GB2537386A

GB2537386A - Gas spring

Info

Publication number: GB2537386A
Application number: GB1506326.6A
Authority: GB
Inventors: Montebello Antony
Original assignee: Nutberry Ltd
Current assignee: Nutberry Ltd
Priority date: 2015-04-14
Filing date: 2015-04-14
Publication date: 2016-10-19
Also published as: GB201506326D0

Abstract

A gas spring 2 is provided. The gas spring 2 comprises: a container 4; a ram 10 which is extendable and retractable in a direction out of and into the container; a first outer chamber 18 defined in the container and having a flexible wall provided by a first bellows 26; a second inner chamber 20 defined in the container and having a flexible wall provided by a second bellows 32; a first pressure applying medium 38 in the first chamber 18 for applying pressure to extend the ram 10; and a second pressure applying medium 40 in the second chamber 20 for applying pressure to retract the ram 10. At least one of the first and second pressure applying media 38, 40 comprises a substance which in use of the gas spring undergoes a phase change to become a gas as the temperature of the gas spring increases from ambient temperature. The gas spring 2 is for use at temperatures equal to or in excess of 200˚C and thus can be used in a sliding refractory gate valve assembly.

Description

Gas Spring This invention relates to a gas spring, such as for a sliding refractory gate valve, to a refractory gate valve assembly comprising a gas spring, and to apparatus for making a gas spring.

Gas springs (also known as Thermo Dynamic Elements, TDEs) are used in sliding gate valve mechanisms which control the flow of molten steel during the continuous to casting process. The springs held in spring pockets in the mechanism are used to clamp relatively sliding refractory plates together. The plates each have a bore in the middle; when the two bores are aligned the flow of steel is at its maximum. Depending on the position of these plates the aperture of the bore increases or decreases thereby determining the flow rate of the steel.

The springs must supply sufficient force to ensure that molten steel cannot get between the plates (which is known as finning), which could then potentially lead to a breakout of molten steel, which can be extremely dangerous as well as costly. The sliding of the refractory plates is powered by hydraulics.

A gas spring (TDE) consists essentially of a metal ram connected to a metal bellows (usually stainless steel), with an inert gas in a sealed chamber and applying pressure to the ram. In one form the gas is inside the bellows which thus expand to extend the ram as the gas expands; in another the bellows are sealed into an outer container and the gas is outside the bellows and ram and inside the container, thereby to compress the bellows and extend the ram as the gas expands.

The springs need to be of compact design whilst providing adequate force. Gas springs have proven capable of satisfying this requirement.

Examples are shown in GB-A-I 457 708.

During the casting process the temperature near the spring pockets can exceed 400 °C.

Gas springs have the advantage that at higher temperatures the gas pressure increases to apply a greater force on the sliding plates, thereby preventing any finning. They also provide an extra element of safety, since the normal failure mode is due to the bellows fatiguing. As such, it would be extremely unlikely for all the gas springs within the mechanism to fail at the same time, unlike other types of spring which may have a failure mode related to temperature and thus may all fail together at the same temperature.

Known designs of gas spring can provide a guaranteed number of heats (cycles) at a maximum temperature of e.g. 500 °C. However, there have been many cases where the gas springs have been operating in temperatures above 700 °C.

The force applied increases with temperature. For certain mechanisms the force applied at high temperatures is too great to allow the sliding mechanism to operate, or is too small at low temperature.

This problem has been addressed in the gas spring described in WO 2011/023928.

This discloses a gas spring comprising a container, a ram which is extendable out of the container, a bellows connected to the ram, a gas within the container and acting to extend the ram, and a mechanical compression spring interposed between the ram and the container and also acting to extend the ram. This gas spring is referred to as a hybrid spring, because it uses both a gas chamber and a mechanical spring.

