EP1727730A1

EP1727730A1 - A method and a plant for purification of oil-contaminated bilge water and ship equipped with a plant for purification of blige water

Info

Publication number: EP1727730A1
Application number: EP05722215A
Authority: EP
Inventors: Hans Sivertsson
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-03-22
Filing date: 2005-03-16
Publication date: 2006-12-06
Also published as: SE526811C2; SE0400755D0; WO2005090151A1; SE0400755L

Abstract

The invention relates to a method for purification of oil-contaminated bilge water. The method comprises an initial separation step, in which the main part of the oil in the bilge water is separated from the water to obtain purified water. The thus purified water is led to a boiler in which water is evaporated. The method also comprises a plant for purification of oil-contaminated bilge water, as well as a ship equipped with the plant according to the invention.

Description

A METHOD AND A PLANT FOR PURIFICATION OF OIL-CONTAMINATED BILGE WATER AND A SHIP EQUIPPED WITH A PLANT FOR PURIFICATION OF BILGE WATER

TECHNICAL FIELD

The present invention relates to a method and a plant for purification of oil- contaminated water, in particular oil-contaminated bilge water. The present invention also relates to a ship equipped with a plant for purification of oil-contaminated water, in particular oil-contaminated bilge water.

BACKGROUND OF THE INVENTION

The present invention relates to purification of bilge water, in particular purification of bilge water on a ship at sea. Large amounts of oil are used for various purposes on ships, such as a cargo-ships. Examples of used oils are heavy oil, lubricating oil and diesel oil. The so called heavy oil (or heavy fuel oil) is a low grade oil often used as fuel for ships, and is combusted in the main engine of the ship. Since heavy oil is not liquid at room temperature, it must first be heated in order to be used as a fuel. On ships at sea, there are also large water flows, particularly cooling water flows. Often, several separate cooling water systems exist. For example, ships usually have a so called HT- system (HT = High Temperature) for the cooling of the ship's main engine or engines. Furthermore, there may be a so called LT-system (LT = Low Temperature) for the cooling of lubrication oil and auxiliary machines. Yet another water flow may be a sea water flow that is used in heat exchangers to cool the water of the HT-system and the LT-system. Occasionally, a sea water flow is used as an alternative for the LT-system. During operation of the ship, there will be a leakage of oil, such as heavy oil and lubrication oil. Moreover, water will leak primarily from the cooling systems of the ship, such as the HT-systeni and the LT-system. Of course, every endeavour is made to minimize the leakage, by use of sealings. In practice, it may however be hard in these contexts to achieve completely tight sealings. One reason for this is that on large ships, vibrations from the ship's main engine are very strong. The strong vibrations may then make it difficult to achieve efficient sealing of the ship's oil and water system. When leaks are discovered, it may also be that the leak can not be sealed immediately, since at sea there are most often other essential tasks given a higher priority than a leak that may be considered as being "small".

Oil and water leaking from their respective systems will flow downwards in the ship and end up in the so called drainage pit. In the drainage pit, a continuous accumulation of water strongly contaminated by oil, so called bilge water, takes place. In many countries, environmental legislation prohibits discharge of bilge water with oil contents exceeding a certain limit. In the Baltic Sea, for example, it is not allowed to discharge bilge water having more than 15 ppm of oil. Yet another requirement for allowing discharge of bilge water, is that the ship is running. If the amount of oil-contaminated water is small, the ship may store it onboard until the ship reaches a harbour. In the harbour, the contaminated water can be pumped out from the ship in order to be processed by a purification plant on land. According to international treaties, the port authorities must accept contaminated water. However, they prefer not to do so. When large amounts of bilge water are formed during short time periods, or when the distance to the closest harbour is large, it is not practical to store the bilge water onboard. Then, the bilge water has to be dumped. In order to be able to do that, the bilge water must be purified at sea, such that the oil contents of the discharged water are kept within the specified limits. One method used today, is to try to separate oil from water by using the density difference between water and oil. One way of achieving a separation based on density differences is disclosed in US Patent No. 4,299,703. The document suggests a method in which two liquids of different densities are separated by centrifugation. It is stated that the method can be used on bilge water. It has also been suggested that bilge water can be stored in a tank in which the oil, due to its lower density in relation to water, tends to separate on top of the water. The intention is that water and oil thereafter can be withdrawn separately from the tank, such that the oil is separated from the water. By such methods can be achieved a far-reaching separation of oil and water. It has been shown, however, that it is difficult by such methods to achieve a complete separation between oil and water. In worst case, it may even happen that the water thus purified still contains more oil and/or other contaminants than prescribed by the environmental legislations. In that case, the water can not be heaved overboard without additional purification. It has been suggested in US Patent No. 4,066,545 that water, that after separation by using differences in specific gravity between water and oil, can be led to a hot stack that leads away exhausts formed at combustion in an engine. It is described that water conduits lead to a stack spray unit, and that the water is instantly flash vaporized in the stack. It is described that the vaporized water then leaves the stack together with combustion exhausts. In one example of such a method, it is stated that the exhausts reached a temperature of 612 °F (about 322 °C). It is also stated that the water handled can have oil contents of up to 30 or 40 ppm, without having a negative influence on the vaporization in the stack, hi WO 81/01141, a process for handling bilge water is suggested. In that document, it is described that bilge water is led to an evaporator ("Nerdampfer"). A part of the water is evaporated in the evaporator. It is stated that the evaporation of a part of the water results in an increased salt content in the remaining bilge water, and that the increasing salt content in the bilge water results in a water-oil emulsion being broken. Then, bilge water is pumped from the evaporator to a subsequent station ("Feinentδler"), in which oil is separated from water by methods said to be conventional.

It is an objective of the present invention to provide a method and a plant for purification of oil-contaminated bilge water, thus enabling efficient purification of the bilge water.

