The natural gas production capacity of Huabei Oilfield is mainly concentrated in the fourth oil production plant, which integrates natural gas production and sales units. In 1985, it began to supply natural gas to the Beijing area. Qing and other cities and counties supply gas. Whether it is internal natural gas energy measurement or external natural gas sales measurement, flow meters have a wide range of practical applications. This paper analyzes the practical application and precautions of flowmeter in natural gas energy measurement from the production and application of the fourth oil production plant of Huabei Oilfield (hereinafter referred to as “our plant”).
1. The actual production characteristics
The natural gas in our plant is mainly condensate gas reservoir gas and oilfield associated gas. After the natural gas is produced, it enters the gas gathering and treatment station through high pressure (16MPa), medium pressure (3.5MPa), and low pressure (0.5MPa) transportation methods for dehydration. , Dehydrocarbon treatment, and the treated dry gas will be exported to Beijing, Tianjin and Langfang for sale.
Natural gas energy measurement in our factory is divided into internal energy measurement and external sales measurement. Internal energy measurement is mainly used for single well gas production measurement, heating furnace, generator and other equipment self-consumption gas, natural gas inflow and outflow, etc., using precession vortex flowmeter and orifice flowmeter, the accuracy is 1.5, The nominal diameter is DN (25～300), the working pressure is between (6.4～0.5) MPa, and the flow range is 500m3/d～1.8 million m3/d. Since the wellhead temperature of some gas wells is as high as 80°C or higher, the flowmeters used for gas production in these gas wells must be of high temperature resistant type.
Orifice flowmeter, turbine flowmeter and precession vortex flowmeter are used for external sales of natural gas, with an accuracy level of 0.5 to 1.0, a nominal diameter of DN (25 to 200), and a working pressure of (2.5 to 0.5) MPa. , the flow range is 300m3/d ~ 500,000m3/d, since the temperature of the external air supply is generally close to the normal temperature level, there is no special requirement for the temperature of the flowmeter.
2. The choice of flowmeter
According to different working conditions, choosing the right flowmeter is very important to improve the measurement accuracy and the service life of the flowmeter. Our factory has a gradual selection and use process in the selection of flowmeters.
Therefore, whether it is internal metering or external metering, orifice flowmeters are used. However, with the continuous expansion of natural gas production capacity in our factory, the demand for natural gas measuring instruments has increased. Due to the high one-time investment and large maintenance of orifice flowmeters, our factory began to try to use precession vortex flowmeters in single wells. . The flowmeter does not have high requirements for gas quality, and is suitable for occasions where the gas production gas conditions of a single well are poor and the requirements for measurement accuracy are not high. Subsequently, the flowmeter was also extended to the measurement of self-consumption gas in the station and the measurement of external gas supply. Due to the poor anti-interference ability of the precession vortex flowmeter during use, sensitivity to airflow disturbance, and abnormal measurement results at small flow rates, measurement disputes often occur. Therefore, according to the requirements of trade settlement metering, the turbine flowmeter is selected to be used. Although the turbine flowmeter has higher requirements on air quality and higher price, it has higher accuracy and is more suitable for the external sales measurement of the treated dry gas.
3. Use and maintenance of flowmeter
(1) Orifice flowmeter
1. Common factors affecting the accuracy of the orifice flowmeter
Since the orifice flowmeter has multiple measurement units, there are many factors affecting its measurement accuracy (such as the processing error of the orifice plate, the installation error, the calculation error of the measurement software, etc.). In addition, under the existing working conditions, because the impurities in the medium have a certain impact corrosion effect on the orifice plate, it is easy to cause the zero point drift of the differential pressure transmitter, especially when the natural gas treatment effect is not ideal, the impact on the measurement bigger. Therefore, the use and maintenance of throttling device and differential pressure transmitter is a key point. Attention should be paid to the following actual operation:
(1) When the natural gas treatment effect is not ideal, dirt will be deposited on the upstream end face of the orifice plate. It will not only reduce the service life of the orifice plate, but also cause a large measurement deviation.
(2) The function of the pressure guiding pipe of the transmitter is to introduce the pressure signal before and after the orifice plate into the differential pressure, and the differential pressure value is measured to participate in the flow calculation. The upstream and downstream pressure guiding pipes will make the differential pressure small (large), resulting in a flow rate. Small (large). In winter, the phenomenon of pressure guiding pipe freezing and blocking is common. If the flow value fluctuates greatly, it is likely that the pressure guiding pipe is filled with liquid or frozen.
(3) Orifice plate rubber ring is deformed. Since the orifice rubber ring is easily deformed under the immersion of clean oil (this situation is particularly prominent in summer), when the natural gas processing unit is out of operation, it is necessary to check the deformation of the rubber ring. Once the orifice plate is loose, it should be replaced immediately. Otherwise, not only will the leakage of the rubber ring cause a large measurement error, but also the orifice plate will fall off and be difficult to remove, and the production must be stopped for maintenance.
(4) When the natural gas is not cleanly treated, the dust and hydrate in it will have a strong erosion and corrosion effect on the orifice plate, which will form pits on the surface of the orifice plate and make the right-angle edges dull. Therefore, the orifice plate should be checked frequently. Replace or the accuracy will be reduced.
(5) The zero point drift of the differential pressure transmitter is not only related to the stability of the instrument itself, but also the liquid in the pressure guiding pipe will also have a great impact. Since the flow rate of the orifice flowmeter and the differential pressure value have a square root relationship, the positive and negative drift of the zero point of the differential pressure transmitter will directly cause the accumulated flow to be too large or too small.
(6) The input of some key parameters in the flow computer is incorrect or the update is not timely. For example, the opening diameter of the orifice plate appears in the form of squares. Since the opening diameter of the orifice plate will change with the season and operating time, the opening diameter of the orifice plate must be measured regularly and updated in the flow computer in time.
The change of natural gas composition not only affects the relative density, but also affects the overcompression coefficient. For a metering system without an online chromatograph, the average value of the natural gas composition for a week is generally input into the flow computer every week when the composition changes little, but when the natural gas composition changes greatly, the natural gas composition must be measured every day. Components are tested and updated.
Post time: 15-03-22