Abstract: Through the analysis of the water content in natural gas, combined with the domestic long-distance pipeline of natural gas, the equipment is damaged due to the water content in the natural gas, the measurement is not accurate, the operating conditions of the centrifugal compressor are unstable, and the pipeline is blocked. Impact, discuss prevention and resolution measures and apply them in practice.
The water in natural gas is mainly derived from three ways, which exist in the form of free water and water molecules in the gas phase. The first way is that the natural gas that is mined underground will carry water. After purification, 100% of the water can not be removed. The second way is that after the pipeline construction is pressurized and dried, some pipelines are low or have water left in the bend. The pipeline is purged and dried, but there is still a large amount of water in the special area. The longer the pipeline distance, the more complicated the terrain is, the more water remains in the pipeline. The third way is when the pipeline is inspected in individual sections. Because the pipeline is exposed to the air, the humid air enters the pipeline, and when it is put into operation again, the moisture contained in the air remains in the pipeline.
The temperament standard for natural gas in China is clearly defined in GB50251-2003 "Code for Design of Gas Transmission Pipeline". The water dew point of natural gas should be lower than the minimum ambient temperature of conveying gas by 5 °C under the maximum conveying pressure, according to China's commercial natural gas. The relevant standards stipulate that there is no water in the commercial natural gas, but in the actual operating water dew point monitoring of the pipeline, a certain amount of water is contained in the operation. The following five aspects are used to analyze the influence of water content in natural gas on equipment.
2.1. Impact on metering equipment:
First, when the water content flows through the ultrasonic flowmeter, the moisture will gradually accumulate on the inner wall of the flow meter pipe and adhere to the ultrasonic probe, which will affect the emission and reception of the ultrasonic wave, thereby affecting the accuracy of the ultrasonic flowmeter measurement; second, the moisture content will adhere. In the temperature and pressure measurement pressure pipe, the pressure pipe will be blocked in severe cases, thus affecting the accuracy of the ultrasonic flow meter measurement.
2.2, The impact of pressure regulation:
First, due to the water in the gas, the pressure shut-off safety shut-off valve, the monitoring regulator valve failure, the downstream pressure reaches a given value, the monitoring pressure regulating valve can not be turned off, so that the downstream pressure super high safety shut-off valve is shut off, thereby affecting Downstream gas. Second, due to the accumulation of water and ice, the pressure regulating tube is blocked and the valve is turned off by mistake. The pressure regulating impulse pipe valve is a rigid connection. In order to ensure that it is not damaged during operation or leakage at the connection point, there is a certain degree of curvature. The gas contains moisture, which is very easy to collect at these bends, plus the temperature in the north. Lower, and then form an ice block, so that the control system can not timely and accurately monitor the signal, issue a wrong command, resulting in valve malfunction.
2.3, Tthe impact on the compressor:
First, the impact on the compressor inlet electromagnetic flowmeter: (1) due to the pressure difference between the upstream and downstream of the electromagnetic flowmeter, causing corrosion and damage to the electromagnetic; (2) under the condition of the pressure difference between the upstream and the downstream of the electromagnetic flowmeter (3) The electromagnetic flowmeter is blocked or blocked by ice, causing differential pressure changes, affecting the operating conditions of the centrifugal compressor, triggering the surge signal, causing the anti-surge valve to malfunction. Cause the unit to return to stop. Second, the cone filter is blocked by ice, which causes a large amount of water vapor to pass through the filter screen hole, causing interception, causing the natural gas temperature to decrease, depositing moisture on the mesh, and even freezing the frost, causing the cone filter to increase in pressure difference. The compressor intake air volume is reduced, the unit surges and stops; the third is the influence on the compressor body: when the water content is large, it affects the dry gas seal filter of the hot standby unit with relatively large pressure. The outlet pressure is 9 MPa. It must be adjusted to about 4 MPa by the dry gas seal pressure regulating valve to be used as a dry gas sealed medium gas. At this time, there will be a temperature drop of nearly 30 °C. If the temperature falls below the dew point of the natural gas, the water will be deposited on the filter. When it is cold, it will form ice, causing the filter to freeze, causing the filter to lose its function, and let the gas containing a large amount of water enter the dry gas seal static ring. The sealing surface causes foaming and damage to the seal when it is severe, and the natural gas contains water and the internal components of the impeller and shaft are easily damaged when the compressor rotates at a high speed.