ACCOUNT OF THE INVENTION

The present invention relates to a method for purification of oil-contaminated bilge water. In the method according to the invention, an initial separation step is conducted first, in which a main part of the separation between oil and water is done. It is understood thereby that the main part, i.e. more than 50 %, of the oil, is separated from the water. In preferred embodiments of t e invention, more than 50 % of the oil is separated from the water in the initial separation step. Preferably, at least two thirds of the oil is separated from the water, and even more preferred, at least three fourths of the oil is separated from the water, by the initial separation step. The initial separation step can be performed by methods known per se. h preferred embodiments of the invention, the initial separation step is performed by utilising the density differences between oil and water, but the separation step can also be otherwise performed. For example, oil and water can be separated from each other by binding the oil to a heavier substance that sinks to the bottom of a tank, pulling the oil with it. By such methods as are mentioned above, it is most often possible to separate much more than 50 % of the oil from the water. Hence, it may be realistic to expect that more than 90 % of the oil, or more than 95 % of the oil, is separated from the water in the initial separation step. According to the method of the invention, a boiler is further provided. Water purified in the initial purification step is supplied to the boiler, such that the boiler is at least partly filled by water. Water in the boiler is heated such that at least a part of the water in the boiler evaporates and leaves the boiler, and at least a part of the remaining contaminants sink to and settle at the bottom of the boiler. The contaminants that have sunk to the bottom, are discharged from the bottom of the boiler. Preferably, all or essentially all of the water supplied to the boiler is evaporated, where after the thus evaporated water is led away from the boiler. In an advantageous embodiment of the invention, the water that has left the boiler is continuously replaced by new water having gone through the initial separation from oil, such that the water level in the boiler remains constant at least during a period. Preferably, the water level in the boiler is constant or essentially constant during the entire time of the process. The boiler may be arranged to communicate with a control tank from which new water can be supplied to the boiler. Then, the water level in the boiler can be controlled by regulating at least one of a supply of water to the control tank, and a removal of water from the control tank. The control tank may suitably be connected, via at least one valve, to a settling tank in which initial separation of oil and water takes place by utilising the density difference between oil and water. Then, water having undergone an initial purification by utilisation of the density difference, can be led from a lower part of the settling tank to the control tank. It is realised that the settling tank can be replaced or supplemented by a device that conducts an initial separation according to other principles. So for example, the settling tank can be replaced or supplemented by a device that separates oil and water by centrifugation. Suitably, water can be led in separate steps from the settling tank to a buffer tank, and from the buffer tank continuously to the control tank. It is to be understood that the method is primarily intended to be used on a ship, in which case the water evaporated from the boiler is removed from the ship. It is to be understood however, that the method can be applied also in connection with plants on land, having received contaminated bilge water from a ship, for example. In advantageous embodiments of the invention, the evaporated water from the boiler is condensed and is led overboard as a liquid. Suitably, water in the boiler can be evaporated by being heated by heat from the propulsion of the ship. In advantageous embodiments of the method according to the invention, a continuous evaporation and a continuous supply of new water to the boiler, is conducted. It should be understood however, that variations of the invention are conceivable, in which water is evaporated in batches in the boiler. Evaporation can take place at atmospheric pressure, but variations are also conceivable in which evaporation takes place at a pressure below atmospheric pressure. The invention also relates to a plant for purification and removal of oil-contaminated bilge water. The plant according to the invention comprises a boiler in which contaminated water can be separated by evaporation from remaining contaminants such as oil residues and chemicals. The plant also comprises a condenser for condensing water evaporated in the boiler, as well as a discharge conduit for the removal of water condensed in the condenser, hi advantageous embodiments, an oil gauge (particularly a ppm gauge) may be arranged in connection with the discharge conduit. It should also be noted in this connection that it normally is a requirement according to current law and regulations that an oil gauge is connected to the discharge conduit. The boiler is provided with an inlet for receiving contaminated water, as well as an outlet for contaminants, which outlet is positioned at the bottom of the boiler. In advantageous embodiments of the invention, the plant may comprise a control tank connected to the inlet of the boiler such that the control tank and the boiler form communicating vessels. In that case, the control tank may be provided with means for controlling a liquid level in the control tank, such that the water level in the boiler can be controlled by controlling the water level in the control tank. It is realised however, that other ways of controlling the level in the boiler are conceivable.Jn advantageous embodiments, the plant comprises a settling tank in which initial separation of oil and water can take place by utilising density differences between oil and water. Then, the settling tank is connected to the control tank via a lower outlet, and via an upper outlet to an oil collecting tank. An oil sensor is suitably arranged in connection with the settling tank, in order to detect whether liquid on a given level of the settling tank is mainly oil or mainly water. Then, the settling tank is preferably arranged such that it can be chosen if one of the upper outlet and the lower outlet is to be opened as a function of a signal from the oil sensor. Preferably, only one of the upper and lower outlets is open at any given time. It is however conceivable that the upper and the lower outlet are altematingly opened, until it is time to refill with new bilge water. A buffer tank can be arranged to receive water from the lower outlet of the settling tank, and to continuously discharge water to the control tank. In an especially preferred embodiment, the boiler comprises a helical heating coil positioned to be lying inside the boiler, such that a centre axis for the heating coil is essentially horizontal. The invention can also be defined in terms of a ship that comprises the inventive plant for purification of bilge water.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a ship at sea. FFiigg.. 22 is a diagrammatic cross-section of a portion of the ship shown in Fig. 1.

Fig. 3 is diagrammatically showing a plant for purification of bilge water.

Fig. 4 is diagrammatically and in cross-section showing a detail of the plant shown in Fig. 3.

Fig. 5 is diagrammatically showing another detail of the plant shown in Fig. 3. FFiigg.. 66 is diagrammatically showing a second embodiment of a plant for purification of bilge water.

Fig. 7 is diagrammatically showing another embodiment of the invention.

Fig. 8 is diagrammatically and in cross-section showing a detail of the plant shown in Fig. 3. FFiigg.. 99 is showing a variation of the embodiment shown in Fig. 6.

