GB2145344A - Agitating settled sludge in a storage tank - Google Patents

Abstract

Apparatus for use in clearing sludge from a storage tank containing a liquid such as oil comprises one or more sludge dislodging machines (15) mounted inside the tank (1), a readily transportable unit (19), including a pump, resting freely on the ground closely adjacent to the wall of the storage tank, suction (4) and delivery conduits (5) both passing through a removable access plate (3) in the wall of the tank, and releasable means (35) for disconnecting the transportable unit (19) from the or each machine (15). Oil from the storage tank is drawn in through suction conduit (4) and pumped, by pump (22), back to the machine (15) which produces two sweeping jets of high pressure oil to emerge, which cause the sludge in the tank to be re-suspended. The suspension can then be withdrawn through an outlet from the storage tank. The apparatus avoids the need for long pipe-runs and the transportable unit (19) can be disconnected and then used at another desludging location after the tank has been desludged. The agitating machine (15) may be removable with the pipes and replaced by a manhole. <IMAGE>

Description

SPECIFICATION Agitating settled sludge in a storage tank This invention concerns apparatus for agitating sludge on the bottom of a storage tank and more particularly though not exclusively to removing settled sludge by agitating it to form a suspension and then withdrawing the suspension.

The accumulation of sludge on the bottom of crude oil storage tanks results in a number of operational problems, for example the capacity of the storage tank is reduced, 'dams' formed by the sludge deposits may trap pools of water which later form water slugs in the outflow from tank, the sludge causes uneven landing of the legs of the floating roof and alternative use of the tank for other oil types and products is prevented. The sludge accumulates despite the operation of normal tank mixers and it must be periodically removed by physically entering the storage tank.

This is costly, a potential hazard to personnel and gives rise to problems with the disposal of large amounts of sludge.

A machine suitable for removing sludge from the bottom of a storage tank is described and claimed in our U.K. patent application 2083764A. This comprises a central body, a casing rotatable about said body and provided with two substantially diametric nozzles arranged so that liquid emerging therefrom sweeps substantially only in one plane, a turbine for rotating the casing about the central body and means ensuring that when the casing is continuously rotated, alternately one nozzle is closed for substantially 1800 rotation whilst the other nozzle is open.

The sludge removing machine can be one of a plurality mounted within the storage tank at intervals about the inside of the tank wall. The nozzle is blanked for the half-revolution when it is pointing generally towards the tank wall so as to avoid damage to the adjacent wall areas and waste of power. Each sludge removing machine is supplied with oil from the tank itself, which is pumped under pressure to the sludge removing machine to cause its casing to rotate, and the jets of oil emerging from the nozzles re-suspend the sludge in the tank. The suspension can thereafter be removed, so as to clear the tank of sludge.

The power required to operate each sludge removing machines is large, typically needing a 600 HP pump. In practice, to provide the pumping power requirements of the sludge removing machines, spare pumping capacity which is generally available at an oil refinery would be used. Thus, typically, a single off-site pump usually at a considerable distance from the sludge dislodging machines would be used and this involves lengthy runs of pipework to the several sludge removing machines which can be extremely costly. Additionally, there is the effort and time required to install such pipework, and the subsequent maintenance.

Furthermore, each sludge removing machine has definite delivery pressure and flow requirements which cannot usually be satisfied by whatever available refinery pumping capacity there may be at that time.

We have now devised apparatus which can simply and effectively be fitted to an existing storage tank, which is compact and enables sludge to be dislodged, which requires a minimum of pipework and is adaptable for suspending sludge at a plurality of sites.

According to this invention from a first aspect, there is provided apparatus for use in agitating liquid in a storage tank such as an oil storage tank, comprising an agitating machine mounted inside the tank, said machine having a liquid inlet for liquid under pressure and a liquid outlet discharging into the tank interior, a readily transportable unit, including a pump, resting freely on the ground, a delivery conduit passing through a removable access plate in the wall of said storage tank and interconnecting the delivery side of said pump and the liquid inlet of the agitating machine, a suction conduit passing through said access plate and interconnecting the suction side of said pump with the interior of said tank to enable liquid from the tank to be drawn in by the pump and be pumped to the agitating machine, and releasable means for enabling the transportable unit to be disconnected from the agitating machine.

