Determination of the level of accumulated solid particles

Abstract

A device 10 for determining the level of accumulated solid particles comprising a hollow elongate guide 12, contact members 18 & 20 mounted on and displaceable along the exterior of the hollow guide f

Description

The present invention relates to a device for determining the level of accumulated solid particles and to a hollow vessel comprising such a device. In many industrial processes, and in particular in oil and gas extraction, large quantities of particulate waste solids are produced. The waste solids are removed by one or more separation processes which may include, for example, the use of cyclone separators. The waste solids accumulate temporarily in a storage vessel from which they can be subsequently discharged. However, the conditions in which such waste solids are produced are often very harsh and, for example, often involve high pressures and / or temperatures. Therefore, it is difficult to establish the level of accumulated solids reliably as the operating conditions often preclude the use of conventional level-determining measures such as sight glasses and the like. In addition, any device which involves moving parts for establishing the level of accumulated solids typically encounters problems in sealing when transferring the motion of the moving parts to the exterior of an accumulation vessel in order to measure the level, particularly if the vessel is pressurised. Consequently, it is common for the accumulated solids to be discharged periodically from the vessel, but this can result in the solids being discharged too soon, which results in an undesirable and unnecessary pressure drop, or being discharged too late, in which the accumulated solids have exceeded the capacity of the vessel. In accordance with a first aspect of the present invention, a device for determining the level of accumulated solid particles comprises: a hollow elongate guide; a contact member for contacting the surface of the accumulated solids, mounted on the exterior of the hollow elongate guide and displaceable along the guide; a body located within and displaceable along the hollow elongate guide; and displacement means for determining the position of the body along the hollow elongate guide; the contact member and the body being magnetically attracted to each other and the contact member being displaceable along the exterior of the hollow elongate guide by displacement of the body within the guide. By displacing the body downwardly along the hollow elongate guide, the magnetic attraction with the contact member causes it also to be drawn downwardly along the exterior of the guide until it contacts the surface of the accumulated solid particles. By determining the position of the body at that point, the level of the accumulated solid particles can be determined. The hollow elongate guide can be reliably and sealingly mounted in the wall of a pressurised accumulation vessel, as it does not move with respect to the vessel. The body can therefore be displaced relatively freely within the hollow elongate guide without encountering the conditions within the vessel to which it is fitted. The contact member can be moved along the hollow elongate guide by moving the body, but there is no requirement for a mechanical connection between the contact member and the exterior of the vessel, which therefore avoids any problems of sealing of moving parts with respect to the vessel. By periodically displacing the body downwardly along the elongate hollow guide to bring the contact member into contact with the surface of the accumulated solid particles and then displacing it upwardly to disengage the contact member from the accumulated solid particles, a periodic indication of the level of the accumulated solid particles can be obtained, which will assist in determining an appropriate time to discharge the particles from a vessel in which the device is fitted. Preferably, the contact member is slidably mounted on the hollow elongate guide. In one embodiment, the contact member comprises a base mounted on the exterior of the hollow elongate guide and displaceable along the guide, and one or more projections extending from the base. In such an arrangement, the projections rather than the base can be configured to engage the accumulated solid particles, which will help to reduce accumulation of solid particles between the base and the hollow elongate guide, which might otherwise interfere with the movement of the base. The or each projection may comprise an arm. The or each arm may be pivotally mounted. Pivotally mounting the arms can assist in passing the device through an aperture in a wall of a vessel into which the device is to be installed. The or each arm may be spring-biased. This allows the arms to be pivoted in order to pass the device through an aperture in a wall of a vessel and to spring back into a deployed position after passing through the aperture. Preferably, the base is complementarity shaped with the exterior surface of the hollow elongate guide. For example, the base may be annular. The magnetic attraction between the body and the contact member may be achieved in three ways, namely: (a) the body located within and displaceable along the hollow elongate guide is magnetic; (b) the contact member is magnetic; (c) both the body and the contact member are magnetic. In one embodiment, the body is attached to an elongate member and the device comprises means for controlling the longitudinal position of the elongate member. The