GB2128638A - Heat treatment method and apparatus - Google Patents

Abstract

An envelope (1) possibly made of stainless steel foil, having an expandible construction is inserted within the vessel (2) to be heat treated. The envelope (1) is caused to enlarge, by supplying a heated stream of fluid, e.g. supplied by a high velocity gas burner (11), so that at least a part of the envelope (1) makes contact with an internal wall portion of the vessel (2). The temperature of the heated stream of fluid is controlled for a predetermined period in order to heat treat the wall portion of the vessel (2). The invention is particularly useful to anneal a reactor vessel which has become embrittled by fast neutron bombardment, since it facilitates installation and reduces exposure of personnel to radiation. <IMAGE>

Description

SPECIFICATION Heat treatment method and apparatus This invention relates to apparatus and a method for heat treating a vessel. The invention may be applied, for example, to anneal a reactor vessel, which has become embrittled as a result of fast neutron bombardment, whereby the vessel may be restored to a serviceable condition.

Known annealing techniques include, for example, processes wherein resistance heating elements are attached to the walls of a vessel to be heat treated, the power supplied to the elements being controlled over a given period in order to carry out the required annealing process.

Such techniques, however, suffer from the disadvantage of requiring a multiplicity of elements to be attached by hand and the time and labour required to carry out the installation (particularly where the elements must be fitted to the interior of the vessel). Moreover, in the case of a reactor vessel, personel are exposed to radiation in both fitting and removing the resistance elements at the start and finish of the process.

An alternative technique for annealing a reactor vessel may involve circulating heated gases within its interior, the interior being sealed by stainless steel insulation to provide a gas circulation chamber. However, this technique requires the installation of the stainless steel insulation to define the gas circulation chamber and thereby has disadvantages which are similar to the resistance element technique described above.

A further annealing technique may involve the use of a heat exchanger comprising coiled pipes.

However a disadvantage of this indirect heating method is that insufficient temperature control can be exercised, particularly at low temperatures, to ensure uniform heating of the vessel walls during the annealing process. The present invention, particularly according to its preferred embodiment, seeks to overcome problems of the above-mentioned type.

According to the invention, a method of heat treating a vessel comprises the steps of: a) providing an envelope having a structure such that its size, in at least one plane, can be enlarged in an operational state; b) inserting the envelope into within the vessel in an initial state; c) supplying a heated stream of fluid to the envelope to cause it to enlarge, so that at least a part of the envelope makes contact with an internal wall portion or portions of the vessel, and d) controlling the temperature of the heated stream of fluid for a predetermined period in order to heat treat said wall portion or portions of the vessel.

The invention also provides apparatus for heat treating a vessel, the apparatus comprising an envelope having a structure such that its size, in at least one plane, can be enlarged in response to a heated stream of fluid to enable at least a part of the envelope to make contact with an internal wall portion or portions of the vessel, the apparatus further including means to supply a heated stream of fluid to the envelope.

An inflatable envelope may be employed for use in the above-mentioned method and apparatus according to the invention; means being provided for inflating the envelope with a heated gas stream. Suitably, such an envelope is made of substantially compliant material, such as stainless steel foil, which is heat resistant and thermally conductive. The method may include first constructing a sealed inflatable envelope having gas inlet and outlet means, a wall portion or portions of the envelope being made of substantially compliant material and having a complimentary shape and dimensions to an internal wall portion or portions of the vessel.Then collapsing the envelope to facilitate its insertion through an opening or access port of the vessel, and inserting the collapsed envelope through the opening or access port and positioning the envelope within the vessel so that the wall portion or portions of the envelope are substantially opposite an internal wall portion or portions of the vessel. A stream of heated gas is then admitted and maintained through the gas inlet means to inflate the envelope, whereby the wall portion or portions of the envelope make contact with the internal wall portion or portions of the vessel. The envelope is thereby heated, and hence so is the interior of the vessel, the gas leaving the envelope through the gas outlet means. The temperature and/or flow rate of the heated gas is controlled, in accordance with a predetermined heat treatment schedule to carry out the heat treatment process.

Instead of using an envelope made of metal foil, the method and apparatus of the invention may employ the heat exchanger comprising an expandible envelope including substantially rigid wall portions coupled together such that the size of the envelope can be enlarged, in response to a heated stream of fluid, to enable at least a part of the envelope to make contact with an internal wall portion or portions of the vessel.