2.4. Impact on the valves and instruments of the station:
First, free water in natural gas will adhere to pressure gauges, pressure gauges, thermometers, temperature changes and other instrumentation pressure tubes, affecting data measurement, even under extreme environmental conditions, causing freezing of the instrument's impulse pipeline; second, free water in natural gas It will cause erosion on the sealing surface of the valve and the damaged valve, and at the same time corrode the sealing structure of the valve, causing the seal to fail. Third, the presence of liquid water will cause the throttling at the valve to be intensified. In severe cases, the valve may freeze, the water freezes, and the volume The expansion causes the valves and pipelines to rupture, which affects safe production.
The electromagnetic flowmeter is a high-range differential pressure flow device composed of a standard orifice plate and a multi-parameter differential pressure transmitter (or differential pressure transmitter, temperature transmitter and pressure transmitter), which can measure gas and steam. , liquid and natural gas flow, widely used in petroleum, chemical, metallurgy, electric power, heating, water supply and other fields of process control and measurement.
First, technical parameters:
1. Nominal diameter: 15 mm ≤ DN ≤ 1200mm
2. Nominal pressure: PN ≤ 10MPa
3. Working temperature: -50 °C ≤ t ≤ 550 ° C
4. Turndown ratio: 1:10, 1:15
5. Accuracy: 1.5, 1.0
Second, the structure:
Magnetic circuit system: its role is to produce a uniform DC or AC magnetic field. The DC magnetic circuit is realized by a permanent magnet, which has the advantages of simple structure and less interference by the alternating magnetic field, but it is easy to polarize the electrolyte liquid in the measuring duct, so that the positive electrode is surrounded by negative ions, and the negative electrode is positive ion Surrounding, that is, the polarization phenomenon of the electrode, and causing an increase in internal resistance between the two electrodes, thus seriously affecting the normal operation of the meter. When the diameter of the pipe is large, the permanent magnets are correspondingly large, bulky and uneconomical, so the electromagnetic flowmeter generally adopts an alternating magnetic field and is generated by the excitation of a 50HZ power frequency power source.
Measuring catheter: its function is to let the conductive liquid to be tested pass. In order to make the magnetic flux diverted or short-circuited when the magnetic flux passes through the measuring catheter, the measuring catheter must be made of non-magnetic, low electrical conductivity, low thermal conductivity and mechanical strength. Non-magnetic stainless steel, FRP, high strength can be selected. Plastic, aluminum, etc.
Electrode: Its function is to extract and sense the induced potential signal. The electrodes are typically made of non-magnetically conductive stainless steel and are required to be flush with the liner so that the fluid passes without obstruction. It should be installed in the vertical direction of the pipe to prevent deposits from accumulating on it and affecting the measurement accuracy.
Enclosure: Made of ferromagnetic material, it is the cover of the distribution system excitation coil and isolates the interference of external magnetic field.
Lining: A complete electrical insulation lining on the inside of the measuring tube and on the flange sealing surface. It directly contacts the liquid to be measured, and its function is to increase the corrosion resistance of the measuring catheter and prevent the induced potential from being short-circuited by the metal measuring tube wall. Most of the lining materials are PTFE plastics and ceramics that are resistant to corrosion, high temperature and wear.
Converter: The induced potential signal generated by the liquid flow is very weak and is greatly affected by various interference factors. The function of the converter is to amplify and convert the induced potential signal into a unified standard signal and suppress the main interference signal. Its task is to amplify the induced potential signal Ex detected by the electrode into a unified standard DC signal.
Third, the installation:
1. Check whether the number and size of the throttling device meet the requirements of the pipe installation position before installation.
2. Newly installed piping system, the flowmeter must be installed after flushing and sweeping the pipeline.
3, pay attention to the installation direction "+" should think about the flow rate.
4. The center of the flowmeter should coincide with the centerline of the pipeline. The concentricity error must not exceed the value of 0.015 (1/β-1).
5. When installing the flowmeter in the pipeline, ensure that its end face is perpendicular to the pipe axis, and the verticality error shall not exceed ±1°.
6. The sealing gasket used for clamping the flowmeter shall not protrude into the inner wall of the pipeline after clamping.
