Pay attention to management and standards to improve the measurement level of offshore crude oil

The level of crude oil export measurement is related to the fairness and fairness of crude oil trade, and also to the company's business integrity. The dynamic measurement of offshore crude oil is the main measurement method for the transfer of offshore crude oil, and the instruments used are mainly flowmeters. The national standard GB9109.1-88 "General Principles for Dynamic Measurement of Crude Oil" requires that the basic error of the flowmeter used for custody transfer is not more than ±2‰. In order to achieve better measurement accuracy, relevant parties need to work hard on equipment selection, construction and installation, standard use, and periodic verification, in order to fundamentally ensure the accuracy of offshore crude oil external measurement.

I. Management of domestic offshore crude oil transportation measurement methods

At present, the domestic marine terminal generally adopts a volumetric flowmeter, and is regularly calibrated by the national large flowmeter metering station (valid for half a year), but there are certain differences in the specific use management methods.

1. Whether to perform the "online solid liquid" check of the flow meter. The periodic verification of the flowmeter is a management and supervision behavior implemented by the competent national authority, and the verification is a specific measure for implementing the measurement accuracy control within the enterprise. Since the flowmeter is affected by factors such as the physical properties of the measured fluid and the actual operating conditions, the installation and the on-site environment during the actual use, the flowmeter coefficient will change, and the offline calibration is difficult to completely reproduce the external transmission. actual status. Therefore, the flowmeter that has been calibrated should be used to obtain the flowmeter coefficient in the actual oil delivery state and participate in the calculation through the "online solid liquid" calibration to ensure high accuracy.

At present, domestic offshore oilfields mainly use standard volumetric tubes for on-line calibration of external flowmeters. However, in some pipelines, the standard volumetric tubes of the oilfields do not take into account the requirements of “on-line solid-liquid” calibration of external flowmeters. As a result, it is impossible to conduct an "online solid liquid" calibration of the external flowmeter during the measurement of crude oil trade.

2. Water content measurement. The basis for settlement between the two parties is the net amount of crude oil. Therefore, taking representative samples for accurate water measurement is an important part of the quantitative identification. At present, the domestic use of flow meter weighted external transmission terminals generally use automatic sampler to collect samples, and according to the implementation of the AQSIQ "SN / T 0186-1993 import and export commodity weight identification procedures flow meter weight" using distillation Determine the moisture content.

Second, the specific operation mode of offshore measurement

Where conditions permit, the identification of the amount of offshore crude oil shipments is usually identified separately using flow meter weighting and tanker cabin weighting.

The flowmeter weight refers to the continuous measurement of the crude oil being measured by the meter or instrument, and the calculation of the indication and correction coefficient of the instrument or meter to obtain a weighting method of the crude oil quantity.

The weight of the tank of the tanker refers to the measurement of the height (space) and temperature of the crude oil in the tank when the crude oil in the tank is static. The volume of the crude oil is calculated according to the tank capacity table of the tanker and is measured according to the measured temperature. Volume correction, which ultimately results in a weighting method for the amount of crude oil.

The external measurement difference rate is the statistics of the difference between the flow meter weighting result and the cabin measurement result in the external transmission operation, and is one of the parameters reflecting whether the measurement result of the external transmission operation is within the allowable range. Strictly speaking, the absolute true value of the measured value is never known, and the measurement result can only be infinitely close to the true value.

Third, the flowmeter selection construction use standard recommendation

(1) Selection of external flowmeter

1. Requirements for measurement accuracy levels. The level of the flow meter in a metering system should be determined based on the measured object and the intended use of the result. For external flowmeters used for crude oil trade settlement, priority should be given to the use of equipment with good repeatability and linearity and high accuracy.

2. The fluid properties of the medium. When selecting a flowmeter, the characteristics of the crude oil fluid to be concerned include viscosity, relative density, and corrosivity. The performance of different types of flow meters is affected by the degree of fluid properties. Taking viscosity as an example, in the volumetric flowmeter and turbine flowmeter commonly used in domestic offshore oilfield terminals, the accuracy of the volumetric flowmeter is less affected by the viscosity change, and is more suitable for high viscosity fluids.

3. Flow range. The flow rate range of the flowmeter should be determined according to the scale of the offshore production and storage tanks and the number of single external transports. In general, the optimum flow rate is 70% 80% of the upper limit of the flow meter.

4. The external flowmeter shall have the conditions for performing the "online liquid" calibration. Since the flowmeter will wear when the flowmeter is in use, and the temperature, pressure, viscosity and flow rate of the measured liquid will be different each time it is sent out, these changes will directly affect its metering characteristics, so that the actual flowmeter coefficient Changes occur, and the “online liquid” calibration can effectively avoid the influence of the above various factors on the accuracy of the measurement results, including: correction of the influence of physical parameters, correction of the influence of operating conditions, installation conditions and Correction of the actual external environmental impact.

(II) Precautions for installation and use of external transmission measurement

The flowmeter autosampler should pay special attention to the following two points when designing and constructing:

1. The position of the sampling probe. Where conditions permit, the sampling probe should be installed in the vertical section first, and the pumping speed should be higher than the settling speed of water and sediment.

2. A static stirring device should be set up before the sampling point of the autosampler to make the crude oil fully stirred before passing through the sampling point. The extracted crude oil sample is more representative, which is more important for light crude oil.

In addition, the operator should also pay attention to the following during the use of the flowmeter:

1. Fluid flow conditions. Crude oil should maintain a stable fluid state in the external measurement, especially at the beginning and end of the external transportation. Because the pipeline is not filled or the liquid level in the production tank is low, it is easy to cause the pipeline to generate gas through the metering system. Affect the measurement accuracy.

2. The change in temperature. The change of temperature will have a great influence on the flowmeter coefficient. Therefore, in an external transmission terminal that does not have online calibration conditions, the external temperature should be kept as high as possible at the temperature of the flowmeter.

3. Pay attention to changes in pressure, flow, etc. The staff should perform proper operation and regular maintenance of the flowmeter to ensure the accuracy and stability of the instrument.

4. Flowmeters, standard volumetric tubes, temperature and pressure transmitters, and flowmeter computers are periodically verified in strict accordance with relevant national standards.

5. The measurement process of crude oil external transportation shall be carried out in strict accordance with the industry standard "SN/T 0186 Import and Export Commodity Weight Identification Regulation Flowmeter Weighting". With the continuous development of China's marine crude oil industry, the General Administration of Quality Supervision, Inspection and Quarantine is formulating the "Regulations for the Inspection and Appraisal of Offshore Crude Oil" for the measurement of offshore crude oil. After the official announcement, the implementation standards for this work will be further standardized and improved.

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