GB2293992A

GB2293992A - Treatment of particulate material

Info

Publication number: GB2293992A
Application number: GB9519710A
Authority: GB
Inventors: Neville Ernest Lange
Original assignee: Serck Baker Ltd
Current assignee: Cameron Products Ltd
Priority date: 1994-09-28
Filing date: 1995-09-27
Publication date: 1996-04-17
Also published as: GB9419484D0; GB9519710D0; WO1996009895A1

Abstract

Treatment of particulate material mixed with contaminants is performed by mixing the material with a fluid, feeding this slurry to a hydrocyclone 2 so that the contaminant is separated from the material and removed with the fluid through the hydrocyclone overflow 10 and the particulate material removed from the underflow 9. The particulate material may be fed into the hydrocyclone 2 to remove more contaminant material. The treatment particularly applies to the removal of oils and waxes from sand which collects in various types of vessels used in an offshore oil production facility. The sand is jetted with water to form a slurry which is fed to the hydrocyclone 2. <IMAGE>

Description

TREATMENT OF PARTICULATE MATERIAL This invention relates to the treatment of particulate material. In particular, it relates to a method and an apparatus for removing particulate material from a vessel, cleaning oil or other loosely adhering material from the surface thereof and conveying the material to a point of collection or discharge and optionally treating the water into which the removed oil has been taken.

The invention particularly relates to the treatment of the particulate material such as sand and scale that accumulates in the various separation and residence vessels that are found in oil production facilities, and particularly offshore oil production facilities. In these facilities the separations achieved in such vessels are primarily as a result of the gravity separation occurring during the time that the fluids are resident in the vessel and the vessels are carefully designed so that the flows of the fluids therein are such as to minimise the distances over which the gravity separation must occur and to minimise turbulence and high velocities within the fluids which would tend to re-entrain separated materials.The accumulation of the particulate material in a particular vessel will reduce the residence time and the flow paths of the fluid in the vessel, both effects thus tending to reduce the efficiency of the separation achieved. The excessive accumulation of particulate material can also lead to malfunction of and damage to instrumentation fitted to the separator vessel that are necessary to control its operation, and make it safe to operate. Particulate material will also collect by gravity separation in any vessel in which the fluids reside for any length of time and the present invention may be advantageously applied thereto.

In the treatment of well fluids a further problem is generated by the propensity of the water phase to produce scales, which can often exhibit a degree of radioactivity. The radiation levels in such scales are usually very low and do not pose a great danger unless ingested. The chance of ingestion of the radioactive particles increases markedly if the scales are allowed to dry and become airborne. These scales will also collect in separator vessels and they can pose hazards to personnel who have to handle or come into contact with them, which may be unavoidable in separator vessels that have no inbuilt solids removal systems and consequently have to be cleaned by persons entering the vessels.It is clearly preferable in this instance to have a handling system that avoids the requirement for human handling of any of these accumulated particulate materials and avoids the possibility of the material drying and becoming airborne.

The particulate material may be removed from the separator vessels by a process known as "jetting" whereby high velocity jets of fluid are directed into the accumulations of particulates to fluidise them, and this fluidised material is allowed to flow from the vessel via a connection adjacent the zone of fluidisation. This allows material to be removed from the vessel without human contact. On an offshore oil production facility this material may be directed immediately to the sea but because the water conveying the material, and the material itself may contain quantities of oil that would cause undesirable pollution of the sea, this practise is seldom acceptable.

In accordance with the invention there is provided an apparatus for the treatment of particulate material having contaminating material mixed therewith comprising means for mixing fluid with the contaminated particulate material, hydrocyclone means, having an underflow outlet and an overflow outlet, for receiving the mixture of fluid and contaminated particulate material, collecting means connected to the underflow outlet of the hydrocyclone for collecting the particulate material from which at least some of the contaminating material has been removed from the hydrocyclone means, and discharge means for safe disposal of said particulate material.

