WO1992014560A1

WO1992014560A1 - Improvements in or relating to washing systems for oil tanks

Info

Publication number: WO1992014560A1
Application number: PCT/GB1992/000253
Authority: WO
Inventors: Victor Russell Gibson
Original assignee: Notac Limited
Priority date: 1991-02-13
Filing date: 1992-02-12
Publication date: 1992-09-03
Also published as: GB9102988D0; GB2252719A

Abstract

A washing system for oil tanks comprises a washer head (16) for mounting within an oil tank, and a conduit (22) for delivering fluid under pressure to nozzles (26) on the washer head which are movable so as to distribute the fluid emerging under pressure from the nozzles over the internal walls of the tank. A cam and follower device (27, 32) is arranged to sense the continuing operative movement of the nozzles, and to transmit to the exterior of the tank, through a fluid filled conduit (33), pressure pulse signals indicative of said continuing movement, means (38) being provided to monitor the signals and produce an output in response thereto.

Description

"Improvements in or relating to washing systems for oil tanks"

The invention relates to washing systems for oil tanks and is particularly, but not exclusively, applicable to washing systems for the tanks of sea-going vessels for the bulk transport of crude oil, commonly known as oil tankers.

When crude oil is discharged from an oil tank, a significant residue is normally left adhering to the walls and floor of the tank. Such residues can contain a measurable percentage of the total contents, so that if they are allowed to remain in the tanks the whole cargo will not be completely discharged. Furthermore, if the residues are allowed to accumulate it is necessary to clean the tanks manually at periodic intervals, starting with digging out the comparatively solid sludge residues, followed by water washing.

To avoid these two disadvantages tank washing machines have been developed. Such machines are usually permanently installed in the tanks and comprise a washer head mounted on conduits which extend downwardly from the top of the tank or upwardly from its floor. Washing fluid, usually crude oil itself, is delivered under pressure to the washer head, which carries one or more nozzles which are movable so as to distribute fluid emerging under pressure from the nozzles over the internal walls and floor of the tank. The washing is carried out during discharge of the tanks, or i mediately after the discharge of each tank. The washing action returns the oily residues into suspension, allowing them to be discharged from the tank. A common form of such tank washing machine comprises a main body part which is rotatable about a vertical axis. Mounted on the main body part, for rotation about a horizontal axis, is a double-ended arm having an outlet nozzle at each extremity thereof. Crude oil under pressure is delivered through the main body part, into and along the rotating arm to emerge from the nozzles. The washer head is so designed that the flow of oil through it causes the main body part to rotate about its vertical axis and the nozzle arm to rotate about its horizontal axis. This rotation about two axes at right angles causes the jets of oil emerging from the nozzles to sweep over every internal surface of the tank.

However, such tank washing machines are prone to blockage, and such blockage may cause the main body part to cease to rotate about its vertical axis. If this happens the nozzle arm will normally cease to rotate, but even if the nozzle arm continues to rotate the jets will in either case scour only very small areas in line with the position where the head happened to stop.

It is at present difficult to determine whether or not a tank washing machine is operating correctly. It is not sufficient to monitor the pressure in the conduits leading to the machines since, as explained above, the oil may still be emerging under pressure from the nozzles. The usual technique currently employed, therefore, is for a person to listen on the outside of each tank in the immediate vicinity of the washing machine. If the machine is operating correctly, a distinctive swishing noise is heard as the jets pass over the internal surfaces of the tank. Such system is inefficient and may be unreliable. Furthermore, there may be some forms of oil tank where access to an appropriate listening position outside the tank may not be possible. For example, designs are currently under consideration for a so- called "mid-deck" oil tanker. This proposed design of vessel has ballast tanks at the sides of the ship leaving space between the ballast tanks for two centre tanks side by side. These tanks are divided horizontally at a level below the loaded draft. It is believed that such designs will carry less risk of oil pollution of the sea than conventional designs, since in the event of the bottom tanks being holed, little or no oil should be discharged into the sea since the upper level of the oil in each tank will be below water level. In such a design, however, listening access to the walls of the tank cannot readily be obtained. The present invention therefore provides an improved tank washing system which is particularly suitable for solving the problems associated with such designs of oil tanker, but it may also be of more general application.

According to the invention there is provided a washing system for oil tanks, comprising a washer head adapted for mounting within an oil tank, conduit means for delivering fluid under pressure to one or more nozzles on the washer head, which nozzles are movable so as to distribute the fluid emerging under pressure from the nozzles over the internal walls of the tank, the system further comprising means for sensing at least a component of the continuing operative movement of said nozzles, means for transmitting to the exterior of the tank from said sensor a signal indicative of said continuing movement, and monitoring means providing an output in response to said signal.

