Field application of CMF pure oil mass flowmeter

1 Introduction

Crude oil production measurement is an important basic work of oil enterprises. Its purpose is to provide accurate data for the production and operation of enterprises and scientific and technological oil production. Jiangsu Oilfield Pilot Production Plant No. 1 currently has 35 crude oil transfer and gathering stations of different production scales. In order to accurately assess the completion of the task indicators of key production blocks, scientifically evaluate the implementation effect of technical solutions, and improve the measurement accuracy of crude oil and sewage mixed liquids during production, storage and transportation, two E series CMF pure oil mass flow meters (CMF for short) produced by MicroMo2tion Company of the United States were introduced in Caoqi and Shaqi metering stations in April 2005.

2 Application of CMF mass flow meters

2.1 Main technical parameters of CMF

(1) Temperature measurement range: 125℃; Pressure measurement range: 15MPa;
(2) Maximum pressure loss ≤0.05MPa;
(3) Accuracy: 0.2%; Repeatability: 0.02%;
(4) Power supply voltage: 220AC; Power consumption of the whole machine: 34W.

The CMF300 mass flow meter consists of two parts: the CMF sensor and the LTD3020 flow computer. The CMF sensor adopts a "U"-shaped tube structure, which significantly shortens the travel of the measured medium in the sensor and requires a lower straight pipe section.

2.2 Working principle of CMF

The flow sensor consists of a vibrating tube, an electromagnetic drive coil installed at the end of the vibrating tube, a temperature sensor and other components. It uses the Coriolis force principle to directly measure the three main parameters of the liquid: mass flow rate, comprehensive density and temperature. When the fluid flows through the vibrating tube, it is twisted. The amount of distortion is proportional to the mass flow rate, and the vibration frequency of the vibrating tube is proportional to the density of the fluid. The MVD core processor transmits the detected frequency and temperature signals to the LTD3020 flow computer using the Modbus protocol to process and display the relevant parameters (see Figure 1 for the workflow).

Figure 1 Schematic diagram of metering process

The LTD3020 flow computer was developed by Shandong Land Technology Co., Ltd. After accurately testing and inputting the standard density of pure oil and sewage, it can display 16 parameters such as pure oil, pure water flow, mass water content, etc. in Chinese through program calculation. It has display and operation functions; it can send TCP/IP data to the host computer through Ethernet to monitor the entire system in real time. CMF has the following characteristics:

(1) The LTD3020 flow computer adopts a dual CPU structure to improve operational stability and reliability. It has a 10Mbps Ethernet port and can be directly connected to the enterprise, local area or wide area network.
(2) It uses 64-bit double-precision floating point numbers to improve the calculation accuracy of metering data. The effective data bit is 15 bits. When the total amount reaches one million tons, the counting error is only 12.5mg.
(3) It realizes the online calculation of water content and pure oil content. It uses the GB1885-1998 "Petroleum Special Meter" database, and the density measurement accuracy is <±0.0001g/cm3. [page]

2.3 On-site application of CMF and improvement of process measures

During normal use of CMF, the standard density of pure oil and pure water must be input into the LTD3020 flow computer to accurately measure the output of pure oil and pure water. According to the formula for the water content of the mixed liquid, when CMF is measured on site, the gas content in the mixed liquid is required to be ≤5%, otherwise it will lead to a decrease in the overall density and an increase in the output of pure oil, which is the phenomenon of "gas increasing oil", increasing the measurement error.

The formula for the water content of the mixed liquid is:

Where: ρm—mixed liquid density, g/cm3;
ρw—pure water density, g/cm3;
ρo—pure oil density, g/cm3;
η—volume percentage of water, %.

When CMF was used at Caoqi Station, the water content was significantly lower than the actual value because the saturated natural gas content in the mixed liquid was ≥5%. To this end, the technicians took the following measures:

(1) Control the pressure of the oil entering the station. The production pressure difference of the 60m3 gas and liquid buffer tank in the station was reduced from 0.35MPa to 0.25MPa to improve the gas-liquid separation effect.
(2) Smoothly control the flow of oil entering the station. Install a set of external oil pump frequency conversion control system, use the (4~20)mA differential pressure signal output by the buffer tank liquid level sensor to control the flow of the external oil pump, and have high and low liquid level alarm functions, so that the buffer tank liquid level is changed from manual operation to automatic control. This measure not only significantly reduces the gas-liquid ratio in the mixed liquid, but also avoids fluctuations in gas content.

The flow measurement error of CMF mixed liquid is ≤±0.2%. The crude oil measurement accuracy is determined on site mainly through instrument display and manual testing of water content, ensuring that the measurement error of crude oil in teams and sub-districts is ≤±2%. Table 1 is the comparison data of CMF and manual testing of water content for 5 consecutive days in April 2005 at Caoqi Crude Oil Transfer Station after the above-mentioned process measures were adopted.

From the comparison data, it can be seen that the average difference in water content is <±0.5%, which meets the measurement requirements.

3. Analysis of the application effect of CMF

(1) CMF pure oil mass flowmeter is the main measuring instrument for oilfield teams and sub-districts. It can accurately measure and display the standard volume and mass of pure oil and pure water, as well as parameters such as water content and mixed liquid density. Its function cannot be replaced by any single flow meter. When the gas content exceeds the rated range, process measures must be taken to improve the gas-liquid separation effect, or the input pure oil and pure water standard density must be corrected to ensure that the accuracy meets the measurement requirements.

(2) When installing CMF, support should be added to the outside of the measuring straight pipe section to eliminate the stress that affects the sensor measurement; at the same time, the high-power motor and alternating magnetic field running near the sensor will generate pulse interference and resonance, so it is necessary to stay away from magnetic field interference or install pulse attenuators and shielding facilities.

(3) CMF must be calibrated for zero flow before use and after it is moved to the installation site; for CMF used intermittently, the line must be swept with hot water at about 80℃ for 3 to 5 minutes before each shutdown to prevent scaling and clogging inside the sensor; the sensor and transmitter should be powered by independent power supplies; the LTD flow computer should be kept away from heat sources such as heating pipelines; CMF should be protected from lightning strikes in outdoor work places.

References
1 Cai Wuchang et al. Flow measurement methods and instrument selection. Chemical Industry Press, 2001.
2 Liu Yongyuan, Gong Zhongwei. Application of mass flow meters in crude oil gathering and transportation systems. Petroleum Industry Technical Supervision, 2007, 5.