Fig. 10 is showing another variation of the plant shown in Fig. 3. Fig. 11 is diagrammatically showing a component that in advantageous embodiments can be combined with the rest of the equipment. Fig. 12 is schematically showing an alternative embodiment of the detail shown in Fig. 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. 1, a ship 1 is shown at sea. In Fig. 2 the ship 1 is diagrammatically shown in cross-section. As is shown in Fig. 2, the ship 1 has an engine 37 driven for example by heavy oil or diesel oil. The engine 37 can drive the ship by delivering power to a propeller 38, via a propeller shaft 39. hi Fig. 2 it is shown how the engine 37 is partly placed below a floor plate 40 of the ship 1. In the ship 1, there are also different water flows, mainly in the form of cooling water systems, such as cooling water systems (not shown) for the engine 37. During operation of the engine 37, it can not be completely avoided that water and oil leaks from various parts of the system. Water and oil having leaked out will then flow downwards in the ship 1 and end up in the drainage pit 36 (or drainage pits 36). The oil-contaminated water in the drainage pit 36, is called bilge water. From the drainage pit 36, bilge water is led to a bilge water tank 35, which collects bilge water to be purified.

It is now refened to Fig. 3. From the bilge water tank 35, bilge water can ^"be pumped up by a not shown pump, to a so called day tank 29 at a higher position. From the day tank 29, the bilge water can be led to a plant 2 for purification of bilge water. In the embodiment of the inventive plant 2 shown in Fig. 3, the day tank 29 is connected by a conduit 33 to a settling tank 17. In Fig. 3, it is symbolically shown how the settling tank 17 can be provided with a float 27, used to monitor the liquid level in the settling tank 17. When the float 27 indicates that a liquid level in the settling tank 17 is below a predetermined level, a valve 32 is opened in order for bilge water to flow via conduit 33 to the settling tank 17. Preferably, the float 27 is areanged to indicate low level when the settling tank 17 is half empty (i.e. half full). The float 27 can be connected to valve 32, such that valve 32 opens automatically when the float 27 indicates low level. In principle, it may also be conceived that ship personnel reads the indication of the float 27 in order thereafter manually to open or close valve 32. When the float 27 indicates high level in the settling tank 17, the valve 32 is closed, either automatically or manually.

The settling tank 17 is intended to be used for initial separation of oil and water, by utilising the density differences between oil and water. Due to its lower density, the oil will tend to position itself on top of the water, such that the bilge water forms a lower water layer and an upper oil layer. It is realised however that in the normal case, t ie separation is incomplete whereby the water layer may still contain an unacceptably high content of contaminants, such as oil residues as well as other contaminants such as chemicals and heavier particles from the oil.

The settling tank 17 is, via at least one lower outlet 18, connected with a control tank 10. Via at least one upper outlet 19 and a conduit 34, the settling tank 17 is connected with an oil collecting tank 20. The upper outlet 19 is intended to serve as an oil outlet, since the oil tends to end up on top of the water. The upper outlet 19 is preferably positioned at a level conesponding to half the height of the settling tank 17, such that oil is drained from the middle of the settling tank 17 and not from the bottom or the upper part of the tank. An oil sensor 21 may suitably be arranged in connection with the settling tank 17, in order to detect whether liquid on a given level of the settling tank 17 is mainly oil or mainly water. The settling tank 17 is ananged such that it can be chosen if one of the upper outlet 19 and the lower outlet 18 is to be opened as a function of a signal from the oil sensor 21. If the oil sensor 21 indicates oil at the predetermined level, a valve 26 is opened which is connected to the upper outlet 19, such that oil may flow from the upper part of the settling tank 17, via the upper outlet 19 and down into the collecting tank 20. If the sensor 21 instead indicates water at the predetermined level, the valve 26 remains closed. Instead, a valve 25 is opened, which is connected to the lower outlet 18, such that water can flow out through the lower outlet 18 and further to the control tank 10. Valves 25 and 26 can be solenoid valves connected to a control and regulation unit, such as a control cabinet or a computer that in advantageous embodiments is also connected to the sensor 21. It is also conceivable that valves 25 and 26 are manually operated after reading of the oil sensor 21. When float 27 indicates that the level is low in the settling tank 17, shutting takes place of the one of valves 25 and 26 that is open at that moment, such that the flow from settling tank 17 ceases. Instead, the settling tank 17 is filled with new bilge water from day tank 29. It is to be understood that normally not more than one of valves 25, 26 is open at any given moment, such that only oil or only water leaves tank 17 at any given moment. However, it may alternate which of valves 25, 26 that is open, such that water and oil, respectively, are altematingly led away from the settling tank 17. A timer (not shoΛvn) can be connected to the plant, in order for the bilge water 17 to get enough time to layer. Suitably, the timer is part of the same control- and regulation unit that is connected to the oil sensor 21. When the settling tank 17 has been filled from the day tank 29, the timer starts countdown. After a predetermined time period Ti, the timer signals either valve 25 that is connected to the lower outlet 18, or valve 26 that is connected to the upper outlet 19. Which one of the valves 25, 26 that is ordered to open depends on which signal that is received from the oil sensor 21. The time period Ti is suitably set to coreespond to the time it takes for the water in a buffer tank 22 to be drained to control tank 10. The buffer tank 22 and its function will be explained further below.

A venting is suitably provided in the top of the settling tank 17.

It is realised that if the float 27 would be too late in indicating low level, i.e. at a too low level in the settling tank, more oil could get out through the lower outlet 18. On the other hand, if the float 27 would be too early in indicating low level, the oil and water emulsion will have less settling time, since refill must take place more often. In that case, the oil and water would have less time to form layers and the separation would be less efficient. The present inventor has found that it is suitable to let the float indicate low level when about half the draining tank 17 has been drained.

If oil sensor 21 senses water, valve 18 for the lower outlet will open, as mentioned above, whereby settling tank 17 starts to be drained of water. Then, valve 26 for the upper outlet 19 is closed. It is realised that there is a layer of oil on top of the water. Accordingly, the oil layer will move downwards when water disappears from the settling tank 17, until the oil sensor detects oil. Then, valve 25 for the lower outlet 18 will close and instead valve 26 for the upper outlet 19 is opened and oil starts to flow through conduit 34 for oil, down to the oil collecting tank 20.

