ACFM

Many of the structures, piping, and processing equipment, used in the petrochemical field, require routine inspection. Conventionally, this is done by using Magnetic Particle Testing, Penetrant Testing, and Visual Inspection. Each of these techniques requires the removal of paint and coating or extensive surface preparation, but unfortunately, these techniques can’t tell you the severity of the defects alone. The Alternating Current Field Measurement (ACFM) technique has been developed for the accurate detection and sizing of defects through coatings. To do this you would need to know both the length and depth of the defects. Knowing the dimensions of the defect, along with fracture mechanics, you can determine if the component tested is safe for continued operation.

ACFM saves the cost of buffing and recoating, and due to the mobility of the newer units, makes it an ideal technique. Using rope access methods can also save on the cost of building scaffolding. The results from use of ACFM have been a cost and time savings of about 8 to 65 percent.

View of 2 Cracks in the Clockwise Direction

Start of Cracks = Red Arrows
End of Cracks = Blue Arrows

The ACFM technique is an electromagnetic non-contacting technique. ACFM has been developed to be able to detect and size surface breaking defects in a range of different materials, and through coatings of varying thickness up to 0.250”, and material type. The two components of the magnetic field used in ACFM are Bx and Bz. (See figure 1) The Bx measures the current density across the surface of the material. This in turn measures the depth of the defects. The Bz measures the current density between the probe and the material. The indication length is found by, a response at either end of the defect caused by current generated poles. In order to aid in the interpretation the Bx and the Bz components are plotted against each other when complete loop is produced. This confirms the presence of cracking. This is known as the butterfly plot. (See figure 2).

Figure. 1 Current flow propagation around a defect using ACFM

The ACFM technique was developed for the offshore industry. However, Since the early 1990’s it has been used in many other applications such as roller coaster inspection for theme parks, Bridge inspections, pressure vessel inspection of both transverse and circumferential cracking, coke drum inspection, inspections of duplex and super duplex material, Reactor inspection, distillation tower inspection, Ring joint flanges, high temperature monitoring of cracks, and many other applications. This is due partly because this technique in been utilized from –20° C to 500° C.

The above picture is of the minimum equipment required to perform an

Figure. 2 Function test on a crack with butterfly plot

The ACFM technique has been accepted by major organizations in the oil and gas, petrochemical, nuclear, aerospace, and theme park industries. Some of the companies that have approved ACFM are Lloyds Register, ABS, BV, DNV, and OCB Germanischer.