GB2130303A - Stripper for tank holds of ships - Google Patents

Abstract

A stripper for ships' cargo tanks is arranged to operate without pumps. The stripper comprises a drain sump 9 having a discharge line 6 and an inlet 11 for inflow from the cargo hold. A compressed air line 13 extends into the drain sump. The inlet 11 can be closed by an automatic float element 12, by flotation, when there is sufficient liquid to fill the sump 9, and by the pressure of compressed air being supplied to the sump, via the pipe 13 when there is only sufficient liquid to fill the drain sump partly. The float 12 has a blind bore afforded by a tube 15, which bore receives loosely the outlet end part 14 of the line 13 so that the part 14 and tube 15 effectively operate like a piston and cylinder. <IMAGE>

Description

SPECIFICATION Stripper for tank holds of ships The invention relates to a stripper for tank holds of ships, the stripper comprising a drain sump, a discharge line extending therefrom and an inlet for inflow from the tank interior.

Large pumps are used to discharge liquid cargoes from the tank holds of ships. The effectiveness of such pumps depends upon the inflowing liquid being at or above a critical minimum height above the pump intake orifice. A certain amount of liquid is therefore bound to be left in the hold since it cannot be discharged by means of the large cargo pumps. However, stripping facilities which are associated with cargo pumps and which are used to strip or drain the residual liquid from the holds are known. Stripping facilities for strippers of this kind comprise a drain sump in the hold floor or bottom and a stripper or draining pump which is much smaller than the cargo pump and whose inlet is disposed in such sump. A residue of liquid which the stripper pump cannot convey is therefore left in the sump too.If the cargo is a liquid whose vapours tend to form a highly explosive mixture, this residue causes difficulties because it is very much greater than the residue permissible for safety reasons.

Automatically controlled strippers for stripping or draining holds in preparation for the receipt of dry cargo are also known. A system of this kind is disclosed in German Offenlegungsschrift 2751325 wherein a drain sump is disposed in the hold bottom, the liquid collecting in the sump and being evacuated thereform by means of a pump which is much smaller than the cargo pump. Patent Specification No. 49804 of the German Democratic Republic discloses strippers for stripping or draining ships' holds in preparation for the reception of dry cargo, which are operated by compressed air. Federal German Republic Offenlegungsschrift No. 251 6680 discloses strippers for the same purpose which are operated by vacuum. According to the book published in the German Democratic Republic by K.Lettow, Schiffbauliche Rohrleitungen, VEV-Verlag Technik, of Berlin, pp. 173/75, similar facilities are used to discharge sewage from collecting tanks on board ships, float control systems being provided to start and stop the pump and to operate the shut-off elements. Floating shut-off elements, more particularly float closure elements, are generally known, more particularly as check valve devices.

It is an object of the invention to provide a stripper for cargo tanks which makes stripping pumps unnecessary and which can remove all residue from a tank. It is another object of the invention to provide a stripper for cargo tanks which can readily be made in a form which will resist chemically aggressive liquids and requires no maintainance.

According to the invention, there is provided a stripper for tank holds of ships, the stripper comprising a drain sump, a discharge line extending therefrom and an inlet for inflow from the tank interior, the drain sump having a connecting line for the supply of compressed gas thereto and the drain sump inlet having a closure member or valve actuatable independently of the liquid level in the drain sump.

Advantageously, the valve member for closing the drain sump inlet is a combination of an automatic buoyancy valve with an additional facility for positive operation.

In the operation of a preferred form of stripper embodying the invention, when a cargo liquid is being received in the cargo tank, the drain sump fills up with the cargo liquid. After the emptying of the cargo tank, the sump is kept closed by the valve member which has floated up into its closed position, so that gas under pressure can be injected through the connecting line into the sump to drive the liquid therefrom through the discharge line. After the supply of compressed gas ceases, the valve member reopens the drain sump inlet so that residual liquid can drain from the hold into the drain sump until the valve member recloses the same and the drain sump can again be emptied by its interior being charged with compressed gas once more.This operation can, if necessary, be repeated a number of times until the residual liquid draining into the drain sump when the valve is open is insufficient to fill the drain sump completely. The inlet to the sump is then closed by moving the valve member into engagement with the inlet by means of the positive operating facility, whereupon the final residue can be expelled from the drain sump by compressed gas.