Advantageously, the settling tank 17 is relatively high, in order for the layer-forming to get enough time. Preferably, the settling tank 17 should be of at least 0.5 m height. Embodiments are conceivable in which the settling tank has a height of considerably more than 0,5 m. For example, a height of 1 m, or 2 m, or even more, is conceivable. It should be realised however that settling tank 17 can be of a height of considerably less than 0.5 m. Accordingly, the value of 0.5 m is not to be seen as a minimum value, but only as value that denotes what is advantageous. Of course, the total volume is also important in order for the time in the settling tank 17 to be long enough to enable efficient layer-forming. If the volume is small, the time for layer-forming will be short if the process is continuous. In one practicable embodiment considered by the present inventor, the settling tank 17 may have total volume of 20 L. It should be realised however that the settling tank 17 can have a volume less than 20 L, but also a volume considerably more than 20 L. Accordingly, in practicable embodiments the settling tank 17 may have a volume of 20-100 L, e.g. Embodiments are also conceivable in which the volume of the settling tank is considerably more than 100 L, and it may even be of several cubic metres.

Fig. 3 shows how the water can flow via conduit 48, to control tank 10. Optionally, a filter 41 is arranged in connection with conduit 48. The filter 41 is intended primarily to catch any dirt particles that otherwise may clog subsequent valve(s). hi a prefereed embodiment, a throttle valve 24 is arranged in conduit 48, such that water can flow only at limited velocity to the control tank 10. Suitably, the throttle valve 24 is arranged downstream the filter 41. Preferably, the throttle valve 24 is adjustable, and it should be set to be open enough for the control tank 10 level to be just below the level of repletion. It may be added that primarily it is the throttle valve 24 that is to be protected by the filter 41. In an advantageous embodiment of the invention, a buffer tank 22 is arranged to receive water from the lower outlet 18 of the settling tank 17, and continuously to discharge water to the control tank 10. When water flows out via the lower outlet 18 of the settling tank 17, only a minor portion of the water can go directly to the control tank 10, since throttle valve 24 limits the flow. Instead, the major part of the water will go to the buffer tank 22. The buffer tank 22 is shown in Fig. 3 at the side of settling tank 17. It should be understood however that Fig. 3 is a diagrammatic sketch and that the buffer tank 22 can be positioned also at a lower level than the settling tank 17, partly or completely. For example, buffer tank 22 may be positioned vertically below settling tank 17. When valve 25 for the lower outlet 18 is closed and the flow from the lower outlet 18 is interrupted, water will continue to flow from buffer tank 22 to control tank 10. Thereby, a uniform water flow to control tank 10 can be achieved. Buffer tank 22 acts as a buffer tank when valve 25 for the lower outlet 18 is closed. It is advantageous to position the buffer tank in a position that is as high up as possible, in order to give a uniform pressure for the throttle valve 24 of the control tank 10. Suitably, the buffer tank 22 volume is at least equally large as the volume of water that can be drained from the settling tank 17 during one cycle. Suitably, buffer tank 22 has a volume that is at least half of the total volume of settling tank 17. However, in advantageous embodiments of the invention, the buffer tank is even larger in relation to the settling tank 17, such that water supply to control tank 10 can be continuous even if during one or more cycles only oil or mainly oil is supplied. Embodiments are also conceivable in which the control tank 10 is relatively large, whereby it completely replaces the buffer tank 22. In that case, control tank 10 should have a volume that is at least half of the volume of settling tank 17. h advantageous embodiments, control tank 10 can be provided with a lid (not shown), that can be opened for inspection of control tank 10.

Control tank 10 is connected to an inlet 7 of a boiler 3. P eferably, the connection is such that the control tank 10 and the boiler 3 form cornm-Tinicating vessels. Then, the liquid level in boiler 3 will be equal to the liquid level in control tank 10. By controlling the water level in control tank 10, it is accordingly possible to control the water level in boiler 3. For that purpose, control tank 10 may be provided with means 11, 12 for controlling a liquid level in the control tank 10, such that the water level in the boiler 3 can be controlled by controlling the water level in the control tank 10. Fig. 8 shows diagrammatically and principally a possible embodiment of a device for controlling the water level in control tank 10. Here, control tank 10 is shown to be provided with a tube

11 extending essentially vertically up through the bottom of control tank 10. When the water level in control tank 10 is high enough, water will finally reach the level of the upper mouth of tube 11. This level can be adjusted by arranging a socket 12 that is threaded on the tube 11. By screwing the socket 12, it can be moved up or down in the direction of arrow A. Thereby, it is possible to determine at which level water should begin to flow out via tube 11. The tube 11 can lead to the bilge water tank 35. When the ship is at sea, and the sea is high, socket 12 is suitably adj usted downwards. This is because at high sea, the risk of cascades from boiler 3 increases. Accordingly, in bad weather it may be suitable to adjust socket 12 downwards , such that the liquid level in boiler 3 is lowered. During more calm conditions, when ttie plant 2 is not moving, socket 12 can be adjusted upwards for a more optimal evaporation.

The connection between the control tank 10 and the boiler- 3, is preferably arranged at the bottom of control tank 10 or lowermost on the side of control tank 10. By the low positioned connection to boiler 3, the advantage is achieved that small particles can be drained to the boiler 3 instead of remaining in control tank; 10.

It should be understood that water that reaches boiler 3 still can be expected to contain contaminants, primarily in the form of oil residues forming an emulsion with water, chemicals and heavy particles from the oil. hi the boiler 3, contaminated water can be separated by evaporation the from remaining contaminants, when heat is supplied to the boiler. For this purpose, the boiler 3 can be provided with a helical heating coil 23 preferably positioned to be lying inside the boiler 3, such that a centre axis for the helical heating coil 23 is essentially horizontal, which is diagrammatically shown in Fig. 4. The present inventor has found that this by this design, the risk of splashes during boiling is diminished in comparison with the case with a standing coil. It should be understood however that the invention can be practiced also with a standing heating coil 23, and also with other types of heating devices. Most ships have a steam surplus from the flue gas boiler, and excess steam may be used for heating the heating coil 23. If dirt (such as oil residues) gets burned and sticks to the heating coil 23, this may negatively affect the function of the heating coil 23. Therefore, it is suitable from time to time to clean the coil 23. This can be done by mechanical cleaning e.g., or by ultrasound. A brush can be used for mechanical cleaning. Such a brush can be automatic, and be fixedly arranged in connection with coil 23. An automatic brush can for example be controlled electrically or pneumatically. It should be realised that other types of cleaning devices than brushes can be provided in connection with the coil 23.