In the latter heat exchanger, the expandible envelope preferably includes overlapping wall portions, which are substantially rigid, e.g. made of thin gauge stainless steel, which slide over each other. In order to seal any gap between the sliding wall portions, sealing members can be attached to each adjacent pair of wall portions, each sealing member having either compliant, or flexible structure which enables the respective adjacent wall portions to slide over each other. For example, the sealing means may be formed by convoluted strips, e.g. made of thin gauge stainless steel, which yield or flex to enable the sliding movement. If the envelope has a body section which is closed at each end, the end closures preferably have at least a section which is conically shaped and includes substantially rigid plates which are interconnected by either compliant, or flexible sealing members.Suitably, the latter sealing members (of the end closures) are connected to the sealing members of the adjacent wall portions of the body. The plates of the end closures are also connected to, or form part of, the corresponding wall portions of the body section.

Also in the latter heat exchanger, it is preferred to use thermally responsive means to promote or assist the enlargment of the enevlope when the heated stream of fluid is supplied to the envelope.

Such means may comprise bimetallic elements which alter their shape or configuration on heating, and which are attached to the envelope so as to exert respective forces on its rigid wall portions when the elements respond to the heated fluid. Such bimetallic elements may be bimetallic springs.

Advantages of the invention are that the envelope can be easily installed to carry out the heat treatment, and that sufficient temperature control can be exercised during heat treatment, despite the use of indirect heating, to ensure uniform heating of the vessel. In the case of a reactor vessel, the envelope can be constructed away from the vessel and it can then be easily inserted into the vessel (simply by being lowered in, (e.g. by means of a hoist), thereby substantially reducing or eliminating exposure of personel to radiation.

An example of the invention will now be described with reference to the accompanying schematic drawings, in which: Fig. 1 is an elevation, in section, of part of a vessel containing an envelope according to one embodiment of the invention, Fig. 2 is a perspective view of the envelope shown in Fig. 1, Fig. 3 illustrates apparatus for heat treating a vessel and which employs the envelope of Figs. 1 and 2, or Figs. 5-8, Fig. 4 is an elevation, in section, of part of a vessel containing an envelope according to another embodiment of the invention, Fig. 5 is a plan view of the envelope of Fig. 4, Fig. 6 shows part of the envelope of Fig. 4 in perspective, Fig. 7 SilOWS a detail of envelope of Fig. 4, and Fig. 8 is a plan view of the detail shown in Fig.

7.

A description will first be given of one embodiment of the invention as illustrated in Figs.

1-3.

Referring to Figs. 1-3, an envelope 1 made of substantially impervious, heat resistant and compliant material, such as stainless steel foil, includes a main cylindrical portion 1 a and end portions 1 b, 1 c. The cylindrical portion 1 a has a diameter which is slightly greater than that of the internal diameter of a cylindrical vessel 2 to be heat treated. For example, with a vessel having a diameter of about 140 cm., the diameter of the cylindrical portion 1 a is about 142.5 cm. This ensures that when the envelope is inflated (as described below), the sides of the cylindrical portion 1 a make contact with the confronting wall portions of the cylindrical vessel 2.

Referring to Figs. 1 and 2, each end portion 1 b, 1 c of the envelope is formed by pieces or "petals" 3 of foil which are welded together along seams 4 and are welded to respective circumferential end portions of the cylindrical portion 1 a. The cylindrical portion 1 a is made from a sheet which is formed into a cylinder and welded down seam 5. The upper end portion 1 b is secured to a metal disc 6, i.e. by welding the inwardly directed ends of the petals 3 to the surface of a circumferential portion of the disc 6. Similarly, the petals 3 (not shown) of the lower portion 1 c are secured by welding, to a metal disc 9. Disc 6 provides support for inlet and outlet duct 7, 8 which pass through apertures in disc 6. Disc 6 is attached to disc 9 by means of for example, four equidistant spacer rods 10.The ends of the inlet and outlet ducts 7, 8 within the envelope 1, are vertically spaced in order to promote the flow of a heated fluid through the interior of the envelope 1, i.e. from the orifice of the inlet duct to the orifice of the outlet duct. The inlet duct receives a stream of heated fluid, e.g. gas from a high velocity gas burner 11 and fan 12.

The outlet duct 8 is vented to atmosphere, as shown in Fig. 3, and it contains bufferfly valve 13 to regulate the outflow of vented gas.