elongate member may comprise a rod. The elongate member may comprise a cable. In one embodiment, the body is sealingly and slidably mounted in a guide tube positioned within the hollow elongate guide, and the device comprises means for selectively applying fluid pressure to the interior of the guide tube. The device may comprise means for selectively applying fluid pressure to the interior of the guide tube on opposite sides of the body. The device may comprise a fluid conduit positioned between the hollow elongate guide and the guide tube and in fluid communication with a portion of the guide tube below the body. The device may comprise means for selectively applying fluid pressure to the interior of the guide tube on one side of the body. The device may comprise spring means on the opposite side of the body to which fluid pressure is selectively applied, for example a gas spring. Preferably, the longitudinal axes of the hollow elongate guide and the inner elongate guide tube are parallel. For example, the longitudinal axes of the hollow elongate guide and the inner elongate guide tube may be coaxial. The device may comprise a fluid conduit positioned between the hollow elongate guide and the guide tube and in fluid communication with a portion of the guide tube below the body. In accordance with a second aspect of the invention, a hollow vessel comprises an enclosing wall defining a cavity for accumulation of solid particles, an inlet in the enclosing wall, an outlet in the enclosing wall at a level below the inlet and a device in accordance with the first aspect of the invention secured in position with respect to the enclosing wall, wherein the hollow elongate guide extends into the hollow vessel. Preferably, the hollow elongate guide is oriented substantially vertically. The vessel may also comprise a valve for controlling flow from the outlet. The inlet may be arranged at an uppermost portion of the enclosing wall. The outlet may be arranged at a lowermost portion of the enclosing wall. In one embodiment, the enclosing wall comprises an aperture in which the hollow elongate guide is located. Preferably, the hollow elongate guide is sealed with respect to the exterior of the enclosing wall. Preferably, the internal space within the hollow elongate guide is isolated from the accumulation cavity. By way of example only, specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a first embodiment of solids level indicator in accordance with the invention; Figure 2 is horizontal cross-section through the solids level indicator of Figure 1, looking in the direction of arrows II - II; Figure 3 is a side view the solids level indicator of Figure 1; Figure 4 is a vertical cross-section through a solids accumulation vessel to which the solids level indicator of Figure 1 is fitted, shown with the indicator in a first position; Figure 5 is a vertical cross-section through a solids accumulation vessel to which the solids level indicator of Figure 1 is fitted, shown with the indicator in a second position; Figure 6 is a vertical cross-section through a solids accumulation vessel to which the solids level indicator of Figure 1 is fitted, shown with the indicator in a third position; Figures 7(a), 7(b) and 7(c) are vertical cross sections through a second embodiment of solids level indicator in accordance with the invention, shown in first, second and third positions respectively, with the portion A of Figure 7(a) also shown at an enlarged scale; Figure 8 is a vertical cross-section through a solids accumulation vessel to which the solids accumulation vessel of Figure 6 is fitted; and Figure 9 is a vertical cross-section through a third embodiment of solids level indicator in accordance with the invention, which is a modification of the second embodiment and which is shown in a position corresponding to Figure 7(c) of the second embodiment. The first embodiment of solids level indicator 10 shown in Figures 1 to 3 comprises a straight stainless steel tube 12 of circular cross-section which is sealingly closed at a lower end by an end cap 14 and which has an annular sealing collar 16 secured to the opposite end, as will be explained. A base in the form of an annular stainless steel carriage 18 having an internal diameter equal to or very slightly larger than the external diameter of the metal tube 12 is slidably mounted on the tube 12. The annular carriage 18 carries three identical straight, elongate, outwardly projecting arms 20, equally angularly spaced around its periphery. The arms extend radially outwardly with respect to the longitudinal axis of the tube 12 when viewed from above, and are also inclined downwardly to the horizontal at an angle of approximately 18°. In this particular embodiment, the inner end of each of the arms 20 is pivotally mounted to the carriage 18 between two parallel mounting flanges 22. Each arm 20 is biased upwardly into the position illustrated in Figures 1 to 3 by means of a spring (not visible) and can be pivoted downwardly against the restoring force of the spring into a position parallel to the longitudinal axis of the tube 12 (shown in chain dot in Figure 3) to facilitate fitting of the indicator, as will be explained. A cylindrical body 24 having an external diameter slightly smaller than the internal diameter of the tube 12 is secured to one end of an elongate straight rod 26. The body 24 is positioned within the tube 12 and its longitudinal position with respect to the tube can be adjusted by means of an actuator (shown schematically at 28) connected