GB2198665A

GB2198665A - Magnetic particle collection

Info

Publication number: GB2198665A
Application number: GB08619743A
Authority: GB
Inventors: Douglas Cecil Gifford
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1986-08-13
Filing date: 1986-08-13
Publication date: 1988-06-22
Anticipated expiration: 2006-08-13
Also published as: GB8619743D0; GB2198665B

Abstract

A magnetic particle collection apparatus efficiently concentrates the liquid component of a two phase liquid/gaseous flow 27 - such as lubrication oil - near the magnetic particle collector 25 so that a greater proportion of magnetic debris in the flow can be trapped and retained for examination. The apparatus comprises a duct 21 fitted with the collector 25, the duct acting to turn the flow progressively through an angle of at least 90 DEG , and preferably a helical angle of 360 DEG as shown, and the collector 25 being mounted at or near the end of the region of turning flow on its radially outer side within that region of the duct where maximum concentration of the liquid component of the flow occurs as a result of the acceleration of the liquid through the region of turning flow. <IMAGE>

Description

iQ!'#GNETIC PARTIaL: DETECTION The present invention relates to improvements in the field of magnetic particle detection apparatus such as is used in the lubrication systems of prime movers and other machinery, and is concerned with improving detection of magnetic debris particles in two phase liquid/gaseous fluid flow as found particularly in the oil systems of gas turbine engines.

Magnetic chip detectors are used extensively in the oil scavenge lines of gas turbine engines to catch, and hold for subsequent examination, small particles of debris from bearings and gears. The amount and type of debris collected is examined at regular intervals to give an indication of the mechanical health of these items.

Such detectors are usually positioned at points in the scavenge lines where it is convenient for ground staff to remove and inspect them, and usually consist of a small bar magnet which protrudes through the wall of the scavenge pipe into its interior by an amount which is typically 20iso of the pipe diameter from the wall.

The oil flows in the scavenge lines are usually a two phase mixture of lubricant and air, the volume percentage of lubricant in the total flow varying between 0 and 100%. However, the predominant flow pattern is annular (in which the liquid component forms an annuls around a central core of gas and vapour) interspersed with larger "slugs" of liquid which disturb the more regular annular flow. Any debris present is of course carried in the liquid component, but the annular mode of liquid flow causes a problem in that only about 10 of the lubricant passes over the magnetic chip detector. Hence it is desireable to arrange for the liquid component to be concentrated in the region of the detector so that a greater proportion of debris can be captured.One way of achieving this is to site the magnetic chip detector at the small diameter end of a conical cyclone arrangement, cyclone apparatus being well knoçn as a means of separating out and concentrating components of flow having different densities.

Unfortunately, this method involves large pressure losses in the flow through the cylone separator, which are not desirable in the lubrication systems of gas turbine engines.

The present invention accelerates the dual phase flow in such a way as to temporarily concentrate the liquid component in the region of the magnetic chip detector whilst avoiding excessive pressure losses in the process of doing so.

According to the present invention, a magnetic particle detection apparatus for detecting magnetically attractable particles in dual phase liquid/gaseous fluid flow, comprises a duct fitted with a magnetic attractor device, the duct being adapted progressively to turn the flow direction of the dual phase flow through an angle of at least 900 and the magnetic attractor device being mounted in the duct wall at or near the end of the region of turning flow on the radially outer side thereof within that region of the duct where maximum concentration of the liquid component of the dual phase flow occurs as a result of acceleration of the liquid component through the region of turning flow.

In order to increase the efficiency with which the liquid component of the fluid flow is concentrated, it is desirable to maximise the acceleration experienced by the fluid flow by minimising the radius of curvature of the bend. A particularly efficacious construction to facilitate such minimisation is to make the duct crosssection at the bend approximately rectangular or trapeizoidal. If the duct cross-section is trapeizoidal at the bend, the narrow side of the duct should be on the outside of the bend.

An alternative to the use of bends in ducts to achieve flow turning is to incorporate in the duct a cylindrical chamber having an inlet and an outlet which cause the flow to enter and exit the chamber in generally tangential directions, the flow turning through an angle between said inlet and outlet due to interception of the flow by the wall of the cylindrical chamber.

It is preferred that the angle through which the dual phase flow is turned is a helix angle and advantageously is an angle of 3600. In the case of the cylindrical chamber the turning of the flow within the chamber may be assisted by a helical insert therein.