Once sludge in the tank has been suspended, the suspension can be removed or withdrawn. Alternatively, the sludge can merely be agitated periodi cally with the agitation machine so that the sludge is substantially maintained in suspension with minimum settling of the sludge. The transportable unit can then be disconnected from the agitating machine and transported to a new location alongside another storage tank and reconnected to the agitating machine in that storage tank. It will be appreciated also that the agitating machine can be used instead as a mixer for two or more substantially immiscible oils of different density present in the same storage tank. The description which follows relates for convenience to re-suspending settled sludge, but is not to be construed as limiting the invention only to such use.

The sludge dislodging machine (agitating machine) can be mounted upon the removable access plate in the wall of the storage tank. For example, the access plate can be bolted to the wall of the tank, so that by releasing the bolts, the access plate/sludge dislodging machine assembly can be removed from the tank for maintenance or inspection, or transportion of the sludge dislodging machine/access plate assembly for use in another oil storage tank. Because the sludge dislodging machine is located proximate the end of the suction conduit passing through the access plate, any settled sludge in the vicinity of the suction conduit end will tend to be re-suspended before it is likely to cause a blockage. Alternatively, the sludge dislodging machine can be centre mounted on the bottom of the tank.

Suitably, the sludge dislodging machine comprises a chamber having an inlet and an outlet, a nozzle housing rotatable about the chamber and means for rotating the housing about the chamber.

There is at least one nozzle secured to said housing with the longitudinal axis of the nozzle being at an angle to the axis of rotation of the housing and the construction of the chamber, nozzle housing and nozzle are such that liquid flowing out of the outlet of the chamber can emerge through the nozzle(s).

Using this machine oil or liquid from the storage tank may be recirculated through the machine and the jet produced by the rotating nozzle re-suspends the sediments in the crude oil and thus facilitates removal or disposal by subsequent processing.

In order that the nozzle housing can rotate about the chamber the cross-sections of the housing and chamber perpendicular to the axis of rotation will generally have to be circular. In practice the chamber and nozzle housing will both be cylindrical or have substantial portions which are cylindrical.

Although the machine will function with only one nozzle usually there will be two such nozzles, preferably located diametrically with respect to the cross-section of the nozzle housing. Since it is intended that the machine be located near the tank wall then preferably liquid only emerges from one nozzle at any one moment and when a nozzle is pointed towards the tank wall liquid is prevented from emerging from that nozzle at that particular moment. The machine described and claimed in our U.K. patent specification 2083764A is one such machine.

The or each nozzle must be secured to the nozzle housing so that the longitudinal axis of the nozzle is at an angle to the axis of rotation of the housing.

This angle is usually substantially 90".

The construction of the chamber, nozzle housing and nozzle are such that liquid entering the inlet of the chamber can emerge through the nozzle and out into the tank and towards the sludge located therein which it is desired to dislodge. This means that the outlet or outlets of the chamber will register with an inlet or inlets in the nozzle housing at some time during the complete rotation of the nozzle housing about the chamber. The inlet or inlets in the nozzle housing will in turn communicate with the nozzle or nozzles attached to the nozzle housing.

There are means for rotating the nozzle housing about the chamber Usually a turbine will be used and this is conveniently located in the chamber downstream of the inlet thereof. A gearing system can translate rotation of the turbine blades into rotation of the nozzle housing about the casing.

The shape of the nozzles typically is circular or could be elongated, but when the orifice of the nozzle is elongated it is convenient if the nozzles are shaped like truncated cones tapering towards their extremities, the taper ensuring that the jet of liquid emerging has a comparatively small angle of spread. The cross-section of the cones can be circular but it is convenient if it is substantially the same as the shape of the nozzle, e.g., rectangular or elliptical.

The machine forming part of the overall apparatus is that which has been described and claimed in U.K. patent specification 2084763A. In this embodiment the chamber comprises a central body which will inevitably be circular in cross-section and is conveniently a disc which is stationary when the machine is in use. This body, e.g., disc, is usually bolted to the inlet pipework. Since the machine is designed to be suspended above but near the floor of a storage tank, this pipework will usually be fixed to an access plate bolted to the wall of the tank, as already mentioned.

