Pipelines are susceptible to different kinds of damage from internal and external corrosion, cracking, manufacturing flaws, and other third party damage. The leakage of contents into the pipelines could be harmful to the environment, especially if the pipes are carrying gas or chemicals.
Therefore, there is a need for the millions of miles of pipelines that run across the States underground and above the ground that carries everything from water to crude oil to be inspected periodically for any signs of damage to prevent any environmental disasters.
This inspection of pipelines could range from visual inspection to non-destructive testing. Non-destructive testing (NDT) includes a variety of analysis techniques to assess the properties of the materials without causing any damage to the material in the process. Some of the tools used in performing NDT of pipelines utilize X-rays, magnetic particles and ultrasonic sound waves.
While visual inspections or use of X-rays can be used to inspect pipelines above the ground, pipelines that are buried under ground or are hard to access require the use of devices known as “pigs” to perform the inspections.
Available in various sizes, pigs have about the same diameter as the inside of the pipeline and are carried out by the flow of air or liquid. These devices are put into one of the pipelines and allowed to travel towards the other end of the pipeline to record valuable data that will be transmitted for further analysis of any flaws in the pipeline. These pigs have the potential to travel several hundred kilometers in a single run.
Pigs that use the magnetic flux leakage method use a strong magnetic field that is established in pipelines by magnets, or by using an electrical current to detect damage. The array of sensors housed in the pigs detect the magnetic flux leakage at the damaged areas and provide details about the area of damage, which can then be taken care of by the Engineers.
While most of the pigs use magnetic flux, some utilize ultrasound technology to detect damage present within the pipelines. Pigs that use ultrasound technology have an array of transducers that emit high frequency ultrasound perpendicular to the pipe wall and record the time interval between the reflected sound (echo) from the inner surface and the outer surface to calculate the wall thickness, thereby identifying areas of damage in the pipeline.
Some of the NDT Methods include the following;
1) Dye Penetration Inspection: Dye Penetrant Inspection (DPI), also called Liquid Penetrant Inspection (LPI) or Penetrant Testing (PT), is one of the oldest and simplest NDT methods. Liquid penetrant inspection is used to detect any surface-connected discontinuities such as cracks from fatigue, quenching, and grinding, as well as fractures, porosity, incomplete fusion, and flaws in joints.
Principles: DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. The penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, a developer is applied. The developer helps to draw penetrant out of the flaw where an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used – fluorescent or nonfluorescent (visible).
The test surface is cleaned to remove any dirt, paint, oil, grease, or any loose scale that could either keep penetrant out of a defect or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination. Note that if media blasting is used, it may “work over” small discontinuities in the part, and an etching bath is recommended as a post-blasting treatment.
- Application of Penetrant:
The penetrant is then applied to the surface of the item being tested. The penetrant is allowed “dwell time” to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the penetrant being used, the material being tested, and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature, one must be careful not to apply solvent-based penetrant to a surface that is to be inspected with a water-washable penetrant.
- Excess Penetrant Removal:
The excess penetrant is then removed from the surface. The removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are common choices. Emulsifiers represent the highest sensitivity level and chemically interact with the oily penetrant to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can remove the penetrant from the flaws. If excess penetrant is not properly removed, once the developer is applied, it may leave a background in the developed area that can mask indications or defects. In addition, this may also produce false indications severely hindering your ability to do a proper inspection.
- Application of Developer:
After excess penetrant has been removed a white developer is applied to the sample. Several developer types are available, including non-aqueous wet developer, dry powder, water suspendable, and water-soluble. The choice of the developer is governed by penetrant compatibility (one can’t use a water-soluble or suspendable developer with water-washable penetrant), and by inspection conditions. When using non-aqueous wet developer (NAWD) or dry powder, the sample must be dried prior to application, while soluble and suspendable developers are applied with the part still wet from the previous step. NAWD is commercially available in aerosol spray cans and may employ acetone, isopropyl alcohol, or a propellant that is a combination of the two. Developers should form a semi-transparent, even coating on the surface.
