Tubular NDT
Encyclopedia
Tubular NDT is the application of various technologies to detect anomalies such as corrosion
and manufacturing defects in metallic tubes. Tubing can be found in such equipment as boiler
s and heat exchanger
s. To carry out an examination "in situ
" (i.e. examination of the tubes in position, where they are installed), a manhole
cover is usually removed to allow a technician access to the tubes. Alternatively, a tube bundle may be removed from a heat-exchanger and transported by forklift
to a maintenance area for easier access.
The usual means of examination is to insert some type of probe into the tubes, one at a time, while data is recorded for later interpretation. The technologies listed below (ECT, RFT, IRIS, and MFL) are all able to detect defects on the outside of the tube from the inside. The tubes must be clean enough to allow passage of the probe: deposits of debris, rust, or scale may have to be removed by chemicals or by high pressure water-jet
. In water-tube boiler
s, the tubes may be examined from the outside when the boiler is shut down, often using ultrasonic testing
.
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
and manufacturing defects in metallic tubes. Tubing can be found in such equipment as boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
s and heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s. To carry out an examination "in situ
In situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...
" (i.e. examination of the tubes in position, where they are installed), a manhole
Maintenance hole
A manhole is the top opening to an underground utility vault used to house an access point for making connections or performing maintenance on underground and buried public utility and other services including sewers, telephone,...
cover is usually removed to allow a technician access to the tubes. Alternatively, a tube bundle may be removed from a heat-exchanger and transported by forklift
Forklift truck
A forklift is a powered industrial truck used to lift and transport materials. The modern forklift was developed in the 1920s by various companies including the transmission manufacturing company Clark and the hoist company Yale & Towne Manufacturing...
to a maintenance area for easier access.
The usual means of examination is to insert some type of probe into the tubes, one at a time, while data is recorded for later interpretation. The technologies listed below (ECT, RFT, IRIS, and MFL) are all able to detect defects on the outside of the tube from the inside. The tubes must be clean enough to allow passage of the probe: deposits of debris, rust, or scale may have to be removed by chemicals or by high pressure water-jet
Water jet cutter
A water jet cutter, also known as a waterjet, is a tool capable of slicing into metal or other materials using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance. The process is essentially the same as water erosion found in nature but greatly accelerated...
. In water-tube boiler
Water-tube boiler
A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes...
s, the tubes may be examined from the outside when the boiler is shut down, often using ultrasonic testing
Ultrasonic inspection
In ultrasonic testing , very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials...
.
Common Methods
- Eddy-current testingEddy-current testingEddy-current testing uses electromagnetic induction to detect flaws in conductive materials. There are several limitations, among them: only conductive materials can be tested, the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of...
(ECT) is commonly used on non-[ferromagnetic] metals and alloys such as copper, brass, and copper nickel. Variations on ECT are partial saturation ECT and magnetic biased ECT, both of which use magnets to allow ECT to operate in lightly ferromagnetic materials or in thin-wall ferromagnetic tubes. - Remote field testingRemote field testingRemote field testing is an electromagnetic method of nondestructive testing whose main application is finding defects in steel pipes and tubes. RFT may also referred to as RFEC or RFET...
(RFT) is used on [ferromagnetic] materials such as carbon steel. - IRIS (Internal rotary inspection system)Internal rotary inspection systemInternal rotary inspection system is an ultrasonic method for the nondestructive testing of pipes and tubes. The IRIS probe is inserted into a tube that is flooded with water, and the probe is pulled out slowly as the data is displayed and recorded...
can be used on all types of metal tubes. IRIS is very slow, but very accurate, and is often used as a back-up to a remote field examination. - Magnetic flux leakageMagnetic flux leakageMagnetic flux leakage is a magnetic method of nondestructive testing that is used to detect corrosion and pitting in steel structures, most commonly pipelines and storage tanks. The basic principle is that a powerful magnet is used to magnetize the steel. At areas where there is corrosion or...
(MFL) testing is used on carbon steel tubes, although it tends to be less accurate than remote field testing.