The nozzle housing rotatable about the central body is preferably cylindrical and is provided with two substantially diametric nozzles. These nozzles should preferably be situated so that when the unit is attached to an inlet, e.g., a manhole, of a storage tank and the nozzles rotate, the jet of liquid is between 20 cm and 40 cm above the floor of the tank.

The nozzles are arranged so that liquid emerging therefrom sweeps in a stream substantially only in one plane. When the machine is operating it is preferable that the jets are substantially parallel to the bottom wall of the tank and so the nozzles should be designed to project substantially at right angles to the longitudinal axis of the machine.

When the machine is in use liquid emerges substantially only from one nozzle at a time. This is because the sludge dislodging machine is located near the wall of the tank and it is highly desirable to prevent a jet of liquid emerging from a nozzle impinging on the tank wall at close quarters with possible damage to the tank wall. Accordingiy it is preferred that when the machine is operating, substantially no liquid impinges on the side wall to which the machine is adjacent.

This blanking mechanism can take various forms but one simple form is to extend the central body, e.g., disc, along the longitudinal axis of the machine with a substantially half cylinder which is also housed within the casing, the half cylinder being large enough to shut off the inlet to one of the nozzles as the nozzle housing rotates. This means that liquid entering the machine and flowing within the housing and towards the nozzles will only be able to emerge laterally from the housing over an arc which is generally no more than 1800.

It is only when one of the nozzles rotates through this arc that liquid can emerge from the machine, i.e. through one of the nozzles. In practice it is preferred that the half cylinder be somewhat greater than a half cylinder, i.e. extend through an arc of 1800 to 200 . However, in some cases the arc could be anything between 1600 to 2000.

An alternative arrangement is for the central body to be in the form of a cylinder with a window therein extending round the wall of the cylinder for approximately 1800 and being positioned so that when the housing rotates about this cylinder liquid can emerge from the window and through a nozzle.

In order to be able to rotate the nozzle housing about the central body, a turbine is preferred and it is preferred that the central body houses the turbine which is rotated by flow of liquid through the machine. The turbine shaft usually has a gear and through a gear train the nozzle housing is caused to rotate about the central body. In the preferred embodiment the turbine is located in the upper part of the machine above the disc constituting the central body and within the substantially half cylinder. The turbine shaft extends downwards through an aperture in the disc and at its lower end is provided with a gear, for example, a worm which engages with a gear train, rotation of which causes the casing to rotate about the central body, e.g. the disc.

The speed of rotation of the machine is fairly critical and in practice it is found desirable that when used for removal of sludge from the bottom of a storage tank, the nozzle housing makes one complete revolution in between 2 and 4 hours, e.g.

about 3 hours.

The other main component of the sludge removing apparatus comprises a pump capable of pumping liquid under pressure. The pump is operatively connected to the sludge dislodging machine and to this end there is usually a delivery conduit interconnecting the pump and machine, and also a suction conduit for the pump. It is convenient that liquid be capable of being pumped by the pump through the nozzle(s) of the machine and being returned from the tank, usually from the bottom of the tank, back to the pump.

Because the delivery and suction conduits pass both through the access plate, this avoids the need for providing a special sealed through-point by which the conduit could otherwise have to pass through the wall of the storage tank. Additionally, the conduit runs can be minimised if the transportable unit is positioned closely adjacent the wall of the storage tank, and this can represent a considerable cost saving in view of the typical high cost per unit length of the conduits, especially any flexible sections in these conduits. The distance between the transportable unit and the access plate preferably should be not greater than the diameter of the storage tank in the case of a cylindrical storage tank, more preferably not greater than one half of the diameter of the storage tank, and even more preferably not greater than one-quarter of the tank diameter.There may, however, be instances where it is undesirable or not appropriate to position the transportable unit at such close proximity to the tank but in such cases the additional cost of the extra conduit runs will of course arise.

The or each conduit connected to the pump may include a section, between the access plate and the pump, which is provided at at least one end with releasable connecting means. This facilitates disconnection of the transportable unit prior to transportation to the next site. Conveniently, each releasable connecting means comprises a pair of flanges bolted together in face-to-face relationship.

Preferably, the or each said conduit section is flexible. In this way, the exact location of the transportable unit relative to the next storage tank is not particularly critical because, during re-connection of the conduits, they can be flexed within a reasonable degree into the desired end positional relationship.