It will be appreciated that the force extending the ram results from both the gas pressure and the force of the mechanical compression spring. By selecting the properties of the mechanical compression spring and by selecting the initial gas pressure, it is possible to tailor the initial force at room temperature, the force at higher temperatures, and the slope of the force/temperature curve. By using a lower initial gas pressure, the increased gas pressure inside the hybrid gas spring as the temperature increases is less than a gas spring where the total force is made up of gas alone. Depending on the ratio between the gas force and the mechanical spring force, the force/temperature curve can be manipulated to provide the desired shape.

A problem with the hybrid spring is that the mechanical compression spring is at risk of relaxation or creep at higher temperatures. This can mean that after exposure to higher temperatures, particularly over an extended period of time, its length when unloaded will be shorter than it was when undamaged. The force applied to the ram at a given temperature may thereafter be smaller than the intended design force.

Viewed from a first aspect the invention provides a gas spring comprising a container, a ram which is extendable and retractable in a direction out of and into the container, a first chamber defined in the container, a second chamber defined in the container, a first pressure applying medium in the first chamber for applying pressure to extend the ram, and a second pressure applying medium in the second chamber for applying pressure to retract the ram, at least one of the first and second pressure applying media comprising a substance which in use of the gas spring undergoes a phase change to become a gaseous phase as the temperature of the gas spring increases from ambient temperature The gas spring may be for use at temperatures equal to or greater than 200°C.

The gas spring may be for a sliding refractory gate valve. this it may be suitable for use in applying a force to a sliding refractory gate valve, for example urging one valve plate against another. The force may be applied by the ram.

Viewed from a second aspect the invention provides a refractory gate valve assembly comprising first and second plates in relatively slidable face to face contact with each other, and at least one gas spring urging one of the plates towards the other plate, the gas spring being in accordance with the first aspect of the invention.

The refractory gate valve assembly can be used to control the flow of molten steel during the continuous casting process. The at least one gas spring is used to urge one of the plates towards the other plate. The other plate may be fixed in the urging direction, so that the relatively slidable plates are clamped together under the force of the gas spring. The force is exerted by the ram. The plates may each have a bore therethrough. When the two bores are aligned the flow of steel is at its maximum. Depending on the position of these plates the aperture of the bore increases or decreases thereby determining the flow rate of the steel.

In a typical arrangement, there is provided a fixed plate and a slidable plate in contact therewith, the at least one gas spring urging the slidable plate towards the fixed plate.

The substance which in use undergoes a phase change may be a liquid, such as water, or a solid, such as carbon dioxide, at ambient temperature. In use of the gas spring in a sliding refractory gate valve, the temperature of the gas spring will be increased from ambient (e.g. 20°C) to the operating temperature of the gate valve (e.g. a temperature in the range 400°C to 600°C). At lower temperatures, the substance in the form of a liquid or solid makes a negligible contribution to the pressure applied in the chamber, but as it heats up it undergoes a phase change to a gaseous phase and then contributes to the pressure in the chamber. At such elevated temperatures, the pressure applying medium comprising the substance which has undergone a phase change can apply a pressure and hence a force on the ram which opposes the force on the ram caused by the pressure applied by the pressure applying medium in the other chamber.

In other words, in use of the gas spring, when the substance changes to the gaseous phase, the pressure in the chamber containing that substance applies a 'bite to the ram which opposes the force applied by the pressure in the other chamber to reduce the net force on the ram. The two pressures are balanced. When the gas spring is in a refractory gate valve assembly, for example, the net force on the ram acts in turn on the slidable refractory plate. -5 -

It is therefore possible to apply force to the ram which initially increases with rising temperature up to a certain temperature, for example 200°C, and then with further rising temperature above the certain temperature, for the rate of force increase to reduce, i.e. to become smaller, zero, or negative. The generation of too much force on the ram at high temperature, as with a conventional gas spring, can thus be avoided, but without the use of a mechanical spring which is vulnerable to relaxation or creep at high temperatures.

The pressure applying medium in the chamber, which acts oppositely to the pressure applying medium comprising the phase changing substance, may be a gas at ambient temperature. In general, therefore, the pressure applied to the ram by the medium in this chamber will increase substantially linearly as the temperature rises from ambient.