We believe the previous practice of mounting the bar magnet type of magnetic attractor device so that it intrudes substantially into the duct may be in error as regards the present invention and we prefer to mount it in the wall of the duct such that its debris-attracting end does not substantially disturb the flow of liquid therepast. This avoids a scrubbing action by the flow of liquid on the debris-attracting end of the magnetic attractor device.

Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a simple form of the invention in diagrammatic manner; Figures 2a and 2b show an alternative more elaborate form of the invention, Figure 2b being a view on the arrow bin Figure 2a; Figure 3 shows a form of the invention which is particularly easy to manufacture; and Figures 4A and 4B show a modified version of the device shown in Figure 3.

Referring now to Figure 1, there is shown a magnetic debris detector apparatus 1 in which a circular crosssection pipe 2 has a 1800 bend 3 between its inlet and outlet ends 5 and 7 respectively. This pipe 2 is incorporated as part of a lubricating oil scavenge line in a gas turbine engine (not shown). The oil is aerated by the hard duty to which the oil is subjected in lubricating the various parts of the engine.

Consequently the fluid flow along the horizontal inlet portion 5 of the pipe comprises a dual phase oil/air flow 9 in which the oil component forms a liquid annulus 11 around a central gaseous core 13 of air and oil mist or vapour, the oil tending to flow along the inside surface of the pipe due to surface tension effects. The ratio of oil to air is about 1 to 10 in the present case, although it can vary between 100% oil and 100% air under certain circumstances.

A magnetic attractor device 15, commonly knows as a magnetic chip detector, comprises a bar type of magnet which is mounted in the wall of the pipe so that its debris-attracting end 17 protrudes significantly into the interior of the pipe 2. The position which the detector 15 occupies with aspect to the bend 3 is important. This is because the bend causes the fluid flow to experience a radial acceleration with the result that as the two phases of the flow go round the bend, the denser liquid phase is flung as shown to the outside radius of the bend, thereby temporarily destroying the liquid annular flow and concentratins the oil on one side of the pipe. Some concentration of debris particles within the oil flow near the wall of the pipe also occurs. We have found that the area 19 of greatest concentration of oil occurs at or shortly after the end of the bend. Since maximum concentration of oil implies maximum concentration of any debris particles carried along with the oil, the magnetic chip detector 15 is positioned at or near the end of the bend in the area 19 on the outside of the bend in order to catch as many niagneticaily susceptible particles as possible.

Although Figure 1 illustrates a 1800 bend in the pipe 2, the oil separationlconcentration effect will also occur to a lesser extent with a 900 bend, whilst a continuous 3600 bend will not only concentrate the oil on one side of the pipe, but will also more efficiently stratify debris particles near the pipe wall within the oil flow, ensuring that a greater proportion of debris particles actually impinge on the magnetic chip detector 15 or pass very near it. A 360 bend, which must of course be a helical arrangement to prevent the pipe fouling itself, is also preferred from the point of view of ready incorporation of the pipe/detector assembly in an existing run of straight pipe.

The degree to which the oil and the particles within it are concentrated at the outside of the pipe bend depends both upon the amount of time for which the radial acceleration is experienced, and also upon the magnitude of the acceleration. It is desirable to maximise the acceleration and its effects and Figures 2a and 2b show a particularly effective form of the invention from this standpoint.

In Figure 2 we see a magnetic debris detector apparatus comprising a pipe 21 having a very tight 3600 bend 23 in it and fitted with a magnetic chip detector 25 at a point very near the end of the bend with respect to the direction of flow 27. The pipe 21 is of normal circular section A-A at its inlet end, but over a length L1 transitions to a rectangular (actually square) crosssection B-B. Over a further length L2 it continues to change to attain a trapeizoidal shape shots at section C-C which is at the beginning of the bend. The trapeizoidal section C-C is maintained constant around the 360 bend, which finishes just before the position of section D-D.Then, over the distances L3 and L4, the trapeizoidal section transitions back to square and round sections respectively.