to the rod 26 by means of which the position of the rod 26, and therefore the position of the body 24, can be adjusted and measured. The annular carriage 18 and the body 24 are configured so that they are magnetically attracted to each other and the magnetic attraction is sufficiently strong to be able to adjust the position of the carriage 18 along the exterior of the stainless steel tube 12 by adjusting the position of the body 24 within the tube 12. This can be achieved in three ways. In a first variant, the body 24 is magnetic. In a second variant, the annular carriage 18 is magnetic. In a third variant, both the body 24 and the annular carriage 18 are magnetic. In the following description of Figures 4 to 6, reference will be made to the first variant, in which the body 24 comprises a strong cylindrical magnet. However, the invention is equally applicable to the second and third variants referred to above. Figures 4 to 6 illustrate the solids level indicator as fitted to a spherical accumulator vessel shown generally at 32. In this particular example, the accumulator vessel 32 comprises a hollow spherical wall 34 having an inlet aperture 36 at its uppermost point and an outlet aperture 38 at its lowermost point to which a controllable outlet valve 39 is attached. A cylindrical through aperture 40 is provided in the wall 34 of the vessel 32 adjacent to, and offset from, the inlet aperture 36 and is aligned with a tubular mounting sleeve 42 having the same internal diameter as the cylindrical aperture 40 and which is secured to and projects from the exterior face of the spherical wall 34. The longitudinal axes of the aperture 40 and the mounting sleeve 42 are aligned with a vertical axis, i.e. the axis A - A passing through the inlet and outlet apertures 36, 38. In use, the arms 20 of the solids level indicator 10 are pivoted downwardly against the restoring force of the springs so that they lie approximately parallel to the longitudinal axis of the stainless steel tube 12 (as shown in chain dot in Figure 3). The solids level indicator 10 can then be fed through the mounting sleeve 42 and the aperture 44 of the accumulator vessel until the undersurface of the annular sealing collar 16 abuts the upper end of the mounting sleeve 42. The solids level indicated 10 is then sealingly secured in position in the mounting sleeve 42. After the carriage 18 has passed through the aperture 40 and into the interior of the accumulator vessel 32 the arms 20 spring back into their deployed position shown in Figures 1 to 3. The magnet 24 is then lowered into the tube 12 by means of the rod 26 and, as explained previously, its position can be adjusted within the tube 12 in order to displace the annular carriage 18, and the arms 20 mounted on the carriage 18, along the tube 12. In operation, the position of the magnet 24 is initially adjusted to withdraw the annular carriage 18 and the arms 20 carried by it to their maximum uppermost extent, as shown in Figure 4. During this period, solids will enter the accumulator vessel 32 through the inlet aperture 36 and will accumulate at the base of the vessel 32 as shown at 50. Periodically, the magnet 24 is lowered slowly further into the tube 12, which in turn draws the annular carriage 18 and the arms 20 downwardly along the length of the tube 12. As the arms 20 come into contact with the accumulated solids 50, it eventually becomes impossible for the carriage 18 and the arms 20 to be moved further downwardly along the tube 12 and any attempt to do so will result in a resistive force on the rod 26 which can be detected by the rod actuator 28, for example by an increase in the current and / or voltage in a motor of the actuator. The depth of the accumulated solids 50 can also be determined by measuring the extent to which the magnet 24 and rod 26 are inserted into the tube 12 when the increase in force is detected. Figure 5 shows the arms 20 in contact with the accumulated solids 50 when the vessel 32 is approximately half full. The position of the magnet 24, and therefore the position of the carriage 18 and arms 20, is varied periodically between its uppermost, withdrawn position shown in Figure 4 in which the arms 20 lie above the level of the accumulated solids 50 and a position shown in Figure 5 in which the arms are in contact with the surface of the accumulated solids. Depending on the fill level of the accumulated solids 50 as detected by the actuator 28, the valve 39 can be actuated to discharge the accumulated solids from the vessel 32. In the lowermost portion of the carriage, shown in Figure 6, the level of accumulated solids 50 is below the lowermost ends of the arms 20 and one of the arms 20 touches the interior face of the vessel wall 34. A second embodiment of the invention is illustrated in Figures 7 and 8. The second embodiment is a modification of the first embodiment and in Figures 7 and 8 and corresponding features are identified with the same reference numerals increased by 100. As for the first embodiment, annular carriage 118 and the body 124 are configured so that they are magnetically attracted to each other and the magnetic attraction is sufficiently strong to be able to adjust the position of the carriage 118 along the exterior of the stainless steel tube 112 by adjusting the position of the body 24 within the tube 112. This can be achieved in three ways. In a first variant, the body 124 is magnetic. In a second variant, the annular carriage 118 is magnetic. In a third variant, both the body 124 and the annular carriage 118 are