The washer head may comprise a main body part pivotable about a first axis, and a subsidiary part pivotable relatively to the main body part about a second axis transverse to the first axis, said nozzles being provided on the subsidiary part. Preferably the two axes are at right angles.

As previously mentioned, the main body part and subsidiary part may be each continuously rotatable in one direction. However, the invention does not exclude arrangements where one or both parts reciprocate back and forth through a predetermined angular movement. The aforesaid sensing means are preferably adapted to sense operative movement of the main body part.

Any appropriate form of sensing means may be employed. Preferably, however, the sensing means includes an element which is mechanically coupled to a movable part of the washer head so as to move in synchronism therewith, said element being coupled in turn to^• means for transmitting to the exterior of the tank a signal indicative of the movement of said element. Said signal transmitting means may comprise a fluid-filled conduit, said element being coupled to means, such as a piston and cylinder device, for transmitting pressure pulses through the fluid in the conduit in response to movement of the element, means being provided at the opposite end of the conduit for sensing said pressure pulses and providing an audible and/or visual output in dependence thereon. The fluid- filled conduit may contain a liquid or a gas, such as air.

Said element may comprise a cam follower which cooperates with a cam surface on the washer head, and preferably on the main body part thereof, so as to reciprocate in response to rotational or other operative movement of the washer head.

The following is a more detailed description of an embodiment of the invention, reference being made to the accompanying drawings in which:

Figure 1 is a diagrammatic vertical transverse section through a mid-deck oil tanker, Figure 2 is a diagrammatic representation of a tank washing system in accordance with the invention, and

Figure 3 is an enlarged diagrammatic view of a part of the system.

Referring to Figure 1, the hull 10 of the tanker includes ballast tanks 11 and 12 along its opposite longitudinal sides. Between the ballast tanks 11 and 12 there are two tank assemblies arranged side by side, each tank assembly being divided horizontally by a mid-deck 13 so as to •provide upper tanks 14 and lower tanks 15.

As previously mentioned, the mid-decks 13 are located below water level when the tanker is fully loaded so that if either of the lower tanks 15 is holed, there is little chance of a significant quantity of oil being discharged into the sea.

Mounted in each of the upper tanks 14 is a tank washing system comprising a washing head 16 which is mounted at the lower end of conduits 17 which extend downwardly from the top wall 18 of the tank. Crude oil is arranged to be delivered under pressure through the conduit 17 and the washer head 16 rotates, in a manner to be described, to deliver jets of oil over the internal walls of the tank to clean off the oil residues, once the tank has been emptied, bringing them into suspension so that they may be discharged with the washing oil from the tank. Similar washing heads 16 are mounted in the lower tanks 15.

Figures 2 and 3 illustrate in further detail one form of tank washing system in accordance with the present invention. Referring to Figure 2, the washing head 16 comprises a main body part 19 which is mounted on a flanged support 20 so as to be rotatable about a vertical axis. The flange 20 is bolted to a corresponding fixed flange 21 which is secured to the lower end of a rigid conduit 22 which extends downwardly through and below the upper wall 23 of the oil tank in which the washing machine is mounted.

A hollow boss 24 is mounted on the main body part 19 for rotation about a horizontal axis. Diametrically opposed arms 25 extend radially outwardly from the boss 24 and end in outwardly facing nozzles 26. The hollow arms 25 are in fluid communication with the hollow boss 24 which is, in turn, in fluid communication with the interior of the main body part 19. In operation fluid, usually crude oil, is delivered under pressure through the conduit 22 to the interior of the washer head 16, passing through the main body part 19, the hollow boss 24, and outwardly along the hollow arms 25 to emerge from the nozzles 26. The washing head 16 is designed internally so that the pressure of oil flowing through it causes the main body part 19 to rotate about its vertical axis and the boss 24 and arms 25 to rotate about their horizontal axis. As the head and nozzles rotate, therefore, the jets of oil sweep over the entire interior surfaces of the tank. The detailed design of the interior of the washer head 16 does not form a part of the present invention and will not therefore be described in further detail.

As previously mentioned, this type of washer head is prone to blockage and where this occurs it may result in non-rotation of the main body part 19. According to the present invention, therefore, the tank washing machine is provided with a monitoring system so that an operator can be satisfied that the main body part 19 of the washer head is rotating and the machine is therefore operating correctly.

Referring to Figure 3: the casing of the main body part 19 of the washer head is formed with a fixed cam surface 27. This may be integrally cast into the body of the casing or may comprise a separate element screwed onto it. A fixed cylinder 28 is mounted vertically on the flanges 20, 21 (or other adjacent fixed part of the machine) . Reciprocable within the cylinder 28 is a piston assembly 29 which is biased downwardly by a helical compression spring 30 disposed between the lower end of the cylinder 28 and a flange 31 on the piston rod. A cam follower in the form of a roller 32 is rotatably mounted on the lower end of the piston rod and cooperates with the cam surface 27.