The second pressure applying medium may comprise the substance which in use undergoes the phase change. In this arrangement, the gas spring may be configured so that the ram provides a net extension force, which in use may for example be applied to a valve plate. The second pressure applying medium can act in a direction to retract the ram at higher temperatures. With such an arrangement, the force applied to extend the ram will increase as temperature increases from ambient, but after a certain level of temperature this force will be opposed by the force of the second pressure applying medium in the second chamber, thereby preventing a further large increase in the net extension force on the ram with increasing temperature.

In an alternative embodiment, the first pressure applying medium comprises the substance which in use undergoes the phase change. In this arrangement, the gas spring may be configured so that the ram provides a net retraction force, which in use may for example be applied to a valve plate. The first pressure applying medium can act in a direction to extend the ram at higher temperatures. With such an arrangement, the force applied to retract the ram will increase as temperature -6 -increases from ambient, but after a certain level of temperature this force will be opposed by the force of the first pressure applying medium in the first chamber, thereby preventing a further large increase in the net retraction force on the ram with increasing temperature.

At least one of the first and second chambers may have a flexible wall connected to the ram and to the container to allow relative movement of the ram and the container.

The or each flexible wall may be a bellows.

By providing at least one of the chambers with a flexible wall such as a bellows a fully gas-tight chamber can be provided whilst permitting the relative movement of the ram and the container. The connection of the flexible wall to the ram and the connection of the flexible wall to the container need not involve any relatively slidable parts, so that sealing integrity at the connection can be generally guaranteed. Each connection may be a fixed and permanent connection, and may comprise a weld.

Each of the first and the second chambers may have a flexible wall connected to the ram and the container to allow relative movement of the ram and the container. Thus the gas spring may comprise a first flexible wall connected to the ram and the container, and a second flexible wall connected to the ram and the container. One of the flexible walls may be exposed on one side thereof to the pressure inside one of the chambers and on the other side thereof to the pressure inside the other of the chambers. Such a flexible wall effectively provides a wall for both chambers. The other flexible wall may be exposed on one side thereof to the pressure inside said one of the chambers and on the other side thereof to atmospheric pressure. For example, a radially outer bellows may form a flexible wall between a radially outer chamber and a radially inner chamber, and a radially inner bellows may form a flexible wall between the radially inner chamber and an inner region of the gas spring at atmospheric pressure. The ram may be positioned in the inner region for axial extension and retraction.

In the gas spring described above, the flexible wall which is exposed on its opposite sides to each of the respective chambers may be configured to fail before the other flexible wall. This can be achieved by configuring the flexible wall to have a shorter fatigue life than the other flexible wall. The fatigue life can be determined by calculations specified by EJMA (Expansion Joint Manufacturers Association) using commercially available software, and can be tested using known procedures. The geometry and/or material of each flexible wall may be configured so that one fails before the other over a period of use. In general, by adding one or more convolutions to a bellows, and/or by adding one or more plies to a bellows, fatigue life is increased.

In the event of failure, the two chambers become joined and the pressure in the combined volume can continue to act on the ram, whether extending it or retracting it depending on the setup.

The first chamber may have a first flexible wall comprising a first bellows, and the second chamber may have a second flexible wall comprising a second bellows. The first and second bellows may be arranged concentrically with each other.

The or each flexible wall may have a first annular periphery connected to the ram and a second annular periphery connected to the container. Each connection may be a fixed and permanent connection, and may comprise a weld. If the or each flexible wall comprises a bellows, then the first annular periphery may be at one end of the bellows and the second annular periphery may be at the other end of the bellows.

The or each flexible wall may be made of steel, such as stainless steel. The fatigue life of a flexible wall, especially at high temperatures, may be increased by using nickel alloys such as inconel 625. With this material, its strength tends not to be weakened at higher temperatures. -8 -

The ram may comprise a shaft, and a portion projecting radially from the shaft. The radially projecting portion may be in the form of a ring which is welded to the shaft. The shaft may have an annular flange providing an axial abutment for an inner periphery of the ring. The first and second chambers may be configured so that in use the first and second pressure applying media apply pressure to the radially projecting portion of the ram in opposite directions. Thus one side of the ring may be exposed to pressure in the first chamber and the other side may be exposed to pressure in the second chamber.