In accordance with a more limited aspect of the invention there is provided an apparatus for cleaning sand contaminated with contaminants comprising water jetting means to separate the sand from the interior surface of a vessel wherein the sand is situated to form a slurry, hydrocyclone means, having an inlet, an overflow outlet and an underflow outlet, into which inlet the slurry from the vessel is fed at high velocity to form a vortex and thereby separating the contaminants from the sand, a collecting vessel for receiving mainly clean sand and some water from the underflow outlet, water removing means for removing excess water from the collecting vessel, and disposal means for disposing the clean sand safely.

In accordance with the invention also there is provided a method for the treatment of particulate material having contaminating material mixed therewith comprising mixing fluid with the contaminated particulate material, feeding the mixture of fluid and contaminated particulate material to a hydrocyclone, means at high pressure to form a vortex, feeding the particulate material from which at least some of the contaminated material has been removed in the hydrocyclone means to a collecting means, and discharging the material therefrom safely.

In accordance with a more limited aspect of the invention also there is provided a method for cleaning sand contaminated with contaminants comprising jetting water against the interior surface of a vessel wherein the sand is situated, to separate the sand therefrom, forming a slurry of water and said separated sand, feeding the slurry to a hydrocyclone means at high velocity to form a vortex, thereby separating the contaminants from the sand, feeding the mainly clean sand to a collecting vessel, removing excess water from the collecting vessel, and disposing of the clean sand safely.

One embodiment of the invention will now be described by way of example only with reference to the single drawing, Figure 1, showing schematically part of an offshore oil production facility. In this description "sand" shall be taken to mean all types of particulate material that may accumulate in a separator. "Separator" shall mean any vessel through which the fluid containing the "sand" passes.

The system shown comprises several separator vessels 1 (four shown), a first hydrocyclone unit 2, a collection vessel 3, and a second hydrocyclone unit 4.

The removal and treatment of the sand from one separator vessel 1 will be considered. In this particular example clean water, i.e. water which should contain less than the maximum amount of oil that it is permitted to discharge into the sea, is available for use in the system, this clean water comes from a degasser vessel 5 - a part of this facility. The separator vessel 1 is fitted with a jetting system 6 that enables the accumulated sand to be fluidised and removed from it via one or more slurry outlet connections 7 on the vessel 1. The pressurised water source used to provide the flow of fluid to jet the separator vessel 1 may be provided by a pump 8 as shown in Figure 1. This pump 8 is termed the "jetting pump" and the pressurised water it provides is termed the "jetting water".The jetting water is raised to a pressure greater than the pressure in the separator vessel 1 so that it will flow into the vessel. The flow of jetting water is controlled by a jetting water control valve set 25. The removal and treatment of the sand from only one separator vessel 1 will be described; it will be clear that this method could equally well be applied to other vessels.

The system includes a first hydrocyclone unit, which contains one or more hydrocyclones designed for the separation of solid particles from liquids, which can discharge material into a collection vessel 3, the collection vessel being a further part of the facility. The hydrocyclone unit 2 is designed and operated so that it separates the sand from the incoming sand and water slurry, and passes the bulk of the sand with sufficient water to enable it to be conveyed in to the collection vessel 3 from its underflow outlet 9, and the bulk of the water containing some residual sand from its overflow outlet 10.In passing through a hydrocyclone in the unit 2 the particles that emerge from the underflow have in almost all cases been thrown to the interior wall of the hydrocyclone by the centrifugal forces some distance from the underflow outlet 9 and have been pushed and spun along the interior wall to the underflow port by the fluid flows inside the hydrocyclone. The action of the particles being pushed along the wall of the hydrocyclone serves to scrub and abrade oils, waxes and other loosely adhering materials (termed the "contaminants") from the surface of the particles. The size and design of a hydrocyclone affects the size of the particles it will separate and the severity and duration of the scrubbing that the particles are subjected to and hence the size and design of the one or more hydrocyclones incorporated in the hydrocyclone unit 2 must be chosen to suit the sizes of the particles and their adhering contaminants that it is anticipated will collect in the separator 1. The contaminants that are removed from the incoming particles will be separately transported by the liquid in the hydrocyclone. Those contaminants that are more dense than water will tend to emerge from the underflow outlet of the hydrocyclone unit, and those that are less dense, i.e. oils and waxes, than water will tend to emerge from the overflow outlet.It is unlikely that any significant concentration of the components of the contaminant will occur in the underflow outlet or overflow outlet of the first hydrocyclone unit as it has not been primarily selected for that duty, therefore because the bulk of the water exits from the overflow outlet of the first hydrocyclone unit 2, that water will carry the bulk of the contaminants.