In order to heat treat a vessel, the envelope 1 is first constructed with reference to a drawing of the vessel showing its internal dimensions. In the example described, this enables the shape and dimensions of the cylindrical portion 1 a to correspond substantially to the shape and dimensions of a selected zone of the interior wall portion of the vessel 2 to be heat treated. In this case, the diameter of the cylindrical portion 1 a was slightly greater than the internal diameter of vessel 2 In order to ensure "contact" (as described below). After construction, the envelope is slightly collapsed or deflated, to facilitate its insertion through an end opening or access port of vessel 2, and the envelope is lowered into the vessel until the outer wall of the cylindrical portion 1 a is opposite the interior cylindrical wall of vessel 2 at the required heat treatment zone.Assuming that the ducts 7,8 are arranged as shown in Fig. 3, with duct 7 ready to receive a heated gas stream from burner 1 , the envelope 1 is inflated by the heated gas stream so that the outer surface of the cylindrical portion 1a of the envelope makes contact with the interior surface of the internal cylindrical wall of the vessel 2 at the heat treatment zone. The envelope is maintained in its inflated condition by the continuous stream of heated gas passing from the inlet duct 7 to the outlet duct 8.

As shown in Fig. 2, thermocouples 14, for example, arranged in three rows of four, are attached to the outer surface of the cylindrical portion 1 a. These thermocouples are connected to a conventional controller 15, which includes temperature indicators and recording devices and which controls the output of the gas burner 11.

The heat input to the vessel 2 may be controlled by controlling the flow rate of the combustion gas supplied to the burner 1 and/or by controlling the fan 12 to control the flow rate of the heated gas stream.

As shown in Fig. 3, the reactor vessel 2 is normally below floor level and its outer wall portions are thermally insulated (18), for example, with a 12 inch (30 cm) thickness of mineral wool.

Whilst an example has been described with reference to a cylindrical vessel, the envelope 1 may have different shapes and dimensions to suit the shape and dimensions of the vessel to be heated. In some cases, the natural compliance of the material from which the envelope is made is sufficient to cause it to sag or collapse to facilitate insertion through the end opening or access port of the vessel. However, the envelope 1 may be deflated by connection to a vacuum pump, if necessary, in order to reduce its overall dimensions to facilitate insertion through, e.g. a small opening.

A description will now be given of a further embodiment which is illustrated in Figs. 4-8. It will be understood, however, that Fig. 3 schematically illustrates apparatus which can be used with the embodiment of Figs. 4-8 (see below).

As shown in Figs. 4-8, an expandable envelope 1 made of substantially impervious, heat resistant material includes a substantially cylindrical body section 1 b which is closed at each end by substantially conically shaped sections or end closures 1 a, 1 c. The body portion 1 b comprises a plurality of substantially rigid curved plates 21, made of thin gauge stainless steel.

Adjacent edge portions of the plates 21 slidably overlap one another. The gap between the overlapping edge portions of the plates 21 are sealed (i.e. to seal the interior of the envelope 1) by convoluted strips 22 extending along the length of the respective overlapping edge portions.

These sealing members 22 are also made of thin gauge stainless steel and are sufficiently flexible to allow the edge portions of the plates 21 to slide over one another.

Each end closure 1 a, 1 c comprises a plurality of substantially rigid paltes 24 which are interconnected by means of sealing members 23 having convoluted triangular shape. These sealing members 23 are also sufficiently flexible to enable the plates 24 to move away from one another.

Plates 24 and sealing members 23 are also made of thin gauge stainless steel. As shown in Fig. 6, the lower ends of plates 24 are either attached to, or form an extended part of, the corresponding plates 21 of the body section. The side edges of plates 24 are attached to the sides Qf the convoluted triangular sealing members 23, the convoluted base of the triangle being attached to the convoluted upper edge of the corresponding sealing member 22 of the body section as shown at an angle of about 45 . The upper end portions of plates 24 are attached to a circular plate 26 which can be of a heavier construction than thin gauge stainless steel to provide support for the envelope and for inlet and outlet ducts 7,8.The end closure 1 c has a similar construction to end closure 1 a, except that it is attached to the body section in the reverse position and that the circular plate 27 may be made of thin gauge stainless steel since it neither supports the weight of the envelope, nor the inlet and outlet ducts 7, 8.

The sealing members 22, 23 are preferably attached to the plates 21,24 by welding.

Similarly, the end plates 26, 27 are attached to the plates 24 of the respective and closures 1 a, 1 c by welding. The ducts 7, 8 are also welded in place on the end plate 26.