The ram may have a stroke of 25 mm or less, 20 mm or less, 15 mm or less, 10 mm or less, or 5 mm or less. In some embodiments, the stroke is 10 mm. The stroke is defined by the range of movement of the ram between a fully extended position and a fully retracted position. The fully extended position may be limited by a first abutment, provided on the ram (for example a shoulder), engaging with a second abutment, provided on the container (for example a portion of a container lid). The fully retracted position may be limited by a third abutment, provided on the ram (for example the base of the ram), engaging with a fourth abutment, provided on the container (for example a base of the container).

In general, by providing a gas spring in which at operating temperature the force on the ram is generated by gas alone, large amounts of force can be generated with a relatively small spring. This is unlike the situation when a mechanical spring is used, because a large force requires a large spring. This creates a requirement for a larger container to be used, adding to costs. In situations where a large force is required but there is only a small amount of space available to apply it, then mechanical only or hybrid gas and mechanical springs cannot always be used, whereas a gas spring having first and second chambers as described herein can be made relatively small whilst providing the required larger forces. When gas is being used to provide the force, it is relatively easy to tailor the amount of force without being subject to physical size constraints.

The container may have a first access passage for introducing into the first chamber the first pressure applying medium for applying pressure to extend the ram, and a second access passage for introducing into the second chamber the second pressure applying medium for applying pressure to retract the ram. The access passages may be closed off after pressure applying medium introduction, for example by insertion of a plug in each passage. The plugs may be welded in place.

A gas may be introduced into one of the chambers at the required pressure. This may be done through an access passage and using a load cell to set the pressure at ambient temperature, e.g. 20°C. The pressure is chosen taking account of the desired pressure to be reached at the higher operating temperature. A check valve may be provided in the access passage, and is beneficial to prevent escape of gas from the chamber after its introduction. In the case of the chamber which is to contain the substance which is to undergo a phase change, a check valve may also be used, for example if the substance is introduced into the chamber in liquid form.

however, the substance could be inserted in solid form, for example as ice in the ease of water, or as dry ice in the case of carbon dioxide. It is then not necessary to have a check valve. Rather, a precise amount of the substance needs to be inserted. The amount is chosen taking account of the desired pressure to be reached at the higher operating temperature.

Viewed from a third aspect the invention provides apparatus for making a gas spring as described herein, the apparatus comprising the container, the ram which is extendable and retractable in a direction out of and into the container, the first chamber defined in the container, the second chamber defined in the container, the first flexible wall comprising the first bellows connected to the ram and to the container to allow relative movement of the ram and the container, and the second flexible wall comprising the second bellows connected to the ram and to the container to allow relative movement of the ram and the container, wherein the container has a first access passage for introducing into the first chamber the first pressure applying medium for applying pressure to extend the ram, and a second -10 -access passage for introducing into the second chamber the second pressure applying medium for applying pressure to retract the ram.

The apparatus may be used by introducing to the respective chambers the first and second pressure applying media. The access passages may be closed off after pressure applying medium introduction, for example by insertion of a plug in each passage. ['he plugs may be welded in place. The apparatus may therefore include first and second plugs for closing off the first and second access passages respectively.

As discussed herein, the gas spring may be for a sliding refractory gate valve. It may also be suitable for other applications where the gas spring is used over a range of temperatures, such as from ambient temperature up to several hundred degrees centigrade, i.e. high temperatures. The gas spring can be used to maintain a relatively constant force over a range of temperatures of 100°C or more (e.g. 200°C to 300°C or more), or 200°C or more (e.g. 200°C to 400°C or more), or 300°C or more (e.g. 200°C to 500°C or more), unlike conventional gas springs for which the force of the spring increases as it heats up.

