It's often referred to as a "stop & check" valve. It's essentially a check valve, with a positive shut off capability. It's got a handwheel and spindle arrangement so that it can be closed, even in the presence of a flow that would keep a normal check valve open, and this also allows it to be used as a stop valve for isolation purposes on the boiler. As I'm sure you know, you should never rely on a check valve for isolation purposes.
The asme section I boiler code now requires that all drum boilers that have a manway i the drum must have 2 stop valves ( plus bleed line)on the steam line leading to the common header system, if the boiler is installed in parrallel with other boilers.
In the past ( prior to about 1993) ASME required 1 stop valve plus 1 non-return valve for boilers on a commmon header system. Some rookie boiler engineers incorrectly interpreted a non return valve to mean a simple check valve, and several boilers were installed without the double stop valve capability. After a lawsuit on the issue of what a non-return valve is supposed to really mean, ASME sidestepped the issue by changing the code words to require a double block and bleed ass'y.