The transportable unit may include skids by which it rests freely on the ground. In this arrangement, the unit can be raised by crane onto a trailer or the like for transportation to a new site. Alternatively, the unit may include wheels by which it rests freely on the ground. A dual-purpose unit can have removable wheels on a chassis or sub-structure in the form of skids, so that by removing the wheels, the transportable unit may rest on the ground on its skids. The transportable unit may be towed to a new site or it can be self-propelled. In either case, especially the latter, the unit may have steerable wheels at a front end.

Preferably, the suction and delivery conduits include valves which can be closed off prior to the transportable unit being disconnected from the sludge dislodging machine to prevent escape of any liquid or sludge from the storage tank.

Where there is a plurality of sludge dislodging machines at separate locations within a single storage tank, each machine may be provided with a respective transportable unit. This avoids having to choose a transportable unit whose pump capacity matches the number of sludge dislodging machines used, which would be necessary with a single pump supplying the several machines.

However, instead, a single transportable unit may be operatively used in conjunction with each sludge dislodging machine in turn.

The sludge dislodging machine need not necessarily be mounted on the removable access plate adjacent to the wall of the tank. For example it could be centre-mounted in the tank. In that case, it would suitably be mounted on a bracket on the floor of the tank and connected, through a delivery line, to the delivery conduit in the access plate. It will be appreciated that the blanking of the nozzles would not be required or even desirable, and usually in the centre-mounted arrangement, at least one nozzle produces a continuous jet of liquid throughout each complete revolution of the sludge dislodging machine.

Although in some cases one unit may be quite sufficient for cleaning the sludge from the bottom of a storage tank it may often be desirable or even necessary to use more than one such unit. One convenient arrangement when cleaning a tank having a cylindrical side wall is to use two but preferably three, substantially equi-spaced units projecting into the tank above the floor of the tank and adjacent to the wall. The sweeping stream from the nozzle from each machine will cover the whole of the floor of the tank with little overlap of each sweep. Generally, the number of machines required depends on the size of the tank and the pumping capacity available.

In the embodiments of this invention although the turbine is usually powered by recirculating the oil, the turbine could be supplied with water under pressure, e.g. 6 to 14 kg/cm2. This may if desired be heated and may contain a detergent, a chemical emulsifier or demulsifier. If it was then desired to use the tank again for oil storage all traces of water would have to be removed before re-using the tank.

This invention also concerns the method itself of dislodging sludge from the bottom of a storage tank. The method is one wherein apparatus as defined in accordance with the first aspect of the invention is operated such that liquid is circulated by the or each pump to the respective sludge dislodging machine to re-suspend the sludge. The suspension may be withdrawn from an outlet of said storage tank.

The method may comprise circulating a liquid through the sludge dislodging machine located near the bottom of the storage tank, and recycling the liquid from the tank back to the machine. The nozzles are caused to rotate and sludge is thereby dislodged and sediments are re-suspended in the stored liquid. Usually the axis of rotation of the or each nozzle is substantially vertical and usually the plane of rotation of the nozzle is substantially parallel to the plane of the floor of the tank. However, if the tank floor slopes slightly downward towards its centre as is quite often the case, the plane of rotation of the nozzle will usually be horizontal so that the emerging jet of liquid will usually be substantially horizontal. The jet of liquid expands, however, and therefore a substantial component of the emerging jet flow will be substantially parallel to the tank floor.

In many instances, the sludge dislodging machines are left in the tanks at all times. However, to significantly reduce the number of machines required for desludging a plurality of tanks at various locations, the or each machine in each tank can be removed from the tank after desludging, the manhole cover refixed, and the machine transported to the next tank together with the transportable pump unit for use at the next tank.

According to the invention from a second aspect, there is provided a method of agitating liquid in a plurality of storage tanks at various locations, each tank having an agitating machine inside the tank with a liquid delivery conduit leading from outside the tank to the inlet of the machine and a suction conduit leading from inside the tank to its outside, wherein a transportable unit, including a pump, is transported to each of said tanks in turn, into a location closely adjacent the external end of the liq uid delivery and suction conduits, and operatively connected to the respective agitating machine through those conduits to cause liquid in that tank to be agitated.