The gas spring may be used to apply a static force. In such an application, the ram applies a force to a subject to force it against an object in a static manner, i.e. without movement of the subject and object, except for taking up tolerances if necessary. A static force is generally required when the gas is used for a sliding refractory gate valve. In that case, a static force can be applied by the ram to a subject comprising a valve plate, to urge the valve plate against an object comprising another valve plate.

The gas spring may be used to apply a force of at least 300 kgk or at least 400 kgf, or at least 500 kgf.

Certain embodiments of the invention will now be described by way of example only and with reference to the drawings, in which: Figure 1 is a sectional view of a gas spring in accordance with the invention; Figure 2 is a sectional view of a refractory gate valve employing a plurality of the gas springs of Figure 1; and Figure 3 is a graph showing the variation of force with temperature of three examples using the gas spring of Figure 1 in comparison to two prior designs.

Rethning to Figure 1, the gas spring 2 comprises a metal cylinder 4 with a base 6 welded to the lower end of the cylinder. At the upper end of the cylinder 4, a lid 8 is welded thereto.

The cylinder 4, the base 6 and the lid 8 together form a container 3. A first, outer chamber 18 and a second, inner chamber 20 are provided in the container 3.

A ram 10 comprises a shaft 9 extending axially through an aperture 12 provided centrally of the lid 8. At its lower end the shaft 9 is provided with a blind bore 14 in which is received a guide bush 16 which projects upwardly from the base 6.

The ram has a shoulder 11 providing a first axial abutment which limits extension of the ram by being engageable with an inner peripheral portion 13 of the lid which forms a second axial abutment. The stroke of the ram, between a fully retracted position in which the base of the ram engages with the base 6 of the container 3, and a fully extended position in which the shoulder 11 engages with the inner peripheral portion 13 of the lid, is 10 mm in this embodiment.

When the gas spring is used as part of a refractory gate valve assembly, the ram will be at a position between the fully extended and fully retracted positions, which is determined by the constraints imposed by the rest of the assembly, such as the distance between the floor of a pocket receiving the gas spring and a valve plate on which the ram acts. In this embodiment the ram is intended to be at a position 4 mm -12 -from the fully extended position, in the direction of retraction. Because of the constraints, the ram moves very little when in operation.

The ram 10 comprises a radially projecting portion in the form of a comiecting ring 22 welded to the base of the shaft 9 of the ram 10. An inner periphery of the connecting ring 22 is welded in a recess defined at the base of the shaft 9 by a radially projecting flange 24.

A flexible wall in the form of first stainless steel bellows 26 extends axially and radially outwardly of the ram 10 between an outer periphery of the connecting ring 22 at the base of the ram and the lid 8 at the top of the container 3. The bellows 26 has a first annular periphery 28 at its lower end welded to the connecting ring 22 and a second annular periphery 30 at its upper end welded to the lid 8. Thus a fixed and permanent connection is provided between the bellows 26 and the connecting ring 22 of the ram 10, and a fixed and permanent connection is provided between the bellows 26 and the container 3.

Another flexible wall in the form of a second stainless steel bellows 32 extends axially, radially outwardly of the ram 10, and radially inwardly of the first bellows 26. The second, inner bellows 32 has a first annular periphery 34 at its lower end welded to the flange 24 of the ram 10. The bellows 32 has a second annular periphery 36 at its upper end welded to the lid 8. Thus a fixed and permanent connection is provided between the second bellows 32 and the connecting ring 22, and a fixed and permanent connection is provided between the second bellows 32 and the container 3.

It will be seen that the lower ends of both the first, outer bellows 26 and the second, inner bellows 32 are fixedly connected to the ram 10, whilst the upper ends of the first and second bellows are fixedly connected to the lid of the container, thereby allowing relative movement of the rain and the container. The inner bellows 32 -13 -effectively provides a flexible wall for both the outer chamber 18 and the inner chamber 20.