According to an inventive feature which can be used independently, the valve member is actuatable positively by the dynamic pressure of the compressed gas in the connecting line.

Consequently, when the sump interior is charged with compressed gas the valve member closes the sump inlet automatically, regardless of how full of liquid the sump is.

Preferably, the drain sump together with the compressed gas connecting line and with the liquid discharge line from the sump is introducible as a unit having structural integrity into an appropriate aperture in the bottom of a cargo tank. This feature facilitates a high degree of workshop prefabrication and more particularly the fitting of the novel stripper into an existing ship. In the case of liquid cargoes of which the vapours tend to form explosive mixtures with air, an inert gas may be used, instead of compressed air, as the gas under pressure, so that the system is completely explosion-proof.

An embodiment of the invention is described below by way of example with reference to the accompanying drawings wherein: Figure 1 is a half-side view in cross-section through the hold of a liquid cargo ship; Figure 2 is a view in vertical section through a drain sump and stripper embodying the invention, installed in the ship of Fig. 1.

Figure 3 is an enlarged view in vertical section of the drain sump inlet and closure element or valve member.

Referring to Fig. 1, a hold 1 of a liquid cargo carrier ship 2 can be emptied by means of a cargo pump the drive element 3 of which is disposed on the deck out of contact with the cargo liquid and the pump part 4 of which is disposed, immersed in the cargo liquid, near a horizontal wall 5 which forms the bottom of the cargo tank, for liquid, defined by the structure of the ship, which wall 5 is spaced above the bottom of the ship's hull. In the following, to distinguish the wall 5 from the bottom of the hull, the wall 5 is referred to as the tank bottom. The intake orifice of pump 4 extends into a trough-like recess 6 in the tank bottom 5 to boost the inflow to the pump 4. A stripper 7 is disposed in the bottom of the recess 6 so as to extend into a space 8 between the bottom of the hull and the tank bottom 5.

The stripper, which has the general reference 7 in Fig. 1, comprises a drain sump 9 in the form of a closed tank. The top of said drain sump is in the form of a plate 10 (Fig.

2) which can be fitted subsequently into an appropriately prepared opening in tank bottom 5-i.e., in the bottom of the recess 6-and can be secured there, for instance, by welding. A drain sump inlet 11, disposed substantially at the centre of the plate 10, can be closed by an element or valve member in the form of a float 1 2. Compressed air or inert gas under pressure can be supplied through a pipe-line 1 3 to the interior of sump 9 to produce an internal pressure in the sump 9 when the inlet 11 is closed.For constructional reasons the line 1 3 is also secured, at its upper end, in the plate 10, communicating with an inlet opening in the plate 10 via which the line 1 3 can be connected to an inflow line (not shown) extending from a source of air or other gas under pressure (not shown). The pipe-line 1 3 is U-shaped in the embodiment shown so as to have a vertical outlet part 14 disposed substantially centrally below the drain sump inlet 11, said outlet part 14 being open at its upper end to provide an outlet port bounded by a horizontal annular surface. In the arrangement shown, the float 12 is substantially cylindrical and extending diametrally vertically therethrough is a tube 1 5 which is open at its lower end and is closed at its top end by a horizontal plate or end wall.The tube 1 5 is fitted over the outlet part 14 of the pipe 13. The internal diameter of the tube 1 5 in the float 1 2 is a little larger than the outer diameter of the part 1 4 of the line 13, so that the tube 1 5 is a loose sliding fit on the part 14. When the sump 9 is empty and the line 1 3 is unpressurised, the float 1 2 occupies a lower position in which it is spaced downwardly from inlet 11 and is supported by the pipe part 14 by engagement of the closed upper end of the inner tube 1 5 with the free end of part 14.

The cooperation between tube 1 5 and part 14 also provides guidance of the float 1 2 on the part 14 for vertical movement. A discharge line 6 is connected at the bottom to the sump 9 and for constructional reasons-i.e., to facilitate subsequent fitting of the stripper to existing structures--extends from the bottom of the sump, in a bend, to pass through the interior of the sump 9 and is also secured at its upper end, in the top plate 10, where it communicates with an outlet opening in plate 10, so that discharge line 6 can be connected, at the plate 10, to a drain tube or the like (not shown).