The developer draws penetrant from defects out onto the surface to form a visible indication, commonly known as bleed-out. Any areas that bleed out can indicate the location, orientation, and possible types of defects on the surface. Interpreting the results and characterizing defects from the indications found may require some training and/or experience.
The inspector will use visible light with adequate intensity (100 foot-candles or 1100 lux is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after 10 to 30 minute development time, depends on product kind. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye. It is also good practice to observe indications as they form because the characteristics of the bleed out are a significant part of the interpretation characterization of flaws.
- Post Cleaning:
The test surface is often cleaned after inspection and recording of defects.
The primary advantages and Disadvantages when compared to other NDT methods are:
- High sensitivity (small discontinuities can be detected).
- Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected).
- Rapid inspection of large areas and volumes.
- Suitable for parts with complex shapes.
- Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
- Portable (materials are available in aerosol spray cans)
- Low cost (materials and associated equipment are relatively inexpensive)
- Only surface-breaking defects can be detected.
- Only materials with a relatively nonporous surface can be inspected.
- Pre-cleaning is critical since contaminants can mask defects.
- Metal smearing from machining, grinding, and grit or vapor blasting must be removed.
- The inspector must have direct access to the surface being inspected.
- Surface finish and roughness can affect inspection sensitivity.
- Multiple process operations must be performed and controlled.
- Post cleaning of acceptable parts or materials is required.
- Chemical handling and proper disposal are required.
2) Magnetic Particle Testing:
Magnetic particle inspection (often abbreviated MT or MPI) is a nondestructive inspection method that provides detection of linear flaws located at or near the surface of ferromagnetic materials. It is viewed primarily as a surface examination method.
Magnetic Particle Inspection (MPI) is a very effective method for the location of surface-breaking and slightly sub-surface defects such as cracks, pores, cold lap, lack of sidewall fusion in welds, etc in magnetic materials.
There are many different techniques. The most versatile technique is using a 110v AC hand-held electromagnetic yoke magnet, a white strippable paint as contrast background, and a magnetic “ink” composed of iron powder particles in a liquid carrier base.
The area is magnetised with the yoke magnet. In the event of a surface or slightly sub surface defect being present, the lines of magnetic force will deform around the defect.
The magnetic ink is applied and the iron powder particles will bridge the gap caused by the defect and give a visible indication against the white contrast background.
Magnetic Particle Inspection (MPI) provides very good defect resolution and is used extensively on: Welded fabrications in magnetic material, Castings, Locating fatigue cracks in items subject to cyclical stress.
Magnetic Particle Inspection is performed in four steps:
- Induce a magnetic field in the specimen
- Apply magnetic particles to the specimen’s surface
- View the surface, looking for particle groupings that are caused by defects
- Demagnetize and clean the specimen
Advantages of Magnetic Particle Inspection
- Can find both surface and near sub-surface defects
- Some inspection formats are extremely portable and low cost
- Rapid inspection with immediate results
- Indications are visible to the inspector directly on the specimen surface
- Can detect defects that have been smeared over
- Can inspect parts with irregular shapes (external splines, crankshafts, connecting rods, etc.)
Limitations of Magnetic Particle Inspection
- The specimen must be ferromagnetic (e.g. steel, cast iron)
- Paint thicker than about 0.005″ must be removed before inspection
- Post cleaning and post demagnetization is often necessary
- Maximum depth sensitivity is typically quoted as 0.100″ (deeper under perfect conditions)
- Alignment between magnetic flux and defect is important
3) Ultrasonic testing:
Ultrasonic nondestructive testing, also known as ultrasonic NDT or simply UT, is a method of characterizing the thickness or internal structure of a test piece through the use of high frequency sound waves. The frequencies, or pitch, used for ultrasonic testing are many times higher than the limit of human hearing, most commonly in the range from 500 KHz to 20 MHz.