The first, outer chamber 18 is defined between the outer bellows 26, the cylinder 4, the base 6 and the lower end of the ram 10. An axially extending access passage 38 is provided in the base 6. This access passage is initially provided with a check valve (not shown) through which an inert gas, such as argon or nitrogen, is charged into the first chamber 18 at room temperature. This charging is done using a load cell so that the pressure in the chamber 18 at a known temperature can be set as appropriate.

Once the first pressure applying medium, in the form of an inert gas for example, is charged into the first chamber 18 via the access passage 38 and the check valve, the access passage 38 is closed by a plug (not shown) which is welded into place.

In use, at operating temperature, the pressure in chamber 18 acts on the lower faces of the connecting ring 22 and the ram 10 to urge the ram upwardly, to the right as seen in Figure 1.

The second chamber 20 is defined by the first bellows 26, the second bellows 32, an upper surface of the connecting ring 22 and a lower surface of the lid 8. The chamber 20 has a radially outer wall defined by the first bellows 26 and a radially inner wall defined by the second bellows 32. The space within the chamber 20 is exposed to the inside surface of first bellows 26 and the outside surface of second bellows 32.

The lid 8 is provided with an axially extending access passage 40. During assembly of the gas spring, the access passage 40 is provided with a check valve (not shown) permitting charging into the second chamber 20 of a second pressure applying medium. In the examples described herein, this medium is water. The volume of water is chosen as desired, taking account of the pressure which is to be attained in the second chamber 20 at operating temperature.

-14 -Once the second pressure applying medium has been introduced into the second chamber 20 via the access passage 40 and the check valve, the opening is closed by a plug (not shown) which is welded into place. Since the second pressure applying medium is a liquid such as water, or a solid, the use of a check valve in the access passage 40 is not essential, and the check valve may be omitted.

In use, the pressure in chamber 20 acts on the upper face of the connecting ring 22 to urge the ram 10 downwardly, to the left as seen in Figure 1. The pressure is applied by the water in a gaseous phase, having undergone a phase change to the gaseous phase as the temperature increased from ambient to the operating temperature.

thus, a sealed unit is created, comprising a first chamber with a first pressure applying medium therein, and a second chamber with a second pressure applying medium therein. The first pressure applying medium acts on one face (the lower face) of the connecting ring to extend the ram 10, i.e. urge it in an upward direction, to the right as seen in Figure 1. The second pressure applying medium acts on the opposite face of the connecting ring (the upper face) and acts to retract the ram by urging it downwardly, or to the left as seen in Figure 1. The ram is axially guided at its lower end by the guide bush 16 and at its upper end by the axial aperture 12 in the lid 8. Radially inwardly of the second bellows 32, the space mainly occupied by the ram 10 is at atmospheric pressure.

Figure 2 shows a refractory gate valve assembly 50 comprising a fixed plate 52 and a slidable plate 54 in contact therewith, and a plurality of gas springs 2 each having a ram 10 urging the slidable plate 54 towards the fixed plate 52. The slidable plate 54 and the gas springs 2 are together provided on a carriage 62 capable of sliding laterally, to the left and right as shown in the Figure. Fixed plate 52 is formed with a bore 56 and the slidable plate 54 is formed with a bore 58. The bores 56 and 58 are shown in alignment, allowing molten steel to pass from a vessel (not shown) above the valve assembly out through a nozzle 60. Depending on the position of the -15 -slidable plate 52 the aperture provided by the two bores increases or decreases thereby determining the flow rate of the steel from the vessel and through the nozzle. The gas springs 2 provide upward force on the slidable plate 54 so that its upper face is in a high pressure engagement with the lower face of the fixed plate 52 to ensure that molten steel cannot get between the plates.

Referring again to the gas spring as shown in Figure 1, in an example, the first chamber 18 was charged with nitrogen gas at 20°C to a desired pressure, and the second chamber 20 was charged with a specific volume of water. The valves were plugged and sealed by welding. The temperature was increased. For temperatures below 200°C the water in the second chamber has no, or very little, effect on the overall force exerted by the ram. Thus, the force is exerted mainly as a result of the increase in pressure in the first chamber as the nitrogen heats up. However, once the temperature starts to rise above 200°C, the pressure within the second chamber starts to increase, due to vaporisation of the water at the higher temperature. Pressure in the first chamber also increases but the build-up of pressure in the second chamber balances the pressure differential between the two chambers.