A generally rectangular type of pipe cross-section is employed for the bend 23 because the bend radius can be more satisfactorily minimised with such a section, rather than a circular section. Thus to take an exaT;ple, for a cylindrical pipe of 15 mm diameter, a square section of equivalent cross-sectional area would be 13.3 mm on a side and this by fabrication can be turned through a radius of approximately 13.3 mm to the outside of the bend. This is about half the radius that could be achieved by bending, the cylindrical pipe through the maximum radius that can be achieved without significant distortion, and of course results in twice the radial acceleration with a corresponding increase in efficiency of separation of flow components.

For the configuration shown in Figure 1, there is only a small amount of stratification of the debris particles near the wall of the pipe on the outside of the bend. The embodiment of Figure 2 improves on this separation of the particles by virtue of the tighter bend and its greater length, and also improves on presentation of the particles to the magnetic field of the magnetic chip detector 25 by chain~ the pipe cross-section.Since the particle separation is related both to the strength of the local "gravity field? and to the time spent in it, extended time in the acceleration zone caused by the full 3600 bend 23 leads to greater particle separation towards the end of the outside of the bend, and the separated particles are more effectively channeled near the magnetic chip detector 25 by the concentration effect of the narrow side of the trapeizium shape, which is on the outside of the bend. Note that in order to minimise drag losses in the fluid flow, the internal corners of the square and trapeizoidal pipe sections are radiused.

A further feature enhancing the effectiveness of the Figure 2 embodiment is the magnetic chip detector 25 itself, best seen in section D-D and Figure 2a.

As mentioned already, this is installed on the outside of the bend very near; but just after, the end of the bend 23. Note that there is some latitude in the positioning of the magnetic chip detector with respect to the bend, but the teaching given in respect of Figure 1 concerning the zone of maximum concentration of the liquid flow should be borne in mind. Due to tightness and length of the bend 23, the debris particles are for the most part being transported by the oil quite close to the outside wall of the bend when they encounter the magnetic chip detector 25.Consequently, there is no need to cause the debris capturing end 29 to protrude very far into the duct and in fact we believe that it is desirable in this embodiment to keep the end 29 almost flush with the wall as shown so as to reduce the possibility of large particles being only temporarily captured and then washed off the end 29 by turbulence in the main stream this could happen in certain conditions of flow in the pipe 21, in which intermittent larger "slugs" of oil disturb the annular flow pattern in the entry to the apparatus of Figure 2.

The circular section bar-magnet detector 25 is held in a plug 31 which is screwed into a seating 33 in the pipe wall.

The plug and seating structure 31,33 for holding the magnetic detector 25 does not form part of the present invention and is therefore shown in simplified form. In practice, the detector 25 is mounted in a bayonet-type plug device, the seating being provided with a self-sealing valve which operates when the bayonet plug is withdrawn in order to prevent leakage of oil from the pipe. Such devices are well known in the aero industry and will therefore not be further described.

By virtue of the funnelling effect of the trapeizoidal cross-section and the concentration and stratification of debris particles near the outside wall of bend 23, the majority of the particles pass over or near the detector 25 and are captured by the concentrated magnetic-field there.

By virtue of the above arrangements a greater proportion of magnetic particles in the flow can be captured than is possible with previous non-cyclone devices.

Although in Figure 2, the head loss between inlet and outlet of the apparatus due to the tight bend and the extra wall periphery of the trapeizoidal pipe sections will be significantly greater than in a straight round section length of pipe, it will be significantly less than in a cyclone separator apparatus because it avoids the more major head losses associated with the constriction of the vortex cone and subsequent reverse flow in the inner vortex. Consequently the expected head loss for the apparatus of Figure 2 would be less than half that for a cyclone separator.

Turning now to Figure 3, a version of the invention is sketched in perspective. This version is simple to manufacture. It consists of a cylindrical chamber 40 fabricated from suitable sheet or extruded tubing material such as stainless steel, and having rectangular section inlet and outlet ducts 43,45 whose sides 47,49 respectively join the wall of cylinder 40 tangentially. Connection between the inlet and outlet ducts 43,45 and the interior of the cylinder 40 is made through cutouts 51,53 in the wall of the cylinder. It is assumed that the transition from circular cross-section pipe to rectangular section and back again occurs beyond the boundaries of the figure. The position of the magnetic chip detector is decided on the same principles as mentioned previously and is indicated at 55.