magnetic. In the following description of Figures 7 to 10, reference will be made to the first variant, in which the body 124 comprises a strong cylindrical magnet. However, the invention is equally applicable to the second and third variants referred to above. The principal modification is that whereas the magnet 24 of the first embodiment is displaced mechanically, in the second embodiment the magnet 124 is displaced by application of pneumatic or hydraulic pressure. The second embodiment of solids level indicator 110 shown in Figures 7 and 8 comprises a straight stainless steel tube 112 of circular cross-section which is sealingly closed at its lower end by an end cap 114 and which has an annular sealing collar 116 secured to the opposite, upper end, as will be explained. A base in the form of an annular stainless steel carriage 118 having an internal diameter equal to or very slightly larger than the external diameter of the metal tube 112 is slidably mounted on the tube 112. The annular carriage 118 is identical to the annular carriage 18 of the first embodiment and will not be further described. A further stainless steel tube 160 is mounted coaxially within the tube 112, and a strong cylindrical magnet 124 having an external diameter slightly smaller than the internal diameter of the inner tube 160 is slidably disposed within the tube 160 and is a sealing fit within the inner tube 160 by means of spaced-apart annular O-ring seals 162 fitted into corresponding annular grooves in the outer surface of magnet 124. The lower end of the inner tube 160 is closed off by the end cap 114 and the upper end of the inner tube 160 is closed off by an end cap 166 having a fluid port 168. A further stainless steel tube 170 is located between the outer and inner tubes 112, 160 and operates as a conduit for pressurised air or hydraulic fluid. The lower end of the tube 170 is closed off by the end cap 114 and the upper end of the tube 170 has a closed end connected to a fluid hose 172. It will also be observed that the interior volume of the portion of the tube 160 below the magnet 124 and the conduit 170 are fluidly connected to each other by means of a connecting passage 176 just above their lower ends. A 3-port, 3-position valve 180 has outlets connected respectively to the tubes 160,170 through the end caps 166 and the fluid hose 172 and its position is adjustable to selectively connect a source 182 of fluid pressure to the tubes 160, 170, as will be explained. In this example, the source 182 of fluid pressure is a source of hydraulic pressure, but it may instead be a source of pneumatic pressure. The pressure in the tube 160 is also monitored by means of a pressure sensor 184. By controlling the pressure in the tubes 160, 170, the longitudinal position of the magnet 124 with respect to the inner tube 160 can be adjusted and measured. In a first position of the valve 180, shown in Figure 7(a), the source 182 of fluid pressure is applied to the tube 170 and the upper end of tube 160 is connected to a sump or drain 186. Consequently, fluid pressure within the tube 170 is transferred via the connecting passage 176 into the volume of the tube 160 below the magnet 124, which causes the magnet 124 to be displaced upwardly within the tube 160, which in turn draws the carriage 118 and the arms 120 carried by it upwardly with it. If pneumatic fluid is used instead of hydraulic fluid, the sump or drain 186 can be omitted and the upper end of the tube 160 can be vented to atmosphere. In a second position of the valve 180, shown in Figure 7(b), the source 182 of fluid pressure is isolated from the interior of the tubes 160,170, which retains the magnet 124 - and therefore the carriage 118 and the arms 120 carried by it - in a fixed position. In a third position of the valve 180, shown in Figure 7(c), the source 182 of fluid pressure is applied to the region of the tube 160 above the magnet 124 and the upper end of tube 170 is connected to the sump or drain 186. Consequently, fluid pressure within the tube 160 causes the magnet 124 to be displaced downwardly within the tube 160, which in turn also displaces the carriage 118 and the arms 120 carried by it downwardly along the tube 112. If pneumatic fluid is used instead of hydraulic fluid, the sump or drain 186 can be omitted and the upper end of the tube 170 can be vented to atmosphere. As in the first embodiment, the solids level indicator 110 is fitted to a spherical accumulator vessel shown generally at 132 in Figure 8. As for the first embodiment, the accumulator vessel 132 comprises a hollow spherical wall 134 having an inlet aperture 136 at its uppermost point and an outlet aperture 138 at its lowermost point. A cylindrical through aperture 140 is provided in the wall 134 of the vessel 132 adjacent to, and offset from, the inlet aperture 136 and is aligned with a tubular mounting sleeve 142 having the same internal diameter as the cylindrical aperture 140 and which is secured to and projects from the exterior face of the spherical wall 134. The longitudinal axes of the aperture 140 and the mounting sleeve 142 are aligned with an axis A - A passing through the inlet and outlet apertures 136,138. As for the first embodiment, in use, the arms 120 of the solids level indicator 110 are pivoted downwardly against the restoring force of the springs so that they lie approximately parallel to the longitudinal axis of the stainless steel tube 112. The solids level indicator 110 can then be fed through the mounting sleeve 142 and the aperture 144 of the