Re: Correct Procedure for blowing down a sightglas *LINK*

Typically a water column is not as susceptible to plugging the passages from the boiler and therefore doesn't have shutoff valves in those lines. There is the possibility of stuff accumulating at the bottom of the column which is what the drain valve is for.
I have never heard of blowing stuff up out of the glass. With the drain valve on the bottom, the direction should always be down. Blowing the glass with the column drain would not be effective because of the large connections between the boiler and the column.
The important point is to individually clear each glass passage. Pay attention to the discharge from each passage since often you can hear if a passage is partially blocked.
If a column is blocked in one passage or the other, it could be difficult to detect with a blowdown procedure, but if it is completely blocked, the glass will not read correctly and that will need to be ascertained by comparing to the second glass or gauge cocks. If the gauge cocks are on the column they would read incorrectly as well.