For economical and other reasons it is important that high capacity pumps are available for pumping a liquid product from large tanks or containers, e. g. in tankers barges or other vessels. Such large capacity pumps, which may, for example, be centrifugal pumps, are usually not suited for pumping the last amount of liquid from the container or tank, because they are not suited for pumping a mixture of liquid and gas. For economical and environmental reasons a tank in tankers and other ships must be almost completely emptied before the tank can be flushed in preparation for reuse. Therefore, it is conventional to use two separate pumps for emptying a large container or tank, namely a centrifugal pump for pumping as much of the liquid content as possible and a piston pump for pumping the residual amount of liquid from the tank, because unlike the centrifugal pump the piston pump is able to pump a mixture of liquid and gas.

FR 548,486 discloses a pumping arrangement comprising a centrifugal pump and a piston pump, which pumps are driven by a common driving shaft. The piston pump is active during the first part of a pumping operation for sucking liquid to a level above the centrifugal pump. Thereafter, the piston pump is inactivated, and the centrifugal pump becomes active. However, the driving shaft constantly drives both of the pumps.

WO 97 27403 discloses an unloading pump arrangement positioned in a well of a cargo tank. This pump arrangement comprises a main pump, which is a centrifugal pump, and an auxiliary pump in the form of a conical sleeve. The sleeve-like auxiliary pump extends downwardly from and is driven by the impeller of the main pump. Also in this case the auxiliary pump is driven also when the main pump is operating and vice versa.

The present invention provides a pumping arrangement, which is able to pump liquid from a container or tank and to empty it almost completely, and which may be made and/or operated more simple and economical than the known pumping arrangements discussed above.

Thus, the present invention provides a pumping arrangement for pumping a liquid product from a tank or container and comprising a rotary, high capacity main pump to be submerged in the product, a rotary driving motor for driving the rotary main pump via a driving shaft, an auxiliary positive pump for pumping a residual amount of said liquid product from the tank or container, and means for selectively drivingly connecting the auxiliary pump to the driving shaft of the main pump.

Thus, no separate driving motor for the auxiliary pump is needed, and the auxiliary pump may be connected to the driving shaft only when the auxiliary pump is needed. This is possible because operation of the high capacity main pump and of the auxiliary pump is not required at the same time.

The main pump may be any high capacity rotary pump. In the preferred embodiment, however, the main pump is a centrifugal pump, but may, for example, also be a gear pump. The auxiliary pump may be any pump, which able to efficiently pump a mixture of liquid and gas. However, preferably the auxiliary pump is a piston pump, but may, alternatively, be any other positive pump.

The common driving motor may be a pneumatic or hydraulic motor, which may be submerged in or positioned above the upper level of the liquid product being pumped. In the preferred embodiment, however, the driving motor is an electric motor. Such motor may be submerged, but is preferably arranged above the liquid level in the tank or container. The auxiliary pump need not be submerged in the liquid product, but may be positioned above the upper liquid level provided that the height, in which the auxiliary pump is positioned above the bottom of the tank or container or above the inlet end of the suction tube of the pump, is sufficiently small to allow the pump to efficiently pump a liquid product from the bottom of the tank. It is preferred, however, that the auxiliary pump is adapted to be submerged in the liquid product to be pumped.

The means for interconnecting the auxiliary pump to the driving shaft of the main pump may be of any kind, such as mechanical and/or electrical. Thus, the means for bringing the driving shaft into and out of driving engagement with the pump may be operated mechanically or electrically by an operator, or they may be operated automatically, for example by a level detector generating an activating signal when the liquid level has reached a predetermined low value.

The main pump may be disengaged from the driving motor when the auxiliary pump is drivingly interconnected with the driving shaft. Alternatively, the main pump and the auxiliary pump may be operating at the same time. In the preferred embodiment the means for interconnecting the auxiliary pump to the driving shaft of the main pump may be activated by reversing the direction of rotation of the driving shaft. In this case the impeller of the main pump will rotate, but not be active.

The pumping arrangement may comprise a device for driving the auxiliary pump. This device may be adapted to automatically become disengaged from the driving shaft when this shaft is being rotated in one direction to drive the main pump, and to become engaged with the driving shaft when the direction of rotation is reversed. Thus, when the driving shaft is reversed the auxiliary pump is activated while the main pump is idling.

