The answer is when it is pure art.  I noticed this on the side of  a building in South Austin.  I really liked the colors and took the picture.  I looked further and found that what appeared to be a functioning valve was not connected to anything and had not been for a long time.  But, it makes for a nice “arty” decoration.

I almost wrote that this is not functioning, but that would have been incorrect as it is functioning as art.