Remote field testing (RFT) is an electromagnetic method of nondestructive testing whose main
application is finding defects in steel pipes and tubes. RFT may also referred to
as RFET (remote field electromagnetic technique). An RFT probe is moved down the inside
of a pipe and is able to detect inside and outside defects with approximately equal sensitivity
(although it can not discriminate between the two)
The basic RFT probe consists of an exciter coil (also known as a transmit or send coil) which
sends a signal to the detector (or receive coil). The exciter coil is pumped with an AC current and
emits a magnetic field. The field travels outwards from the exciter coil, through the pipe wall, and
along the pipe. The detector is placed inside the pipe two to three pipe diameters away from the
exciter and detects the magnetic field that has travelled back in from the outside of the pipe wall
(for a total of two through-wall transits). In areas of metal loss, the field arrives at the detector
with a faster travel time (greater phase) and greater signal strength (amplitude) due to the
reduced path through the steel. Hence the dominant mechanism of RFT is through-transmission.
EDDY CURRENT TESTING (ETC)
The two major applications of eddy current testing are surface inspection and tubing inspections. Surface inspection is used extensively in the aerospace industry, but also in the petrochemical industry. The technique is very sensitive and can detect tight cracks,
corrosion and wall loss.
Tubing inspection is generally limited to non-ferromagnetic tubing and is known as conventional eddy current testing. Conventional ECT is used for inspecting steam generator tubing in nuclear plants and heat exchangers tubing in power, oil and petrochemical industries. The technique is very sensitive to detect and size pits. Wall loss or corrosion can be detected but sizing is not accurate.
A technique that is often used involves feeding a differential bobbin probe into the individual tube of the heat exchanger. With the differential probe, no
dsignal will be seen on the eddy current instrument as long as no metal thinning is present. When metal thinning is present, a loop will be seen on the impedance plane as one coil of the differential probe passes over the flawed area and a second loop will be produced when the second coil passes over the damage. When the corrosion is on the outside surface of the tube, the depth of corrosion is indicated by a shift in the phase lag. The size of the indication provides an indication of the total extent of the corrosion damage
MAGNETIC FLUX LEAKAGE FOR TUBULAR (MFL)
MFL is a technique used for the inspection of tubes made of ferrous materials. This technique will normally be applied as a fast screening technique if small diameter pitting is expected. Because of limitations to its sizing abilities the technique is not often used as a stand –alone technique.
MFL can also be used on air n cooler tubes. MFL is sensitive to sharp type defects like pits and grooving. In- and external pits can be detected. Depending on probe con guration MFL can distinguish between in-and external defects and can detect gradual wall-loss. For ID/OD discrimination the probe needs to be equipped with a second coil and to detect gradual defects a Hall-effect sensor in the probe is needed.
NEAR FIELD TESTING (NFT)
Near field testing is used for the inspection of fin fan carbon steel tubing in heat exchangers tubes it was first developed as an alternative to magnetic flux leakage (MFL). Specifically designed to detect internal corrosion, erosion, or pitting on the inside of carbon steel tubing. Because the eddy current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tubes.
NFT is a technology that uses two coils — a transmitter and a receiver. Typically the receiver coil is close to the transmitter coil, taking advantage of the transmitter’s near-field zone — that is, the zone where the magnetic field from the transmitter coil induces strong eddy currents, axially and radially, in the tube wall. NFT is also much more sensitive to defects close to structures such as support plates and tubesheets.
The inspection weld in new fabrication or repair according to the reference codes:
ASME B31.1 and B31.3 for new piping
ASME Section VIII for Pressure Vessels
API 650 for tank
API 1104 for pipelines
AWS D1.1 for structure
INTERNAL ROTARY INSPECTION SYSTEM (IRIS)
Internal rotary inspection system (IRIS) 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. The ultrasonic beam allows detection of metal loss from the inside and outside of the tube wall.
With the IRIS (internal rotary inspection system), we can inspect tube by Heat Exchangers, Boilers, Air Coolers and Feedwater Heaters, This technique uses a water driven rotating mirror to direct an ultrasonic beam, which is re directed 90 degrees to the internal tube wall, The water column generated by the probe assembly acts as a water couplant and transmits sound waves from the ultrasonic transducer to the tube wall. The ultrasonic transducer is mounted axially in the tube and the beam is directed toward the mirror, which is placed at a 45-degree angle to the transducer. The mirror is supported on a water driven turbine that spins on an axis parallel with the probe axis. Using special ultrasonic electronics a B-scan pattern of the (cross-sectional pro le) tube wall is presented.