A condenser 4 is arranged above boiler 3, in order to receive and condense water evaporated in the boiler 3. Suitably, condenser 4 has a cooling coil (not shown) to cool down water evaporated from boiler 3. A cooling medium for the cooling coil may be sea water, e.g. A discharge conduit 5, 5a, 5b for discharge of water condensed in the condenser 4, leads from the condenser 4. Fig. 3 shows how an oil gauge 6 is arranged in connection with a portion 5b of the discharge conduit. The oil gauge 6 is advantageous and is included in prefened embodiments of the invention. Embodiments without the oil gauge in discharge conduits 5a, 5b, are however also conceivable. It is realised that the object of oil gauge 6 is to control that the water does not have a prohibited high amount of oil. It is to be realised that the oil gauge 6 can be seen more generally as a gauge for contaminants in general, and not only for oil. As stated above, the boiler 3 is formed with an inlet 7 to receive contaminated water. Moreover, the boiler 3 has an outlet 8 for contaminants, such as oil residues and heavy particles from the oil, which outlet 8 is positioned at the bottom 9 of boiler 3. In advantageous embodiments, the boiler 3 has a safety valve 28 that is able to handle a free flow of steam from a broken heating coil 23, e.g.

The plant according to the invention operates in the following way. Contaminated water is supplied to the boiler 3, such that boiler 3 is at least partly filled with water. The contaminated water in the boiler 3 is heated in order to evaporate at least a part of the water in the boiler 3, in order to leave the boiler 3. The present inventor has found that thereby, at least a part of the contaminants in the water will sink to the bottom 9 of the boiler 3, and settle on the bottom 9 of the digester 3 as a sediment of contaminants, in particular an oil sediment. Instead of lying as a top layer, the contaminants will accordingly sink to the bottom. Thereafter, the contaminants can be lead away from the bottom 9 of boiler 3. Fig. 3 shows how a valve 31 is provided in connection with the oil outlet 8 on the bottom 9 of the boiler 3. hi a preferred embodiment of the invention, valve 31 is a valve that can be rapidly opened and shut, and that has a relatively large area, such that outlet 8 can be open during a short time, for example for 0.5-3 seconds, whereby contaminants on the bottom 9 of boiler 3 can be rapidly drained from boiler 3, optionally by aid of a certain overpressure inside boiler 3. Preferably, each moment of opening should not be longer than two seconds. It should be realised that if the valve area (area accessible for flowing through) is large enough, a rapid draining can be achieved without an inner overpressure in boiler 3. For example, it may be mentioned that the present inventor in one embodiment has considered a valve with an opening of 15 mm in diameter. Of course, other sizes are conceivable. The smaller the area of the valve 31, the more often it has to be opened. The draining from boiler 3 can preferably be automatic and optionally be controlled by a timer (not shown).

In prefened embodiments of the inventive method, water that leaves boiler 3 is continuously replaced by new water, such that the water level in boiler 3 remains constant or constant at least for a period. Preferably, the supplied contaminated water is continuously boiled, so that essentially all water that is supplied to boiler 3 is evaporated and separated from the oil and/or other contaminants, where after the thus evaporated water is led away from boiler 3.

A constant water level in boiler 3 can be achieved as boiler 3 is arranged to communicate with control tank 10, from which new water can be supplied to boiler 3. The water level in boiler 3 can be controlled by regulating at least one of a supply of water to control tank 10, and a removal of water from control tank 10. Removal of water can for example be controlled by the arrangement shown in Fig. 8. Supply of water to tank 10 can be controlled e.g. by controlling by the throttle valve 24 shown in Fig. 3.

As mentioned above, control tank 10 is communicating with settling tank 17, via at least one valve 24. It is realised that water is led in separate steps from settling tank 17 to buffer tank 22, and continuously from buffer tank 22 to control tank 10.

It is also realised that even if control tank 10 has been described in connection with settling tank 17, it is conceivable that there is no dedicated control tank 10 between settling tank 17 and boiler 3. Also, embodiments are conceivable in which the bilge water undergoes an initial separation step in settling tank 17 and is then led directly to boiler 3. Water evaporated in boiler 3 goes upwards and is led to condenser 4. Fig. 5 shows schematically how steam V passes a labyrinth seal 30 on its way from boiler 3 to condenser 4. The object of labyrinth seal 30 is to prevent contaminated water from splashing up into condenser 4 during boiling. As mentioned above, the risk therefore increases in connection with heavy seas. It is prefened that only steam V makes its way to condenser 4. hi condenser 4, the steam is cooled and condensed again to water that is now separated from oil residues and other contaminants. Fig. 3 shows that the condenser has a conduit 5 for discharge of condensed water. Fig. 3 shows conduit 5 divided in an upstream portion 5a and a downstream continuing portion 5b. hi the event of heavy sea, there may be a risk that cascades of contaminated water will pass labyrinth seal 30. In order to prevent contaminants from moving on in that case, the water can be made to pass an additional filter 44. However, embodiments without this additional filter 44 are conceivable. The upstream portion 5 a of conduit 5 leads to a condensate tank 43. Fig. 3 shows how an oil gauge 6 is provided in a portion 5b of conduit 5 positioned downstream condensate tank 43. If oil gauge 6 indicates that the oil content is within allowed limits (it is most often required that the oil content should be below 15 ppm), the condensed water can be led to an outlet 45 where it is heaved overboard, as is also indicated in Fig. 1. Reference numeral 75 indicates an overboard valve. According to the regulations, purified bilge water can only be heaved overboard from a running ship. The time, the position of the ship and the amount of water that is let out, must be noted. The pumping out of purified bilge water can be automatic or manual. If the oil content is above the allowed limit, water can be sent to the bilge tank 35. Fig. 3 shows oil gauge 6 positioned downstream condensate tank 43. It should be realised however that the oil gauge 6 can be positioned between condenser 4 and condensate tank 43. Fig. 3 shows that the discharge conduit 5a continues 5b downstream the condensate tank 43, and that the oil gauge 6 is positioned in the continuing portion 5b. As is evident from above, embodiments are however also conceivable in which the oil gauge 6 is positioned in the portion 5a of discharge conduit that is upstream condensate tank 43.

