Magnetic flux lines 1) follow the path of least resistance, 2) always form closed loops, and 3) never cross. They also CANNOT be nondestructively measured in a ferrous tube in standard field tests; only the magnetic field in AIR in the coil or around the tube can be measured.
When a magnetizing coil is turned on, the magnetic lines of force will travel in air around the coil (perpendicular to the current direction.) – FIG. 1. In pipe inspection, whenever a ferrous piece of pipe is placed in a magnetizing coil, magnetic lines of force within the coil will travel in the material as the ferrous material offers a lesser path of resistance. With a given test configuration (coil, fill-factor, pipe placement within the coil, etc.), the number of magnetic flux lines that travel in the pipe and not in the air is dependent on the permeability of the pipe and the total amount of pipe (mass) in the coil – FIG. 2.
There are 2 kinds of EMI Wall measurement systems commonly used – 1) An AC-coupled wall system, and 2) A DC-coupled wall system.
AC-coupled – The output of a hall-element is directly proportional to the input. If a hall-element is excited with a 5 volt power supply, the output voltage (OV) will be approximately 2.5 volts. The OV changes when the hall-element is placed in a magnetic field and oriented at an optimum angle to the magnetic field direction produced by the coil. The change in the magnetic field strength inversely affects the OV of the sensor. On AC-coupled systems, a capacitor is commonly used to remove or “zero” the OV of the hall-element – FIG. 3. This limits the AC-coupled systems to detecting only those changes in material cross sectional area or permeability that occur quick enough so as not to be absorbed by the capacitor. In other words, a wall loss can only be detected dynamically and not statically.
DC-coupled – The OV of a Hall-element is read directly. When this occurs, the capacitor used in the AC-coupled method is removed – FIG. 4. This allows for a measurement of the cross sectional area and permeability of a pipe where the change is either quick or slow (dynamically and statically).
AC-coupled Benefits – 1) The base line appears more consistent as the capacitor is constantly “zeroing” the signal that goes to the chart. 2) The visual effects of magnetic flux-leakage at the end of a tube (commonly called the “end-effect”) are minimized for the same reason.
AC-coupled Disadvantages – 1) Small and/or large areas of wall loss that occur “gradually” over an area are not detectable. 2) Extreme wall differences between one tube and another (as with different pipe weights) are not detectable.
DC-coupled Benefits – 1) Ability to detect areas of wall loss that occur “gradually” (such as OD wear in used drill pipe). 2) Ability to detect differences in pipe wall thickness or pipe weights between one joint and another. 3) Ability to detect large differences in pipe permeability thus giving an indication of a grade change.
DC-coupled Disadvantages – 1) Magnetic end-effects are more noticeable. 2) The base line fluctuates as permeability and wall thickness vary – possibly giving an indication of a drifting baseline.
Example: In Fig.5, the flux-density reading (in air) would be less than the flux-density reading in Fig.6 due to thinning (loss of mass) on the ferrous tube (figures are exaggerated and flux density shown is not proportionate or to scale).
The following 2 charts ran on a piece of 3 1/2″ drillpipe. The same buggy head, coils, and cables were used. The same type of EMI system was used – however – one system had an AC-Coupled EMI Wall system and the other had a DC-Coupled EMI Wall system. Note the mid-tube gradual OD wall loss illustrated on the DC-Coupled wall system not shown on the AC-Coupled wall system. This is indicative of a typical joint of used drill pipe that wears more in the center than towards the ends due to the whipping motion in the rotary drilling process. [Large spikes on left indicate upset.]
Essentially, EMI Wall systems are capable of showing a change (either gradual or more abrupt) in the magnetic flux density. This may or may not be due to wall thickness. Example – An N80 grade of pipe with 12-1/2 % wall loss may give the same indication as a P110 grade of pipe that has nominal wall.
Misconception/Myth #2 –
EMI Wall loss systems can detect wall loss in quadrants.
As the flux density changes due to a change in thickness on one side of the pipe, it changes throughout the active area of the magnetizing coil. It does NOT simply change in one spot.
|What actually occurs-as described in Item #3 above- is that the thickness change (usually somewhat abrupt-as a grind or a machined area) creates substantial flux-leakage in the localized area. This causes the EMI Wall loss sensor in that area to respond differently than the other wall loss sensors – see FIG. 7. This could be considered an undesirable aspect of EMI Wall loss measurement as it can interfere with the true flux-density reading and it is redundant as the EMI Flaw detection system is already detecting and locating localized areas of flux-leakage in the transverse system. Example – a 10% or 20% wall loss grind that occurs over a 4″ or 6″ area will usually give a much higher indication than the same wall loss whereas the grind is contoured out over a much larger area and the flux leakage from the grind is less drastic.