Abstract : This paper analyzes the reasons for the occurrence of repetitive over-characteristics in the verification of the newly commissioned flowmeters for crude oil exporting stations, and proposes countermeasures to solve the relevant problems in the implementation of flowmeter coefficient handover measurement. In order to revise the relevant regulations in the future, the flowmeter coefficients Under the condition of handover, the reference to break the basic error limit is provided.

Key words: Flowmeter coefficient crossover difference adjuster 0 INTRODUCTION Sinopec Pipeline Co., Ltd.'s external metering stations, the vast majority of which are equipped with liquid volumetric flowmeters for the measurement of crude oil transfer and volumetric tubes for verification flowmeters, are installed in the stations. . According to the relevant national regulations, the accuracy of the flow meters used for trade settlement should be 0.2, that is, the basic error should not exceed ±0.2%. Therefore, the accuracy of the flowmeters of the external metering stations that the pipeline company and the refinery exchanges with reaches 0.2%, and the two parties can directly use the indicated value of the flowmeter as the settlement value. This indicator is indeed a considerable amount of data for both the unit that exports oil from outside the metering station and the oil-receiving unit, and the impact is still very large. In order to improve the accuracy of handover measurement, as close as possible to achieve zero error transfer, according to the requirements of the China Petrochemical Corporation and over the years the experience of carrying out the handover of some metering stations, most of the pipeline companies have adopted the flow meter coefficient method for crude oil Transfer.

The transfer of the flow meter coefficient uses the flow meter coefficient to correct the flow meter. According to JJG667-1997 "Liquid volumetric flowmeter" regulations, the flowmeter should be cycled within a period of no more than 6 months. If the basic error exceeds ±0.2%, the flowmeter's differential adjuster must be adjusted. Within 6 months, the supply and demand sides test the flowmeter coefficient transferred every 1 to 2 months, and calculate the amount of oil actually transferred during the period based on the flowmeter coefficient measured twice before and after, and determine the amount of compensation. This approach will solve the "± 0.2%" problem. Therefore, the realization of the coefficient transfer method not only imposes requirements on the reliability of the on-site flowmeter operation, but more importantly, it raises the accuracy of the measurement standard for the on-site verification flowmeter (volumetric tube flow standard device) and the stability of long-term operation. The request.

1 Requirements for Standard Equipment and Flowmeters in the Transfer of Coefficients 1.1 Standards The standard equipment is generally a volumetric tube flow standard device (hereinafter referred to as volumetric tube). The volumetric tube is a metering standard for on-site liquid flow verification flowmeters. Its structure is a circular tube with a certain length and the same inner diameter. After the inner surface is precision machined, the measuring section, displacement device and the position of the displacer can be precisely determined. The composition of the detection switch. The principle is that when the crude oil flows through the inspected flow meter 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 flow meter to determine the flow rate of the flow meter. Point error and flow coefficient.

The standard volumetric value of the volumetric tube is based on the verification of JJG209-1994 Volumetric Tube. Use water as the verification medium to determine the standard volume of the standard section and convert the volume of the water standard to the standard volume value of the volume tube. After the volumetric tube has been tested, the repeatability should be better than 0.02% and the accuracy should be better than 0.05%. The calibration period for volume tubes is generally three years. Due to the high accuracy and repeatability of the volumetric tube, the verification flowmeter is stable, reliable, and can realize on-line verification. Therefore, it is widely used as a standard for flowmeters in the measurement of domestic oil products.

1.2 The flow meter used in the field of the flowmeter is generally a liquid volumetric flowmeter, and mainly includes a scraper, a waist wheel, and a dual-rotor flowmeter. A liquid volumetric flow meter is a flow meter that uses a measuring element to continuously divide (segregate) a flowing liquid into a single volume portion for measuring the total volume of a full pipe flow in a closed pipe. Flowmeters are tested in accordance with JJG677-1997 "Liquid Volumetric Flowmeters", which specifies that the basic error limit is ± 0.2%, and the repeatability of each flow point should not exceed 1/3 of the basic error (0.067%). The volumetric flowmeters used for crude oil metering and handover meet the requirements of the existing procedures and do not imply that these flowmeters are fair and reasonable for both parties. In particular, in the metering stations that perform coefficient transfer, the flowmeters have better linearity and repeatability than flowmeters. It is more important to address the basic errors. For example, in a metering station that handles 10 million m3 of oil in one year, if only basic error requirements are considered, a maximum of nearly 20,000 m3 of difference may occur each year. If the coefficient is adopted, the influence of the error can be basically solved, and the fairness is achieved for both parties.

