Inspection of welds and pipes - how is flaw detection of pipelines carried out?
The welding seam of two pipes is the most unreliable section in pipelines. How long the highway will last depends on its quality.To avoid accidents during the operation of the structure, flaw detection of pipelines is carried out. This is especially important for highways located underground.
The content of the article:
General information
There are several methods for detecting defects in pipe welds:
- magnetic;
- acoustic;
- electric;
- optic.
Their task is to determine the tightness of the joints, the strength of the metal in the seams, whether there are stresses and other parameters that determine the reliability of the pipelines. At the same time, flaw detection methods are almost the same for all types of pipelines: heat, gas, water-, oil pipelines.
All of the methods mentioned above belong to the category of “non-destructive” technologies. That is, flaw detection is carried out directly on the construction site. Pipe joints are not destroyed, which reduces the cost of installation work.
Pipeline flaw detection is based on a scanner called a flaw detector. Each technology has its own operating principle for this equipment. The most effective flaw detectors:
- eddy current;
- ultrasonic;
- magnetic powder;
- capillary.
Details about the methods
Flaw detection of pipelines is a procedure that must be carried out after installation of the pipeline. This will avoid its possible destruction during operation. Flaw detection makes it possible to identify any defects in pipes. The operating principle of scanners used in the diagnostic process is different. Therefore, it is worth first studying each of them in more detail.
Eddy current flaw detectors
The operating principle of the device is based on the creation of eddy currents, which are directed from the outer plane of the pipeline to the inner one through the weld seam. The current passing through a homogeneous metal structure does not change its parameters. If there are defects inside the seam, that is, its homogeneity is impaired, the resistance increases, which reduces the strength of the eddy current.
The flaw detector records and deciphers this decrease, determining the quality of the weld metal, defects and heterogeneity.
Advantages of the method:
- high speed of work;
- low result error;
- low cost of operations.
Minuses:
- the thickness of the seam under study is no more than 2 mm;
- The reliability of the device is low.
Ultrasonic flaw detectors
Ultrasonic flaw detection of pipelines is considered the most used technology. It is carried out using five different methods for detecting defects:
- Pulse echo method.
- Shadowy.
- Echo-mirror.
- Mirror-shadow.
- Delta method.
In the first case, ultrasound is sent by the device through the weld layer. If there are defects inside the metal, the pulse is reflected in the form of an echo. That is, the ultrasound comes back. The device records the return time, which determines the depth of the sink or pore.
In the second case, not only a device is used that sends an ultrasonic signal, but also a reflector. The latter is installed on the opposite side of the welded joint of the pipeline. If the distance between the two parts of the device and the sound travel time are known, then by changing the second parameter (magnification) it is possible to determine where the defect is located and what size it is.
The third version of ultrasonic flaw detection resembles the first.Only the flaw detector is supplied with a reflector, which is installed on the upper surface of the seam, just like the signal emitter. Both elements are located parallel to each other. If a signal arrives at the receiver, it means there is a defect inside the metal that reflected the beam.
The next method is similar to the previous one. The difference is that the signaling device and the reflector are located at an angle of 90° to each other.
The fifth method of ultrasonic flaw detection is rarely used. The reasons are the complexity of setting up the equipment and the lengthy deciphering of the results obtained. It is based on the redirection of ultrasonic energy, the direction of which changes the defect of the weld.
In this case, a transverse beam is supplied, transforming into a longitudinal one. Partial mirror reflection occurs. The reflector catches precisely the longitudinal signal, the strength of which determines the size of the flaw in become.
Magnetic Powder Flaw Detectors
This flaw detection is based on the property of steel to change the magnetic field near areas that differ from the main part in low density. Here it becomes weaker. Cracks, cavities or pores inside the metal have a low density due to the air contained in them.
To detect defects in pipelines, magnetic powder, also known as a ferromagnetic substance, is used. It is poured onto the weld, where electric current is supplied using two coils - magnetizing and additional. Electricity inside the metal creates a magnetic field. If there are defects, it weakens around them. This is the reason why magnetic powder is attracted.
If powder collects on the surface during the examination, this indicates one thing - a flaw in the weld has been found in this area.In-line flaw detection of main pipelines is carried out using this method.
There are two options for testing - dry and wet. In the first case, magnetic powder is used. In the second, a suspension of this powder is an aqueous solution.
In order for the quality of the test carried out using the second option to be high, it is necessary to additionally treat the surface being examined with an intermediate material - technical oil, grease and others.
The advantages of magnetic particle flaw detection include:
- clear result, visible without additional devices;
- low price.
Flaws:
- small depth of examination - up to 1.5 mm;
- can only be used on pipelines assembled from ferromagnetic alloys;
- difficulty in demagnetizing large pipes.
Capillary flaw detectors
This technology is used to detect small surface cracks that are not visible to the naked eye. They confirm that the metal at the junction of two sections of the pipeline is heterogeneous.
The pipeline flaw detection process is carried out as follows:
- An indicator substance, penetrant, is applied to the weld seam. It has the ability to penetrate into the smallest defects under the action of capillary forces. Hence the name of the method.
- The treated surface is cleaned of the applied substance, which has already penetrated deep into the metal.
- A developer in the form of a white powder is applied on top. This may be talc, magnesium oxide, or another substance. It has an important property - adsorption. That is, it can absorb other substances.
- The developer begins to pull penetrant out of the cracks, which forms a pattern of joint flaws on the surface of the white powder.At the same time, the indicator is clearly visible in ultraviolet rays.
This type of pipeline flaw detection is usually used if it is necessary to detect surface flaws in the metal at the junction. It is useless for deep defects. And this is a minus. The main advantage is ease of implementation.
Results
To check pipe joints in main pipelines, it is not necessary to use one flaw detection method. Each area can be examined in one way or another. When choosing the optimal method, you must first evaluate how well the joint corresponds to it. For example, the capillary method is suitable for water pipelines, for oil and gas pipelines only ultrasonic.
Flaw detection of a pipeline is a necessary procedure that allows you to evaluate the quality of the laid pipeline and identify all possible flaws. It is carried out using several methods. It is better to carry out the procedure at the stage of pipeline installation, so that after completion of the work you do not encounter unpleasant “surprises” in the form of leaks or ruptures.
Which method would you prefer to use and why? Write in the comments. Share the article on social networks and save it in bookmarks so as not to lose useful information.
We also recommend watching selected videos on our topic.
Diagnostics of the linear part of main pipelines.
How to perform x-ray inspection of welds in production.