The result is that both chambers maintain a balanced pressure as the temperature increases. That is, the force on the ram caused by the pressure in the first chamber is reduced by the pressure in the second chamber which acts on the ram in the opposite direction. By using a medium in the first chamber which is in the gaseous state throughout the operating range of the gas spring, and a medium in the second chamber which changes phase from liquid to gas over the operating range, a positive net force tending to extend the ram will be maintained at low temperatures, but at higher temperatures the higher pressure in the first chamber does not lead to a higher net force on the ram. This is because of the counteracting force on the ram caused by the pressure in the second chamber as the water turns into water vapour at temperatures above 200°C.

Tests were carried out using the embodiment described in three examples. In Example A the second chamber was charged with 0.6 ml of water, in Example B, -16 -the second chamber with charged with 0.5 ml of water and in Example C the second chamber was charged with 0.4 ml of water. The net force on the ram was measured. An initial force reading at the ambient temperature of 20°C was taken, and further readings were taken at 100°C and at 100°C intervals up to 500°C (600°C for

Example B).

For comparative purposes, corresponding tests were carried out on a known gas spring of the kind described in the introduction above (i.e. with a single gas chamber acting to extend the ram), and on a known hybrid spring of the kind described above (i.e. with a single gas chamber and a mechanical spring acting together to extend the ram). The results are shown in the table below and are plotted in Figure 2.

Tempera! tire Known gas spring (kg!) Known hybrid Example A, 0.6 ml (kg!) Example B, 0.5 ml (kgf) Example C, 0.4 ml (kg!) (°C) spring (kg!) 570 560 570 580 580 725 605 670 678 654 920 660 790 782 766 300 1115 715 650 660 645 400 1309 770 630 635 700 500 1504 825 624 689 800 600 790 The results show that the examples, employing pressure balancing, have a much flatter force/temperature gradient than that of the known gas spring. The change in force between 20°C and 500°C for Example B is 109 kgf, compared to 934 kgf from the known single chamber gas spring. Whilst the known hybrid gas spring also achieved a smaller change than the single chamber gas spring (a change or 265 kgf between 20°C and 500°C), the embodiment used in the example does not make use of a mechanical spring to achieve a smaller change result and therefore avoids the vulnerability to relaxation or creep of such a spring at high temperatures.

-17 -The gas spring shown in Figure 1 is designed so that outer bellows 26 fails before inner bellows 32. If there is failure of bellows 26 during operational temperatures, the gases in the two chambers combine to provide an increased volume of gas.

However, at the same time there is no change in the overall volume of the combined chambers, so that after failure pressure continues to be applied to extend the ram.

A test was carried out in which bellows 26 was cycled to failure. The force on the ram after failure, from a force reading taken at 500°C, was 790 kgf. Once the gas spring had cooled down to ambient temperature the force reading was 250 kgE The designed in failure mode allows the gas spring to generate the required force during operation even though the bellows 26 fails. Once the failed gas spring has cooled its overall length when unloaded, would be reduced compared to its unloaded length before failure. An operator can therefore become aware of this failure by observing a reduced force reading after cooling, indicating gas spring failure.

When operating a sliding refractory gate valve assembly, it is usual to employ several springs to act on the slidable refractory plate, as shown in Figure 2. In the case of known hybrid springs, these each contain a mechanical compression spring and when these are exposed to temperatures above their normal operating temperatures relaxation tends to cause all the springs to fail at the same time. However, it is very unlikely for any two gas springs as described herein to fail at exactly the same time. Hence these gas springs have this further advantage over the hybrid springs, whilst providing a similar force profile with increasing temperature, i.e. a relatively constant or flat force profile.

