US3695297A

US3695297A - Compensating pressure tank for oil-filled power cables

Info

Publication number: US3695297A
Application number: US146295A
Authority: US
Inventors: Antonio Ferrentino
Original assignee: Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1970-05-23
Filing date: 1971-05-24
Publication date: 1972-10-03
Anticipated expiration: 1989-10-03
Also published as: DE2125661A1; FR2090248A1; GB1284060A; FR2090248B1

Abstract

A tank contains a first chamber having a bellows-type elastic wall and has a normally open valve through which the chamber is connected to an oil-filled cable and when the chamber wall contracts sufficiently the valve is engaged by the wall and closed. The first chamber is surrounded by a fluid filled second chamber which is connected to a source of fluid through a similar valve which is normally open but which is closed by contact with a wall of the first chamber when the wall thereof expands by a predetermined amount.

Description

United States Patent Ferrentino COMPENSATING PRESSURE TANK FOR OIL-FILLED POWER CABLES [72] Inventor: Antonio Ferrentino, Monza, Italy [73] Assignee: Industrie Pirelli Societa per Azioni,

Milan, Italy [22] Filed: May 24, 1971 [21] Appl. No.: 146,295

[30] Foreign Application Priority Data May 23, 1970 Italy ..25005 A/70 [52] U.S. Cl. ..l38/30 [51] Int. Cl ..F16l 55/04 [58] Field of Search 138/26, 30

[56] References Cited UNITED STATES PATENTS 7/1969 Legrand ..138/30 [451 Oct. 3, 1972 Primary Examiner-Henry T. Klinksiek Attomey-Brooks, Haidt & Haffner [57] ABSTRACT A tank contains a first chamber having a bellows-type elastic wall and has a normally open valve through which the chamber is connected to an oil-filled cable and when the chamber wall contracts sufficiently the valve is engaged by the wall and closed. The first chamber is surrounded by a fluid filled second chamber which is connected to a source of fluid through a similar valve which is normally open but which is closed by contact with a wall of the first chamber when the wall thereof expands by a predetermined amount.

13 Claims, 2 Drawing figures PATENTEDucra m2 SHEET 2 [1F 2 I IN VENTOR. flwromo @Pcwrwo 147 Tole/V678 COMPENSATING PRESSURE TANK FOR OIL- FILLED POWER CABLES The object of the present invention is to provide a pressure tank for the compensation of the volume of fluid oil, especially suited for installation alone, or in parallel with other pressure tanks of the same type, on power cables subject to very high pressures.

The pressure tanks for the compensation of the variations in the volume of oil, due to the variations in the temperature of the fluid oil in power cables, have, up till now, been constructed with groups of lenticular elastic cells, arranged in parallel, each communicating with the other, filled with gas at atmospheric pressure or .preloaded, and immersed in a container filled with the cable oil and therefore, sensitive on their surface to the variations in the oil pressure.

In other types, the cells are filled with oil and immersed in gas or in oil on which a suitable pressure is exerted.

These cells, which serve the double purpose of being elastic separators between fluids of the same or different natures and elements for the transmission of the pressure exerted by the containing or contained fluid, are limited in their application in that they give good results so long as the pressure does not exceed levels of about 4 atmospheres. With the improvement in cable techniques and methods of production, the tendency is to make cables, with the same electrical characteristics and therefore the same transversal dimensions in all of their parts, longer and longer, especially for underwater installations. It follows, therefore, that in order to ensure the presence of insulating oil in every point of the cable, even higher pressures must be reverted to, sometimes reaching or exceeding atmospheres.

Under these conditions, the use of pressure tanks with lenticular cells becomes critical since beyond a certain limit of pressure there occur in the cell walls, which are subject to fatigue stress, some localized deformations, which exceed the elasticity limit permitted by the material of which they are composed. Such deformations cause radial ridges to form which may lead to a break. It is well known that a break in one cell not only endangers the life of the pressure tank but also leads, in most cases, to the contamination of the cable insulating oil with serious prejudice to its dielectric properties. Moreover, the typical curve which, plotted on a pair of cartesian axes, indicates the varia tion in the cell volume along the abscissa in accordance with the variation in pressure applied to it along the or dinate is rather sloped with respect to the horizontal and this slope increases with the increase in pressure. This means that for the desired variations in volume corresponding considerable variations of pressure are required, and therefore, due to the above behavior of the cells at high pressure, it is understandable that their use is limited to modest volume variations.

