Definition of NDT
Every fixed equipment construction, use NDT as quality control tools. The purpose of NDT activity is to detect defects/discontinuity (above the surface, beneath the surface, and in a material). NDT measure the geometry of the objects and determine the chemical composition of materials. In general, Non Destructive Test is an analysis technique that is carried out to evaluate a material without damaging the function of the test object. Some types of NDT that used in oil and gas industry include:
- Radiography Test
- Magnetic Particle Inspection
- Dye Penetrant Test
- Ultrasonic Flaw Detector
- Vacuum Test
- Holiday Detector
Non Destructive Test (NDT) testing is carried out against several materials such as concrete, steel, metal, iron, and other materials or objects. NDT has several types or methods that each type has advantages, limitations, and also shortcomings. Each NDT type must use different instruments so that the results obtained from each test are also certainly different. Detail of NDT can be read in ASME Boiler and Pressure Vessel Code Section V.
1. Radiography Test (RT)
Radiography is a part of the Non Destructive Test that uses x-rays or gamma rays that can penetrate almost all metals. RT reveal defects or discrepancies behind metal walls or within the material itself.
Radiography Test Principles of Work
Radiation intensity will change depending on the thickness of the material and Density Material. Radiation will produce a different shadow on the Radiography Test film.
Advantages of Radiography Test :
- Able to detect weld metal surface defects (welds) or Raw Materials.
- Can present scattered data
- The defects that appear in film 1: 1
- Can be operated in difficult positions
Deficiency/disadvantages of Radiography Test :
- Required Personnel (people) who are already qualified (Certified Personnel as ASNT requirements)
- Testing costs are more expensive than DPT, UT, and MT
- The danger of X-ray and Gamma Rays
2. Magnetic Particle Inspection / Testing
This test is used to detect defects that are located on the surface or slightly below the surface, on objects that are ferromagnetic (have high magnetic properties).
Detects the formation of new magnetic fields (leakage fields) due to magnetic lines cut off by discontinuity so that they will attract magnetic particles to gather around the leakage field.
Advantages Magnetic Particle Inspection / Testing:
- Easy to do
- Does not require special expertise to operate it
Disadvantages Magnetic Particle Inspection / Testing :
- Use is limited to materials that are ferromagnetic
- The possibility of defects that are not detected due to orientation defects in the direction of the magnetic field lines
3. Dye Penetrant Test
Dye Penetrant is an NDT method to determine the presence or absence of cracks in welds (welds). This test is very easy to do and the implementation is also very short.
The working principle of the Dye Penetrant Test method is to use penetrant fluid by utilizing its ability to pass through discontinuous gaps as well as the work of the developer to recover fluid that is absorbed in cracks, so defects in the material can be detected
Advantages of the Dye Penetrant Test :
- Easy to Apply
- Cheap in financing
- Not affected by the magnetic properties of the material and its chemical composition
- The scope of the examination is quite extensive
Disadvantages of the Dye Penetrant Test :
- Cannot be carried out on porous objects or material powder metallurgy products. This will cause excessive absorption of penetrant fluid so that it can indicate a false defect.
4. Ultrasonic Flaw Detector
Ultrasonic Flaw Detector is the oldest and the most common. Since the 1940s, physical laws governing the propagation of sound waves through solid material have been used to detect hidden cracks, voids, porosity, and other internal discontinuities in metals, composites, plastics, and ceramics.
The Working Principle of Ultrasonic Flaw Detector:
The principal works are to utilize the ultrasonic wave propagation issued by the transducer on the work-piece and then the return wave is captured by the receiver. The received waves can be measured in intensity, the propagation time, or resonance caused so that in general the ultrasonic examination is based on the difference in the intensity of the received wave and the propagation time.
Advantages of Ultrasonic Flaw Detector:
- The permeation depth to detect defects is very accurate compared to other NDT methods
- Only need 1 side of the test specimen
- Displays distance information on the CRT screen
- Simple test specimen preparation
- Can be used in addition to detecting flaws
Disadvantages of Ultrasonic Flaw Detector:
- The surface must be accessible to probes and couplings
- Required skills and training is higher than other methods
- Surface finishing and roughness affect inspection results
- Difficult to inspect thin objects
- Requires reference standards
5. Vacuum Test
Inspector usually used Vacuum test during offline tank inspection to find any leakage at tank bottom plate. Vacuum Test is a test carried out on a path that has been welded (welding seams) to detect a leak or crack. This Vacuum Test is carried out only on welding seams that are found on flat plates (not curled) and not on pipes.
The Principle of Vacuum Test
The basic principle of this Vacuum Test is to detect welding leaks by making the air around the object to be tested into vacuum. This NDT using a media such as a tube of transparent material. Leakage will be detected through a measuring instrument attached or visible directly in the presence of foam/bubbles from liquid soap that appears in the tube. The working principle of the Vacuum Test is the opposite of the working principle of the Air pressure Test.
Advantages of Vacuum Test :
- The power supply is easy to get because it is only compressed air in the compressor engine
Disadvantages of Vacuum Test
- The dimensions of the test instrument determine how long to test the plate
- The shape of the device to be tested is adapted to the existing vacuum
- For testing in places with vertical position and overhead, a little extra energy is needed to hold the weight of the vacuum device used.
- Vacuum tests cannot be carried out on curved plates let alone pipes
6. Holiday Detector
Holiday Detector is a tool that has a function to detect the presence of holes or the majority of a material, such as welding on a pipe that has an uneven surface, causing vacancies, here is the function of holiday detectors to detect these empty gaps (porosity).
Holiday Detector Working Principle
The Holiday Detector Principle conducts an electric current to the material that has been coated either new or old where the tool will sound or give a signal if there is a small hole or pinhole in the coating (either new or old) for a new coating usually in the form of bubbles or porosity.
For older coatings, there is usually a stretch between the coating and the metal. A holiday detector will explain for the application of how many KV voltages is applied to certain coating thickness.
Advantages of Holiday Detector :
- Can know the exact location of the material contained porosity
- Can detect the location of a small point (hole) in the material
Disadvantages of Holiday Detector :
- The checking process that uses brushes takes quite a long time on large materials