In use, while there is sufficient liquid in the hold 1, the float 1 2 keeps the drain sump inlet 11 closed after the sump 9 has filled with cargo liquid, so that at any time internal pressure can be built up in the sump 9 by supplying compressed air or other gas thereto through the line 13, in order to expel liquid from the sump via the discharge line 6. When the pressure in the sump subsequently drops by reason of termination of the supply of compressed gas, the float 1 2 drops under gravity to open the drain sump inlet 11 so that further liquid from the hold 1 drains into the drain sump 9, and this sequence can be repeated to strip from the hold the residue of cargo liquid.When the quantity of cargo liquid draining into the sump is less than the sump capacity the float 1 2 will not, of course, be lifted into engagement with inlet 11 by flotation. However, in this case, the float 1 2 is lifted into engagement with inlet 11 by the dynamic pressure of the gas under pressure supplied through the line 13, to the sump 9, and once lifted is pressed into its position in which it closes the drain sump inlet 11, by the static gas pressure within sump 9. That is to say the clearance between the exterior of part 14 and the interior of tube 1 5 is sufficient to allow the flow of gas through said clearance from pipe 1 3 to the sump interior, at an acceptable rate, but is not so great as to lead to such a drop in gas pressure, in the region of the upper closed end of the tube 15, when said upper end has been lifted from the upper end of part 14, that the gas pressure in said upper end would be insufficient to lift the float 12, all having regard, inter alia, to the flow capacity and pressure of the source, not shown, of the pressurised gas and to the flow resistance of the pipe 1 3. The inset at the lower left-hand corner of Fig. 3 illustrates the simultaneous support of the tube 1 5 (and float 1 2) in its raised position and supply of gas (as indicated by arrows) to the sump 9 via the clearance gap between tube 1 5 and part 14. The construction of the part 14 and of the tube 15 pushed over it after the fashion of a piston and cylinder thus ensures that there is always adequate pressure available to make the float 1 2 close the inlet 11 even when the sump 9 is partly filled. The pressure medium, e.g. compressed air or an inert gas, is supplied through a known multiway valve 1 6 (Fig. 13), which in one position connects the pipe line 1 3 with a source of gas under pressure and in another position (shown) connects the pipe line 1 3 with an exhaust line and seals off the outlet from the source of gas under pressure, respectively to build up pressure in the sump 9 and to reduce pressure therein.

The stripper described with reference to the drawings operates without attention or maintenance and without outside energy, apart from the pressurised gas source, since the drain sump inlet is closed automatically in the final stripping because the sump is charged with the pressure gas for expelling the liquid from the sump.

Claims (9)

1. A stripper for tank holds of ships, the stripper comprising a drain sump, a discharge line extending therefrom and an inlet for inflow from the tank interior, the drain sump having a connecting line for the supply of compressed gas thereto and the drain sump inlet having a closure member or valve actuatable independently of the liquid level in the drain sump.

2. A stripper according to claim 1, in the form of a unit having structural integrity which can be introduced as a unit into an aperture provided in the bottom of a tank provided in a ship's hold.

3. A stripper according to claim 1 or 2, in which the valve is actuatable by the supply of gas under pressure to said connecting line.

4. A stripper according to any of claims 1 to 3, in which said connecting line for the supply of compressed gas comprises an end portion, within the sump, which is. coaxial of the orifice of said inlet and terminates in a port at its end nearer said inlet, said end portion extending into a blind bore in the valve member.

5. A stripper according to claim 4, in which the valve member is guided for sliding movement on said end portion of the pressure gas connecting line.

6. A stripper according to claims 5, in which said blind bore is provided by a tube closed at its top end and received in said valve member, in which said valve member is in the form of a float, and in which said end portion of said connecting line extends into said tube from below.

7. A stripper according to claim 6, in which the float bears by way of the closed end of the tube on the end face, bounding said part, of the pressure gas connecting line.

8. A stripper substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

9. Any novel feature or combination of features disclosed herein.