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Sealing Threads

Sealing threads on steam pipes

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Alan Donovan26/06/2019 17:08:51
61 forum posts
29 photos

Hello All.

It has been necessary to replace the regulator on a 'Betty', all the parts are remade or refurbished and I now need to install these within the boiler. The original regulator does not follow the drawings, but I am trying to keep to the existing 'design and build' as much as possible. It is similar to the 'Simplex' regulator arrangement. I have three threaded connections, two are within the boiler itself and the third is in the bulkhead (smokebox end). The smallest thread is 3/8" OD. Connections are copper to bronze or bronze to bronze.

Reference to 'Maisie - words and music' (p72) suggests using 'plumbers jointing' to make the joints steam-tight. I have a few questions relating to this.

1) Are we talking about 'Boss White' or similar here? 2) Does it need to have hemp added? 3) Previous posts have suggested Hylomar (I assume Blue Hylomar) - which I feel more confident about as it has an aviation pedigree and possibly a higher working temperature. 4) Would you add hemp to the Hylomar - in this case?

What does the forum think? I am open to any guidance and any alternative suggestions.

I assume the joint sealing products and system will have to be approved by a boiler inspector, so I would welcome an inspector's point of view as well.

Best wishes to all and 'Thank You' in advance.


Edited By Alan Donovan on 26/06/2019 17:28:42

Neil Wyatt26/06/2019 18:54:48
18990 forum posts
734 photos
80 articles
Posted by Alan Donovan on 26/06/2019 17:08:51:

1) Are we talking about 'Boss White' or similar here? 2) Does it need to have hemp added? 3) Previous posts have suggested Hylomar (I assume Blue Hylomar) - which I feel more confident about as it has an aviation pedigree and possibly a higher working temperature. 4) Would you add hemp to the Hylomar - in this case?

1) Yes

2) No

3) Boss White 200C up to 7bar. Blue Hylomar 250C and 35bar

4) no.

Boss white will do the job for less cost, but I can see no other reason for not using blue hylomar.

You'd only use hemp for reinforcing packing of loose fitting joints.

You might also consider PTFE tape.


Edited By Neil Wyatt on 26/06/2019 18:55:07

alan-lloyd26/06/2019 19:10:32
176 forum posts

What about Loctite tread sealant?

Alan Donovan27/06/2019 08:56:09
61 forum posts
29 photos


Thank you for your input. It is very much appreciated.

I will certainly have a look into the Loctite option before I make my decision on which product to use.


Hopper27/06/2019 10:11:37
6188 forum posts
319 photos

Loctite make a teflon pipe sealant that is a good substitute for teflon plumbers tape.

Personally I would just use plumber's tape though.

Nigel Graham 209/07/2019 09:05:25
2009 forum posts
27 photos

Plumber's Tape. That would have been my suggestion.... once.

A boiler-testing seminar I attended in Taunton last year (? 2017?) said NO to PTFE tape, admittedly for boiler mountings and test-plugs, because small shreds can escape and cause trouble by blocking small steam and water ways. Where the fitting is up to a shoulder, as it usually is, use a thin fibre washer. I'd add, or where the design allows, an O-ring in a correctly-sized groove in the joint face.

I've certainly noticed a lot of people use umpteen turns of PTFE tape on a finely-threaded fitting as if more means better, but it never looks quite right. It does force surplus tape out of the joint.

Wondering the professional Plumber's view, I asked a friend who is a central-heating installer and accredited gas fitter. He says he uses pipe sealant, but very rarely uses PTFE tape, less than one small reel a year.

I have heard it's really a thread lubricant rather than sealant, but this probably in pipe connections that rely on BSP Taper threads, so tightly metal-to-metal. That is all right for heavy steel pipe but not robust enough for the much more delicate fittings on our miniatures.

norm norton09/07/2019 10:09:36
183 forum posts
9 photos

Hi Alan

Search for Rocol Steam Seal which is designed for this very job. Suppliers on eBay, and Heritage Steam are a good company.


Lainchy09/07/2019 10:47:54
271 forum posts
103 photos

Loctite 542?

Andrew Evans09/07/2019 11:52:41
342 forum posts
8 photos

Rocol steam seal will seal it well, once it's gone off it won't be easy to undo the joint though. It remains soft for a few hours so you can get the position exactly right without panicking. It's used on full scale high pressure fittings.

Neil Wyatt09/07/2019 14:05:58
18990 forum posts
734 photos
80 articles

Boss white and hylomar are also designed for just this sort of application, but don't set to allow easy dis-assembly.


Alan Donovan17/07/2019 22:33:59
61 forum posts
29 photos

Hi all.

Thank you all for your input. Here is some feedback after your comments and suggestions.

After considering all responses I have decided to go for, what may be, a slightly unusual approach.

This is the refurbishment of an existing locomotive and I have found that the two bulkheads upon which the regulator mechanism is mounted are neither parallel to each other or axially inline. Therefore it has been necessary to 'manipulate' a section of the steam pipework to set the regulator housing in the correct alignment for a (hopefully) successful assembly with the regulator and stem. This requires the exact positioning of the three components that make up the regulator housing assembly.

Therefore I have chosen to use Hylomar with hemp packing for the two threaded joints on the copper pipe. Hylomar for the higher temperature and pressure rating and hemp to provide a 'tight fit' to the threads so that a firm but exactly positioned joint will result.

The finished joints were cleaned up after assembly to remove superfluous sealant and hemp to ensure it shouldn't become detached and block any steam passages.

All being well there should be a photo of the 'regulator housing assembly' below. The rule shows how the copper pipe has had to be manipulated to align the regulator housing (the RH end) with its mating components



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