According to the invention from a third aspect there is provided a method of agitating liquid in a plurality of storage tanks at various locations, each tank having a manhole in the tank wall closed by a removable cover, wherein a readily transportable unit, including a pump, is transported to each of said tanks in turn, into a location closely adjacent the manhole of each tank, and also an access plate, on which an agitating machine is mounted, is secured in position in place of the cover of that manhole with the agitating machine located inside the tank and in operative connection with said pump through delivery and suction conduits both passing through said access plate, and the pump and agitating machine are operated together to cause liquid in that tank to be agitated.

The invention is now described, by way of example, with reference to the drawings in which: Figure 1 is a horizontal sectional view through a manhole in the wall of a storage tank showing a transportable pump unit located adjacent the tank wall and supplying a sludge dislodging machine located inside the tank, Figure 2 is a side view, partially cut-away, of the transportable unit seen from its far side, Figure 3 is a plan view of the transportable unit, Figure 4 is a vertical sectional view taken through the tank wall, showing how the sludge dislodging machine is supported within the tank, Figure 5 is a vertical sectional view through the manhole in the tank wall, showing the pipework externally adjacent the manhole, Figure 6 is a view of the pipework of Figure 5 seen from the end, Figure 7 is a diagrammatic plan view showing three sludge dislodging machines mounted inside a single tank with their respective transportable pump units, Figure 8 is a view similar to Figure 7, but showing a centre-mounted sludge dislodging machine, and Figure 9 is a perspective view showing how the centre-mounted sludge dislodging machine can be supported above the tank bottom.

Referring to Figures 1 to 7 in general and especially to Figure 1, there is shown a portion of the wall 1 of a generally cylindrical storage tank having a manhole 2 which ordinarily is closed by a manhole cover (not shown). The tank 1 is for storing oil, but it is to be understood that the tank could be used for storing other liquids in which sludge is likely to settle in the bottom region.

Secured to the manhole 2 in place of the manhole cover is an access plate 3 through which pass suction and delivery conduits 4, 5. The suction conduit comprises a rigid pipe section 6 which is secured to the access plate 3, a flexible pipe section 7, and a shut-off valve 8 connected between the rigid and flexible pipe sections. Similarly, delivery conduit 5 comprises a rigid pipe section 9 secured to the access plate 3, a shut-off valve 10 in the pipe section 9, a flexible pipe section 11 and a non-return valve 12 connected between the flexible pipe section 11 and the rigid pipe section 9. The access plate 3 is removably bolted to the mounting flange 13 of the manhole, so that by unfastening the bolts 40 (Figure 6), the access plate can be detached from the mounting flange.

As best shown in Figure 4, the rigid pipe section 9 comprises a downwardly angled portion 14, to the lower end of which is connected the inlet of a sludge dislodging machine 15, exactly as described in U.K. patent specification 2083764A and having nozzles 16, 17 (Figure 6). The wall of the straight, horizontal section of rigid conduit 9 is strengthened by external strengthening ribs 18, so as to support the weight of the sludge dislodging machine 15 without risk of fracturing the rigid pipe section 9.

A flexible drive 33 is coupled at one end to the casing of the sludge dislodging machine through step-up gearing (not shown), and is itself coupled at its other end to an external indicator 34, so as to provide an indication of when the sludge dislodging machine is rotating to an external operator.

The external indicator can, for example, be as disclosed in our published U.K. patent application 2101321A.

Reverting to Figure 1, a readily transportable unit 19, including a pump, is located closely adjacent to the manhole cover and access plate. As shown in Figure 2 and 3, transportable unit 19 comprises a chassis or frame 20 on which is mounted an engine 21, including a clutch, which can suitably be diesel powered, driving a pump 22, mounted at a rear end of chassis 20, via a drive shaft 41 with a flexible drive coupling 23. The chassis 20 can be carried on wheels 24 whereby unit 19 rests freely on the ground but is readily transportable to any desired new location. The front wheels can be steerable, as indicated in Figure 3, and a towing rod 25 may be provided at the forward end of the chassis 20. Alternatively, instead of using wheels 24, the chassis 20 may take the form of skids, by which the transportable unit 19 rests freely on the ground.In this instance, for transporting the unit 19 to a new location, the unit needs to be raised using a crane or other lifting device and placed on a trailer or transporter. For this purpose, the chassis is provided at each end with anchoring points 26, and a lifting beam 27, with which the hook of the crane can be engaged, is linked to anchoring points 26 by means of tension cables or chains 28.

