The results of 70 cycles of rigorous testing for resistance to thermal cycling of the medium are presented – temperature reduction of the supplied water δTin.30 sec = 262°C over the first 30 sec of the process for two check valves of the type S23307-0160-65, DN 65 (body material St12Kh18N10T) as well as four of the same thermal cycles for a bellows-type stop valve S.KZSA-100-00-00-E-03, DN100 (body material St20). Sealing gaskets containing thermally expanded graphite are installed in the body–cap connector of all valves. At the completion of the tests, all three types of gaskets retained their integrity and serviceability. However, if the stop valve DN100, which structurally has an axisymmetric temperature distribution of the metal in the above-flow part of the body, retained its tightness throughout all thermal cycles, then out of 140 thermal cycles the check valves DN65, which do not have such an axisymmetric temperature distribution dynamics, had 18 brief (40–50 sec) weak seal failures in the front portion of the valve connector. The average difference of the temperature of the metal above the flow portion of the valve body in the front and back sectors was estimated as ΔTav ~ 40°C in the presence of seal failure.