Compression nut identification

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Sorry, I'll try again:

Peter

It looks like the hex portion is a slimmed down nut, but I'd expect any normal compression nut/olive would fit ... it's not my floor that gets flooded if it doesn't, that's why I keep a heap of manky old towels in the shed.

Looks like a standard (if old) nut on the copper pipe, with an adaptor between it and the drain fitting. There's a bit of thread visible, compare that with a normal compression fitting ?

Sort of OT, but I was wondering about dissimilar metals (copper/iron) touching and perhaps electrochemical activity increasing the likelihood of corrosion. Would it help in cases like this to use several wraps of PTFE tape around the pipe thread to help "insulate" between the metals? Or would it make little or no difference?

I think it would be hard to prevent metal to metal contact, as the point of the PTFE is to conform to occupy the gaps between the threads. Where there is no gap the metal will cut through the soft PTFE and touch. If you were really concerned you might have to insert a length of insulated pipe.

But if you have an unvented heating system (closed loop under pressure, ie no header tank) then there should be little oxygen in the system to cause corrosion. You can additionally control the pH and scale to keep it in the sweet spot for corrosion minimisation. VDI2035 is a modern standard for doing that, and it's fairly simple to do:

If you measure the OD of that section of exposed thread (to the peaks, not into the thread), what size is it?

a 1/2" BSP thread will ~21mm.

I would guess the drain valve has a male BSP stub on each end, and the transition fitting is a compression to BSP female adaptor. Likely from

At least two different threads have been used on 15mm compression nuts to my knowledge, though new ones seem to be all the same now. The adapter into the tee almost certainly has a male taper half inch BSP thread, but replacing this part, with the 15mm pipe loose in the compression fitting would take minutes rather than seconds, so is not an attractive proposion.

I would be prepared to rapidly salvage and use the old nut if the new one didn't fit.

From the age it looks like it will be 1/2" pipe.