As seen in Figure 3, the pump 22 has a suction side 29, and also a delivery side 30 which is connected to an elbow pipe 31 through a flow-control valve 42. Referring to Figure 1, one end of flexible pipe section 11 is connected to elbow pipe 31 and one end of the other flexible pipe section 7 is connected to a basket filter 32, including a drain cock 50, which is mounted on a mounting flange on the suction side 29 of pump 22. Each flexible pipe section 7, 11 is provided at each end with releasable connections, which can take the form of abutting flanges in face-to-face relationship, clamped together by bolts (not specifically shown). The connections 35 can easily be released merely by releasing the bolts so that the mating flanges can be separated.

Figure 7 indicates, diagrammatically, a typical arrangement in which three sludge dislodging machines 15 are arranged within the oil storage tank 1, mounted on respective access plates 3 bolted to manholes 2 equi-angularly spaced about the circumference of the tank. Each sludge dislodging machine 15 is provided with a respective transportable unit 19 with its pump set connected through suction conduit 4 to the interior of the tank 1 via access plate 3 and its delivery conduit 5 connected to the inlet of the sludge dislodging machine 15.

Although three sludge dislodging machines are shown in Figure 7, it is to be understood that it would equally be possible to use a different number of sludge dislodging machines, e.g. one, two or four, and it is not essential that the machines form a symmetrical arrangement relative to the central upright axis of the storage tank.

In operation, the three pumps are started up and oil from the storage tank 1 is drawn in through each suction conduit 4 and returned under pressure through delivery conduit 5 to the inlet of each sludge dislodging machine 15. This causes the nozzles 16 and 17 to rotate about the vertical axis of the machine 15, the blanking mechanism of the machine ensuring that oil emerges only from the machine 15 in a direction generally away from the adjacent wall regions 1. The action of each machine will cause the sludge to be dislodged and resuspended as previously described. On emptying the storage tank through an outlet pipe (not shown), the suspension, comprising oil and suspended sludge, will be withdrawn, through the outlet pipe, thereby removing the sludge from the storage tank.

When the desludging operation has been completed, the shut-off valves 8, 10 in the suction and delivery conduits 4, 5 are closed-off, and, with valve 42 open, pipe sections 7, 11, pump 22 and the associated pipework are drained off through drain cock 50. Then, the releasable connections 35 between each flexible pipe section 7, 11 and the pipework secured to access plate 3 are disconnected, to enable the three units 19 then to be transported to the next tank to be desludged, where the flexible suction and delivery sections of each transportable unit are connected-up to the pipework of the next respective sludge dislodging machine. Desludging of the next tank is put into operation in the manner described for the first tank.

In the arrangement of Figure 7, it is not essential that a respective transportable unit be connected up to each of the three dislodging machines. For example, one transportable unit could be operatively used with each sludge dislodging machine in turn. This, of course, saves on the number of transportable units which are needed to desludge the tank, but will increase the overall time required.

Referring to Figures 7 and 8, there is shown a modification involving a single centre-mounted sludge dislodging machine 15'. This machine is identical to machine 15 in the Figures 1 to 7 embodiment, except that it incorporates no blanking mechanism, so that each nozzle produces high pressure jets of oil throughout each complete revolution of the machine casing, and also the machine 15' is mounted the other way up, on an upstanding end portion of a rigid delivery line 43, which end portion is supported on a mounting bracket 36 fixed to the bottom wall of tank 1.

Delivery line 43 is directed radially outwardly from machine 15' and is angled upwardly, close to the tank wall, and connected at its outer end to an inwardly directed end portion, within tank 1, of delivery conduit 5. The connection can be a releasable flanged connection employing clamping bolts, or any other suitable connection. In this way, the connection can be released to permit the access plate to be unbolted for enabling the inside of the tank to be inspected through the manhole.

In operation, the sludge dislodging machine agitates and re-suspends the oil throughout the whole of its 3605 rotation and the suspension is withdrawn from the tank.