Fig. 11 shows diagrammatically another oil separator 80 that can be used in preferred embodiments of the invention. Here, the oil separator 80 is shown as a vessel 80 suitably positioned in conduit 5a, between condenser 4 and filter 44 in the embodiments shown in Figs. 3, 6, 9 and 10. When water comes from condenser 4, it ends up first in vessel 80 in which any remaining oil forms a top layer 81. Then, water is led from a lower outlet and further via conduit 5a to filter 44. A venting pipe in the top of vessel 80 is denoted 86. Oil can be drawn from layer 81 via a conduit 84 provided with an openable valve 83. A venting pipe 87 may also be present in conduit 5a downstream vessel 80.

Fig. 12 shows diagrammatically an alternative to the labyrinth seal shown in Fig. 5. Fig. 12 shows a water separator comprising a vessel 90 with holes 91. When it is used, vessel 90 is positioned inside boiler 3. The water separator also has a funnel-shaped part 92, the funnel of which faces downwards. When liquid is evaporated in boiler 3, steam will rise up from the liquid surface LL and go into vessel 90, via holes 91. Then, the steam turns downwards and thereafter upwards, through the upside-down funnel 92. If any contaminated water has entered through holes 91, it is drained back via conduit 93 leading from the bottom of vessel 90 and down below the liquid surface LL in boiler 3. It is realised that the water separator shown in Fig. 12 can be used in all shown embodiments. It is also realised that it can be used independent of if an oil separator as shown in Fig. 11 exists or not.

The method is primarily intended to be used on a ship 1, in which case the water that is evaporated in boiler 3 is removed form the ship 1. h advantageous embodiments of the invention, this takes place by the evaporated water from the boiler 3 being condensed and led overboard as a liquid. It is also conceivable however, that the evaporated water is not condensed but is led overboard as a steam. It should also be realised that the inventive method need not be used on a ship 1, but that the method can be used on land as well. Also, the inventive method and the inventive plant can be used on a rig such as

One possible example concerning the dimensioning of the components of the plant, is that the settling tank 17 as well as the condensate tank 43 could have a volume in the magnitude of about 20 L, while the buffer tank 22 could be of somewhat smaller volume. In one conceivable embodiment, the total height of the plant could be 1-2 m. It should also be understood however that in each individual case, the dimensioning should be adapted to circumstances, and the dimensions mentioned above should only be seen as possible examples.

In advantageous embodiments of the invention, bilge water is evaporated in boiler 3 by being heated by waste heat from the ship's main engine(s) and or auxiliary machines. It is also conceivable however to use one or more separate heat sources to heat the bilge water. Above, the method has been described as a method in which contaminated water is evaporated in a continuous process. It should be understood however, that variants are conceivable, in which water is batch- wise evaporated in the boiler 3.

In advantageous embodiments of the invention, the boiler 3 can be provided with a tube for flushing the walls 3 clean by pure water or some other suitable liquid.

As the method is described above, the evaporation normally takes place at atmospheric pressure. Variants are conceivable however, in which boiling/evaporation takes place at pressures below or above atmospheric pressure.

Fig. 3 shows that boiler 3 and condenser 4 are separated from each other by a certain distance. Accordingly, the condenser 4 is not shown to rest on boiler 3. Thereby, heat exchange from boiler 3 to condensed water in condenser 4, is avoided. Such a design may be advantageous, since it has been shown that the function of the oil gauge 6 may be interfered with if the water passing the oil gauge 6 is of too high a temperature. Some other type of thermal insulation between boiler 3 and condenser 4 is also conceivable. Another way of keeping down the temperature of the water passing the oil gauge 6, can be to cool the water down before it reaches oil gauge 6. For this purpose, the inventive plant 2 can be provided with a cooling device (not shown in the figures), for cooling of the condensed water. Such a cooling device can be placed e.g. in connection with condensate tank 43.