It will be noted that the pressure differential across the outer bellows 26 is the difference between the pressures in the outer and inner chambers 18 and 20, and the pressure differential across the inner bellows 32 is the difference between the pressure in the inner chamber 18 and atmospheric pressure. These pressure differentials are generally less than that across the bellows of a single chamber gas -18 -spring. With lower pressure differentials, a greater fatigue life can be achieved.

Claims

-19 -CLAIMS1. A gas spring for use at temperatures equal to or greater than 200°C, comprising a container, a ram which is extendable and retractable in a direction out of and into the container, a first chamber defined in the container, a second chamber defined in the container, a first pressure applying medium in the first chamber for applying pressure to extend the ram, and a second pressure applying medium in the second chamber for applying pressure to retract the ram, at least one of the first and second pressure applying media comprising a substance which in use of the gas spring undergoes a phase change to a gaseous phase as the temperature of the gas spring increases from ambient temperature.
2. A gas spring as claimed in claim 1, for a sliding refractory gate valve.
3. A gas spring as claimed in claim 1 Or 2, wherein at least one of the chambers has a flexible wall connected to the ram and to the container to allow relative movement of the ram and the container.
4. A gas spring as claimed in claim 1 or 2, wherein each of thc first and the second chambers has a flexible wall connected to the ram and to the container to allow relative movement of the ram and the container.
5. A gas spring as claimed in claim 4, wherein one of the flexible walls is exposed on one side thereof to the pressure inside one of the chambers and on the other side thereof to the pressure inside the other of the chambers.
6. A gas spring as claimed in claim 5, wherein the other flexible wall is exposed on one side thereof to said pressure inside said one of the chambers and on the other side thereof to atmospheric pressure.
7. A gas spring as claimed in claim 6, wherein said one flexible wall is configured to fail before said other flexible wall.

-20 -
8. A gas spring as claimed in any of claims 3 to 7, wherein the or each flexible wall has a first annular periphery connected to the ram and a second annular periphery connected to the container.
9. A gas spring as claimed in any of claims 3 to 8, wherein the or each flexible wall comprises a bellows.
10. A gas spring as claimed in any of claims 3 to 8, wherein the first chamber has a first flexible wall comprising a first bellows, and the second chamber has a second flexible wall comprising a second bellows.
11. A gas spring as claimed in claim 10, wherein the first and second bellows are arranged concentrically with each other.
12. A gas spring as claimed in any preceding claim, wherein the ram comprises a shall, and a portion projecting radially from the shaft, and wherein the first and second chambers are configured so that in use the first and second pressure applying media apply pressure to the radially projecting portion of the ram in opposite directions.
13. A gas spring as claimed in any preceding claim, wherein the second pressure applying medium comprises said substance which in use undergoes a phase change to become a gaseous phase as the temperature of the gas spring increases from ambient temperature.
14. A gas spring as claimed in any preceding claim, wherein the ram has a stroke of 10 nun or less.
15. Apparatus for making a gas spring as claimed in claim 10 or 11, or as claimed in claim 12, 13 or 14 when dependent on claim 10 or 11, the apparatus -21 -comprising the container, the ram which is extendable and retractable in a direction out of and into the container, the first chamber defined in the container, the second chamber defined in the container, the first flexible wall comprising the first bellows connected to the ram and to the container to allow relative movement of the ram and the container, and the second flexible wall comprising the second bellows connected to the ram and to the container to allow relative movement of the ram and the container, wherein the container has a first access passage for introducing into the first chamber the first pressure applying medium for applying pressure to extend the ram, and a second access passage for introducing into the second chamber the second pressure applying medium for applying pressure to retract the ram.
16. A refractory gate valve assembly comprising first and second plates in relatively slidable face to face contact with each other, and at least one gas spring urging one of the plates towards the other plate, the gas spring being as claimed in any of claims 1 to 14.
17. A gas spring substantially as hereinbefore described with reference to the accompanying drawings.

18, A refractory gate valve assembly substantially as hereinbefore described with reference to the accompanying drawings.