Abstract: Based on LabVIEW, the data acquisition card PCIï¼6023E is used to continuously collect pressure, flow, temperature and other parameters to monitor the running status of the pipeline. A real-time communication network is formed through data transmission methods such as telephone lines and local area networks. system. This system has been successfully applied to different scenes.
Introduction The pipeline transportation industry is one of the five major transportation industries that are on par with railway, highway, aviation and water transportation. With the continuous development of the pipeline transportation industry, various monitoring technologies have also developed. Pipeline leak monitoring methods include internal detection method and external detection method. Among them, the external detection method includes flow balance method, pressure difference method, chemical method, stress wave method, real-time model method and negative pressure wave method, etc. The characteristics and applications of these methods are different.
In recent years, China's crude oil leakage is mainly caused by man-made damage, which is characterized by short duration and large leakage, which is a sudden accident. Therefore, we use the negative pressure wave detection method. We use NI's virtual instrument development platform LabVIEW to design and continuously improve the crude oil pipeline leak monitoring system based on virtual instruments. This system has been successfully applied to the gathering and transportation pipeline networks such as Shengli Oilfield and Sinopec Pipeline Storage and Transportation Weifang Oil Transportation Company. On the long-distance pipeline, considerable economic benefits and good social benefits have been achieved.
Figure 1 Schematic diagram of the composition of the monitoring system
Figure 2 VISA property modem communication program diagram
Negative pressure wave leakage monitoring and positioning principle The negative pressure wave method is an acoustic method. The so-called pressure wave is actually an acoustic wave propagating in the pipeline. When the pipeline suddenly leaks, it is equivalent to a negative pressure wave that propagates at a certain speed at the leakage point and propagates to both ends of the pipeline. The negative pressure wave contains information about the leak. By detecting the negative pressure wave, the leakage can be detected. According to the time difference of the negative pressure wave generated by the leakage to the two ends of the pipeline, the leakage point is located. This method has a fast response speed and high positioning accuracy, and is a positioning method that has been widely valued.
The location of the leak point can be calculated by the following formula:
(1)
Where a is the propagation speed of the pressure wave in the pipeline, Dt is the time difference between the upstream and downstream sensors receiving the pressure wave, and L is the length of the pipeline.
The premise of this formula is that the pressure wave velocity is constant. For foreign countries, most of them are light oil, which can be transported at room temperature. The density of crude oil does not change much along the pipeline, so the wave velocity can be regarded as a constant. The crude oil in China has the characteristics of high viscosity, high wax content and high freezing point, which must be transported by heating. As a result, the propagation speed of the pressure wave is not a constant and is greatly affected by temperature. Taking into account the elasticity, density and elasticity of the pipe, the pressure wave propagation velocity a should be rewritten as:
(2)
In the formula:
a — Propagation speed of pressure wave in the tube, m / s;
K — volumetric elastic coefficient of liquid, Pa;
r — density of liquid, kg / m3;
E — the elasticity of the pipe, Pa;
D — pipe diameter, m;
e — pipe wall thickness, m;
C1 — correction factor related to pipeline constraints;
t — temperature, ° C.
In addition to considering the influence of temperature, according to the current situation in China, since only one pressure transmitter can be installed in the station, it cannot be judged by the pressure alone whether the negative pressure wave is generated inside or outside the station. The flow change has different properties for leakage and operation in the station. By analyzing the flow change, the source of the negative pressure wave can be identified.
Based on the above considerations, we adopt the method of combined pressure and flow judgment, and install pressure transmitters, temperature transmitters and flow transmitters with appropriate ranges at the outlet and inlet of the pipeline respectively to continuously collect the pressure, temperature and flow of crude oil. Monitor the operation of the pipeline. We chose the PCI-6023E data acquisition card from NI, using its two analog single-ended inputs and a universal 24-bit counter. Considering factors such as function, price and development cycle, this capture card is very suitable for our requirements.
System Design Take the leakage monitoring system of Cangzhou-Hejian long-distance pipeline as an example to introduce the design of the whole system. The composition of the system is shown in Figure 1. It consists of five sets of sub-station devices installed in Linyi, Dezhou, Dongguang, Cangzhou, and Hehe and the central station device in the Cangzhou dispatching room. The devices at each station include pressure sensors, temperature sensors, industrial computers, signal acquisition cards and modems.
