Abstract: The sealing surfaces of a coke oven door are cleaned with high-pressure fluid by a hydraulically operated high-pressure piston pump which displaces either a hot or a cool conveyed media ladened with solid particles such as a coal mass and a fluid for cleansing the sealing surfaces of the coke oven doors and door frames. The pump includes a closed pump cylinder having an inlet and outlet valve for a conveyed medium at both ends of the cylinder and a partition dividing the cylinder. Inlet and outlet valves for hydraulic fluid are located in the cylinder on each side of the partition. An inner double piston is axially movable to and fro with the cylinder and comprises two output piston portions with a piston rod connected to each portion and located between them guided on bearings in the cylinder. A delivery space is formed between the piston portions and each end of the pump cylinder and the respective delivery spaces at each end simultaneously and oppositely decrease and increase during an operation.