The purpose of the present invention is to overcome the above disadvantages, and to provide a pressure tank especially suited to the high pressure and which will provide a typical pressure-volume curve which is sufficiently flat to produce, with small pressure variations, considerable displacements of volumes.

The present invention provides, moreover, the adthe important quality of meeting high pressure requirements, can, in its overall formulation, be designed for pressures of any value, making the elastic separator have the sole function of separating, without subjecting it to differences in pressure between its inner and outer surfaces and hence, prolonging its fatigue resistance to the maximum irrespective of its mechanical strength.

More precisely, the object of the present invention is a compensation pressure tank for oil-filled power cables in which one single outer cover contains at least a first and a second chamber, the first chamber, having an elastic wall, being filled with the degasified oil of the cable communicating with it and being immersed in the second chamber and sealed off from it, this second chamber being filled with a liquid, preferably oil and being in communication with a third expansion chamber, such tank being characterized by the fact that the first chamber has a fixed and rigid base plate, an elastic, bellows-type peripheral surface, and a mobile rigid closing plate, the communication between the first chamber and the cable being achieved by means of at least a first stop-valve fitted on the base plate in line and in mirror relationship with respect to a second stop-valve fitted on an upper plate of the second chamber. The first and the. second stop-valves, in their normal positions, are at a distance from said mobile plate and are in positions such as to form, with the plate, a hydraulic cut-off and are operated by the same alternately with compression or expansion of the first chamber.

The attached drawings show, as examples, two preferred embodiments of the invention and in such drawings:

FIG. 1 is a longitudinal cross-section of a compensation pressure tank of the invention with the second expansion chamber contained in a single unit and a gas supply tank; and

FIG. 2 shows a longitudinal cross-section of a compensation pressure tank of the invention with the second expansion chamber as a separate unit.

The compensation pressure tank in FIG. 1 includes a tank enclosing a volume divided into a first elastic chamber A, a second chamber B and a third expansion chamber C.

The first elastic chamber A is mainly within the second chamber B and is defined by a fixed and rigid base plate 6, .coinciding with the base of the tank 1, a wall 2 of stainless steel having a bellows-type peripheral surface which acts as an elastic membrane, a flange 4 welded to the wall 2, the flange 4 engaging sealing washers and being held in fluid-tight relation between the hollow cylindrical wall 5 of the outer cover 1 and the fixed base plate 6, and a movable rigid closing plate 3, preferably of stainless steel, secured to the upper end of the membrane 2.

The first elastic chamber A, which is sealed off from the second chamber B, contains degasified insulating fluid oil and communicates with the cable (not shown) through a stop-valve 7, inserted in the center of the fixed base plate 6. The valve 7 comprises a hollow cylindrical body 8, subdivided into a first cavity 10 and a second cavity 11.

The first cavity 10 has an end opening 9 for providing a connection with the cable and contains, coaxially, a first helical spring 12 which rests on the surface surrounding the'opening 9 and is connected at the other end with a first shutter 13 fitted with a ring washer 23 which forms a fluid seal when in contact with seat 14 of said first shutter 13.

The shutter 13 is axially movable in the second cavity 11 which contains second helical spring 15, stiffer than the first spring 12 and resting at one end on said shutter valve 13, while the other, or free end fits in a piston 16 joined by a small shaft to a second shutter 17 outside the valve housing and having on its own inner surface a seal or ring washer 18. The shutter 17 is made to fit into a corresponding seat 20 formed at the free edge 19 of the valve housing. The piston 16 has a plurality of channels 28 at the end thereof, which permit oil to flow past the piston 16 when it is against the stop ring 29.

The second chamber B is full of a liquid, preferably oil, and is separated from the expansion chamber C, by a top plate 22. A second stop-valve 7, fitted in the center of plate 22, puts the second chamber B in communication with the expansion chamber C.

The stop-valve 7', the corresponding parts of which are marked with the same reference numbers, but primed, as the corresponding parts of the first stopvalve 7, is the same, in line with and exactly mirrors the first valve 7. In their normal positions,

valves

7 and 7 are at a distance from the movable plate 3 and in a position such as to form with the plate 3 a hydraulic cut-off. The

valves

7 and 7 are operated alternately by the plate 3 due to the compression or expansion of the elastic chamber A. The expansion chamber C is partially filled with oil and has a greater volume than the oil flowing into it for maximum migration of oil from the cable to the elastic chamber A. The remaining space in chamber C is filled with a gas under pressure such as air, nitrogen, etc. The oil contained in the chamber C is, in volume, such as to guarantee its presence, whatever the displacement of oil from the Chamber A to the cable.

