The present invention relates to a level control device for a liquid in a sump, such as liquid from a gas-liquid contacting device, in particular for liquid in a sump of a scrubber for the removal of liquid soluble gases from mixed gas streams. The device of the present invention is particularly suitable for use with an onboard system for scrubbing marine exhaust gases, where the scrubbing liquid is seawater.

In such systems, it is important to control the level of the liquid in the sump. If the level falls too low or the sump becomes empty then the gas/vapour could enter a downstream liquid treatment system. This could damage to pumping devices and, due to the lower heat capacity of gases, could also cause heat damage to the devices. If the level rises too high then devices above the sump, such as a scrubbing column could be flooded, which would prevent efficient scrubbing operation.

Aeration of liquids during removal from sumps or tanks is common. It can result from the level control device utilised, typically a weir. The turbulent movement over this device causes the entrainment of air into the liquid. A certain amount of aeration may be desirable, for example, if the liquid is water that is to be subsequently discharged into a sea, river or lake. For example, aeration can cause the oxidation of harmful sulphides into .harmless sulphates. However, too high an amount of aeration can be problematic in downstream liquid treatment devices. High levels of aeration can result in bubble expansion and cavitations upon entry of the liquid into a pumping device, such as a hydro-cyclone. This can cause damages to the equipment such as impeller damage, vibration and permanent pump failure.

Subsequent de-aeration of a highly aerated liquid flow requires further steps, such as the use of a settling vessel. Such steps may be expensive and highly undesirable as, for example, space is already very limited on the boats in which these scrubbing devices are utilised. The Applicant has realised that the prior art level control devices suffer from certain drawbacks. The present invention provides an improved apparatus and method for controlling the level in a sump tank.

In accordance with a first aspect of the present invention, there is provided a system for controlling the level of liquid in a sump. The system comprises a sump and a first conduit connected to the sump at a first level. The first conduit defines a first flow path for liquid flowing from the sump. The first conduit extends to a second level, which is at a higher level than the first level, and from the second level to a third level which is at a lower level than the first level. In use therefore, the system may be configured to set a desired residual liquid level in said sump at said second level such that when the liquid level in said sump is higher than said residual level, liquid flows along said first flow path from said first level up to said second level and to down to said third level. In an equilibrium condition, the second level may then provide a way of setting a residual liquid level in the sump.

The term 'conduit' should be understood to cover pipes, tubes etc. whether rigid or flexible and of various diameters.

The term 'sump' should be understood to cover any tank or vessel which is used to collect a liquid. Terms such as up, down, vertical, horizontal, higher and lower should be understood to refer to properties of the system when it is installed in normal operating conditions.

The present invention therefore provides a system for controlling the level of liquid in a sump where the liquid flowing from the sump travels along a first flow path up to a raised point (or peak) and then falls to a point which is lower than the point at which the fluid leaves the sump. This arrangement provides a gentle weir- like flow over the raised point, which only flows when the liquid level in the sump exceeds a certain, predetermined residual level. If the level of the liquid in the sump is at or below the second level, no liquid will flow out of the sump to the second level. The residual level of liquid in the sump can thus be set by selecting the height of the second level. Preferably, the first conduit has a substantially horizontal portion at the second level.

Preferably, the first conduit extends no higher than the second level. In other words, the highest point of the first conduit is at the second level. Preferably, the system is configured such that liquid flowing along the first fluid flow path from the second level to the third level is aerated. This can be achieved by designing the first conduit to extend downwardly between the second and third levels at a high gradient. This accelerates the liquid rapidly and if the conduit is not completely filled with said liquid, some of the air contained within the conduit will be entrained/adsorbed in the liquid flowing through the conduit. As previously discussed, aeration of the liquid can be desirable.

Preferably, to ensure that the first conduit between the second and third levels is not completely filled with liquid, the system is configured to maintain a permanent liquid level at the third level. This can be maintained by providing a back pressure from downstream devices.

