GB2313913A

GB2313913A - Non-destructive testing for surface flaws on massive electrically-conductive components

Info

Publication number: GB2313913A
Application number: GB9611961A
Authority: GB
Inventors: Michael Keith Bowling
Original assignee: Somerset Technical Laboratories Ltd
Current assignee: Somerset Technical Laboratories Ltd
Priority date: 1996-06-07
Filing date: 1996-06-07
Publication date: 1997-12-10
Anticipated expiration: 2016-06-07
Also published as: GB9611961D0; GB2313913B

Abstract

A method of non-destructive testing for surface flaws on massive, electrically-conductive components in situ in a food processing plant, comprises probing the surface with an electronic flaw-detection probe and analysing its electrical output. A highly stressed and massive component such as a pump block in a dairy homogeniser is tested for surface flaws by the use of a device which detects variations in eddy currents induced when a hand-held probe connected to a matched electronic unit is moved about on the surface of the component.

Description

METHOD AND APPARATUS FOR NON-DESTRUCTIVE TESTING FOR SURFACE FLAWS ON MASSIVE ELECTRICALLY-CONDUCTIVE CONTAINERS This invention relates to the detection of flaws in massive components in process plant, non-limiting examples of which might be centrifugal separators and homogeniser pump cylinder blocks.

Much plant used in the dairy process industry typically contains components which are made from electrically conducting austenitic stainless steel, and are thus weighty, or massive. Such components are made thus to ensure sufficient strength and resistance to corrosion. The mechanical stress in operation is considerable, and the tolerance to flaws reduces as the flaw size increases. Thus early detection of defects is essential.

Existing test methods necessitate removal from the plant, thorough cleaning, penetrant inspection, decontamination, and re-installation. On removal, the components, despite conscientious attention to plant hygiene, are usually covered in clinging, greasy, lactic residues, which may be impossible to remove from any minute cracks present. Liquid penetrant is used for this job, and it will not penetrate or drive out these residues.

Moreover, flaws may be so "closed" in the unstressed condition that they may be impenetrable by present chemical methods.

The result of the above shortcomings is that, at present, only limited faith can be placed in the structural integrity of these components while under stress in service, particularly when they are of some age. The probable results of catastrophic failure are dire An aim of the present invention is to overcome these problems.

It is known that surface and close sub-surface defects will cause variations in response to electrical signals in the region of the defects. Thus if a coil carrying alternating current is placed close to the surface of the component under test, eddy currents induced in the component will affect the current in the coil by mutual induction, detectable by measurement of the coil current.

Examples of this technique are found in US Patents Nos.4095181, 4107605, 4219774, 4445089, 4677379, 4799010 and 5021738. Whilst eddy current probes have been used for example to test rivets in aircraft skins, it had not previously been proposed to develop a corresponding technique for massive components in food processing plants.

The present invention provides a method of non-destructive testing for surface flaws on massive, electrically-conductive components in situ in a food processing plant, comprising probing the surface with an electronic flaw-detection probe and analysing its electrical output.

In order that the invention may be better understood, a preferred embodiment will now be described, by way of example only, with reference to the accompanying drawing, in which: Figure 1 is a perspective view of a centrifugal separator used in a food processing plant; and Figure 2 is a perspective view of an homogeniser pump block.

In a centrifugal separator such as that shown in Figure 1, certain areas are particularly prone to cracking, such as the typical area illustrated in Figure 1. In the homogeniser pump block of Figure 2, in which pump cylinders are created by means of internal bores as shown, the crack-susceptible areas are the inner walls of the bores.

In this embodiment of the invention, an eddy current detector probe (not shown) in hand-held form, and including at least one AC coil, is placed in contact with the massive, electrically-conductive component of food processing plant, in situ and without the need to clean the plant. The probe is placed in contact with the component at the point where defects are suspected. The surrounding area is then scanned with the probe normal to the surface, at a speed not greater than 75mm per second, with the adjacent tracks at 3mm pitch. An internal electronic control circuit, connected by cable to the probe, includes a signal generator which energises the coils in the probe. It also includes detection circuitry for responding to signals from the coils caused by eddy currents induced in the adjacent surface. Defects are presented aurally, by converting the electrical output of the probe to an audible signal. The defects are also indicated visually on a screen integral with the main electronic unit which controls the detector probe. A typical electronic control circuit for the probe is shown in US patent 4107605, which provides an oscilloscope trace as a visual output.

Claims

CLAIMS:

1. A method of non-destructive testing for surface flaws on massive, electricallyconductive components in situ in a food processing plant, comprising probing the surface with an electronic flaw-detection probe and analysing its electrical output.

2. A method according to claim 1, in which the flaw detection probe generates an electric field in the component under test, by means of a movable AC coil.

3. A method according to claim 2, in which the movable AC coil is enabled to detect changes in the strength of the eddy currents resulting from the electric field generated in the component by the movable coil.

4. A method according to claim 3, in which the probe is a hand held probe, used to scan a suspect area of the component under test in a predetermined and controlled fashion.

5. A method according to claim 4, comprising providing a visual and an aural output indicative of the changes in strength of the eddy currents induced in the component under test.

6. A method of testing for surface flaws, substantially as described herein with reference to the accompanying drawings.