Author: Deng Xiaojun
The leader of the underground pipeline detection operation team knows that the pipeline positioning method mainly has two methods: peak method and zero value method. In practical work, the peak method is used more often, but the zero value method is rarely used. In the following, I will talk about the wonderful use of the zero value method in underground pipeline detection in the implementation of the Yancheng project in December 2021 to help the new detection team leader solve the problem.
Before using the zero value method to detect underground pipelines, let's first understand the detection principle of metal pipeline detectors.
As we all know, the pipeline detector is mainly composed of two parts: the transmitter and the receiver, the role of the transmitter is to transmit a specific frequency of the current signal, the receiver is to detect the magnetic field signal of this specific frequency. Receiver built-in upper, middle and lower three coils to detect the magnetic field signal, of which the upper and lower two coils are placed horizontally, used to detect the horizontal component of the magnetic field of the underground pipeline, the middle coil is placed vertically, used to detect the vertical component of the magnetic field of the underground pipeline, just above the pipeline, the horizontal coil receives the largest horizontal magnetic flux component, and the signal is displayed on the receiver screen is the largest; the vertical coil receives the smallest vertical magnetic flux component, and the signal is displayed on the receiver screen is the smallest, according to this principle, the peak mode is used to detect, The collected signal is like a mountain peak, which is called the peak method, and the zero value mode is used to detect, and the collected signal is like a valley, which is called the zero value method. Determining the position of the pipe by detecting the horizontal or vertical component of the magnetic field is the detection principle of the pipeline detector. These two signal detection methods can be flexibly selected during the actual detection process to achieve a better detection effect.
Receiver built-in coil structure diagram
Plot of the horizontal and vertical components of the magnetic field
Team leaders who have used the Reddy instrument detection pipeline know that the zero value method of the Reddy instrument has a very intuitive detection indicator arrow. During the detection process, it can indicate the location of the pipeline to the operator, and the operator can quickly find the approximate location of the pipeline according to the pointing of the arrow. For pipeline detection beginners, this method is fast to get started, and you can learn to locate the pipeline in a few minutes after debugging the equipment.
Schematic diagram of zero-value detection
From the above figure, we can see that when using the zero value method to detect, when the detector is located on the left side of the pipeline, the instrument will appear a right-facing indicator arrow; when the detector is located on the right side of the pipeline, the instrument will appear a left-facing indicator arrow; when the detector is directly above the pipeline, the left and right arrow indicators will appear at the same time. The probe can determine the position of the pipeline based on the simultaneous appearance of the left and right arrows. Theoretically, when the double arrow appears at the same time, the position where the receiver stops is the projection position of the underground pipeline on the ground. But in practice, when the receiver is not directly above the pipe but 20-30cm away from the real position of the pipeline, the double arrow indication will also appear at the same time. It can be seen that the error is very large in this way to locate. The reason is that the single vertical coil has poor filtering effect, low signal-to-noise ratio, and is highly susceptible to side interference. Its advantages are: it can be used to quickly track the direction of the pipeline, and the pipeline can also be tracked when the pipeline signal is weak, and the location of the pipeline is more intuitive.
Let's talk about the clever use of this method based on the actual situation of the Yancheng project.
In early December last year, according to the work arrangement of the business department, I came to the Yancheng project to guide the construction of two new groups and help them solve the practical problems encountered in the detection.
One afternoon I went to Century Avenue between the Yanjing Expressway and the Qingnian Road Elevated Highway, where I met Zhang Fan, the head of the detection team, who was exploring a 500mm diameter water supply pipe. I watched from the sidelines for more than half an hour, and seeing that he didn't have any progress in his work, I picked up the device and started various detection methods in peak mode. What side induction method, pressure line method, 45 degree inclination method, lateral scanning method, exhausted my lifelong learning, still found nothing, and finally failed to find the location of the water supply pipe. I didn't think this would work, so I asked Zhang Fan in detail what pipelines were nearby and their approximate locations, and also learned that this water supply pipe was 400-500 meters away from the current location, and there were valve wells and river crossings. After obtaining this information, the brain has a basic detection scheme: prepare to use the zero value mode, starting from the known points at both ends.
We rode the electric car to the east pipeline crossing, he set up the transmitter, and then placed it on the water supply pipe; I took the receiver, set the zero-value detection mode, and began to probe forward 10 meters from the transmitter. As I tracked the signal forward, he carried the transmitter and followed my detection trail closer to the ground.
This has two advantages: one is that the pipeline signal will not be easily lost when tracking; the other is to always maintain a certain transceiver distance detection, without adjusting the receiver's gain according to the distance from the transmitter, so as to ensure that the receiver can directly receive the electromagnetic field signal from the pipeline.
After tracking and detecting a certain distance (generally 50 meters) forward, the two people stop at the same time, and then change the receiver to peak mode, using the 5-wire method to locate and fix the depth. Mark the ground, record the various data of the pipeline point with the mobile phone and draw a map, and then continue to track and detect, and in less than half an hour, the original work stagnation was detected.
Using this method, the detection starts at the valve well at the other end, and the remaining pipeline tracking is quickly completed. The next day, using the zero value method detection mode, he helped him solve the problem of an 800mm water supply in the north of the road in more than an hour, which he dealt with several times and did not solve it well.
The key reasons that cannot be solved are the following points in my analysis:
1, the pipeline buried too deep (3-4 meters), the use of peak method detection, the strongest part of the signal range is too wide, in the range of the amplitude of the hand swing can not see the obvious maximum value, can not determine the pipe position according to the maximum value;
2. The position of the transmitter is too far away from the receiver, and the magnetic field signal on the pipeline is not enough to be transmitted to the ground and detected by the receiver;
3, in the process of tracking, the transmitter has gradually deviated from the pipeline or too far from the pipeline, the magnetic field excited by the transmitter can not be sensed to the pipeline or the signal on the pipe is very weak, resulting in a small current on the pipeline, the generated magnetic field is not enough to conduct to the ground and be detected by the receiver. The end result is that it cannot be detected.
There are many methods of pipeline detection, we must continue to learn and summarize, blindly and mechanically use one or more methods, can not solve the various difficult problems encountered in pipeline detection. The group leaders who are good at learning and summarizing have rapid technological progress, their ability is also very strong, and their income is naturally high; those group leaders who do not love learning and do not want to progress naturally stop their skills, and they often need help from others to solve problems when they encounter problems.