What is this?

SeanBeans

On the 8th floor of a 15 story building, heating stack.

Steamhead

Maybe some badly encrusted insulation?

ratio

Steamhead said:

Maybe some badly encrusted insulation?

That's what it looks like to me, with some mineral deposits from whatever water has been spraying on it for years.

Can you poke into it with a probe of some kind?

SeanBeans

It’s solid metal and it was leaking from somewhere inside the bigger metal sleeve.

It has a tag!
You can access the valves from the cover, it steamed up to the 11th floor and leaked down to the 1st

EzzyT

Looks like a expansion fitting

KC_Jones

EzzyT said:

Looks like a expansion fitting

I would agree with that, I marked this up a bit to complete the letters that could be seen. I see Expansion Joint on the label. And yes I know my "handwriting" is terrible.

KC_Jones

Here you go.

KC_Jones

And this.
https://www.ebay.com/p/Robertshaw-Model-I-1291-Packless-Expansion-Joint/1028302806

SeanBeans

Can anyone find any literature on it?