Boiler drainage pipes are laid by the installation unit according to the site conditions, and most of them are laid along the boiler. The leakage of this kind of pipeline has the following situations: the welding condition on the back of the pipeline laying joint is poor, and there are many welding defects, which leads to leakage; There are many leaks at the joint between the pipeline and the valve, mostly because the pipeline is not slotted, the counterpart is not concentric, offset and strongly butted. The leakage of butt joint or welding stop line between header pipe joint and pipeline is mainly due to the fact that the pipeline is fixed on the steel frame, the header expands with the boiler, and the joint fatigue is caused by frequent start and stop of the boiler; The pipe burst due to internal and external corrosion thinning, which is mainly caused by poor internal drainage and external rain corrosion. For this kind of leakage, we can check the boiler drainage pipe by spectrum and thickness measurement, replace the thinned pipe, and weld all the installation joints again for nondestructive testing. Re-adjust the pipeline with poor expansion.
The desuperheating water pipes of superheaters and reheaters will also leak, and there are the following situations: the desuperheating water flow orifice plate leaks, because the original desuperheating water flow orifice plate of the boiler is flange-type, the layout is compact, and the flow and temperature of each branch pipe are unequal; Pipeline leakage is mostly due to the fact that desuperheating water pipes are generally laid side by side, and the gap between pipes is very small or even no gap, which leads to wear and leakage due to vibration in operation; The leakage caused by medium scouring and pipe wall thinning mainly occurs at the elbow; Pipeline weld leakage is mainly caused by lack of groove and many welding defects. In order to solve the above problems, the following measures can be taken: (1) change the flange flow orifice to welding type, and properly distance it to facilitate maintenance and operation; Check and measure the thickness of the desuperheating water pipe, replace the pipe with thin wall, and re-weld all joints without groove; Arrange and fix the piping system reasonably to avoid friction, and take rain-proof measures for heat preservation to avoid external corrosion.
Because most of the temperature sleeves of the boiler main and reheat steam systems and water supply systems are threaded, with the increase of start-stop times after operation, the medium flow in the tubes will cause vibration, which will lead to leakage at the threaded part of the temperature sleeve. It is necessary to replace the temperature sleeve during welding repair or unit mediation, which will bring certain threats to safe and economic operation. The treatment measure is to change the threaded temperature measuring sleeve into the welded temperature measuring sleeve through the overhaul of the unit.
This paper analyzes the leak detection methods of oil pipelines at home and abroad, and probes into the anti-theft monitoring methods of oil pipelines in oil fields. Aiming at the problem of oil pipeline anti-theft monitoring, this paper points out that the key technology of oil pipeline anti-theft monitoring system is pipeline leakage detection and alarm and accurate location of leakage point, and introduces the application of oil pipeline leakage monitoring system in Shengli Oilfield.
Key words:
Leakage monitoring and anti-theft of oil pipeline
Leakage is the main fault of oil pipeline operation. Especially in recent years, oil leakage accidents caused by oil pipeline perforation and corrosion perforation occur frequently, which seriously interferes with normal production and causes huge economic losses. The annual economic loss of Shengli Oilfield alone is as high as tens of millions of yuan. Therefore, the research and application of oil pipeline leakage monitoring system has become an urgent problem in the oil field. Advanced automatic monitoring technology of pipeline leakage can find the leakage in time and take measures quickly, thus greatly reducing the occurrence of oil theft cases and oil leakage losses, which has obvious economic and social benefits.
Present situation of leakage monitoring technology for 1 oil pipeline at home and abroad
The automatic monitoring technology of oil pipeline leakage has been widely used abroad, and the legislation of developed countries such as the United States requires that the pipeline must adopt an effective leakage monitoring system.
There are three main methods for oil pipeline leakage detection: biological method, hardware method and software method.
1. 1 biological method
This is a traditional leak detection method, which mainly uses people or trained animals (dogs) to walk along the pipeline, check the abnormal situation of pipeline accessories, smell the smell released in the pipeline, listen to the sound and so on. This method is direct and accurate, but it has poor real-time performance and consumes a lot of manpower.
1.2 hardware method
There are mainly visual detectors, acoustic detectors, gas detectors and pressure detectors. Vision detectors use temperature sensors, such as multi-sensor cables laid along pipelines, to measure the temperature change at the leak. When the pipeline leaks, when the fluid flows out of the pipeline, the acoustic detector will make a sound. Sound waves propagate at a speed determined by the physical characteristics of the fluid in the pipeline. The acoustic detector detects this wave and finds the leak. For example, the acoustic leak detection system (wavealert) developed by Houston Acoustics Company (ASI) in the United States consists of multiple groups of sensors, decoders and wireless transmitters. The antenna sticks out of the ground to contact the control center. This method is limited by the detection range, and many acoustic sensors must be installed along the pipeline. The gas detector needs to use a portable gas sampler to walk along the pipeline to detect the leaked gas.
