard device) of the on-site calibration flowmeter.

1 Requirements for standard devices and flowmeters in coefficient handover

1.1 Standards

Standards are generally volume tube flow standard devices (hereinafter referred to as volume tubes). The volume tube is a measuring standard for on-site real liquid calibration flowmeters. Its structure is a round tube with a certain length and the same inner diameter. After precision machining, its inner surface is composed of a metering section, a displacer, and a detection switch that can accurately determine the position of the displacer. The principle is that when crude oil flows through the flowmeter to be tested and the volume tube, the volume of fluid displaced by the displacer from the metering section between the two detection switches is compared with the indicated value of the flowmeter to determine the indication error and flow coefficient of each flow point of the flowmeter. The

standard volume value of the volume tube is obtained by calibration based on JJG209-1994 "Volume Tube". Water is used as the calibration medium to determine the standard volume of the standard section, and the water standard volume is converted to the standard state as the basic volume value of the volume tube. After calibration, the repeatability of the volume tube should be better than 0.02% and the accuracy should be better than 0.05%. The calibration cycle of the volume tube is generally three years. Due to the high accuracy and good repeatability of the volume tube, the flow meter is stable, reliable and can be verified online, so it is widely used as a standard flow meter in domestic oil product measurement.

1.2 Flowmeter

The flow meter used on site is generally a liquid volumetric flow meter, mainly including scraper, waist wheel, double rotor flow meter, etc. The liquid volumetric flow meter is a flow meter that uses measuring elements to continuously divide (isolate) the flowing liquid into individual volume parts and is used to measure the total volume of full pipe flow in a closed pipeline. The flow meter is calibrated in accordance with JJG677-1997 "Liquid Volumetric Flow Meter", which stipulates that its basic error limit is ±0.2%, and the repeatability of each flow point should not exceed 1/3 of the basic error (0.067%). For the volumetric flow meter for crude oil measurement and delivery, meeting the requirements of the existing regulations does not mean that these flow meters are fair and reasonable to both parties to the delivery, especially in the metering station where the coefficient delivery is implemented, the better linearity and repeatability of the flow meter are more important than the requirements for its basic error. For example, in a metering station with an annual oil delivery volume of 10 million m3, if only the basic error requirements are considered, the maximum difference may be nearly 20,000 m3 per year. If the coefficient handover is adopted, the impact of the error can be basically solved, and fairness is achieved for both parties of the handover.

2 New problems encountered in the measurement of coefficient handover

In the calibration of a newly commissioned crude oil pipeline external transmission metering station, the standard volume value of the volume tube calibration is 2130.0767L, and the repeatability is 0.012%. The flowmeter is a DN200 Smith scraper flowmeter. In the process of calibrating the flowmeter with this volume tube many times, excluding various other influencing factors, the repeatability is sometimes qualified and sometimes exceeds the tolerance. After analysis, it was found that the one-way standard volume value of this volume tube is 1065L. The flowmeter of this station is equipped with an instrument difference adjuster. Its correction output shaft discharges 100L for each rotation, that is, 1000 pulses are emitted, but the installed instrument difference adjuster makes about 1/4 of the pulses emitted by the flowmeter uneven. The phenomenon of uneven pulses is caused by the acceleration of the transmitter output frequency when the rotor rotates to a fixed area during the rotation of the flowmeter. If the standard volume value of the volume tube is an integer multiple of one rotation of the flowmeter's corrected output shaft (i.e. 100L), no matter where the pulse starting counting point of the flowmeter falls, the area covered by the uneven number of pulses in each calibration will be basically close. Since the standard volume of the volume tube is 10.6 times the transfer ratio of the flowmeter, the area covered by the uneven pulses is different in each calibration stroke. If the area of ​​uneven pulses can be covered each time, the repeatability of the flow point can meet the requirements. Otherwise, the difference in the number of pulses for each calibration is large, and the repeatability can easily exceed the requirements of the regulations. [page] The

same flowmeter is now calibrated with different volume tubes, and the results are shown in Tables 1 and 2.

Table 1 shows the results of calibrating the flowmeter with instrument error adjuster using the volume tube whose standard volume value is not an integer multiple of 100L. It can be seen that the repeatability at the 300m3/h flow point is very good, while the repeatability of the other two flow points is out of tolerance. Table 2 shows the results of calibrating the same flowmeter using the volume tube whose standard volume value is an integer multiple of 100L (about 4000L).

It can be seen that the reason why the repeatability of the same flowmeter varies greatly when calibrating it with different volume tubes is not the flowmeter, but the volume tube.

3 Solutions

In view of the problem that the standard value of the volume tube of the station is not an integer multiple of 100L and cannot guarantee the repeatability of the flowmeter, the following methods are recommended:

1) Remove the instrument error adjuster on the flowmeter and replace it with an empty instrument error, that is, directly use the flowmeter coefficient for handover measurement. Although this method has an out-of-tolerance basic error, if the repeatability is quite good (such as the maximum of 0.022% in the above table), which is much better than the requirement of 0.067%, this method can be used. After removing the differential adjuster, the factors affecting the performance of the flowmeter are reduced accordingly, which is more conducive to the regular analysis of the performance changes of the flowmeter in the future.

Table 3 shows the results of calibrating the flowmeter with an empty differential adjuster using a volume tube whose standard volume value is not an integer multiple of 100L. Although the basic error is larger than the tolerance, the linearity of the flowmeter and the repeatability at each flow point are quite good.

2) Make appropriate modifications to the volume tube and add a short tube to the metering section of the volume tube so that the standard volume value is an integer multiple of 100L (such as 1200L). However, this method is relatively troublesome to implement, and it is necessary to increase investment, clean the volume tube and re-calibrate with water.

3) Since the pulse starting counting point of the flow meter is random, the group calibration method can be used, that is, the arithmetic mean of multiple (3 or more) calibration data for each flow point is taken as a calibration data, and the flow coefficient and repeatability are calculated with such 3 data.

4 Conclusion

It is feasible to remove the instrument differential adjuster on the flow meter and use the flow meter coefficient handover in the handover measurement of crude oil. In fact, in countries that implement API standards, most flow meters are not equipped with instrument differential adjusters, and basically use flow meter coefficient handover. Since the flow meter does not use an instrument differential adjuster, an error source in the flow meter is reduced, making the measurement more accurate. It also saves the investment in replacing the instrument differential adjuster, and correspondingly saves the maintenance work of adding lubricating oil to the instrument differential.

Since the meter adjustment gear is adjusted to 96% when the flowmeter leaves the factory, the relative error δm of the flowmeter is basically around -4.0%. If the instrument error adjuster is not used, the flowmeter cannot meet the basic error requirement of no more than ±0.2% specified in the JJG677-1997 regulation. Therefore, it is recommended to consider adding regulations on the handover of flowmeter coefficients when revising the regulations. For example, for flowmeters that implement coefficient handover measurement, it is stipulated that the maximum and minimum errors of each flow point of the flowmeter shall not exceed 0.3%, and the repeatability of each point shall not exceed 0.05% (stricter than such requirements in the verification regulations), without being limited to the basic error of no more than ±0.2%. (end)