Abstract: A pressure vessel designed to hold a plurality of cylindrical cartridges in the form of cross-flow semipermeable membrane filtration elements having central permeate tubes that are interconnected to form a composite permeate passageway that extends axially of the pressure vessel. Several constructions are shown for compensating for assembly clearance and potential pressure vessel growth and uniquely biasing the column of cartridges towards the downstream end of the pressure vessel to prevent axial shuttling movement that could cause wear and deterioration of seals which would result in leakage and deterioration of product while providing for absorption of shock potentially promulgated from the downstream end should feed pressure be suddenly lost.