The interconnecting means may comprise an annular member arranged loosely around the driving shaft and means for selectively locking the annular member to the driving shaft.

The locking means may be activated manually or automatically as mentioned above. As an example, the outer peripheral surface of the annular member may define an eccentric or a cam surface for reciprocatingly driving a piston of the auxiliary pump.

In the preferred embodiment the automatic activation of the locking means is obtained by means of a ratchet device. Thus, the locking means may be activated automatically by reversing the rotational direction of the driving motor or the driving shaft. More specifically, the pumping arrangement may comprise ratchet teeth formed on an annular end surface part of the annular member and at least one pawl member arranged axially opposite to the ratchet teeth and being connected to the driving shaft so as to rotate together therewith. The pawl members may be of any known type. Preferably, at least one pawl member may be a pin-like member extending from and movably arranged in an axial bore, the pin-like member being biased into engagement with the ratchet teeth.

The auxiliary pump may be of any conventional type, whether single or double acting. In a preferred embodiment each piston of the auxiliary piston pump has a peripheral outer seal dividing the cylinder space into smaller and a larger volume pumping chambers, the larger volume pumping chamber having inlet and outlet valves associated therewith, and the

outlet valve communicating permanently with the smaller volume pumping chamber, whereby the smaller volume pumping chamber may function as a buffer chamber. The discharge flow from a pump of this type is less pulsating than when a conventional piston pump is used.

The invention will now be further described with reference to the drawings, wherein Fig. 1 is a side view of a conventional pumping arrangement for discharging liquid products from tanks in ships or from similar large containers, Fig. 2 is a sectional view of the lower part of an embodiment of the pumping arrangement according to the invention, and Fig. 3 is a sectional view in an enlarged scale of the piston pump shown in Fig. 2.

Fig 1 shows a pumping system or pumping arrangement of the known type used as a cargo pump for pumping a liquid product 10, such as petroleum products or liquid chemicals from a tank or container 11 in a tanker, a barge or another vessel. The pumping system comprises a centrifugal pump 12 submerged in the liquid 10. The pump 12 has an inlet opening 13, which is positioned in a depression 14 formed in the bottom of the tank or container 11. A discharge or outlet tube 15 extends upwardly from the centrifugal pump to a flanged end 16 outside the tank 11. The centrifugal pump 12 is driven by an electric motor 17, which is positioned outside the tank 11, and which is drivingly connected to the impeller of the pump 12 by means of a driving shaft extending trough a vertically arranged cylindrical, oil-filled housing 18. As explained more in detail below, purging conduits 19, which extend from a sealed space within the oil-filled housing 18 and out from the tank 11, renders it possible to check whether the sealing of the oil- filled housing 18 is tight. The whole pumping system forms a unit, which is mounted on a cover or lid 20. The cover 20 is detachably connected to a collar 21 defining a well opening in the top wall of the tank 11. This means that the whole pumping unit may be removed from the tank 11 by releasing the cover 20 from the collar 21.

The centrifugal pump 12 is not able to empty the tank 11 completely. Therefore, in order to comply with international regulations regarding the amount left in the cargo tank when the tank is flushed or cleaned the conventional pumping system described may further be provided with a stripping facility requiring supply of compressed air or nitrogen. As another possibility, the system may include a separate stripping pump, which may be a double-

acting, submerged piston pump (not shown). The outlet of the stripping pump may then be connected to an outlet or stripping conduit shown in Fig. 1.

Figs 2 and 3 illustrate how the pumping system shown in Fig. 1 can be modified so as to form an embodiment of the pumping arrangement according to the invention.

Fig. 2 shows the centrifugal pump 12 having an impeller 23, which is fastened to the lower end of the driving shaft 24, the other end of the shaft being connected to the electric motor 17. The shaft 24 is mounted rotatably within the housing 18 by means of ball bearings 25 and may be divided into separable sections, which are interconnected by a spline connection 26. As mentioned above, the inner space of the cylindrical housing 18 is oil- filled, and an air-fille space 27 is defined around the driving shaft 24 and between an upper sealed oil-filled space and a lower sealed space. The space 27 is connected to the purging conduits 19. A possible leakage may be detected by blowing compressed air through the space 27 via the conduits 19.