The system includes a second hydrocyclone unit 4, which contains one or more hydrocyclones designed for the separation of materials less dense than the water from the water. This unit is located so that it receives the water from the overflow outlet 10 of the first hydrocyclone unit, and acts to separate the water entering it into two streams, a smaller flow rate stream from the overflow outlet of the second unit 4 containing the majority of the contaminants that are less dense than water i.e. waxes and oils, and a larger flow rate stream from the underflow which consists of water from which the majority of the oils and waxes have been removed. The underflow will also contain the majority of the contaminants that are more dense than water.

The overflow stream from the second hydrocyclone unit 4 could, because of its small flowrate, be treated in an existing oily water treatment process on the platform, for example the drains system.

The piping feeding fluid to the first hydrocyclone unit 2 is arranged so that this hydrocyclone can be fed with fluids jetted from the separator vessel 1 or with fluids from the collection vessel 3. Fluids taken from the collection vessel 3 are delivered to the first hydrocyclone unit 2 via a slurry recirculation pump 11. The slurry recirculation pump 11 need only generate sufficient pressure to cause the fluids to flow through the first hydrocyclone unit 2 and therefore wear of the pump by the slurry is minimised. The collection vessel 3 may have one or more connections that are used to deliver fluid to the positioned at the lowest point of the vessel for fully draining the vessel and the second 13 positioned high up the vessel at a position where it would not be covered by the settled accumulated sand.A tapping 14 is taken from the jetting water source into the collection vessel 3, which via a set of internal fitments, is used to fluidise sand that has accumulated in the vessel to form a slurry so that it may be drawn into the slurry recirculation pump 11 through the upper connection 13 on the vessel 3. The lower connection 12 to the slurry recirculation pump 11 may also be opened to allow some sand slurry to be drawn from the bottom of the vessel 3. Other arrangements may be used to produce a pumpable slurry from the accumulated sand in the collection vessel 3.

When the slurry is recirculated to the first hydrocyclone unit 2 by the slurry recirculation pump 11, the first hydrocyclone unit 2 separates most of the water from the slurry and directs towards the degassing vessel 5 via the second hydrocyclone unit 4 and the sand and remainder of the water pass from the underflow outlet 9 of the first hydrocyclone unit 2 back into the collection vessel 3. To maintain the level of the slurry in the collection vessel 3 the flowrate of the water coming form the jetting water source 8 must be set of match the flowrate of water passing from the overflow of the first hydrocyclone unit 3 via the second hydrocyclone overflow control valve set 23 towards the degasser 5. By doing this the sand in the collection vessel 3 may be "scrubbed"and "rinsed" with clean water as many times as is required to clean the sand to the required degree.Figure 1 shows that the water from the overflow outlet 10 of the first hydrocyclone unit 2 may alternatively be directed to the intake of the jetting pump rather than to the degasser by bypass line 15. This may be done to avoid disturbing the internal flows in the degasser, or if it is desirable to avoid mixing the two waters.

To remove the sand from the facility, it is run in recirculation mode and a valve 16 fitted after the slurry recirculation pump is opened to allow the slurry to flow overboard through a pipeline.