An alternative embodiment of the invention will now be explained with reference to Fig. 6. In Fig. 6, the day tank 29, the settling tank 17, the boiler 3, and the condenser 4, have the same functions as described above with reference to Fig. 3. The day tank 29 is connected to the settling tank 17, via a conduit 33. hi conduit 33, there is a valve 32 that can be opened when the float 27 detects that the level is low in the settling tank 17. As in the plant shown in Fig. 3, the settling tank 17 has a lower outlet 18 that can be opened and closed by a valve 25. Furthermore, the settling tank 17 has an upper outlet 19 that leads to a collecting tank for oil, via a conduit 34. The oil flow through conduit 34 can be shut off or turned on by operation of a valve 26. The embodiment shown in Fig. 6 is however intended to operate by evaporation in boiler 3 taking place at a pressure below atmospheric pressure. For this purpose, boiler 3 and condenser 4 are connected to a vacuum source, which is shown in Fig. 6 as an ejector 57 connected to an ejector tank 55. Fig. 6 shows that the connection may be provided with a valve 61 , for opening or shutting of the connection. In this embodiment, the settling tank 17 is, via a conduit 48, connected to a buffer tank 42 that can also be connected to a vacuum source. Fig. 6 shows how the buffer tank 42 is connected to the same ejector 57 as is boiler 3 and condenser 4, via a conduit 60. A valve 65 in the conduit 60 between buffer tank 42 and ejector 57 can be opened and shut as needed. Via an additional valve 66, the buffer tank 42 can be connected with or isolated from atmosphere. A lower outlet 8 is arranged at the bottom of boiler 3, which outlet leads to a drainage tank 46 for contaminants, such as sediments of oil and heavy particles in the oil. A valve 62 in the conduit from outlet 8 to drainage tank 46 can be opened or shut as needed. Drainage tank 46 can be connected to ejector 57, in order to achieve the same negative pressure in the drainage tank 46 as in boiler 3. Fig. 6 shows that the drainage tank 46 can be connected to ejector 57 or some other vacuum source, via a valve 79. When desired, the drainage tank 46 can be disconnected from the ejector 57, the valve 62 can be closed, and the drainage tank 46 can be allowed to change to atmospheric pressure. Via a valve 78 e.g., drainage tank 46 can be connected to or isolated from atmospheric pressure. From drainage tank 46, oil sediment and other contaminants removed from the bottom of boiler 3 can be led to the oil collecting tank 20. For this purpose, there is an openable valve 63 in the conduit between drainage tank 46 and oil collecting tank 20. When drainage tank 46 is below atmospheric pressure, the valve 63 between drainage tank 46 and collecting tank 20 can be opened such that oil sediment from the boiler finally ends up in collecting tank 20. In this embodiment, the method according to the invention operates in the following way. The buffer tank 42 is at atmospheric pressure when liquid is led from settling tank 17 to buffer tank 42. Then, the flow from settling tank 17 is shut off, and buffer tank 42 is connected to a vacuum source, such that the pressure in buffer tank 42 becomes negative. Boiler 3 and condenser 4 are connected to the vacuum source all the time, and hence they are constantly at negative pressures. After the buffer tank 42 has been connected to a vacuum source, a valve 64 is opened in the conduit from buffer tank 42 to the boiler, and the water in buffer tank 42 is led into boiler 3. In boiler 3, water is evaporated at a pressure below atmospheric pressure. Then, evaporation can take place at a lower temperature than if evaporation took place at atmospheric pressure. When buffer tank 42 is once again going to receive water from settling tank 17, valve 65 can be shut in order to disconnect ejector 57. Also, valve 66 is opened such that there will be atmospheric pressure again in buffer tank 42. Since buffer tank 42 can not be in continuous communication with boiler 3, the level in boiler 3 must be otherwise controlled as way compared to the embodiment of Fig. 3. For this purpose, boiler 3 can be provided with a float 76. Via a cable 77 e.g., the float 76 of boiler 3 may be connected with valve 64 in the conduit from buffer tank 42. When the float 76 indicates low level in boiler 3, valve 64 is opened. It should be understood that float 76 also can be connected to a control cabinet or similar that in turn controls valve 64. It should also be understood that the level in boiler 3 may be monitored by other means than a float. Such alternative means can also be connected to valve 64 or a control device that controls valve 64. For the rest, the method is essentially the same as in the embodiment according to Fig. 3. As in the embodiment according to Fig. 3, the discharge conduit 5b leads to an outlet 45, and an oil gauge may be positioned in connection with the discharge conduit. It may be pointed out that in Fig. 6, the condenser is shown to be positioned directly on top of boiler 3. It is realised that in the embodiment according to Fig. 6, boiler 3 may operate at a lower temperature than in the embodiment according to Fig. 3. Accordingly, there is less effect of heat transfer from boiler 3 to the condensed water, and the risk of interference with the operation of the oil gauge 6 is smaller.

Yet another embodiment of the invention will now be explained with reference to Fig. 7. The embodiment shown in Fig. 7 may be suitable for smaller merchant ships or fishing boats. Bilge water can be pumped - possibly by hand - from drainage pit 36 to settling tank 17. The settling tank is left for a while in order for an initial separation to take place by oil and water forming layers, where after oil can be led away via one or more upper outlets 19. Then, water having undergone an initial purification can led away by opening valve 53 and letting out water via at least one lower outlet 18. Hereby, drainage can be done in the open, such that the liquid can be visually assessed. A funnel 54 can be used to lead the liquid into conduit 49 leading to inlet 7 for boiler 3. When boiler 3 is filled and valve 53 is closed, boiler 3 and condenser 4 can be connected by an ejector 57 e.g. to a vacuum source, where after evaporation takes place. During boiling, contaminants such as oil residues and heavier particles sink to the bottom of boiler 3 and are withdrawn via outlet 8. Fig. 7 shows that the outlet 8 is connected to a valve 52 that can be opened when contaminants such as oil sediments are to be emptied from the bottom of boiler 3. It is realised that the plant shown in Fig. 7 can operate also at atmospheric pressure, i.e. without ejector 57. The plant shown in Fig. 7 can also have a vacuum pump instead of an ejector 57. A cut-off valve in conduit 49 is indicated by reference numeral 74.

Fig. 9 shows a variant of the plant shown in Fig. 6. The plant shown in Fig. 9 differs from that shown in Fig. 6, by replacement of the ejector shown in Fig. 6 with a vacuum pump 70. For the rest, the plant shown in Fig. 9 is the same as is shown in Fig. 6, and it operates in the same way.

Fig. 10 shows a variant of the embodiment according to Fig. 3. h the embodiment shown in Fig. 10, the oil gauge 6 is positioned in the portion 5b of the discharge conduit that is positioned downstream the condensate tank 43. A three way valve 71 is provided downstream oil gauge 6. From the three way valve 71, water can be led either to outlet 45 where the water is heaved overboard, or back to the system, such as back to bilge tank 35. A flow meter 72 can be arranged to measure the amount of water that is heaved overboard through outlet 45. The letter P indicates a pump. Also, in the embodiment shown in Fig. 10, the buffer tank 22 is placed at a vertically lower level than the control tank 10, such that water must be pumped from the buffer tank 22 to the control tank 10. The embodiment according to Fig. 10 is conceivable for example for ships that spend more time at the wharf than at sea. Fig. 10 also symbolically shows an inspection viewer 73 positioned before the three way valve 71. The inspection viewer 73 can be used to show the operation to inspecting authorities. The inspection viewer 73 can also be placed after the three way valve 71, such that the inspection viewer 73 and the three way valve 71 will swap places in the figure. It is to be understood that an inspection viewer 71 may be present also in the embodiments shown in the other figures, and then at positions corresponding to Fig. 10.

Even though the invention has been described above in terms of a method and a plant, it is understood that these categories only reflect different aspects of the same invention. Accordingly, the inventive plant is intended to be used to conduct the inventive method. Accordingly, the inventive method may comprise such steps that follow naturally from use of the inventive plant, independent of if such steps are explicitly mentioned or not.

It should be understood that the invention can be defined also in terms of a ship 1 equipped with such a plant 2 for purification of bilge water, as is described above. Furthermore, the invention can be seen in terms of a method of installing the inventive plant on a ship, in which case the method of installation comprises the steps that follow naturally from installing the plant as described above. The invention can also be defined in terms of a rig equipped with the inventive plant.