The envelope 1 is gas tight and expandable. It can be used for heat treating a hollow vessel having an opening or access port to its interior, only the cylindrical wall portion 2 of such a vessel being shown in Fig. 4. The body 1 b has a complimentary shape and dimensions to the internal part of the cylindrical wall 2, but its crosssectional size (in the radial plane of the cylinder), in an initial cold state, is small enough to allow the envelope 1 to be inserted through the opening or access port of the vessel 2. Once within the interior of the vessel, the envelope 1 is caused to expand as described below.

As shown in Figs. 7 and 8, a plurality of thermally responsive elements 25 are attached, at spaced intervals, to adjacent plates 21 along the length of the convoluted strips or sealing members 22. One of more of these elements may also be attached to adjacent plates 24, i.e. bridging the convoluted sealing members 23, if required. Each of these elements, whch may be a bimetallic spring (see more clearly in Fig. 8) alters its shape or configuration on heating, i.e. when heated fluid, such as heated gas, is supplied to the interior of the envelope 1 through the inlet duct 7. As shown in Fig. 8, the bimetallic spring 25 includes leg portions 25a joined to a coil portion 25b. The ends of the leg portions 25a may be welded to the respective plates 21.When the spring 25 is heated, the legs 25a move apart, thus causing the plates 21 to move apart within the limitations imposed by the flexure of the convoluted strips or sealing members 22.

The design of the envelope 1, e.g. the number and disposition of the rigid plates 21, 24 and the configuration of the sealing members 22,23, depends on the diameter of the access port of the vessel and on the internal diameter of its cylindrical body 1 b. More or less sealing members may be required, or a series of convoluted strips, like a concertina or corrugation can be used to obtain the required expansion/contraction of the envelope 1.

As in the previous embodiment described with reference to Figs. 1-3, a stream of heated fluid, such as heated gas is supplied through duct 7 so that it circulates within the envelope 1 and leaves via the exhaust duct 8. The equipment schematically illustrated in Fig. 3 may be used in conjunction with the envelope 1 of Figs. 4-8, the optimum design would be one where the rigid plates 21 move outwardly, in response to the stream of heated fluid supplied to the envelope, so taht they make contact with the confronting internal wall surfaces of the section of the vessel 2 being heat treated. The expansion is such that the plates 21 slide relative to one another, without separation of the free side edges of the plates 21 (since such separation might cause engagement between the side edges).Preferably, there is a minimum overlap between the side edges when the plates 21 contact the interior wall surfaces of the vessel, but sufficient overlap to ensure that the plates 21 can slide relative to one another during contraction of the envelope 1 on cooling.

The envelope 1 of Figs. 4-8 may have a different shape and construction to that shown in the drawings in order to suit the particular requirements for heat treating the design of the vessel.

Instead of using a high velocity gas burner discharging into the inlet duct 7, an indirect air heater, i.e. one having a heat exchanger and being of known construction, can be used. Such a heater employs a high velocity gas burner which discharges into a heat exchanger, the heat exchanger having a process air inlet, fitted with a fan, and an outlet for heated air. The heated air would be supplied to the duct 7 and the gas supply to the high velocity burner would be regulated in order to control the temperature of the heated air output.

Claims (32)

1. A method of heat treating a vessel, the method comprising the steps of: a) providing an envelope having a structure such that its size, in at least one plane, can be enlarged in an operational state; b) inserting the envelope into within the vessel in an initial state; c) supplying a heated stream of fluid to the envelope to cause it to enlarge, so that at least a part of the envelope makes contact with an internal wall portion or portions of the vessel, and d) controlling the temperature of the heated stream of fluid for a predetermined period in order to heat treat said wall portion or portions of the vessel.

2. A method according to claim 1, wherein the envelope is inserted into a vessel in a collapsed condition and is then caused to enlarge by supplying the stream of heated fluid to the envelope.

3. A method according to claim 1, wherein the envelope is caused to enlarge in response to the temperature of the stream of heated fluid, the structure of the envelope having thermally responsive means to assist its enlargement.

4. Apparatus for heat treating a vessel, the apparatus comprising an envelope having a structure such that its size, in at least one plane, can be enlarged in response to a heated stream of fluid to enable at least part of the envelope to make contact with an internal wall portion or portions of the vessel, the apparatus further including means to supply a heated stream of fluid to the envelope.

5. Apparatus according to claim 4, wherein the envelope has a structure which enables it to be inflated in response to the stream of heated fluid.