An alternative method of discharging the sand is as follows. While running in recirculation mode jetting water is introduced into the intake of the slurry recirculation pump 11. As the flow of this jetting water is increased it will prevent slurry being drawn into the pump 11 and circulated to the first hydrocyclone unit 2 and will purge sand out of those parts of the system.

The pump 11 may then be stopped and the flow of jetting water into the pump suction stopped. The sand in the collection vessel 3 may then be fluidised and directed overboard as described previously in the jetting of separators.

Before the slurry recirculation stage is begun, some of the water that has conveyed the slurry into the collection vessel 3 from the first hydrocyclone unit 2 may be drained from the vessel 3. This may be performed if that water contained a large quantity of oil (which could easily occur during the jetting of a separator 1) to reduce the amount of oil that would be recirculated with the slurry.

To jet the separator the following sequence of events occurs. Firstly the valve 16 between the hydrocyclone unit underflow outlet 9 and the collection vessel 3 is closed. The collection vessel 3 is drained down to the level set by the drain connections on the vessel and then the vessel is pressurised with gas to a pressure somewhat below that of the separator 1 that is to be jetted. The jetting pump 8 is switched on and initially the jetting water is injected into the slurry line 18 after the slurry outlet valve 17 on the vessel to be jetted via the conveying water control valve set 19 and attached piping. This water then flows through the slurry line 18 to the first hydrocyclone unit 2 from which it exits via the overflow outlet 10. The flow of water through the first hydrocyclone unit 2 generates a differential pressure in the same manner as an orifice plate, and a differential pressure transmitter which measures this pressure drop is used in conjunction with a flow control valve and flow controller to control the flowrate through the first hydrocyclone unit 2. Initially the flow of water from the jetting pump 8 into the slurry line 18 is set to be slightly higher than the flow through the first hydrocyclone unit 2. This causes the control valve in the conveying water control valve set 19 to go full open to attempt to achieve its set flow, and raises the pressure at the inlet to the first hydrocyclone unit 2 above the pressure of the separator 1. The slurry outlet valve 17 on the vessel 1 is then opened.A flow of water into the vessel occurs equal to the difference in set points of the conveying water control valve set 19 and the first hydrocyclone unit 2. This causes the pressure at the inlet 20 to the first hydrocyclone unit 2 to equalise with the pressure in the separator vessel 1, minus the line losses between the two. The flow into the jetting water manifolds 6 on the separator vessel 1 is then established to as to fluidise the sand within the vessel to form a slurry. The isolation valve 6 between the hydrocyclone unit 2 and the collection vessel 3 is then opened so that the sand separated by the hydrocyclone can be collected, and the flowrate of water introduced after the slurry outlet valve reduced.Because the first hydrocyclone unit 2 is drawing a constant amount of fluid from the vessel, reducing the flowrate of the conveying water below the flowrate of the hydrocyclone unit 2 causes the slurry to be drawn from the vessel 1 via the slurry outlet valves 17 to the first hydrocyclone unit 2 where it undergoes cleaning and separation.

The conveying water also dilutes the slurry and so adjustment of the conveying water flowrate may be used to control the sand content of the slurry sent to the first hydrocyclone unit 2. It may be desirable to do this to reduce the initially high sand concentration that occurs at the beginning of a conventional jetting cycle to reduce the risks of blockage of the slurry line or the hydrocyclone unit 2. A density measuring device (not shown) may be fitted to the slurry line 18 after the point at which the conveying water is injected, that is connected to a density control system that adjusts the flowrate of the conveying water to maintain the density of the slurry at a selected value.Also by controlling the flowrate of jetting water into the separator vessel 1 and slurry drawn out of the separator vessel to be equal or nearly equal it is possible to minimise the disruption to the separator vessel level control system, which if it were upset too severely could cause the separator vessel to lose efficiency and possibly cause it to shutdown, or upset any other process that may be going on inside the vessel 1.