Accordingly, as the main body part 19 of the washer head rotates about its vertical axis the roller 32, and hence the piston assembly 29, are caused to move up and down.

The upper part of the cylinder 28 is gas or liquid filled and a similarly filled conduit 33 leads through the upper wall 23 of the tank to a pressure sensing assembly, indicated generally at 34, at a convenient location. The pressure sensing assembly 34 may be of any suitable kind to detect the pressure pulses in the conduit 33 caused by the up and down reciprocating movement of the piston 29. For example, there may be provided a pressure gauge, as indicated diagrammatically at 35, and a pressure-sensitive electric switch device as indicated diagrammatically at 36. The pressure gauge 35 provides a simple method of monitoring pressure in the line 33 and the dial of the gauge will indicate the continuing occurrence of pressure pulses, indicating that the washer head is still rotating. Should the gauge indicate a cessation or reduction in frequency of the pressure pulses, this will indicate that the washer head has stopped rotating or has slowed down significantly.

Alternatively or additionally, the pressure sensitive electric switch device 36 may provide an input, indicated diagrammatically at 37, to a computer 38 having a visual display 39. The computer may be provided with software arranged to monitor the signals received through the input 37. For example the computer may monitor the frequency of the pressure pulses and cause an alarm to be sounded if the frequency falls below a predetermined value. A single computer may receive inputs from all the oil tanks in the vessel and may thus be arranged to indicate on the VDU the number and location of any tank washing machine which fails. The computer may be arranged to give warning of incipient failure of each tank washing machine by detecting a slowing of rotation. It would also be able to compare the actual speed of rotation of the washer head with the designed speed of rotation, to give other information.

Although the piston and cylinder arrangement described is preferred for providing the pressure pulses, any other suitable means may be employed. For example, rotation of the washer head may be arranged to operate a valve in a pressure line so as to induce a pressure pulse with each rotation.

Although a pressure pulse arrangement is preferred, the invention includes within its scope other arrangements for detecting rotation of the washer head and transmitting to a suitable location a signal indicative of continuing rotation of the head. Preferably the detecting device, and also the means for transmitting the signal out of the tank, are mechanical and/or hydraulic/pneumatic. Electrical/electronic methods will not normally be preferred in view of the dangers of sparking in an environment where there are flammable materials, but a suitably protected electrical/electronic system could theoretically be employed.

The invention therefore allows the remote monitoring of effective operation of washing machines within all tanks of an oil tanker, even though the tanks may not be readily accessible.

Claims

CLAIMS 1. A washing system for oil tanks, comprising a washer head adapted for mounting within an oil tank, conduit means for delivering fluid under pressure to one or more nozzles on the washer head, which nozzles are movable so as to distribute the fluid emerging under pressure from the nozzles over the internal walls of the tank, the system further comprising means for sensing at least a component of the continuing operative movement of said nozzles, means for transmitting to the exterior of the tank from said' sensing means a signal indicative of said continuing movement, and monitoring means providing an output in response to said signal.

2. A washing system according to Claim 1, wherein the washer head comprises a main body part pivotable about a first axis, and a subsidiary part pivotable relatively to the main body part about a second axis transverse to the first axis, said nozzles being provided on the subsidiary part.

3. A washing system according to Claim 2, wherein the first and second axes are at right angles.

4. A washing system according to any of Claims 1 to 3, wherein the main body part and subsidiary part are each continuously rotatable in one direction.

5. A washing system according to any of Claims 1 to 4, wherein the aforesaid sensing means are adapted to sense operative movement of the main body part.

6. A washing system according to any of claims l to 5, wherin the sensing means includes an element which is mechanically coupled to a movable part of the washer head so as to move in synchronism therewith, said element being coupled in turn to means for transmitting to the exterior of the tank a signal indicative of the movement of said element.

7. A washing system according to Claim 6, wherein said signal transmitting means comprise a fluid-filled conduit, said element being coupled to means for transmitting pressure pulses through the fluid in the conduit in response to movement of the element, means being provided at the opposite end of the conduit for sensing said pressure pulses and providing an audible and/or visual output in dependence thereon.

8. A washing system according to Claim 7, wherein the fluid-filled conduit contains air.

9. A washing system according to Claim 7 or Claim

8, wherein the means for transmitting pressure pulses comprises a piston and cylinder device.

10. A washing system according to any of Claims 6 to 9, wherein said element mechanically coupled to a movable part of the washer head comprises a cam follower which cooperates with a cam surface on the washer head, so as to reciprocate in response to rotational or other operative movement of the washer head.

11. A washing system for oil tanks, substantially as hereinbefore described with reference to the accompanying drawings.