6. Apparatus according to claim 5, wherein the envelope is made of stainless steel foil.

7. Apparatus according to claim 5 or 6, wherein the dimensions of the envelope are slightly greater than the internal dimensions of the vessel at least in the region where contact is made.

8. Apparatus according to any one of claims 5-7, wherein the envelope has a main cylindrical portion made of compliant material, and end portions formed by pieces of compliant material which are secured to one another and to the cylindrical portion.

9. Apparatus according to claim 8, wherein said pieces of compliant material of each end portion are secured to a rigid member, the rigid member at one end of the envelope being spaced from the rigid member at the other end of the envelope by spacing means.

10. Apparatus according to any one of claims 5-9, wherein the means for inflating the envelope comprises inlet and outlet means attached to the envelope, and a source of heated fluid which is supplied under pressure to the inlet means.

11. Apparatus according to claim 10, wherein said inlet and outlet means respectively comprise inlet and outlet ducts, the ends of the ducts being spaced apart in the envelope.

1 2. Apparatus according to claim 4, wherein the envelope comprises a plurality of substantially rigid wall portions which are expandibly coupled together.

13. Apparatus according to claim 12, wherein said wall portions slidably overlap one another.

14. Apparatus according to claim 13, wherein gaps between adjacent sliding wall portions are sealed by compliant or flexible sealing members.

1 5. Apparatus according to claim 14, wherein the sealing members are formed by convoluted strips.

1 6. Apparatus according to claim 4, wherein the envelope has a substantially cylindrical body section which is closed at each end by end closures having at least a section which is conically shaped and which includes substantially rigid plates interconnected by compliant or flexible sealing members, the latter sealing members being connected to the sealing members of the adjacent wall portions of the body section, and the plates of the end closures being connected to, or forming part of, the corresponding wall portions of the body section.

1 7. Apparatus according to any one of claims 12-16, including thermally responsive means to promote or assist the enlargement of the envelope when the heated stream of fluid is supplied to the envelope.

18. Apparatus according to claim 16, wherein the thermally responsive means comprise bimetallic elements which alter their shape or configuration on heating, and which are attached to the envelope so as to exert respective forces on its rigid wall portions when the elements respond to the heated fluid.

1 9. Apparatus according to claim 17, wherein said bimetallic elements are bimetallic springs.

20. A heat exchanger for use in the method according to claim 1 , the heat exchanger comprising an expandible envelope including substantially rigid wall portions coupled together such that the size of the envelope can be enlarged, in a response to a heated stream of fluid, to enable at least a part of the envelope to make contact with an internal wall portion or portions of the vessel.

21. A heat exchanger according to claim 20, wherein the envelope comprises a plurality of substantially rigid wall portions which are expandibly coupled together.

22. A heat exchanger according to claim 20, wherein said wall portions slidably overlap one another.

23. A heat exchanger according to claim 20, wherein gaps between adjacent sliding wall portions are sealed by compliant or flexible sealing members.

24. A heat exchanger according to claim 20, wherein the sealing members are formed by convoluted strips.

25. A heat exchanger according to claim 20, wherein the envelope has a substantially cylindrical body section which is closed at each end by end closures having at least a section which is conically shaped and which includes substantially rigid plates interconected by compliant or flexible sealing members, the latter sealing members being connected to the sealing members of the adjacent wall portions of the body section, and the plates of the end closures being connected to, or forming part of, the corresponding wall portions of the body section.

26. A heat exchanger according to claim 20, including thermally responsive means to promote or assist the enlargement of the envelope when the heated stream of fluid is supplied to the envelope.

27. A heat exchanger according to claim 20, wherein the thermally responsive means comprise bimetallic elements which alter their shape or configuration on heating, and which are attached to the envelope so as to exert respective forces on its rigid wall portions when the elements respond to the heated fluid.

28. Apparatus for heat treating a vessel substantially as herein described with reference to Figs. 1-3 of the accompanying drawings.

29. Apparatus for heat treating a vessel substantially as herein described with reference to Figs. 4--8 of the accompanying drawings.

30. A method of heat treating a vessel substantially as herein described with reference to Figs. 1-3 of the accompanying drawings.

31. A method of heat treating a vessel substantially as herein described with reference to Figs. 4-8 of the accompanying drawings.

32. A heat exchanger for use in the method according to claim 1 and substantially as herein described with reference to the accompanying drawings.