Electricity: measuring and testing – Magnetic – By paramagnetic particles
Reexamination Certificate
1998-04-22
2001-06-12
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
By paramagnetic particles
C324S227000
Reexamination Certificate
active
06246235
ABSTRACT:
TECHNICAL FIELD
This invention relates to the inspection of steel pipes. More particularly it relates to the use of magnetic field generating devices together with magnetic-responsive particles to reveal flaws in ferromagnetic articles such as pipes.
BACKGROUND OF THE INVENTION
Magnetic lines of force in ferromagnetic articles are normally regular and predictable, and this phenomenon has long been used to reveal flaws as part of an inspection process in the production of such articles as pipes. While some techniques have involved the application of an electric current directly to the article to be inspected, in order to generate magnetic lines of force, this practice has been considered dangerous and has other disadvantages, and accordingly the discussion below is restricted to those techniques and configurations wherein magnetic lines of force are induced by the use of electromagnets or other magnetic field inducing devices which need not touch the scrutinized articles. The reader may be interested, however, in the use of eddy current transducers such as employed by Chickering et al in U.S. Pat. No. 4,862,079 for monitoring wear and thickness in control rods of nuclear reactors.
It is known to use large electromagnets to induce magnetic fields in steel pipes and other articles; iron filings or other magnetic responsive particles are spread on the surface of the articles to reveal the patterns of the magnetic lines of force. (Hereafter, in this description, the term pipes may be used interchangeably with “steel pipes and other articles”, as it is clear that magnetic-responsive articles other than steel pipes may be inspected in the same or a similar manner). Placement of the pipe in the magnetic field of an electromagnet causes the lines of magnetic force to pass through the pipe in predictable patterns which, however, are distorted by flaws; the distortions are made clearly visible in disruptions of the patterns of the magnetic particles. Commonly, the particles are colored or coated to reflect ultraviolet light.
To complete the saturation of the test piece with magnetic lines of force, it has been known to use both longitudinal and transverse magnetizing devices. That is, a longitudinal magnetic field is induced typically by electromagnetic coils around the pipe (but not touching it) and the transverse, or orthogonal, field is induced by devices in which the axis between the north and south poles is oriented to be more or less parallel to the pipe.
While solid cores work well in magnetic flux amplifiers, they create hazards such as possible shock and arc burns; they also tend to heat up, reducing their efficiency unless a cooling system is used. If no cooling system is used, the core may burn out prematurely.
Spierer, in U.S. Pat. No. 4,477,776, illustrates several different configurations of magnets around a test piece, including one in FIG. 4 in which the longitudinal field is “vectored” by placing the poles of the magnet in positions other than directly across the test piece. He uses both transverse and longitudinal fields, but does not use magnetic particles, relying instead on magnetic sensing means which generate output signals.
In U.S. Pat. No. 4,694,247, Meili et al rotate and advance the pipe on a bed of dry magnetic particles while the pipe is subjected to magnetization.
Jenks, in U.S. Pat. No. 4,931,731, like Spierer in the patent mentioned above, uses both longitudinal and transverse magnetic fields; unlike Spierer, he reads the flaws by observing disturbances in the resulting patterns of magnetic particles. He employs a particular circuitry for maintaining a predetermined balance between the two magnetic fields. See also Kamimura in Japanese application 56-31557. Both Jenks and Kamimura employ two coils to generate the longitudinal field.
Two coils are also used by Lam in U.S. Pat. No. 5,534,775 to generate the longitudinal field. Lam uses “time-varying” magnetic fields, and passes the current through the two coils in directions which tend to cancel the lines of force which do not contribute to the longitudinal field.
Many of the prior art constructions are bulky and difficult to use in the environment of a production plant. It is desirable to keep the inspection process simple while still thorough.
SUMMARY OF THE INVENTION
Briefly, the inventors designed an apparatus and process for inducing a magnetic field in a pipe to be inspected, and thereafter inspecting the pipe for flaws. The invention is designed to be used primarily on the ends of relatively long pipe.
An end of the pipe is brought to rest just above a magnetizing module designed for the purpose, which is typically about 48 inches long. The magnetizing module and the pipe are encircled by a single coil. In order to induce a magnetic field in the pipe, both the magnetizing module and the coil are energized. The magnetizing module will tend to generate a circular, or transverse, magnetic field concentric with the pipe for approximately the length of the module. The coil will tend to generate a longitudinal magnetic field, running in the direction of the pipe.
Because the two units are energized together, they make a composite, vectored, magnetic field—that is, the composite magnetic field is neither orthogonal to the pipe nor parallel to it, but is helically skewed as a function of differences in the strength of the currents used in the module and the coil. In practice, vectoring skews the magnetic lines of force through angles of up to 70° from the orthogonal, more commonly through 20° on each side of the orthogonal, but can be easily done through 360°.
Various combinations of timing and strength of current may be used to achieve various vectoring effects.
Either at the same time the pipe end is being magnetized, or just before or just after, it is sprayed in a more or less conventional manner with a slurry of colored or UV-sensitive magnetized powder or particles which are capable of forming themselves in the patterns of the magnetic lines of force. It may be sprayed inside as well as outside. The pipe need not be rotated at this stage. The pipe, having been magnetized and containing a residual, composite, vectored magnetic field, is then transported to an inspection booth for inspection of the magnetized end. At the inspection booth, the inspector is able to inspect one end of the pipe with a completely unobstructed view, since there is no structure around at all other than the walls of the booth, which may be used to keep out extraneous light. The pipe is rotated and he looks inside the pipe as well as outside. Flaws which are perpendicular to the pipe axis are more easily seen than with other systems because the magnetic field is vectored.
For inspection of the other end of the pipe, it is transported to a second processing area, where the other end is first magnetized in the same manner as above and then placed in a second inspection booth to receive a similar inspection.
The unique magnetizing module used for the imposing the originally transverse magnetic lines of force has a core of tinned copper electrical cable made of at least 1000 strands; generally, for an industrial pipe mill capable of handling pipe of about 26 to about 50 feet in length and varying in diameter from 4 to 11 inches, a core of tinned copper electrical cable having at least 1400 strands is preferred. The upper limit on the number of strands will be dictated as much by economics as by technical effect; generally more than 2000 strands is not necessary. The cable is manufactured or cut to the desired length for the end segment to be inspected, in our case preferably 48 inches. It is encased in a non-magnetic insulating jacket and sealed in a waterproof compound. While we find the most convenient geometry for the module is a straight unit adapted for placement of the pipe above it, many other configurations could be used so long as the operator is able to utilize the resulting patterns of magnetic lines of force.
REFERENCES:
patent: 3287632 (1966-11-01), Tompkins
patent: 3763423 (1973-10-01), Förster
patent: 4477776 (1
Lowden Donald E.
Sanders Kenneth W.
Krayer William L.
Snow Walter E.
USX Corporation
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