accumulator vessel until the undersurface of the annular sealing collar 116 abuts the upper end of the mounting sleeve 42. The solids level indicator 110 is then sealingly secured in position in the mounting sleeve 42. After the carriage 118 has passed through the aperture 140 and into the interior of the accumulator vessel 132 the arms 120 spring back into their deployed position shown in Figure 6. The inner tube 160 with the magnet 124 located in it and the further tube 170 which acts as the conduit for pressurised fluid are lowered into the tube 112 and secured in position. The outlet conduits from the valve 180 are then connected respectively to the tubes 160, 170. In operation, the valve 180 is operated to apply pressure to the portion of the tube 112 below the magnet 124 in order to displace the magnet 124 upwardly and thereby withdraw the annular carriage 118 and the arms 120 carried by it to their maximum uppermost extent, as shown in Figure 7(a). During this period, solids will enter the accumulator vessel 132 through the inlet aperture 136 and will accumulate at the base of the vessel 132 as shown at 150. Periodically, the valve 180 is operated to apply pressure to the portion of the tube 160 above the magnet 124 in order to displace the magnet 124 slowly downwardly further into the tube 160 and along the tube 112, which in turn draws the annular carriage 118 and the arms 120 downwardly along the length of the tube 112. As the arms 120 come into contact with the accumulated solids 150, it eventually becomes impossible for the carriage 118 and the arms 120 to be moved further downwardly along the tube 112 and any attempt to do so will result in an increase in the pressure in the portion of the tube 112 above the magnet 124, as detected by the pressure sensor 184. The depth of the accumulated solids 150 can then be determined by measuring the amount of fluid supplied to the tube 112 above the magnet 124. Pressure is then applied to the conduit 170 to displace the magnet 124 - and therefore the carriage 118 and arms 120 - upwardly along the tube 112, and the procedure is repeated periodically. A third embodiment of the invention is shown in Figure 9. The third embodiment is a modification of the second embodiment, and the same reference numerals have been used to identify corresponding features. The 3-port, 3-position valve 180 of the second embodiment has been replaced with a 2-port, 2-position valve 180a. In addition, the further tube 170 and the connecting passage 176 of the second embodiment have been omitted and the region of the tube 160 below the magnet 124 is filled with pressurised gas 190, e.g. nitrogen, which is sealed by the O-ring seals 162 mounted on the magnet to form a spring which urges the magnet 124 towards the upper end of the tube 160. As for the second embodiment, in a first position of the valve 180a, shown in Figure 9, the valve 180a is operated to apply pressure to the portion of the tube 160 above the magnet 124 in order to displace the magnet 124 slowly downwardly further into the tube 160 and along the tube 112, which in turn draws the annular carriage 118 and the arms 120 downwardly along the length of the tube 112. As the arms 120 come into contact with the accumulated solids 150, it eventually becomes impossible for the carriage 118 and the arms 120 to be moved further downwardly along the tube 112 and any attempt to do so will result in an increase in the pressure in the portion of the tube 112 above the magnet 124, as detected by the pressure sensor 184. The depth of the accumulated solids 150 can then be determined by measuring the amount of fluid supplied to the tube 112 above the magnet 124. In the second position of the valve 180a, the source of pressure 182 is isolated from the tube 160 and the upper end of tube 160 is connected to the sump or drain 186. If pneumatic fluid is used instead of hydraulic fluid, the sump or drain 186 can be omitted and the upper end of the tube 160 can be vented to atmosphere. However, the gas 190 below the magnet 124 which was previously compressed as the magnet 124 moved downwardly is able to displace the magnet 124, and therefore the carriage 118 and arms 120 upwardly, out of engagement with the accumulated solids. The cycle is then repeated periodically. As an alternative, the nitrogen spring may be arranged above the magnet 124 and the fluid pressure may be applied to the opposite, lower side of the magnet, in which case the further tube 170 and the connecting passage 176 of the second embodiment would be retained. This would result in an arrangement in which the pressure from the fluid source 182 would be applied to displace the magnet 124 upwardly against the restoring force of the nitrogen spring located above it. The invention is not restricted to the details of the foregoing embodiments. For example, although the carriage 18,118 is shown as having three arms extending from it, fewer than or more than three arms may be present and the arms may or may not be pivotally mounted on the carriage. Alternatively, a continuously-extending projection, e.g. a flat annulus or a frusto-conical collar may be mounted on the carriage. Indeed, it may not be necessary for any projections to extend from the carriage, and the surface area of the undersurface of the carriage may sufficient so that it can contact the accumulated solids 50, 150 directly without sinking significantly into the accumulated solids, depending on the circumstances. In addition, the rod 26 may be replaced with a cable, for example a steel cable.