Claims (20)

1. Apparatus for use in agitating liquid in a storage tank such as an oil storage tank, comprising an agitating machine mounted inside the tank, said machine having a liquid inlet for liquid under pressure and a liquid outlet discharging into the tank interior, a readily transportable unit, including a pump, resting freely on the ground, a delivery conduit passing through a removable access plate in the wall of said storage tank and inter-connecting the delivery side of said pump and the liquid inlet of the agitating machine, a suction conduit passing through said access plate and interconnecting the suction side of said pump with the interior of said tank to enable liquid from the tank to be drawn in by the pump and be pumped to the agitating machine, and releasable means for enabling the transportable unit to be disconnected from the agitating machine.

2. Apparatus according to claim 1, wherein the agitating machine is mounted upon said access plate.

3. Apparatus according to claim 1 or 2, wherein the agitating machine comprises at least one nozzle for issuing a jet of liquid under pressure for resuspending sludge in the storage tank.

4. Apparatus according to claim 1, 2 or 3, wherein the or each conduit includes a section, between the access plate and the pump, which is provided at at least one end with releasable connecting means.

5. Apparatus according to claim 4, wherein each releasable connecting means comprises a pair of flanges bolted together in face-to-face relationship.

6. Apparatus according to claim 4 or 5, wherein the or each said conduit section is flexible.

7. Apparatus according to any preceding claim, wherein the transportable unit is positioned closely adjacent the wall of said storage tank.

8. Apparatus according to any preceding claim, wherein the transportable unit includes skids by which it rests freely on the ground.

9. Apparatus according to any one of claims 1 to 7, wherein the transportable unit includes wheels by which it rests freely on the ground.

10. Apparatus according to any preceding claim, wherein the access plate, which covers a manhole opening in the wall of said storage tank, is removably bolted in position.

11. Apparatus according to any preceding claim, wherein the suction and delivery conduits include valves which can be closed off prior to the transportable unit being disconnected from the agitating machine to prevent escape of any liquid or sludge from the storage tank.

12. Apparatus according to any preceding claim, comprising a plurality of agitating machines at separate locations within the tank, each machine being provided with a respective transportable unit.

13. Apparatus for use in agitating settled sludge in a storage tank containing a liquid such as oil as said liquid is withdrawn from said tank, substantially as hereinbefore described with reference to Figures 1 to 7 or Figures 8 and 9 of the accompanying drawings.

14. A method of agitating liquid in a storage tank, wherein apparatus according to any preceding claim is operated such that liquid is circulated by the or each pump to the respective sludge dislodging machine to agitate the liquid.

15. A method according to claim 14 for use in clearing settled sludge from a storage tank, wherein the agitation produces a suspension of sludge in the tank and the suspension is withdrawn from an outlet of said storage tank.

16. A method according to claim 14 or 15 involving the use of a plurality of agitating machines at separate locations within the tank and a single transportable unit, wherein the transportable unit is operatively connected to and operated with each of the agitating machines in turn.

17. A method of agitating liquid in a plurality of storage tanks at various locations, each tank having an agitating machine inside the tank with a liquid delivery conduit leading from outside the tank to the inlet of the machine and a suction conduit leading from inside the tank to its outside, wherein a transportable unit, including a pump, is transported to each of said tanks in turn, into a location closely adjacent the external end of the liquid delivery and suction conduits, and operatively connected to the respective agitating machine through those conduits to cause liquid in that tank to be agitated.

18. A method of agitating liquid in a plurality of storage tanks at various locations, each tank having a manhole in the tank wall closed by a removable cover, wherein a readily transportable unit, including a pump, is transported to each of said tanks in turn, into a location closely adjacent the manhole of each tank, and also an access plate, on which an agitating machine is mounted, is secured in position in place of the cover of that manhole with the agitating machine located inside the tank and in operative connection with said pump through delivery and suction conduits both passing through said access plate, and the pump and agitating machine are operated together to cause liquid in that tank to be agitated.

19. A method according to claim 17 or 18, of clearing settled sludge from said tanks, wherein the aigtating produces a suspension of sludge in each tank and the suspension is removed from the tanks.

20. A method of clearing settled sludge from a storage tank, substantially as hereinbefore described with reference to Figures 1 to 7 or Figures 8 and 9 of the accompanying drawings.