It is the thermosyphon vessel's job on the one hand to supply double mechanical seals with buffer fluid to compensate leakage, and on the other hand to provide cooling or heating for the buffer fluid. Operation can be pressureless or with pressurization. Circulation of the buffer medium is performed by the thermosyphon principle or by forced circulation. Auxiliary equipment make for easier operation and increase the reliability. Thermosyphon vessels are pressure vessels within the meaning of the pressure vessel regulation.