In the preferred embodiment of FIG. 2, the expansion chamber C is a tank separate from that containing the other two chambers A and B, and communicates with the chamber B through an appropriate pipe 27, connected to the second stop-valve 7. The tank containing chamber C may, therefore, be placed at any distance from that enclosing the chambers A and B.

The gas under pressure for the chamber C is supplied from an appropriate tank D (shown in FIG. l-only) separate from the tank 1, although it may be alongside or on top of the expansion chamber C or it may be combined with said expansion chamber C as shown in FIG. 2.

The tank D of FIG. 1 or the combined tank containing the chamber C as shown in FIG. 2, which could moreover be only one or several pressure tanks arranged in parallel, is also fitted with a warning pressure switch 24 which signals insufficient pressure, with a safety valve 25 and a filling cock 26. The tank D has dimensions proportioned so as to restrict the variations of gas pressures within the limits assigned to the oil pressure, and it is preloaded when it is wished to keep the oil pressure always above a certain value.

The oil which, during the working of the cable, enters from the cable into chamber A or, vice versa, travels from chamber A to the cable, lengthens or, respectively, shortens the elastic bellows-type membrane 2 and displaces an equal volume of oil from the chamber B to the chamber C or vice versa. The stroke of the elastic bellows-type membrane or its elongations and shortenings are limited hydraulically by the valves 7 and 7', operated alternately by the movable plate 3, such valves preventing the bellows-type walls of the membrane 2 from sufiering harmful and substantial differences of pressure between the inside and the outside so that the elastic membrane 2 operates exclusively as a separator between the two volumes of oil in the chambers A (degasified insulating oil) and B (oil in contact with the gas of chamber C).

If, for example, due to a cooling of the cable, the degasified oil should flow from the chamber A towards the cable, the elastic bellows-type membrane 2 will contract until it contacts, by means of the movable plate 3, the shutter 17 of the valve 7, the piston 16 of which will slide downwards and by means of the spring 15, stifi'er than the spring 12, will push, in turn, the shutter 13 to rest, with the ring washer 23, on the cor responding seat 14, causing the closure of the valve 7. As can be seen, the spring 15 makes it possible to close the shutter 13 with a pre-established force and not with all the force which the plate 3 is capable of exerting under differences in pressure which may even be very slight. From this moment no more oil may leave the chamber A and in the chambers A and B the liquids are at the same pressure, excluding in this consideration the negligible hydrostatic pressures and the slight negligible difference in pressure between the elastic chamber A and the second chamber B caused by the weak reaction of the bellows-type surface 2 when it is not in its rest position.

If the washer 23 did not ensure a perfect seal, there would be a persistent flow of oil towards the cable which would make the movable plate 3 descend still further due to compression of the spring 15, until the shutter 17 touches, with its washer 18, the corresponding seat 20. This second closure constitutes a guarantee against the possible non-operation of shutter 13.

If, instead, the cable were to heat up, and consequently the oil increase in volume and tend to migrate from the cable to the chamber A, the plate 3 would rise, moving the oil from the second chamber B to the expansion chamber C, opposing the pressure of the gas above the oil, through the stop-valve 7 until, coming in contact with the shutter 17 of the stopvalve 7', it would shut the latter. The operation of the stop valve 7' corresponds to that previously described for the stop-valve 7.

Due to the action of the plate 3 on the shutter 17 the piston 16 pushes up the spring 15 which, being stiffer than the spring 12, forces the latter to compress, putting the shutter 13' with its associated washer 23' into contact with the seat 14', so achieving a first closure of the stopvalve 7 If however the oil continues to flow, due to a poor sea] by the washer 23', towards the expansion chamber C, a further slight displacement of the mobile plate 3 puts the shutter 17' with its respective washer 18' into contact with the seat 20 formed at the edge 19.

The stop-

valves

7 and 7 with their function as hydraulic cut-offs for the movable plate 3 which prevent the elastic bellows-type surface 2 from being subjected to differences in pressure on the outside and the inside, thereby increasing the fatigue resistance to the maximum.

The constructional details of the invention may naturally be varied according to requirements. For example the cable communication valves and the valves communicating with the expansion chamber C may be more than one, or each stop-valve may comprise more than two cavities in series.

