the line for the addition of black liquor bl a sub fraction of the spreader nozzles 10 is activated in the event of the first threshold value being exceeded where the degree of opening may be modulated by pulse width they may be opened for example for 20 of the time span of a period lasting 300 seconds the remaining spreader nozzles 10 may be activated with the same modulation of pulse width 20 of 300 seconds in the event of a second threshold value being exceeded the degree of opening of the spreader nozzles may be increased such that they are held open for example during pulse width modulation for 40 of the time span of a period lasting 300 seconds in the event that a third threshold value is exceeded and the degree of opening can be increased at even higher temperatures by 20 in steps until all spreader nozzles are held continuously open it is an advantage if the cooling effect can be coupled in several stages such that a sudden and rapid cooling effect is not introduced into a superheated gas phase which may cause an uncontrolled and rapid fall in pressure which may even lead to such a severe negative pressure in the steam pre treatment vessel that it risks implosion it will be realised from this example of temperature controlled activation of the cooling effect that also other control principles for the cooling may be implemented the flows in to and out from the steam pre treatment vessel for example may be monitored and if the inflow of cold chips for example should cease or.