Oil pipelines, especially long-distance pipelines, generally have longer distances and longer distances between stations. Taking the Linyi-Cangzhou-Hejian long-distance pipeline as an example, the pipeline has a total length of 260 kilometers. The distance between the stations is between 40 and 50 kilometers. Because Cangzhou Station and Cangzhou Center stand together, the distance between the two computers is only a few hundred meters. The network card is used to form a local area network, and TCP / IP protocol is used for real-time data transmission. The remaining stations and Cangzhou Central Station use internal oil transportation companies. Some microwave communication facilities use high-performance modems for data transmission. In addition to the data stored in the computer of this station, the pressure and temperature of each sub-station are also transmitted to the central station in real time. The central station monitors the running status of each station in real time, and archives the data for management personnel to analyze. A very important feature of the monitoring system is to synthesize the data of each sub-station to determine whether there is a leak, especially the data of the corresponding two stations must be available to locate the leak.
According to the requirements of the monitoring system substation, the collected data is transferred to the central station in real time and the existing communication facilities of the pipeline department. Two remote data transmission methods are used in the system: for substations that have network connections to the central station, computers Form a local area network and use the network TCP / IP protocol for remote data transmission; for the sub-station that communicates through the telephone line, use a modem to connect with the telephone line, and the sub-station transmits the collected data to the central station in real time remotely. The development of network TCP / IP protocol communication and modem communication programs under the LabVIEW platform is more efficient, concise, and faster than other languages.
It is more convenient to use AT command and VISA interface of virtual instrument to develop modem communication program under LabVIEW platform, and the compiled program is more flexible. The AT command establishes the connection between the modems. The computer obtains the modem line status and exchanges data with the serial port VISA feature. Figure 2 shows the computer obtaining the status and reading of the modem line CTS, DCD, DSR, etc. through COM1 (ASRL1 :: INSTR) The block diagram of the bytes in the serial port.
The modem itself has a very good data compression and error correction protocol. For errors that may occur during the data transmission process, according to the characteristics of the system sending data in real time and the amount of data sent each time is not large, the system adopts a simple transmission Data + data checksum + end mark verification method. The actual operation shows that this method can better meet the requirements of the system.
If there are network communication conditions, the communication between the sub-station and the central station can adopt Client / Server structure. In the network, any computer that provides services for other computers is called a Server, and a computer that uses these services is called a Client. The sub station runs the Server program, and the central station runs the Client program. The data collected by the sub station computer is transmitted to the central station computer in real time through the connection established by the Client and the Server program. It is not necessary to know much about the details of the TCP / IP protocol to develop network programs under LabVIEW. Users can write complex programs based on programming references and user manuals.
System software System software includes signal acquisition, leakage judgment, GPS timing, modem communication, leak point location and so on. The signal acquisition part collects data, displays the change of the signal in the trend graph, and saves the data. The leak judgment part uses a variety of pipeline leak detection methods such as negative pressure wave method, flow difference method, and pressure gradient method to analyze and judge the collected working condition data to determine whether there is a leak. In order to unify the computer time in each sub-station, we adopted GPS time calibration technology. The output data structure of the GPS receiver used in the field adopts NMEA-0183 (Ver 2.0) 4800 baud rate, the data level is TTL, and the standard second pulse and data stream containing time information are output every second. We are in the Visual C ++ 6.0 environment The program for reading out the GPS data stream and correcting the computer system time is written below. In order to prevent the drift of the computer's time chip, we call the time correction program every hour in the main program, adopt the method of dynamic call, occupy less memory, and work stable and reliable. The modem communication part is responsible for transmitting the pressure data of the sub-station to the central station. The location of the leak point is a key part of the success of the system. The location of the leak point uses the wavelet analysis and other signal processing techniques provided in the Signal Processing Toolset software package to accurately detect the characteristics of the singular point of the signal, which can be accurately determined in practical applications. The location of the leak.
In the process of programming, the technology of constructing complex multi-task parallel application program on the LabVIEW platform is adopted. For the multi-task scheduling of the platform, dynamic call is used as much as possible to optimize the program structure and reliability.
The user's operation of the system is designed as a menu operation, and each menu is dynamically called through the VI Server method, which not only saves memory, but also makes the program structure clear and modular. There are other PLC systems on site, using DDE (Dynamic Data Exchange) as an interface with other systems.
Conclusion The pipeline leak monitoring and positioning system based on the LabVIEW platform can quickly find leaks and determine the location of the leak. It has been installed and running normally on multiple pipelines. After years of theoretical research and a large number of field tests, we have developed and installed a variety of pipeline leak monitoring systems suitable for different situations. With the further development of the pipeline industry, pipeline leak monitoring and positioning systems will surely have greater application prospects.
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