What is the function of a boiler non-return valve? Is it, as the name implies, a check valve that prevents backflow into the steam drum?

Yes,  Boiler non-return valve (also referedto as a Stop Check valve) is a check valve that can be closed.  This valve has a disc and stem but the stem is not fastened to the disc.  When desired the stem can be turned down to shut off flow.  But when the stem is retruned to the open position the disc will not come with it unless there is pressure under the disc.  

In many boiler application this the valve is an angle globe type and is positioned on top of the boiler in the outlet line. 

The ASME code no longer requires a non-return valve on the steam leads, but does require a double block and bleed on the main steam leads if more than 1 boiler are interconnected on a common header and these boilers have steam drums with manways.State codes may require these devices, though.

The purpose of the non-return was two-fold. Under the interpretation that it is a combined stop-check valve, then it can be used as one of 2 required stop valves that will prevent steam ingress into a steam drum while a maintainence person is inside the drum of an idle boiler, so he doesn't get cooked. The check valve works even if the control system is fouled up and inadvertantly opens a stop valve .

The second reason is to enable startup of a second boiler after the main steam lead is already pressurized. For a drum type boiler to startup and fire at more than a 10% firing rate, it is neccesary that the water in the waterwalls not be excessively "subcooled", that is , the waterwall fluid temp must be close to the saturation temperature associated with the current drum pressure. If teh drum pressure were artificailly raised due to a lack of a closed steam valve, then the water would not boil when a high heat flux is experienced and would lead to tube overheat. Proper startups require an outlet steam valve to be closed and a drum vent opened to ensure the drum pressure is  as close to the saturation pressure that is adssocaited with the  restart waterwall fluid temperature. This is automatically assured if a nonreturn valve is supplied.

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