Preferably, the first conduit between the second and third levels is substantially vertical. This is advantageous as it increases aeration of the liquid.

Preferably, the system further comprises means for introducing a gas into said first flow path. This is advantageous where the liquid flowing along the first flow path is aerated for a number or reasons.

Firstly, as the liquid flows from the second level down to the third level it accelerates. This creates a substantial drop in pressure at this point. The low pressure region, i.e. the partial vacuum, created serves to suck the fluid through the upstream parts of the first conduit, for example the parts of the first conduit extending between first and second levels. This can be problematic as this can cause an the level of the liquid in the sump to drop below the desired level. As previously discussed, if the sump level falls too low, or even empties, then the gas/vapour could travel along the first conduit and enter a downstream liquid treatment system. This could cause damage to the downstream system. Further, if the sump is emptied, a partial vacuum could be created in the sump and in any upstream apparatus, such as a scrubbing tower, which could have undesirable effects.

By introducing gas into the first flow path, i.e. into the first conduit, this compensates for the reduced pressure created by the liquid accelerating from the second level to the third level. In order words, the partial vacuum created sucks gas into the first conduit rather than drawing (or educting) liquid along the first conduit. Thus avoiding the problem of the liquid level in the sump falling too low. The point at which the gas enters the first conduit thus acts as a vacuum break.

Secondly, introducing gas into the first conduit allows the air in the region between the second and third levels to be replenished.

Preferably, the gas is introduced into the first conduit at or near the second level.

Preferably, the gas is air.

Preferably, the means for introducing air into the first conduit is an atmospheric vent, i.e. a vent open to the atmosphere, such that the air is at atmospheric pressure and will be drawn into the first conduit when the pressure therein is less than atmospheric pressure.

Preferably, the first conduit comprises a substantially horizontal first portion at or below the first level and an angled portion at an angle to the first portion, the angled portion extending from the first portion to the second level.

Preferably, the angled portion between the first portion and the second level is substantially vertical. This provides a more compact arrangement.

Preferably, the first portion is located below said first level and the first conduit comprises a portion extending between said first portion and said first level. The connecting portion may be a substantially vertical portion connected to the sump.

Preferably, the system further comprises a second conduit defining a second flow path for liquid flowing from the sump, wherein the second conduit is connected to the first conduit at first and second locations, the first and second locations being located below the second level such that the second flow path is located below the second level.

The term 'connected' should be understood to mean 'in fluid connection with', such that fluid can flow between the connected parts. Preferably the second location is located at a lower level than said first location and/or the second location is located at or below said third level.

Preferably, the first location is at or below said first level.

The second conduit provides a second flow path that bypasses said second level. In other words, it does not extend up to the second level. Such an arrangement provides various advantages.

Firstly, by flowing some of the liquid from the sump along the second flow path, this reduces the amount of liquid flowing along the first flow path (i.e. through the first conduit). This prevents the flow of liquid over the second level becoming too high which could result in excessive aeration of the liquid. Secondly, by bypassing the higher first flow path, any sediment that leaves the sump can be removed. Such sediment may not normally flow up to the second level and could clog the lower parts of the first conduit. Instead, the sediment can flow out of the system through the second conduit.

Preferably, the liquid flowing along said second flow path does not undergo aeration. This can be achieved by ensuring that the second conduit is completely filled with liquid and not open to any gas sources. For example, the point at which it rejoins the first conduit can be below the permanent water level provided by a downstream back pressure as previously discussed. Such an arrangement also ensures sufficient back pressure on the liquid in the second conduit to prevent too high a flow flowing through the second flow path, such that the level in the sump is maintained and some liquid will flow always flow along the second flow path.

Ensuring that the liquid flowing along the second flow path does not undergo aeration allows the amount of aeration of the liquid leaving the system to be chosen. Only liquid flowing in the first flow path will undergo aeration. If a reduced amount of aeration is required, then the system can be designed to also flow a certain amount of liquid along the second flow path, for example by selecting a pipe for the second conduit of a desired internal diameter. At the second location, the first and second flow paths mix, which will the reduce amount of aeration of the resulting flow, i.e. the liquid leaving the system.