The flowrate of fluid into the collection vessel 3 from the underflow outlet 9 of the hydrocyclone unit 2 is controlled by the pressure within the collection vessel 3, previously set somewhat below the pressure of the separator 1 to be jetted. As fluid enters the collection vessel 3 the pressure in the vessel is maintained at the appropriate value by the control valve venting the gas displaced from the vessel so as to maintain the desired flowrate from the underflow of the first hydrocyclone unit 2 into the vessel 3. When the level in the collection vessel 3 rises to a predetermined level the gas outlet control valve closes and after a short period the underflow from the first hydrocyclone unit 2 ceases as the remaining gas in the vessel is compressed to the pressure at the inlet of the first cyclone.The isolation valve 16 between the first hydrocyclone unit 2 and the collection vessel 3 is then closed. This technique of using the gas pressure in the collection vessel 3 is used to achieve flow control of the slurry flowing from the underflow outlet 9 without having to use a flow control valve on that stream. A valve could be used to achieve that flow control but would suffer rapid wear from the sand particles in the fluid passing through it.

The volume of the collection vessel 3 may be such that it can accept the underflow from the first hydrocyclone unit 2 for a long enough period to complete the jetting of the separator 1, or alternatively it may be of a lesser volume which would require the separator to be jetted in two or more stages.

In the latter case just prior to stopping the underflow of the first hydrocyclone unit 2 the flowrate of the conveying water injected after the slurry isolation valve 17 may be increased to stop slurry being withdrawn from the vessel 1 and to purge the sand from the slurry line which might otherwise block the line when the flow was stopped. The supernatant liquid in the collection vessel 3 could then be drained off and the separator jetting cycle could be repeated.

In an application where the separator vessel 1 is not operating at a high enough pressure for the slurry from the separator to flow through the hydrocyclone units, the slurry can be introduced to the inlet of the slurry recirculation pump 11 to boost its pressure prior to entering the hydrocyclone units.

An enhancement to the system is to fit an instrument to measure the density of the slurry entering the first hydrocyclone unit 2 or exiting its underflow and to use this measurement to adjust the underflow flowrate by adjusting the pressure in the collection vessel. This allows the minimum quantity of water to be taken into the collection vessel 3 with the separated sand and will increase the period that a separator 1 can be jetted before the collection vessel 3 fills with slurry. Density measurement can also be used to sense when a vessel 1 has been fully jetted: this may be taken to be when the concentration of sand in the slurry entering the first hydrocyclone unit 2 drops below a predetermined value, and to then stop the jetting of the vessel 1.It is desirable to minimise the time of jetting because jetting usually has an adverse affect on the separation efficiency of the separator, and to minimise the cycle time of the jetting and sand cleaning operation.

Another control strategy to enhance the operation of the system is to vary the pressure in the collection vessel 3 as a function of the time since the beginning of the jetting process, the vessel being at a lower pressure in the early stages of jetting when a high slurry sand concentration is expected and a larger underflow flowrate is required to convey the sand to the collecting vessel, and raising the pressure in the vessel in the later stages of the jetting to reduce the underflow flowrate when the slurry sand concentration is lower. This method of control would not bring as great an improvement as using the density measurement of the slurry to control the underflow flowrate because the flowrates would have to be set to be larger to allow the unit to cope with unexpected surges in slurry concentration that may occur.

Claims

CLAIMS:

1. Apparatus for the treatment of particulate material having contaminating material mixed therewith comprising means for mixing fluid with the contaminated particulate material, hydrocyclone means, having an underflow outlet and an overflow outlet, for receiving the mixture of fluid and contaminated particulate material, collecting means connected to the underflow outlet of the hydrocyclone for collecting the particulate material from which at least some of the contaminating material has been removed in the hydrocyclone means, and discharge means for safe disposal of said particulate material.

