present invention the upper end of the vessel 10 is closed by a domed end cap having a filler neck cylinder 12 of reduced circular section usually 2 ft to 4 ft diameter projecting axially therefrom at the bottom end the vessel is closed by a generally funneled configuration having a blow line conduit 16 issuing substantially along the funnel axis and flow controlled by a motor valve 17 a quantity of wood chips or other appropriate cellulosic fiber source is charged into the vessel 10 interior through a chip chute or channel 14 past a capping valve 20 which seals at least 150 psig steam pressure within the vessel 10 preferably a chip weight or volume flow meter 24 is disposed in the chip supply channel 14 as the source of a chip flow rate related signal from the meter transducer 26 to a process controller 30 also disposed within the chip supply channel 14 is a temperature sensor 22 responsive to temperature within the channel of the character that is indicative of steam escaping from the vessel 10 interior sensor 22 is constructed to transmit a signal to the controller 30 in the event of steam escape from the vessel 10 into the chip supply channel operatively connected to the upper or bottom end.