1.3 software method
It uses the data of flow, pressure and temperature provided by SCADA system to detect leakage through the methods of flow or pressure change, mass or volume balance, dynamic model and pressure point analysis software. Foreign companies attach great importance to the safe operation of oil pipelines, and pipeline leakage monitoring technology is relatively mature and widely used. After long-term research and development, Shell has produced a new pipeline leak detection system with the trademark ATMOS Pine. ATMOS Pine is designed based on the principle of statistical analysis, and adopts the optimized sequence analysis method (sequence probability ratio test method) to measure the overall behavior changes of pipeline inlet and outlet flow and pressure to detect leakage, which has advanced graphic recognition function. The system can detect the leakage of 1.6kg/s without false alarm.
At present, most long-distance oil pipelines in domestic oil fields are not equipped with automatic leak detection system, and mainly rely on manual inspection along the pipeline and manual reading of pipeline operation data, which is very unfavorable to the safe operation of the pipeline. The research on leakage monitoring technology of long-distance oil pipeline in China has been reported since the 1990s, but it is only in the last two years that it has really made a breakthrough and played a role in production. Department of Automation of Tsinghua University, Institute of Precision Instruments of Tianjin University, Peking University and Petroleum University have all done research in this field. For example, the pipeline operation status and leakage monitoring system (pressure wave method) developed by Tianjin University was installed in the section from Puyang first station to hua county of Zhongluo Line (Zhongyuan-Luoyang), and the leakage detection system developed by Tsinghua University (mainly negative pressure wave method combined with pressure gradient method) was applied in Northeast Pipeline Bureau 1993.
Research on pipeline leakage monitoring technology
Through the analysis and comparison of various pipeline leakage detection technologies at home and abroad, combined with the special requirements of oil pipeline anti-theft monitoring, Shengli Oilfield Oil and Gas Gathering and Transportation Company and other units organized extensive and in-depth investigation and research.
The technical key of anti-theft monitoring system solves two problems: one is the alarm of pipeline leakage detection, and the other is the accurate location of leakage point. In view of these two key technologies, the technical idea adopted by Shengli Oilfield is: pressure wave (negative pressure wave) detection method is the main method, and flow detection method is the auxiliary method.
2. 1 system hardware composition
① Computer system: An industrial control computer is installed at the upstream and downstream ends of the pipeline for data acquisition and software processing.
② Primary instrument: pressure transmitter
temperature sensor
flow sensor
③ Data transmission system: two sets of spread spectrum microwave equipment are used for real-time data transmission.
2.2 Leak detection method
2.2. 1 negative pressure wave method
When a long-distance pipeline leaks, due to the pressure difference between the inside and outside of the pipeline, the pressure at the leak suddenly drops, and the liquid around the leak supplements the leak due to the pressure difference, resulting in negative pressure fluctuation in the pipeline. This process spreads upward and downward from the leakage point, decays exponentially, and gradually tends to calm down. This pressure drop fluctuation is very different from normal pressure fluctuation, and it has an almost vertical front. The pressure sensors at both ends of the pipeline receive the transient pressure information of the pipeline to judge the occurrence of leakage, and calculate the position of the leakage point by measuring the time difference between the instantaneous pressure wave generated during leakage and the propagation speed of the pressure wave in the pipeline. In order to overcome noise interference, wavelet transform or correlation analysis, kullback information measure detection based on the difference of random variables and other methods can be used to process pressure signals. The former Soviet Union began to study and use automatic leak detection technology in 1970s, and the popularization of negative pressure wave leak detection system reduced oil pipeline leakage accidents by 88%. The propagation law of negative pressure wave in pipeline is the same as that of sound wave and water hammer wave, and its speed depends on the elasticity of pipe wall and the compressibility of liquid. According to the domestic measurement, when the average oil temperature is 44℃ and the density is 845kg/m3, the propagation speed of water hammer wave in Daqing crude oil pipeline is1029 m/s. For general crude oil steel pipelines, the velocity of negative pressure wave is about 1000 ~ 1200m/s, and the frequency range is 0.2~20kHz. Negative pressure wave method is very sensitive to sudden leakage, which can be detected 3min 3 minutes. It is suitable for monitoring oil stealing by criminals drilling holes in pipelines, but it is insensitive to slowly increasing corrosion leakage.
Negative pressure wave method has fast response speed and high positioning accuracy. Its positioning formula is
Pressure measuring points p 1 and p2 are set upstream and downstream respectively. When the pipeline leaks at X, leakage occurs.
