A well tool device having a movable partition device within a housing, the partition device separating an inner volume of the housing into a first compartment and a second compartment, a pyrotechnic mixture in the first compartment, and a fluid line providing fluid communication between the second compartment and an outside of the housing. A method includes, while running the well tool device into the well, equalizing a pressure difference between the second compartment and an ambient pressure outside of the housing, by means of allowing fluids to enter the second compartment through the fluid line, while running the well tool device into the well, equalizing a pressure difference between the first compartment and a pressure inside the second compartment, by means of the partition device being affected by the pressure inside the second compartment, installing the well tool device in the well, and igniting the pyrotechnic mixture.