In industrial applications, ultrasonic testing is widely used on metals, plastics, composites, and ceramics. The only common engineering materials that are not suitable for ultrasonic testing with conventional equipment are wood and paper products. Ultrasonic technology is also widely used in the biomedical field for diagnostic imaging and medical research.
High frequency sound waves are very directional, and they will travel through a medium (like a piece of steel or plastic) until they encounter a boundary with another medium (like air), at which point they reflect back to their source. By analyzing these reflections it is possible to measure the thickness of a test piece, or find evidence of cracks or other hidden internal flaws.
In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing.
There are two methods of receiving the ultrasound waveform, reflection and attenuation.
In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the “sound” is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection.
In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces.
Advantages: Ultrasonic testing is completely nondestructive. The test piece does not have to be cut, sectioned, or exposed to damaging chemicals. Access to only one side is required, unlike measurement with mechanical thickness tools like calipers and micrometers. There are no potential health hazards associated with ultrasonic testing, unlike radiography. When a test has been properly set up, results are highly repeatable and reliable.
Limitation: Ultrasonic flaw detection requires a trained operator who can set up a test with the aid of appropriate reference standards and properly interpret the results. Inspection of some complex geometries may be challenging. Ultrasonic thickness gages must be calibrated with respect to the material being measured, and applications requiring a wide range of thickness measurement or measurement of acoustically diverse materials may require multiple setups. Ultrasonic thickness gages are more expensive than mechanical measurement devices.
Ultrasonic testing of weld: One of the most useful characteristics of ultrasonic testing is its ability to determine the exact position of a discontinuity in a weld. This testing method requires a high level of operator training and competence and is dependant on the establishment and application of suitable testing procedures. This testing method can be used on ferrous and nonferrous materials, is often suited for testing thicker sections accessible from one side only, and can often detect finer lines or plainer defects which may not be as readily detected by radiographic testing.
4) Radiography testing:
Radiographic Testing (RT or X-ray or Gamma ray) is a non-destructive testing (NDT) method that examines the volume of a specimen. Radiography (X-ray) uses X-rays and gamma-rays to produce a radiograph of a specimen, showing any changes in thickness, defects (internal and external), and assembly details to ensure optimum quality in your operation.
RT usually is suitable for testing welded joints that can be accessed from both sides, with the exception of double-wall signal image techniques used on some pipe. Although this is a slow and expensive NDT method, it is a dependable way to detect porosity, inclusions, cracks, and voids in weld interiors.
RT makes use of X-rays or gamma rays. X-rays are produced by an X-ray tube, and gamma rays are produced by a radioactive isotope.
The method is based on the same principle as medical radiography in a hospital. A piece of radiographic film is placed on the remote side of the material under inspection and radiation is then transmitted through from one side of the material to the remote side where the radiographic film is placed.
The radiographic film detects the radiation and measures the various quantities of radiation received over the entire surface of the film. This film is then processed under dark room conditions and the various degrees of radiation received by the film are imaged by the display of different degrees of black and white, this is termed the film density and is viewed on a special light emitting device.
Discontinuities in the material affect the amount of radiation being received by the film through that particular plane of the material. Qualified inspectors can interpret the resultant images and record the location and type of defect present in the material. Radiography can be used on most materials and product forms, e.g. welds, castings, composites etc.
Radiographic testing provides a permanent record in the form of a radiograph and provides a highly sensitive image of the internal structure of the material.
The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object causes exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film, while inclusions of high density, such as tungsten, will appear as light areas.
All discontinuities are detected by viewing the weld shape and variations in the density of the processed film. This permanent film record of weld quality is relatively easy to interpret if personnel are properly trained. Only qualified personnel should conduct radiography and radiographic interpretation because false readings can be expensive and can interfere seriously with productivity, and because invisible X-ray and gamma radiation can be hazardous.