Preferably, the system comprises means for controlling the amount of liquid flowing along the second flow path. The means may be a valve and in particular a variable valve. In turn, this allows the amount of liquid flowing along the first flow path to be controlled and thus the amount of aeration of the liquid leaving the system to also be controlled. This prevents the liquid leaving the system being too highly aerated. Where the concentration of air in the liquid is sufficiently low at the exit of the device it is possible to mitigate the need for a de-aeration tank, and therefore minimize the space occupied by the system.

The present invention thus provides a system that can control both the level of the liquid in the sump and the level of aeration of the liquid leaving the system. Preferably, the liquid in the system is water. The water may be seawater or freshwater from rivers, lakes etc.

The system of the present invention may be part of an exhaust gas scrubber, where the sump receives liquid that has been used to scrub exhaust gases. The sump may be located underneath a scrubbing tower. In particular, the system may be part of a marine exhaust gas scrubber, where the scrubbing liquid may be seawater. The system may thus be installed on a marine vessel.

The liquid, e.g. seawater, leaving the system, may be discharged to the sea. Preferably, the liquid leaving the system is treated in a downstream treatment facility prior to being discharged.

The present invention extends to the system of any of the embodiments described above further comprising, or in combination with, a liquid contained therein.

The present invention extends to an exhaust gas scrubber comprising the system of any of the embodiments described above.

The present invention extends to a system and method for scrubbing marine exhaust gases emitted by marine engines, where the inlet of the scrubber is connected to the exhaust outlet of the marine engine. The inlet may be indirectly connected to the exhaust gas outlet, e.g. via an intervening gas treatment apparatus, e.g. a scrubber.

The present invention extends to a method of controlling the level of liquid in a sump using the system of any of the embodiments described above, the method comprising passing liquid along said first conduit and setting said residual level in said sump at said second level.

In accordance with a second aspect of the present invention, there is provided a method of controlling the level of a liquid in a sump, comprising the steps of: allowing liquid to exit a sump at a first level and passing liquid along a first conduit defining a first flow path for liquid flowing from the sump, the conduit extending from the first level to a second level, which is at a higher level than the first level, and from the second level to a third level, which is at a lower level than the first level.

Preferably, the method comprises the step of setting the level of the liquid in the sump at a desired residual level, the residual level being at the second level such that when the level of the liquid in the sump is higher than the second level, liquid flows out of the sump along the first flow path up to the second level and down to the third level.

This further aspect of the present invention may include any of the features described with respect to the first aspect.

Preferably, the method further comprises the step of introducing gas into the first flow path.

Preferably, the gas is air.

Preferably, the air is at atmospheric pressure. Preferably, gas is introduced into the first flow path at the second level.

Preferably, the method further comprised maintaining a permanent water level in the first conduit at the third level. As previously mentioned, this arrangement ensures that the first conduit between the second and third levels is not completely filled with liquid. Preferably, the permanent water level is maintained by a downstream back pressure. Preferably, the method further comprises separating a portion of the liquid flowing along the first flow path and flowing the separated portion along a second fluid flow path defined by a second conduit, wherein the second conduit is connected to the first conduit at first and second locations, the first and second locations being located below the second level such that the second flow path is located below the second level.

Preferably, the second location is located at a lower level than the first location, and/or the second location is located at or below the third level, and/or the first location is located at or below the first level. Preferably, the liquid flowing along the second flow path does not undergo aeration.

Preferably, the method further comprises the step of controlling the amount of liquid flowing along the second flow path.

Preferably, the method further comprises the step of aerating the liquid flowing along the first fluid flow path from the second level to the third level.

Preferably, the method further comprises the step of controlling the amount of aeration of the liquid flowing along the first flow path by controlling the amount of liquid flowing along the first flow path.

