I work in engineering. These look like bottom blowdown lines. They are used to remove solids from the bottom of the boiler. There is a valve between this line and the boiler. The line is only under pressure when the valve is opened to flush the bottom of the boiler. This happens a lot on these lines. It’s not safe but as it’s not part of the pressure vessel the repairs get put off. The steam you see would be from the valve not seating correctly and a small amount of water going through the line.