Selecting the right inspection for your tools requires some basic knowledge on the techniques in each Method. Non-destructive testing is divided into various methods, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of Non-destructive testing. Use the table below to choose the inspection technique right for your tools.
Near Surface Cracks
Ferrous tools only
MPI OD transverse Dry
MPI OD Transverse Wet
MPI OD Bi-directional Dry
MPI OD Bi-directional wet
LPI OD Full length
LPI- Liquid Penetrant Inspection MPI - Magnetic Particle Inspection
While Dry Magnetic Particle is the better for finding subsurface discontinuities, it does tend to be less sensitive than Wet Magnetic Particle. Sensitivity in Magnetic Particle inspection breaks down as the smallest findable defect. Currently both are widely accepted in offshore drilling tools. Magnetic Particle inspection is also only performed on Ferromagnetic material. Non-magnetic tools will usually require Liquid Penetrant inspection. Visual Inspection is usually combined with another type of non-destructive testing method. If your tool hasnít been ran since it was last inspected, some companies will agree to accept a visual inspection only. Your company shall outline this in a written practice and be reviewed before the inspection takes place.
The picture illustrates a circular field used to detect longitudinal defects. This technique requires a capacitor discharge unit with a central conductor to use the wet technique, or simply an AC Yoke for the dry technique. The picture below illustrates a longitudinal field using a DC Coil to detect transverse defects. Cracks and discontinuities are most likely found perpendicular to the magnetic field.
Demagnetization is not a technique of NDT however it is often required after Magnetic Particle Inspection is performed. Tools with a magnetic field left in them can cause a catastrophic failure and it is usually recommended that residual magnetism left in the tool post inspection be less than Ī 3 Gauss.