When I walk I rarely look around except for the road ahead so that I do not miss potholes and traffic. That is why it took me so long to see a non-circular flange that was near my home. There have been two instances:

1. On my way home, I saw a Diesel Electricity Generator. Atop the squatly sitting generator, is a vent connection with nice weather shield. However, what caught my attention was the connection it made with generator assembly. The connection was non-circular flange. The pipe size is 3″ (visual guess) and at the flange, it is bolted (4 bolts). I thought that as one off case and decided to ignore.

2. Yesterday, when I was commuting, I saw a diesel-generator-driven-pump at a railway station. Then I saw the flanges, they were squarishly circular or circular square (OK, I give up describing) i.e. non circular flanges. It was small portable pump unit.

My question: what is the reason that non circular flanges are used and do portable units play part to reasoning? I had never thought they existed before I saw them (quite similar to Black Swan problem).