2. Apparatus according to Claim 1 comprising means for returning at least some of the particulate material from which at least some of the contaminating material has been removed from the collecting means to the hydrocyclone for removal of more contaminating material.

3. Apparatus according to Claim 1 or Claim 2 comprising pressurising means for increasing the pressure within the collecting means, the pressure so created being used to control the flow of fluid and particulate material from the hydrocyclone.

4. Apparatus according to any one of the preceding claims comprising a second hydrocyclone means, connected to the overflow of the hydrocyclone means, for receiving fluid containing contaminating material removed from the particulate material, and for separating said fluid from said contaminating material.

5. Apparatus for cleaning sand contaminated with contaminants comprising water jetting means to separate the sand from the interior surface of a vessel wherein the sand is situated to form a slurry, hydrocyclone means, having an inlet, an overflow outlet and an underflow outlet into which inlet the slurry from the vessel is fed at high velocity to form a vortex and thereby separating the contaminants from the sand, a collecting vessel for receiving mainly clean sand and some water from the underflow outlet, water removing means for removing excess water from the collecting vessel, and disposal means for disposing the clean sand safely.

6. Apparatus according to Claim 5, including recirculation means for returning sand from the collecting vessel to the hydrocyclone means for further cleaning.

7. Apparatus according to Claim 5 or Claim 6 comprising pressurising means for increasing the pressure within the collecting means, the pressure so created being used to control the flow of sand and water from the hydrocyclone means.

8. Apparatus according to any one of Claims 5, 6 or 7 comprising a second hydrocyclone means connected to the overflow outlet of the hydrocyclone means for separating the water from the mixture of water and contaminants from the hydrocyclone means.

9. Apparatus according to any one of Claims 5 to 8 comprising a density measuring device to measure the concentration of sand entering the hydrocyclone means thereby to determine when jetting has been completed.

10. Apparatus according to any one of Claims 5 to 8 comprising a density measuring device to measure the concentration of sand entering the hydrocyclone means thereby to adjust the pressure in the collecting vessel so as to maintain an underflow flow rate from the hydrocyclone means that is appropriate to the inlet sand concentration.

11. Apparatus according to any one of Claims 5 to 8 comprising a density measuring device to measure the concentration of sand entering the hydrocyclone means thereby to adjust the flow of conveying water that dilutes the slurry entering the hydrocyclone means.

12. Apparatus according to any one of Claims 5 to 11 comprising means for varying the pressure in the collecting vessel as a function of time thereby to regulate the flow from the underflow outlet of the hydrocyclone means to be appropriate for an anticipated time dependent variation of sand concentration in the slurry fed to the hydrocyclone means.

13. Apparatus according to any one of claims 5 to 12 wherein the disposal means prevents exposure of the sand to atmosphere.

14. Apparatus according to any one of Claims 5 to 13 comprising means for providing a controlled conveying water stream for transporting slurry from the vessel.

15. Apparatus according to Claim 14 wherein the conveying water stream determines the concentration of sand in the slurry entering the hydrocyclone means.

16. Apparatus according to any one of Claims 5 to 15 comprising means for controlling the pressure drop across the hydrocyclone means thereby controlling the throughput thereof.

17. Apparatus according to any one of Claims 5 to 16 comprising means for controlling the flow of jetting water.

18. Apparatus according to any one of Claims 5 to 17 comprising means for balancing the flow of jetting water to the vessel with the flow of slurry from the vessel thereby minimising the disturbance of the contents of the vessel.

19. Apparatus according to any one of Claims 5 to 8 comprising a plurality of vessels, a single hydrocyclone means, a single flow controlled conveying water source and a single flow controlled jetting water source.

20. A method for the treatment of particulate material having contaminating material mixed therewith comprising mixing fluid with the contaminated particulate material, feeding the mixture of fluid and contaminated particulate material to a hydrocyclone means at high pressure to form a vortex, feeding the particulate material from which at least some of the contaminated material has been removed in the hydrocyclone means, to a collecting means, and discharging the material therefrom safely.

