Cylinder unions

[ben]

Hopefully the last query before I get this job done.

Most of the openings on my new (14-year old) cylinder *appear* to be 22mm compression threads. However, the fact that on two of them I can't insert the pipe very far in (a couple of mm, a bit more with force) suggests they may be something else. Maybe I just need to get some wire wool on the inside of the fittings and clean them out, or maybe I am missing something crucial.

Plan was to use 22mm blanking nuts on the solar in/out, and then chunk a compression nut and solve on a piece of pipe on every other fitting. Googling shows the existence of cylinder unions e.g

- but I can't work out if these are simply a time saving device to make it easier to align everything for soldering, or if there is anything special about the connection.

[ben]

Ignore that bit, I can see the difference now - 1" is not 22mm. But I'm assuming that in general a 22mm compression nut is actually a BSPP 3/4"?

[John Rumm]

Cylinders IME often have either a male threaded connection, onto which some kind of coupling similar to that in your link can be screwed on. Or often, the cylinder connection is just a protrusion of stainless / copper pipe of appropriate size, onto which they expect the installer to fix a compression fitting. If yours was previously installed and was of the second type, you may now have a compression fitting with the original outer compression nut and olive missing. That fitting might be

3/4" and not BSP. The outer diameter of 3/4" pipe is slightly smaller than 22mm. That usually means that with some effort you can get a 22mm compression fitting to mate with 3/4" pipe, but you can't get 22mm pipe into a 3/4" compression fitting.

A photo of what you have would help...

[ben]

Thanks,

Ben

[John Rumm]

Well it has the internal bevel you would expect on a compression fitting. The discolouration of the internal surface also looks like something that may have at one time had a pipe in it. Can't see the shoulder / end stop for the pipe though.

If you can measure that ID, and its around 21.5mm, then its expecting a

3/4" pipe.

[Tricky Dicky]

You need some of these;

Or

If you do not want to solder. In either case the male thread on the cylinder needs wrapping with PTFE tape

Richard

[Andrew]

That flat roof looks dodgy. Must have been leaking for a while for OSB to get into that state.

[ben]

See the other thread. Half of it is now stripped off. No shortage of work on this place.

[Roger Hayter]

The first link you mention is a union fitting - the seal is made by a metal to metal seal of the male conical brass part in the fitting and the female conical part of the fitting on the tank. PTFE on the threads won't help make a watertight seal as water can simply pass between the nut and the pipe if the conical union does not seal.

If the surfaces of the union fitting are good enough this is a very reliable seal. If the fitting is distorted or too pitted or worn to seal then the second option you link to can be used, the seal being made to the threads, though preferably with either hemp and sealant or the Loctite thread sealand which comes as a "string" of ptfe and stuff. 1" is very much on the big side for sealing with PTFE tape. More especially since AFAICS neither thread is a taper one. The !" female to compression adapter should be available in a 1" taper BSP female thread which I would prefer if I was doing it. But the proper union is better if it works as there is a risk of damaging the fittings on the tank if over tightened.