Claims

1. A device for determining the level of accumulated solid particles, comprising:a hollow elongate guide;a contact member for contacting the surface of the accumulated solids, mounted on the exterior of the hollow elongate guide and displaceable along the guide;a body located within and displaceable along the hollow elongate guide; anddisplacement means for determining the position of the body along the hollow elongate guide;the contact member and the body being magnetically attracted to each other and the contact member being displaceable along the exterior of the hollow elongate guide by displacement of the body within the guide.

2. A device as claimed in claim 1, wherein the contact member is slidably mounted on the hollow elongate guide.

3. A device as claimed in claim 1 or claim 2, wherein the contact member comprises a base mounted on the exterior of the hollow elongate guide and displaceable along the guide, and one or more projections extending from the base.

4. A device as claimed in claim 3, wherein the or each projection comprises an arm.

5. A device as claimed in claim 4, wherein the or each arm is pivotally mounted.

6. A device as claimed in claim 5, wherein the or each arm is spring-biased.

7. A device as claimed in any of claims 3 to 6, wherein the base is complementarityshaped with the exterior surface of the hollow elongate guide.

8. A device as claimed in any of claims 3 to 7, wherein the base is annular.

9. A device as claimed in any of the preceding claims, wherein the body located withinand displaceable along the hollow elongate guide is magnetic.

10. A device as claimed in any of the preceding claims, wherein the contact member is magnetic.

11. A device as claimed in any of the preceding claims, wherein the body is attached to an elongate member and the device comprises means for controlling the longitudinal position of the elongate member.

12. A device as claimed in claim 11, wherein the elongate member comprises a rod.

13. A device as claimed in claim 11, wherein the elongate member comprises a cable.

14. A device as claimed in any of claims 1 to 10, wherein the body is sealingly and slidablymounted in a guide tube positioned within the hollow elongate guide, and wherein the device comprises means for selectively applying fluid pressure to the interior of the guide tube.

15. A device as claimed in claim 14, wherein the device comprises means for selectively applying fluid pressure to the interior of the guide tube on opposite sides of the body.

16. A device as claimed in claim 14 or claim 15, comprising a fluid conduit positioned between the hollow elongate guide and the guide tube and in fluid communication with a portion of the guide tube below the body.

17. A device as claimed in claim 14, wherein the device comprises means for selectively applying fluid pressure to the interior of the guide tube on one side of the body.

18. A device as claimed in claim 17, comprising spring means on the opposite side of the body to which fluid pressure is selectively applied.

19. A device as claimed in claim 18, comprising a gas spring.

20. A device as claimed in any of claims 14 to 19, wherein the longitudinal axes of thehollow elongate guide and the inner elongate guide tube are parallel.

21. A device as claimed in claim 20, wherein the longitudinal axes of the hollow elongate guide and the inner elongate guide tube are coaxial.

22. A hollow vessel comprising an enclosing wall defining a cavity for accumulation of solid particles, an inlet in the enclosing wall, an outlet in the enclosing wall at a level below the inlet and a device as claimed in any of claims 1 to21 secured in position with respect to the enclosing wall, wherein the hollow elongate guide extends into the hollow vessel.

23. A hollow vessel as claimed in claim 22, wherein the hollow elongate guide is oriented substantially vertically.

24. A hollow vessel as claimed in claim 22 or claim 23, comprising a valve for controlling flow from the outlet.

25. A hollow vessel as claimed in any of claims 22 to 24, wherein the inlet is arranged at an uppermost portion of the enclosing wall.

26. A hollow vessel as claimed in any of claims 22 to 25, wherein the outlet is arranged at a lowermost portion of the enclosing wall.

27. A hollow vessel as claimed in any of claims 22 to 26, wherein the enclosing wall comprises an aperture in which the hollow elongate guide is located.

28. A hollow vessel as claimed in any of claims 22 to 27, wherein the hollow elongate guide is sealed with respect to the exterior of the enclosing wall.

29. A hollow vessel as claimed in any of claims 22 to 28, wherein the internal space within the hollow elongate guide is isolated from the accumulation cavity.

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