BCS NDT

Phased array inspections are done using a probe with multiple elements that can be individually activated. By applying appropriate delays, a Phased Array is capable of generating an ultrasonic beam, modelling it according to the desired characteristics of aperture, angle and focus (electronic beam formation). The focal law may also be dynamically modified between pulses, thereby allowing effective movement of the scan beam (electronic scanning).

Each element of the Array acts as a punctiform source of ultrasonic waves when receiving the energizing pulse. When multiple elements are energized, the individual waves combine according to the principle of interference and generate a resulting wavefront. A flat wavefront is generated if energizing pulses are synchronous. The introduction of time delays in pulsations alters the interference processes between elementary waves. These time delays can be adjusted to give an adaptive Phased Array that generates beams with the required direction (angle) and focus.

An angled beam is achieved by applying a constant pulse time delay to consecutive Array elements (linear focal law).

Focusing is obtained by adjusting the pulse time delays of the elements to achieve a parabola-shaped sequence, symmetrical to the emitting Array (quadratic focal law). This allows the Phased Array to generate a focused beam at any depth (within a near field)

The resulting data can be combined to form a visual image representing a slice through the part being inspected. Data acquired by the Phased Array during scanning can be video displayed in real-time in all conventional formats used by digital instruments: A-scan, B-scan, C-scan and D-scan.

Origins of the method date back to 1959 but practical field trials were not carried out until 1972. At that time operators began to investigate the feasibility of using mechanised welding techniques to replace the traditional manual Shielded Metal Arc Welding process. When evaluating the welding process, it was determined that parts of the weld bevel were poorly inspected by the traditional X-radiography which relies on defect orientation parallel to the beam to permit detection. To address this shortcoming they initiated a program to develop an automated ultrasonic inspection system.

High-frequency sound is introduced into the part being inspected and if the sound hits a material with a different acoustic impedance (material density and acoustic velocity), some of the sounds will reflect to the sending unit and can be presented on a visual display.  By knowing the speed of the sound in the part and the time required for the sound to return to the sending unit, the distance to the reflector can be calculated.

The most common sound frequencies used in UT are between 1.0 and 10.0 MHz, which are too high to be heard and do not travel through the air.  The lower frequencies have greater penetrating power but less sensitivity, i.e. less ability to detect small indications, while higher frequencies don’t penetrate as deeply but can detect smaller indications.

The two most common types of sound waves used in industrial applications are compression (longitudinal) waves and shear (transverse) waves.  Shear waves travel at approximately half the speed of longitudinal waves.

Sound is introduced into the part using an ultrasonic transducer (“probe”) that converts electrical pulses into mechanical waves, and then converts returning sound back into electric impulses. This signal can be displayed as a visual representation on a digital or LCD screen.  The distance from the transducer to the reflector can be determined. The signal can also be interpreted by operators to determine the type of discontinuity (lack of fusions, slag, lack of root penetration, missed edge, porosity, cracks, segregations).  Because ultrasound will not travel through air a “couplant” is used between the face of the transducer and the surface of the part to allow the sound to propagate into the part.

Internal Rotating Inspection System (IRIS)

IRIS is an ultrasonic technique applied for the inspection of the remaining wall thickness of tubes. It is more accurate than other tube inspections and returns information on the geometry of the defect and remaining wall thickness. It is often used in conjunction with Eddy Current or Remote Field Testing.

Electromagnetic testing is a general test category that includes Eddy Current testing, Alternating Current Field Measurement (ACFM) and Remote Field testing. All of these techniques use the induction of an electric current or magnetic field into a conductive part, then the effects are recorded and evaluated.

Eddy Current Testing (ET) & Eddy Current Array (ECA)

Basic principle: when a coil is excited with an alternating electrical current, it produces a magnetic field around itself. This magnetic field oscillates at the same frequency as the current running through the coil. If the coil approaches a conductive material, currents are induced in the material.

Eddy current makes use of array probes (ECA) to improve the versatility of inspection and coverage area. Typical use of Eddy Current is the inspection of heat exchangers, CRA surfaces, tubing, and welds.

Advantages:

• Minimal surface preparation required;
• Can detect surface breaking and sub-surface flaws;
• There is no need for couplant or direct contact;
• Can replace standard surface methods;
• Fast inspection;
• Recordable with encoder and stored digitally.

Magnetic Flux Leakage (MFL)

Magnetic Flux Leakage detects anomalies in normal flux patterns created by discontinuities in ferromagnetic material saturated by a magnetic field. This technique can be used for piping and tubing inspection, tank floor inspection and other applications.

Advantages:

• used on all ferromagnetic materials;
• used to inspect finned tubes;
• fast inspection.

Remote Field Testing (RFT)

A magnetic field is created by one coil and is received by a second coil after travelling through the material. This technique allows for the detection of internal and external flaws, although it’s not possible to distinguish between them.

Advantages:

• used on ferromagnetic materials;
• detection of internal and external flaws;
• detection of wall loss

Magnetic Particle Testing is applied to detect surface and near-surface discontinuities in ferromagnetic materials. The sensitivity is higher for surface discontinuities and decreases rapidly with depth. Typical flaws that can be detected with MPI are cracks, lack of fusions, cold laps, and laminations.

A magnetic field is induced on the area to be examined with a permanent magnet or an electromagnet. Ferromagnetic particles are applied on the surface. When the magnetic field encounters a discontinuity transverse to the direction of the field, the field is forced out of the part and over the discontinuity causing a leakage field that attracts the ferromagnetic particles.

The magnetic field can be generated with permanent magnets (Direct Current, DC), which penetrate deeper into the part, or electromagnets (Alternating Current, AC).

Either dry or wet magnetic powders may be used for magnetic particle testing and they may be coloured with a visible dye (Color Contrast method) or a fluorescent dye (Fluorescent method).

Magnetization techniques are generally one of the following: prods, longitudinal magnetization, circular magnetization, yokes, and multidirectional magnetization.

Liquid penetrant testing is a method used to detect discontinuities open to the surface of nonporous materials. Typical flaws detected with PT are cracks, lack of fusions, cold laps, laminations and porosities.

Principle: a very low viscosity liquid (dye) is applied to the surface to be examined and some time is allowed for the fluid to penetrate discontinuities open to the surface. The excess penetrant is then removed and the part is dried. At this stage the developer is applied: the developer has the function of both absorbing the penetrant trapped in the discontinuities and creating a background contrast to enhance the visibility of indications.

The dyes can be either visible under white light (Color Contrast) or visible under ultraviolet light (Fluorescent). Either of them can be used with one of the following techniques: water washable, post-emulsifying, solvent removable.

The standard technique is applied between 5°C and 52°C temperature. The method might be used with temperatures outside this range, but the procedure has to be qualified according to the specific reference code.