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What is Nondestructive Testing (NDT)?

 



What is the Nondestructive Testing (NDT) Industry?


Nondestructive testing (NDT) is a very specialized type of work that plays a critical function within the Construction and Manufacturing Industries in Canada. The Industry is comprised of highly specialized technicians who use sophisticated equipment and techniques to evaluate areas that are difficult or impossible to examine using the naked eye.

Technicians inspect boilers, vessels, piping, building materials, fabrication & joining processes and evaluate the integrity of welded structures in new repairs to ensure safe, efficient operation in compliance with stringent government codes and strict industry standards.

NDT Contractors provide inspection services to clients involved in the fabrication, construction and maintenance of:

  • pipe mills, pipelines
  • petroleum refineries, oil and gas bulk plants and terminals
  • pulp and paper mills
  • chemical processing and production facilities
  • mines
  • airplanes
  • nuclear and thermo-power generating plants
  • storage tanks and associated facilities

 

NDT Methods

The most common nondestructive testing methods include Radiography, Ultrasonic, Magnetic Particle, Dye Penetrant and Eddy Current.

 

Radiographic Testing (RT)

The traditional radiography method is the process of making a permanent record on radiographic film of test objects in order to detect defects. It is done by exposing the test object to either electrically generated X-rays or gamma-rays from a radiation source. Radiation from the source passes through the object and is recorded on radiographic film. The film is processed, viewed by qualified technicians who are able to detect defects and anomalies in accordance with applicable codes and standards.

This method of testing is used in a number of applications such as: 

  • weld quality control on pipelines and related facilities, using both internal crawlers and external exposure capabilities;
  • material and weld quality control in shop and plant fabrication, for both new construction and maintenance;
  • investigation of equipment internals without disassembly;
  • corrosion investigation and wall thickness determination in operational piping and vessels using "shadow shots"; and
  • radiography of geological cores.


Recent advancements in digital radiography, which does not require the use of expensive film and developing equipment, have allowed radiography to be considered for additional types of work including high volume manufacturing Quality Assurance.

 

Ultrasonic Testing (UT)

This method uses electrically generated sound waves to penetrate through an object in order to detect defects. Sonic reflection, refraction and absorption are then displayed and recorded on a video screen for interpretation. This process requires significantly more skill and experience in order to provide accurate interpretations.

This method of testing is used in a number of applications such as:

  • quality control testing of welds on piping fabrication and installation;
  • corrosion surveys on in-service pipelines and/or facilities to develop life histories on equipment;
  • quality control on welds and material in vessel, bridge and structural fabrication;
  • wall thickness testing in vessels and facilities; and
  • quality control on welding of continuous track rail lines.


Advancements in UT equipment including phased array and three dimensional (3D) technology have greatly increased the flexibility and adaptability of UT for all types of NDT work.

 

Magnetic Particle Inspection (MPI)

This method of testing detects surface or slightly sub-surface defects in ferromagnetic materials. Magnetic field is induced into the test object using an electromagnet. A detection medium is then applied which contains magnetic particles that will group around defects that cause irregularities in the magnetic field. The detection media can be wet or dry or even fluorescent.

The process is done either in the field, using portable magnetic yokes, or in a shop using a magnetic bench. The bench is more efficient for large volumes of work. It is a very simple and cost-efficient process but is limited to ferromagnetic materials.

It is used in a number of applications such as:

  • quality assurance inspection of piping fabrication in construction and maintenance in pulp mills;
  • inspection of repairs on vessels and fittings;
  • inspection of automotive and industrial equipment components for cracks;
  • quality control inspection on valve bodies and assorted castings;
  • preventative maintenance inspection of gas compressors and related facilities; and
  • inspection of critical structural and equipment bolts for cracking.

 

Liquid Dye Penetrant (LPI)

Liquid penetrant is used to detect surface defects in both ferrous and non-ferrous materials. A penetrating dye is applied to the surface of an object and left for a brief period as capillary action draws it to any surface defects. A developer is then applied and defects can be observed by the technician.

This process is simple, versatile and cost efficient and can be further enhanced by using bright coloured or fluorescent dyes. The only drawback is that it can only detect surface defects.

This type of inspection is used in the same types of applications as magnetic particle testing but on non-ferromagnetic materials as well, such as alloys and stainless steel. Other types of applications include:

  • annual inspection of lifting tackle for forest fire helicopter gear; and
  • quality control of welds on stainless steel components in pipe mills.

 

Eddy Current (EC)

The eddy current testing technique is used on non-magnetic and slightly magnetic materials such as brass, copper and stainless steel for crack detection, material thickness measurements, coating thickness measurements and conductivity measurements for material identification, heat damage detection, case depth determination, saddle wear, pitting, transverse cracking, freeze bulges, splits, dents, and heat treatment monitoring. This makes eddy current a useful tool for detecting corrosion damage and other damage that causes thinning of the material on aircraft skins and in the walls of tubing such as heat exchangers and boiler tubes. The eddy current method is very good for surface and near surface defects, is sensitive to small cracks and other defects and requires very little part preparation. It is limited to use on conductive materials, with the surface accessible to the probe.

Eddy current testing is used in:

  • car plants,
  • refineries,
  • air-conditioning and
  • rivet holes in airplanes.
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