I claim:

1. A compensating pressure tank for oil-filled power cables, said tank comprising an elastic wall dividing at least a portion of said tank into first and second closed chambers, a first normally open valve communicating with said first chamber and having an actuating member and means extending through a wall of said tank for connecting an oil-filled power cable thereto and permitting the oil in said cable to flow into and out of said first chamber, said valve being positioned with its actuating member engagable by a portion of said wall upon movement of said wall in a first direction corresponding to a decrease in the oil pressure in said cable to close said valve and a second normally open valve communicating with said second chamber and having an actuating member and means extending externally of said second chamber for connecting a source of fluid thereto and permitting the flow of fluid into and out of said second chamber, said second valve being positioned with its actuating member engagable by a portion of said wall upon movement of said wall in a second direction corresponding to an increase in the oil pressure in said cable to close said second yalve.

2. A tank set forth in claim 1 wherein said source of fluid is an expansion chamber means having a gas therein for applying pressure to fluid therein.

3. A tank as set forth in claim 2 wherein said expansion chamber means is part of said tank.

4. A tank as set forth in claim 2 wherein said expansion chamber means is separate from said tank.

5. A tank as set forth in claim 2 wherein the fluid volume of said expansion chamber means is at least as great as the volume change within said first chamber as said portion of said wall moves from a position adjacent to the actuating member of said first valve to the position in which it closes said second valve.

6. A tank as set forth in claim 1 wherein said elastic wall is a bellows mounted at one end on a wall of said tank and extending therefrom into said tank.

7. A tank as set forth in claim 6 wherein said wall of said tank is rigid and the other end of said bellows remote from said tank wall has a rigid place secured thereto.

8. A tank as set forth in claim 7 wherein at least one of said bellows and said rigid plate is made of stainless steel.

9. A tank as set forth in claim 7 wherein said first valve is mounted within said bellows with its actuating member adjacent to said rigid plate and engagable by the portion of said bellows to which said rigid plate is secured.

10. A tank as set forth in claim 9 wherein said second valve is mounted on a wall of said tank external to said bellows with its actuating member adjacent and engagable by said rigid plate.

11. A tank as set forth in claim 1 wherein each of said first and second valves comprises a hollow cylindrical body having an opening at one end, a seat internally of said body facing away from said one end, said seat bein intermedi te s aced from said one end and the ther end 0 sai d fio y, .a shutter mounted within said body and intermediate said seat and said other end of said body, said shutter being engagable with said seat, a piston mounted intermediate said shutter and said other end of said body, a first spring mounted intermediate said one end of said body and said shutter, and acting between said shutter and said body to urge said shutter away from said seat and normally maintaining it spaced therefrom, a second spring, stiffer than said first spring, mounted intermediate said shutter and said piston and acting therebetween to urge said shutter toward said seat and said piston toward said other end of said body and stop means for limiting the movement of said piston.

12. A tank as set forth in claim 11 wherein said body has an externally facing seat at said other end thereof and said piston carries a further shutter engagable with said externally facing seat but spaced therefrom when said piston is in engagement with said stop means said piston and said further shutter forming said actuating member for the valve.

13. A tank as set forth in claim 12 wherein said first and second valves are mounted co-axially and with their open ends facing oppositely.

Claims

1. A compensating pressure tank for oil-filled power cables, said tank comprising an elastic wall dividing at least a portion of said tank into first and second closed chambers, a first normally open valve communicating with said first chamber and having an actuating member and means extending through a wall of said tank for connecting an oil-filled power cable thereto and permitting the oil in said cable to flow into and out of said first chamber, said valve being positioned with its actuating member engagable by a portion of said wall upon movement of said wall in a first direction corresponding to a decrease in the oil pressure in said cable to close said valve and a second normally open valve communicating with said second chamber and having an actuating member and means extending externally of said second chamber for connecting a source of fluid thereto and permitting the flow of fluid into and out of said second chamber, said second valve being positioned with its actuating member engagable by a portion of said wall upon movement of said wall in a second direction corresponding to an increase in the oil pressure in said cable to close said secOnd valve.

11. A tank as set forth in claim 1 wherein each of said first and second valves comprises a hollow cylindrical body having an opening at one end, a seat internally of said body facing away from said one end, said seat being intermediate and spaced from said one end and the other end of said body, a shutter mounted within said body and intermediate said seat and said other end of said body, said shutter being engagable with said seat, a piston mounted intermediate said shutter and said other end of said body, a first spring mounted intermediate said one end of said body and said shutter, and acting between said shutter and said body to urge said shutter away from said seat and normally maintaining it spaced therefrom, a second spring, stiffer than said first spring, mounted intermediate said shutter and said piston and acting therebetween to urge said shutter toward said seat and said piston toward said other end of said body and stop means for limiting the movement of said piston.