Variants of the inventive method are conceivable, in which the principle of evaporating all the water in the boiler is applied independent of if contaminants are led away from the bottom of the boiler or not.

By using the phenomenon that oil residues tend to sink to the bottom 9 of the boiler 3 when the contaminated water is boiled, and by removing the contaminants from the bottom of the boiler 3, the advantage is attained that a very efficient separation from the water is achieved of oil and other contaminants. If all or essentially all water is evaporated, the advantage is attained that additional separation is normally no longer needed. By using evaporation to full extent, the inventive method and plant can be used independent of the salt content of the water.

By allowing the water to condense, after evaporation of the bilge water, the advantage is achieved among other things that it is easy by an oil gauge to control that the water that is thereafter heaved overboard is within allowed limits.

If the bilge water that is to be purified already at the start has a relatively low oil content, the initial separation step can be eliminated. Accordingly, the initial separation step may be advantageous, but embodiments of the inventive method are conceivable in which no initial separation step is conducted. It should be realised however that if the initial separation step is excluded, the amount of oil in the boiler will be expected to increase in time. Hence, if the initial separation step is completely eliminated, it is likely that the entire boiler has to be emptied from time to time.

Claims

1) A method for purification of oil-contaminated bilge water, which method comprises the following steps: a) an initial separation step, in which the main part of the oil in the bilge water is separated from the water to obtain purified water, b) providing a boiler (3), c) supply to the boiler (3) of water purified in the initial purification step, such that the boiler (3) is at least partly filled by water, d) heating the bilge water in the boiler (3), so that at least a part of the water in the boiler (3) is evaporated and leaves the boiler (3), and remaining contaminants in the water sink to the bottom (9) of the boiler (3) and settle on the bottom (9) of the boiler (3), e) removing contaminants from the bottom (9) of the boiler (3).

2) A method according to claim 1, characteri s ed in that the water that leaves the boiler (3) is continuously replaced by new water, so that the water level in the boiler (3) remains constant.

3) A method according to claim 2, charac teris ed in that the boiler (3) is arranged to communicate with a control tank (10), from which new water can be supplied to the boiler (3), and in that the water level in boiler (3) is controlled by regulating at least one of a supply of water to the control tank (10), and a removal of water from the control tank (10).

4) A method according to claim 3, charac teri s e d in that via at least one valve (24), the control tank (10) is in communication with a settling tank (17) in which initial separation of oil and water takes place by using the density difference between oil and water, and water having undergone an initial purification by using the density difference being led from a lower part of the settling tank (17) to the control tank (10).

5) A method according to claim 4, charac teri s ed in that water is led in separate steps from the settling tank (17) to a buffer tank (22), and continuously from the buffer tank (22) to the control tank (10). 6) A method according to claim 1, characteri s e d in that the method is applied onboard a ship (1), and that the water evaporated in the boiler (3) is removed from the ship (1).

7) A method according to claim 6, charact eri s ed in that the water evaporated in the boiler (3) is condensed and led overboard as a liquid.

8) A method according to claim 6, characteri s ed in that the bilge water in the boiler (3) is evaporated by being heated by heat derived from the propulsion of the ship (1).

9) A method according to claim 1, charact eri s ed in that water is evaporated in batches in the boiler (3).

10) A method according to claim 1, characteris ed in that the evaporation takes place at atmospheric pressure.

11 ) A method according to claim 1, charact eri s e d in that the boiling takes place at a negative pressure.

12) A method for purification of oil-contaminated bilge water, which method comprises the following steps: a) an initial separation step in which the main part of the oil in the bilge water is separated from the water, b) providing a boiler (3), c) supply to the boiler (3) of water that has been purified in the initial separation step, and d) heating the water in the boiler (3), so that essentially all water having been supplied to the boiler (3) is evaporated and separated from the oil, where after the thus evaporated water is removed from the boiler (3).

13) A method according to claim 12, characteri s ed in that the water that is removed from the boiler (3) is continuously replaced by new water, so that the water level in the boiler (3) remains constant during a period at least.

14) A method according to claim 12, charac teri s ed in that the evaporation in the boiler (3) takes place at atmospheric pressure. 15) A method according to claim 12, charact eri s ed in that the method is applied onboard a ship (1) and the evaporated water is removed from the ship (1).

16) A plant (2) for purification of oil-contaminated bilge water, which plant (2) comprises: a boiler (3) in which contaminated water can be separated from contaminants by evaporation, a condenser (4) for condensation of water evaporated in the boiler (3), a discharge conduit (5a, 5b) for discharge of water condensed in the condenser (4), and an oil gauge (6) arranged in connection with the discharge conduit (5a, 5b), the boiler being provided with an inlet (7) to receive contaminated water and an outlet (8) at the bottom (9) of the boiler (3).

17) A plant (2) according to claim 16, characteri s ed in that the plant (2) comprises a control tank (10) connected to the inlet (7) of the boiler, such that the control tank (10) and the boiler (3) form communicating vessels, the control tank (10) being provided with means (11, 12, 24) for controlling a liquid level in the control tank (10), such that the water level in the boiler (3) can be controlled by controlling the water level in the control tank (10).

18) A plant (2) according to claim 17, charact eri s ed in that the plant (2) comprises a settling tank (17) in which initial separation of oil and water can take place by using the density difference between oil and water, the settling tank (17) being connected via a lower outlet (18) to the control tank (10), and via an upper outlet (19) to an oil collecting tank, an oil sensor (21) being arranged in connection with the settling tank (17) in order to detect whether the liquid on a given level of the settling tank (17) is mainly oil or mainly water, and the settling tank (17) being arranged such that it can be chosen which one of the upper outlet (19) and the lower outlet (18) that is opened as a function of a signal from the oil sensor (21).

19) A plant (2) according to claim 18, characteri s e d in that a buffer tank (22) is arranged to receive water from the lower outlet (18) of the settling tank (17), and to continuously discharge water to the control tank (10).

20) A plant (2) according to claim 16, characteri s ed in that the boiler (3) comprises a helical heating coil (23) positioned to be lying inside the boiler (3), such that a centre axis for the heating coil (23) is essentially horizontal. 1) A ship (1), characteri s ed in that the ship (1) comprises a plant (2) according to any one of claims 16-20.