The oil/air mixture enters from inlet duct 43, is helically swirled round within cylinder 40 due to its momentum at entry, its contact with the curved wall of the cylinder, and an assumed pressure gradient between inlet and outlet; it exits through outlet duct 45.

As in the previous embodiments the oil and debris is concentrated in the radialy outer side of the swirling flow and is intercepted by the magnetic chip detector fixture at 55.

Figures 4A and 4B show a further embodiment of the invention in which Figure 4A is an end view on a cylindrical chamber 60, with inlet and outlet ducts 61 and 63, and magnetic chip detector fixture 64 these components being of similar construction to those described in connection with Figure 3. Figure 4B is a view on section line B-B in Figure 4A showing the interior of the cylinder 60, the components within the cylinder however, not being sectioned.

It will be seen that the embodiment of Figure 4 differs from that of Figure 3 in additionally comprising a helix 65 made from a suitable sheet material such as P.T.F.E. or stainless steel, the helix 65 being fixedly mounted on a central cylindrical rod 67 of much smaller diameter than the cylinder 60.

The rod 67 is fixed in the ends of the cylinder 60 as shown. The helix 65 is in fact slightly modified from a perfect helical form in order that its extremity 69 is better aligned with the direction of the oil/air flow entering the cylinder 60, this extremity 69 being positioned to coincide with the centreline of the inlet duct 61. A different configuration and disposition of the helix 65 within cylinder 60 may be more advantageous and could be determined by experiment. For example, a single turn of the helical form could extend from end to end of the cylinder rather than over only the mid-part of its length as shown in Figure 4B.

The purpose of the helix 65 is to stabilise the swirling motion of the oil/air mixture within the cylinder 60 and guide it positively from the inlet to the outlet, this being particularly desire able where the pressure gradient across the device may not be of a suitable value to aid in establishing the desired helical swirling flow within the cylinder 60.

0 Although a full 360 turning angle for the flow turning through the bend of the pipe 21 (Figure 2) or the cylindrical chambers 40 and 60 (Figures 3 and 4) is judged desirable from the point of view of achieving good stratification of debris particles near the wall of the pipe or chamber without overmuch loss of energy from the flow due to drag in the flow at the liquid component over the wall surface, helix angles of appreciably more than 3600 are of course possible, and helical or non-helical turning angles of appreciably less than 3600 may be appropriate in some circumstances.

Claims

Claims:

1. A magnetic particle detection apparatus for detecting r magnetically attractable p#icles in dual phase liquid/ gaseous flow, comprising a duct fitted with a magnetic attractor device, the duct being adapted progressively to turn the flow direction of the dual phase flow through 0 an angle of at least 90 and the magnetic attractor device being mounted in the duct at or near the end of the region of turning flow on the radially outer side thereof within that region of the duct where maximum concentration of the liquid component of the dual phase flow occurs as a result of the acceleration of the liquid component through the region of turning flow.

2. A magnetic particle detection apparatus according to claim 1 in which the turning of the dual phase flow is 0 caused by the duct having at least a 90 bend therein.

3. A magnetic particle detection apparatus according to claim 2 in which the duct cross-section at the bend is approximately rectangular.

4. A magnetic particle detection apparatus according to claim 2 in which the duct cross-section at the bend is approximately trapeizoidal, the narrow side of the duct being on the outside of the bend.

5. A magnetic particle detection apparatus according to Claim 1 in which the turning of the dual phase flow is due to the incorporation in the duct of a cylindrical chamber having an inlet and an outlet which cause the flow to enter and exit the chamber in generally tangential directions, the flow turning through an angle between said inlet and outlet due to interception of the flow by the wall of the cylindrical chamber.

6. A magnetic particle detection apparatus according to any one of claims 1 to 5 in which the angle through which the dual phase flow is turned is a helix angle.

7. A magnetic particle detection apparatus according to claim 6 in which the angle through which the dual phase flow is turned is 3600.

8. A magnetic particle detection apparatus according to Claim 5 in which helical turning of the flow within the cylindrical chamber is assisted by a helical insert therein.

9. A magnetic particle detection apparatus according to any one of claims 1 to 7 in which the magretic attractor device is mounted in the duct such that its debris-attracting end does not substantially disturb the flow of liquid therepast.

10. A magnetic particle detection apparatus substantially as described in the present specification with reference to and as illustrated by any one of Figures 1 to 4 of the accompanying drawings.