Preferably, the method of the present invention is part of a method of scrubbing exhaust gases, where the liquid used to scrub the exhaust gases is collected in the sump.

Preferably, the exhaust gases being scrubbed are marine exhaust gases, e.g. emitted by engines of marine vessels.

The present invention thus provides a method of controlling the level of liquid in a sump where the level of aeration of the liquid leaving the system can also be controlled.

The system and method of this invention are not limited in their application and may be used in any situation where a level control is required for a liquid sump or tank, whilst controlling the resulting level of aeration in the liquid downstream. The present invention in any of the aspects of the present invention may include any of all of the features described in respect of the other aspects and embodiments of the invention to the extent that they are not mutually inconsistent.

A preferred embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawing, of which;

Figure 1 is a cross section through a preferred embodiment of a system for controlling the level of liquid in a sump in accordance with the present invention.

The level control device 1 shown in Figure 1 comprises a sump 2 which empties through a liquid outlet 3. The liquid outlet is located at a first level 23. The liquid outlet 3 comprises a central pipe 4 directed downwards from the base of the liquid sump 1. After the liquid outlet 3, the liquid enters a first conduit 5, which extends downwardly to a 90 degrees bend 6. The first conduit has an internal diameter of approximately 0.25 metres.

After the bend 6, the first conduit 5 is connected to a second conduit 15 at junction 8. The second conduit 16 extends vertically downwardly.

The first conduit 5 then extends vertically upwardly via portion 10 to a 90 degrees bend 11 at a second level 24. The second level 24 corresponds to the level of liquid in the sump 2 when no liquid is flowing from the sump 2. When the level of liquid in the sump 2 rises above this level, liquid flows into the first conduit 5 and over second level 24. The level 24 is between 0.50 and 0.75 metres above the base of the sump 2.

At the second level, the first conduit has a horizontal portion 12. The horizontal portion 12 leads to a downward 90 degrees bend 13. Liquid flowing over this bend 13 falls down vertical portion 14 to a third level 25. At the third level 25 there is a permanent water layer 15 which is maintained by backpressure created by downstream devices.

As the liquid falls down the vertical portion 14 it is aerated. In other words, air is entrained or adsorbed into the liquid flow.

The second conduit 16 has a diameter of approximately 0.10 metres. It comprises a 90 degrees bend 17, a horizontal portion 18 and a valve 19. The valve 19 can be used to control the amount of liquid flowing through the second conduit 16 and thus control the amount of liquid that flows along the first conduit 5 and up and over the second level 24. Thus, the valve can be used to control the amount of aeration of the liquid 26 leaving the system. The second conduit is connected to the first conduit at point 20, which is below level 25. This arrangement ensures that a back pressure is provided on the liquid in the second conduit so that it does not flow too quickly therethrough. Point

20 is approximately 7 metres below the point 8 at which the other end of the second conduit 16 is connected to the first conduit 5. An atmospheric vent 21 is connected to the horizontal portion 12 of the first conduit 5. This vent 21 allows air 22 at atmospheric pressure to enter the first ^' conduit 5 and acts as a vacuum break to the low pressure created within the vertical section 14 of the first conduit 5. Air 22 will be sucked into the system 1 when the pressure in the first conduit 5 drops below atmospheric pressure. This arrangement prevents liquid from upstream parts of the first conduit 5, such as portions 10, 7, being sucked (i.e. siphoned) up to the horizontal portion 12, and maintains the level of the liquid in the sump.

The sump 2 may be the sump of a scrubber, such as an exhaust gas scrubber onboard a marine vessel. The liquid 26 leaving the system 1, may be passed to a downstream treatment device, and then discharged, e.g. into the sea.

By passing liquid from the sump along the first and second conduits 5, 16, the system 1 may be used to control the level of liquid in the sump 2 and to control the amount of aeration of the liquid 26 leaving the system 1 by controlling the relative amounts of liquid flowing through each conduit using valve 19.