2 new problems encountered in the measurement of the handover of the coefficient In the verification of a new crude oil pipeline output station newly put into operation, the standard volume value of the volume pipe verification is 2130.0767L, and the repeatability is 0.012%. The flowmeter is a DN200 Smith screed flowmeter. In the process of calibrating the flowmeter multiple times using this volumetric tube, the repetitiveness is sometimes qualified and sometimes out-of-tolerance in the absence of other various influencing factors. After analysis, it was found that the standard volume value of this volume tube was 1065L. The flow meter of this station was equipped with a differential adjuster. The corrected output shaft discharges 100 L per rotation, that is, it sends 1000 pulses, but the installed device is poor. The regulator makes the pulse from the flow meter about 1/4 uneven. The phenomenon of uneven pulse is caused by the faster output frequency of the transmitter 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 integral multiple of the revolution of the flowmeter correction output shaft (ie, 100L), the area of ​​uneven pulse number covered in each verification will be determined regardless of where the flowmeter's pulse initiation count point falls. Basically close. Since the standard volume of the volumetric tube and the transfer ratio of the flowmeter are 10.6 times, this makes the area covering uneven pulses different for each stroke of the assay. If the area of ​​uneven pulse number can be covered each time, the repeatability of the flow point can meet the requirement. Otherwise, the difference in the number of pulses per test is large, and the repeatability easily exceeds the requirements of the protocol.

Different volume tubes are used to test the same flowmeter. The results are shown in Table 1 and Table 2.
Table 1 shows the result of using a volumetric tube tester with a volumetric tube to calibrate the flowmeter with a volumetric tube that is not an integral multiple of 100L. It can be seen from this that the repeatability at the 300m3/h flow point is good while the other two flows The repeatability of the points is poor. Table 2 shows the results of testing the same flow meter with a volumetric tube (approximately 4000 L) whose standard volume value is an integral multiple of 100L.

It can be seen that the reason why the repeatability change of the same flowmeter is verified with different volumetric tubes is not the flowmeter but the volumetric tube.

3Solutions For the problem that the station volume tube standard value is not an integral multiple of 100L and cannot guarantee the reproducibility of the flowmeter, the following methods are recommended:

1) Remove the flow difference adjuster on the flow meter and replace it with the empty difference, that is, directly use the flow meter coefficient for hand-over measurement. Although the basic error of this method is very bad, if the repeatability is quite good (such as the maximum in the above table is no more than 0.022%), much better than the 0.067% rule, this method can be used. And after removing the difference adjuster, the factors that affect the performance of the flowmeter are also reduced correspondingly, and it is more beneficial to periodically analyze the performance change of the flowmeter.

Table 3 shows the results of an empty balancer flowmeter using a volumetric tube with a standard volume value that is not an integral multiple of 100L. Although the basic error is larger, the flow meter's linearity and repeatability at each flow point are quite good.


2) Make appropriate modifications to the volume tube. Add a short tube to the metering section of the volume tube so that the standard volume value is an integral multiple of 100L (eg 1200L). However, this method is relatively troublesome to implement, but also to increase investment, cleaning the volume tube and re-calibration of water.

3) Since the initial count point of the flowmeter pulse is random, the method of group verification can be adopted, that is, the arithmetic average of the verification data for multiple flow points (3 or more) is taken as one verification data for each flow point. This 3 data calculation flow coefficient and repeatability.

4 Conclusions Removing the flow difference adjuster on the flow meter, it is feasible to use the flow meter factor in the delivery of crude oil. In fact, in countries that implement API standards, most of the flowmeters do not install poor regulators and basically use the flowmeter coefficients to transfer them. Since the flowmeter does not use a differential adjuster, a source of error in the flowmeter is reduced, making the measurement more accurate. It also saves the investment of the changer differential adjuster, and correspondingly saves the maintenance work of adding the lubricant to the dispenser.

Since the flowmeter is shipped from the factory, the instrument adjustment gear is adjusted to 96%, so the relative error of the flowmeter δm is basically around -4.0%. If the adjustment device is not used, the flowmeter cannot meet the basic error specified in the JJG677-1997 procedure. Not more than ±0.2% of the requirement. Therefore, it is suggested that when the regulations are revised, consideration should be given to increasing the regulations regarding the handover of flowmeter coefficients. For example, for flowmeters that perform coefficient transfer measurement, the maximum and minimum error of each flow point of the flowmeter should not exceed 0.3%, and the repeatability of each point should not exceed 0.05% (strictly meeting such requirements in the verification procedures). The basic error does not exceed the limit of ±0.2%.

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