vessel falls below a threshold value whereby the injection is activated water or cooled process fluids from the production process for cellulose pulp is used as cooling fluid these cooled process fluids may be cooled white liquor cooled black liquor or cooled filtrate from a subsequent washing stage etc the amount of cooling fluid that is injected is preferably controlled to be proportional to the degree of risk of blow through and this can take place through activating different numbers of injection nozzles or by using a degree of opening of each activated injection nozzle that is modulated by the pulse width in one simple form of regulation of the cooling the activation of the cooling is controlled as a dependence on the temperature in the volume of chips detected by the measurement probe 32 or by a temperature sensor arranged in the gas phase above the level of chips not shown in the drawing the control means 31 opens the valve 11 to a degree that is proportional to the excess of at least a first or a second threshold value or proportional to the excess of one threshold value the first threshold value may be a pre determined first temperature t lev1 and the second threshold value may be a pre determined second temperature t lev2 where t lev1 lt t lev2 the regulation of the flow of cooling fluid also preferably takes place in combination with the activation of other regulatory measures the supply of steam may be stopped for example when the temperature becomes too high the amount of cold chips that is fed in may also continues or be allowed to establish a higher.