Abstract: Valve for use with a tubular chemical reactor operating at pressures of over 1000 bar and temperatures of more than 250.degree. C. The actual wall thickness of the lower valve part within the area of the inlet/outlet duct is at least 35% less than the wall thickness of adjacent wall areas, the undersize being compensated by the prestress applied by the tightening bolts of the upper valve part. A lens type seal is located between the lower valve part and the upper valve part and simultaneously (1) seals the connection between the valve parts and (2) comprises the valve seat. The thermal conductivity of the seal is at least 50% lower than that of the valve material.