An arrangement of a coke drum unit and of a coke crushing unit for use in a closed, gastight system for gaining sellable petroleum coke pieces out of solidified petroleum coke in a coke drum unit. It comprises a coke drum unit containing solidified petroleum coke; a coke crushing unit for crushing the petroleum coke into sellable petroleum coke pieces arranged below the coke drum unit; a lower funnel connected to the coke crushing unit and having an upper end portion; an upper funnel connected to the lower end of the coke drum unit and having a lower end portion with a smaller diameter than the upper end portion of the lower funnel; the lower end portion of the upper funnel extending into the upper end portion of the lower funnel; and at least one circumferential sealing element providing a gastight sealing between the upper funnel and the lower funnel and allowing for an axial and radial movement of the upper funnel with respect to the lower funnel.