The withdrawal screen section is for impregnation of chips during the manufacture of chemical pulp. Chips are steamed and impregnated in a low pressure impregnation vessel using pressurized hot spent cooking liquor BL. The spent cooking liquor produces most of the steam BLST necessary for steaming the chips. To reduce the need for adding fresh steam for chip steaming is a first withdrawal section in the impregnation vessel modified to be used both as a flash tank for warm pressurized wash filtrate and improved wash-out feature for the spent impregnation liquid behind the screen in the withdrawal section. The withdrawal volume is arranged with an upper part located above the liquid level LIQLEV of the impregnation vessel. Additional pressurized warm wash liquor REC1 is added to the withdrawal volume and flashed off steam is led in a steam duct to the chip volume above the liquid level.