в тексте обратный называют nonreturn valve

И тут же в конце абзаца пишут check valve , а он по словарю тоже обратный и стопорный (запорный)

Что делать ?
писать тоже обратный, ка ки Nonreturn или автора хотели что-то подчеркнуть и это другой уже клапан

контекст:
1.3.4 Non-return Extraction Flap Valves
The non-return extraction flap valves prevent the steam back flow in the turbine in case of sudden drop of turbine load or in case of turbine trip. The flap valves have the pressure air actuator that allow the steam flow in the required direction, but forced closing of the flap valve in case of turbine trip or in case of warning of high condensate / feed water level in the connected heater. The non-return action is maintained even when the servomotor is open. The flap valves can be closed by spring force, together with the air actuator force. The non-return flap valves with actuators for forced closing are installed in all extractions where the operational pressure can be in some operational mode higher then 1.5 bars. For lower pressure levels there are used the non-return flap valves without governing.
Is necessary install check flap valve as close to the turbine as possible.