System to determine a total flow rate Q tot of a washing liquid at a washing liquid inlet of an exhaust gas cleaning unit installed in a marine vessel, the exhaust gas cleaning unit comprising - a scrubber pipe comprising an exhaust gas flowing between an exhaust gas inlet and an exhaust gas outlet; - two or more spraying nozzles (I - VI) mounted at different height levels in the scrubber pipe, wherein each of the spraying nozzles (I - VI) is • adapted to spray washing liquid into the exhaust gas present in the scrubber pipe in order to clean at least part of the SO x out of the exhaust gas; and • operated by a valve (a - f) adapted to open and to close the respective spraying nozzle (I - VI), and comprising an uppermost active spraying nozzle that is opened and activated by its respective valve; characterised in that the system comprises - at least one pressure sensor arranged to measure a pressure P applied to the uppermost active spraying nozzle, expressed in Bar; and - a process controller • calculating the flow rate Q ind of the washing liquid flowing through each of the individual active spraying nozzles, expressed in litre per minute or m 3 /h, using the formula: Qind = K × ( P − pressure correction + height correction), wherein ▪ K is a spraying nozzle factor depending on the type of spraying nozzle (I - VI), ▪ the pressure correction is a correction factor of the pressure P as a function of the elevation of the respective spraying nozzle in the scrubber pipe, expressed in Bar, and ▪ the height correction is a correction factor of the pressure P based on the height difference between the location of the pressure sensor to the respective active spraying nozzle where the flow rate is calculated, expressed in Bar; and • calculating the total flow rate Q tot of the washing liquid at the washing liquid inlet of the exhaust gas cleaning unit, expressed in liter per minute or m 3 /h, by summing up the different flow rates Q ind of the washing liquid through each of the individual active spraying nozzles. - a scrubber pipe comprising an exhaust gas flowing between an exhaust gas inlet and an exhaust gas outlet; - two or more spraying nozzles (I - VI) mounted at different height levels in the scrubber pipe, wherein each of the spraying nozzles (I - VI) is • adapted to spray washing liquid into the exhaust gas present in the scrubber pipe in order to clean at least part of the SO x out of the exhaust gas; and • operated by a valve (a - f) adapted to open and to close the respective spraying nozzle (I - VI), and comprising an uppermost active spraying nozzle that is opened and activated by its respective valve; • adapted to spray washing liquid into the exhaust gas present in the scrubber pipe in order to clean at least part of the SO x out of the exhaust gas; and • operated by a valve (a - f) adapted to open and to close the respective spraying nozzle (I - VI), - at least one pressure sensor arranged to measure a pressure P applied to the uppermost active spraying nozzle, expressed in Bar; and - a process controller • calculating the flow rate Q ind of the washing liquid flowing through each of the individual active spraying nozzles, expressed in litre per minute or m 3 /h, using the formula: Qind = K × ( P − pressure correction + height correction), wherein ▪ K is a spraying nozzle factor depending on the type of spraying nozzle (I - VI), ▪ the pressure correction is a correction factor of the pressure P as a function of the elevation of the respective spraying nozzle in the scrubber pipe, expressed in Bar, and ▪ the height correction is a correction factor of the pressure P based on the height difference between the location of the pressure sensor to the respective active spraying nozzle where the flow rate is calculated, expressed in Bar; and • calculating the total flow rate Q tot of the washing liquid at the washing liquid inlet of the exhaust gas cleaning unit, expressed in liter per minute or m 3 /h, by summing up the different flow rates Q ind of the washing liquid through each of the individual active spraying nozzles. • calculating the flow rate Q ind of the washing liquid flowing through each of the individual active spraying nozzles, expressed in litre per minute or m 3 /h, using the formula: Qind = K × ( P − pressure correction + height correction), wherein ▪ K is a spraying nozzle factor depending on the type of spraying nozzle (I - VI), ▪ the pressure correction is a correction factor of the pressure P as a function of the elevation of the respective spraying nozzle in the scrubber pipe, expressed in Bar, and ▪ the height correction is a correction factor of the pressure P based on the height difference between the location of the pressure sensor to the respective active spraying nozzle where the flow rate is calculated, expressed in Bar; and ▪ K is a spraying nozzle factor depending on the type of spraying nozzle (I - VI), ▪ the pressure correction is a correction factor of the pressure P as a function of the elevation of the respective spraying nozzle in the scrubber pipe, expressed in Bar, and ▪ the height correction is a correction factor of the pressure P based on the height difference between the location of the pressure sensor to the respective active spraying nozzle where the flow rate is calculated, expressed in Bar; and • calculating the total flow rate Q tot of the washing liquid at the washing liquid inlet of the exhaust gas cleaning unit, expressed in liter per minute or m 3 /h, by summing up the different flow rates Q ind of the washing liquid through each of the individual active spraying nozzles.