The pumping arrangement shown in Figs. 2 and 3 comprises a piston pump 29 having a frame 30 which is connected to the outer side of the housing 18 by bolts or similar releasable fastening means. The frame 30 defines a pump cylinder 31 in which a piston 32 may reciprocate. The piston 32 is driven by an annular driving member 33, which is positioned around a section of the driving shaft 24 so as to be rotatable in relation thereto.

The outer peripheral surface of the driving member 33 is cylindrical. However, the axis of the peripheral surface is offset in relation to the axis of the driving shaft 24 so as to define an eccentric. The outer peripheral surface of the driving member 33 co-operates with a surrounding bearing ring 34, which is connected to the piston by means of a piston rod 35 comprising a movable link 36. The link 36 is arranged within a space 37, which is interconnected with the oil-filled space within the cylindrical housing 18, and the piston rod 35 extends out from the space 37 through a bore including annular seals. As best shown in Fig. 3 this bore comprises an air-fille space 38, which may be connected with purging conduits 39 performing a function similar to that of the purging conduits 19 and the space 27 mentioned above.

An annular connecting member 40 is arranged around the driving shaft 24 and is connected thereto by means of a key 41. The connecting member has a number of axial bores opening into the lower end surface of the connecting member 40, and each of these

bores receives a connecting pin 42 with a sliding fit. The pins 42 are biased by gravity and possibly also by spring means positioned within the pin receiving bores towards an extended position in which the outer free ends of the pins are engaging with the adjacent upper end surface of the annular driving member 33. This end surface has an annular arrangement of ratchet teeth 43 formed thereon for co-operation with the connecting pins 42. The ratchet teeth are shaped such that the driving shaft 24 may rotate freely in relation to the driving member 33 in the usual rotational direction during operation of the centrifugal pump 12 while the driving shaft 24 is drivingly interconnected with the annular driving member 33 by the ratchet device 42,43 when the shaft is rotated in the opposite direction. It should be understood that the pins 42 could be replace by other types of pawl members movably mounted on the driving member 33.

The piston pump 29 has a suction tube or an inlet tube 44 having its free end positioned in the depression 14 and an outlet tube 45. The pump 29 is a kind of double-acting pump having two cylinder chambers, namely a pumping chamber 46 and an auxiliary chamber or buffer chamber 47. The pumping chamber 46 is connected to the inlet tube 44 via a suction valve or inlet valve 48 and to the outlet tube 45 via a pressure valve or outlet valve 48. The auxiliary chamber or buffer chamber 47 is permanently connected to the outlet tube 45 via a passage 50. Because the auxiliary chamber 47 is increasing in volume when the volume of the pumping chamber is decreasing and vice versa the chamber 47 may serve as a buffer chamber reducing the pressure pulse height of the pumped fluid flow.

The pumping arrangement or system described above with reference to Figs. 2 and 3 operates as follows: When the liquid product 10 has to be emptied from the cargo tank 11 the electric motor 17 is started so as to operate the centrifugal pump 12 in the usual manner, whereby almost all the liquid 10 is pumped out of the tank 11. When the centrifugal pump 12 is operative the piston pump 29 is inoperative, because the driving shaft 24 is rotating in such a direction that the connecting pins 42 are not in driving engagement with the ratchet teeth 43. When only a residual amount of the cargo liquid 10, which cannot be removed by the main pump 12, is left in the tank 11 the operator may reverse the rotation direction of the motor 17 and of the driving shaft 24. This causes the connecting pins 42 to move into driving engagement with the ratchet teeth 43 such that the eccentric or driving member 33 is forced to rotate with the shaft 24. Rotation of the eccentric 33 cause the piston rod 35 and the piston 32 connected therewith to reciprocate, whereby almost all of the residual amount of liquid may be pumped out of the tank 11.

It should be understood that various amendments and modifications of the embodiment described above could be made without departing from the scope of the present invention as defined in the attached claims. As an example, the main pump need not be a centrifugal pump, but may be any other type of high capacity pump. Similarly, the piston pump may be replaced by any other type of positive pump, and the ratchet device used for coupling the auxiliary pump 29 to the driving shaft 24 may be replace by any other mechanical and/or electrical coupling means which may be activated manually or automatically.