The negative pressure wave propagates to both sides at a certain speed α, and is detected by sensors p 1 and p2 at T and t+τ0, and the pressure signal is processed, where α is the wave velocity and L is the distance between p 1 and p2.
When there is no leakage, the correlation coefficient φ (τ) remains near a certain value; When leakage occurs, φ (τ) will change, and when τ = τ 0, φ (τ) will reach the maximum value.
Theoretically, the positioning formula is as follows:
Where: x the distance m between the leakage point and the head-end pressure measuring point.
L total length of pipeline m
Propagation velocity m/s of pressure wave in pipeline medium
Time difference s between upstream and downstream pressure sensors receiving pressure waves
As can be seen from the above formula, in order to achieve accurate positioning, it is necessary to accurately calculate the propagation speed a of pressure wave in pipeline medium and the time difference between upstream and downstream pressure sensors receiving pressure wave.
① Determination of propagation velocity of pressure wave in pipeline medium
The propagation speed of pressure wave in pipeline depends on the elasticity of liquid, the density of liquid and the elasticity of pipeline.
Where α is the propagation speed of pressure wave in the pipeline, m/s;
K—— Volume elastic coefficient of liquid, Pa;
Rho-density of liquid, kg/m;
E- pipeline elasticity, pa;
D—— the diameter of the pipeline, m;
E-wall thickness, m;
C —— correction coefficient related to pipeline constraint conditions;
In the formula, the elastic coefficient k and density ρ vary with the temperature of crude oil. Therefore, the influence of temperature on the negative pressure wave velocity must be considered, and the negative pressure wave velocity should be corrected by temperature. On the basis of theoretical calculation and repeated field tests, the wave velocity of negative pressure wave can be accurately determined.
② Determination of time difference of pressure wave
To determine the time difference of pressure waves, it is necessary to capture the inflection points of pressure waves at both ends and adopt effective signal processing methods, such as Kullback information measurement, correlation analysis and wavelet transform.
③ Application of pattern recognition technology
Normal operation of pumps, valves, pouring, etc. will also produce negative pressure waves. In order to eliminate the interference of these negative pressure waves, the system adopts advanced pattern recognition technology. According to the difference between the waveforms of leakage wave and negative pressure wave produced by production operation, the alarm accuracy of the system is improved and the false alarm is reduced after repeated simulation tests on site.
Flow detection
Under the normal operation of the pipeline, the input flow and output flow of the pipeline should be equal, and when leakage occurs, the flow difference will inevitably occur, and the flow of the upstream pumping station will increase and the flow of the downstream pumping station will decrease. However, due to the influence of many factors such as the elasticity of the pipeline itself and the change of fluid properties, the flow change at the first and second ends has a transitional process, so this method is not accurate enough to determine the location of the leakage point. The system has been installed and used in Tarr crude oil pipeline in Germany. The ultrasonic flowmeter is clamped outside the pipeline to measure, and then the total amount in the pipeline is calculated according to the temperature and pressure changes of the pipeline. Once there is an imbalance, it means there is a leak. Japan also stipulates the use of this leak detection system in the commercial law of oil pipeline, which stipulates that it will give an alarm when the leakage exceeds 80L within 30s. The flow difference method is not sensitive enough, but it has high reliability. When it is combined with pressure wave, it can greatly reduce false alarm.
3 Application effect and promotion
After expert acceptance and field test organized by Shengli Oilfield, the main technical indexes of the system are as follows:
① Minimum leakage monitoring sensitivity: 0.7% of total throughput per unit time;
② Alarm point positioning error: ≦ 2% of the measured pipe length;
③ Alarm response time: ≦200 seconds.
The overall level of oil pipeline leakage monitoring and alarm system in Shengli Oilfield is in a leading position in China, with good application effect and promotion scale. At present, the leak detection system has been widely used in the oil pipeline of oil and gas gathering and transportation company in Shengli Oilfield, and has achieved obvious benefits. It has arrested oil thieves and saboteurs many times, effectively cracked down on oil theft crimes, reduced economic losses for the oilfield by more than 6,543,800 yuan every year, and provided a guarantee for the safe operation of the pipeline.
4 conclusion
4. 1 It is effective and reliable to monitor oil pipeline leakage by using negative pressure wave combined with flow;
4.2 Real-time data transmission based on oilfield local area network can improve the response speed of leakage monitoring system and realize automatic leakage monitoring, alarm and location;
4.3 Installing a pipeline leakage monitoring system on the oil pipeline can ensure the safe operation of the pipeline and obviously reduce the occurrence of oil theft accidents, which has obvious social and economic benefits.