7. The installation of the flowmeter must be tight and no leakage is allowed. Therefore, the installation work must be carried out before the pipeline is pressure tested.
8. The pressure guiding pipe should be laid vertically or obliquely, and its inclination should not be less than 1:1. For fluids with higher viscosity, the slope should also increase. When the differential pressure signal transmission distance is greater than 3 meters, the pressure guiding tube should be inclined in sections, and the collector and settler should be respectively at the high and low points.
9. In order to avoid distortion of differential pressure signal transmission, the positive and negative pressure guiding tubes should be laid as close as possible, and anti-freezing measures should be taken in severe cold areas. Electrothermal or steam insulation can be used, but to prevent the superheated vaporization of the measured medium and the generation of gas in the pressure guiding tube to cause a false differential pressure.
10. The length of the straight pipe before and after the installation of the electromagnetic flowmeter is unclear. You can ask the manufacturer or install it according to the 10D after the first 20D.
11. The pressure guiding tubes from the positive and negative pressure take-up ports should be kept parallel under any circumstances.
Adverse effects of electromagnetic plate flowmeter on the long-distance natural gas pipeline
2.5, the impact on the gas pipeline:
First, the presence of free water in the pipeline will affect the efficiency of pipeline transportation. Second, the average temperature in the northern region is below minus 10 °C. The low temperature lays the foundation for the formation of hydrates and facilitates the blockage of natural gas pipelines. Conditions, the free water in the pipeline in the higher pressure, lower temperature, the free water in the pipeline will produce a white crystalline solid, the appearance is similar to ice, ice blocking the pipeline. Third, due to the electrochemical and other forms of corrosion in the free water in the pipeline, it accelerates the aging of the pipeline and affects the safe production. At the same time, the hydrogen sulfide and carbon dioxide gas in the pipeline react with water to form sulphuric acid and hydrocarbon acid, which strongly affects the pipeline. Corrosion, shortening the service life of the pipeline.
1) Reducing the water content in natural gas, making the natural gas in the pipeline meet the transportation standard, is the most direct way to avoid the formation of water, that is, after the natural gas is extracted from the gas well, it needs to undergo sand removal, purification, dehydration, dehydrogenation, etc. After the series process completes the gas treatment, it is piped to each user;
2) Strengthen the station to monitor the filter separator, manifold, valve, compressor and other equipment, timely discharge the sewage, and periodically replace the filter element according to the requirements of the field equipment;
3) In the case where the station allows, it can increase the gas delivery temperature and reduce the gas pressure, and can also avoid the formation of hydrates;
4) Do a good job of winter insulation and increase the laying of electric heating cable for the metering device, pressure regulating 撬, instrument pressure tube, electromagnetic flow meter and other equipment, and strengthen the equipment insulation;
5) Inject hydrate inhibitor into the pipeline to reduce hydrate formation. Commonly used hydrate inhibitors are methanol, ethylene glycol or diethylene glycol. Methanol has a thawing effect on the formed hydrate, while other alcohols do not.
1) Discharge method for the instrument's impulse tube: First, close the instrument root valve, open the vent valve to vent and use hot water to heat the discharge, and secondly, remove the pressure tube when the first step cannot be removed. The blockage is removed indoors, and the pressure pipe needs to be replaced in case of seriousness.
2) Pipeline plugging solution: First, the suppression method, in the cold winter conditions, inject chemical inhibitors (methanol, ethylene glycol, etc.) into the plugging point to remove the blockage; second, stop production and vent, use the venting method to unblock the pipeline, Use the pressure difference between the two sides of the pipeline to discharge the hydrate; the third is to solve the plugging by other methods. By increasing the gas delivery temperature and lowering the gas pressure to remove the blockage, you can use electric heat tracing, electromagnetic heating, steam purging, and plugging. The hot method removes the blockage.
In recent years, with the progress of China's economy and the rapid development of society, natural gas energy has risen rapidly, and its mining and transportation technology is gradually improving, and with the rising demand, it faces new challenges. The first station of natural gas long-distance pipeline is the starting station of a pipeline. It has functions such as emergency cut-off, filtration, metering, splitting, pressurization and pigging. This paper mainly analyzes the causes of water content in natural gas during the actual production operation and the hazards of the equipment in the station, and proposes prevention and treatment methods, which has great guiding significance in actual production.