21. A method according to Claim 20 comprising the step of returning at least some of the particulate material from which at least some of the contaminating material has been removed from the collecting means to the hydrocyclone means for removal of more contaminating material.

22. A method according to Claim, 20 or Claim 21 comprising increasing the pressure on the collecting means thereby controlling the flow of fluid and particulate material from the hydrocyclone.

23. A method according to any one of Claims 20 to 22 comprising feeding fluid containing contaminating material from the hydrocyclone means to a second hydrocyclone means and separating said fluid from said contaminating material therein.

24. A method for cleaning sand contaminated with contaminants comprising jetting water against the interior surface of a vessel wherein the sand is situated to separate the sand therefrom, forming a slurry of water and said separated sand, feeding the slurry to a hydrocyclone means at high velocity to form a vortex, thereby separating the contaminants from the sand, feeding the mainly clean sand to a collecting vessel, removing excess water from the collecting vessel, and disposing of the clean sand safely.

25. A method of cleaning sand according to Claim 24 comprising returning sand from the collecting vessel to the hydrocyclone means for further cleaning.

26. A method of cleaning sand according to Claim 24 or Claim 25 comprising increasing the pressure within the collecting means and controlling the flow of sand and water from the hydrocyclone means by means of the pressure so created.

27. A method of cleaning sand according to any one of Claims 24, 25 or 26 comprising separating water from a mixture of water and oils and waxes from the hydrocyclone means in a second hydrocyclone means.

28. A method of cleaning sand according to any one of Claims 24 to 27 comprising measuring the concentration of sand entering the hydrocyclone means and thereby determining when jetting has been completed.

29. A method of cleaning sand according to any one of Claims 24 to 28 comprising measuring the concentration of sand entering the hydrocyclone means and thereby adjusting the pressure in the collecting vessel so as to maintain an underflow flow rate from the hydrocyclone means that is appropriate to the inlet sand concentration.

30. A method according to any one of Claims 24 to 27 comprising measuring the concentration of sand entering the hydrocyclone means and thereby adjusting the flow of conveying water that dilutes the slurry entering the hydrocyclone means.

31. A method according to any one of Claims 24 to 30 comprising carrying the pressure in the collecting vessel as a function of time and thereby regulating the flow from the underflow outlet of the hydrocyclone means to be appropriate for an anticipated time dependent variation of sand concentration in the slurry fed to the hydrocyclone means.

32. A method according to any one of Claims 24 to 31 comprising disposing of the cleaned sand without exposure thereof to atmosphere.

33. A method according to any one of Claims 24 to 32 comprising providing a controlled conveying water stream for transporting slurry from the vessel.

34. A method according to Claim 33 wherein the conveying water stream determines the concentration of sand in the slurry entering the hydrocyclone means.

35. A method according to either of Claims 33 or 34 comprising using the conveying water to flush sand from pipelines carrying slurry from the vessel thereby to avoid blockage due to sand deposition.

36. A method according to any one of Claims 24 to 35 comprising controlling the pressure drop across the hydrocyclone means and thereby controlling the througput thereof.

37. A method according to any one of Claims 24 to 36 comprising controlling the flow of jetting water to the vessel.

38. A method according to any one of Claims 24 to 37 comprising balancing the flow of jetting water into the vessel with the flow of slurry from the vessel and thereby minimising the disturbance of the contents of the vessel.

39. A method according to any one of Claims 23 to 26 comprising separating sand from a plurality of vessels utilising a single hydrocyclone means, a single flow controlled conveying water source and a single flow controlled source of jetting water.

40. Apparatus for cleaning sand substantially as described herein with reference to and as illustrated in the accompanying drawing.

41. A method of cleaning sand substantially as described herein with reference to and as illustrated in the accompanying drawing.