5" gauge Britannia (a builder's diary)

[runner42]

Hi Steve,

what a gigantic post you provided in 2020, I don't know how I missed it, but better late than never I have added my like. Let me say that your photographic work and narratives are excellent matching the workmanship that you have achieved throughout. I noted from your personal post that the construction has had a 15 year hiatus but that gap has been all to the good because we late comers have had the ability to see your work since this forum possible didn't exist at the very beginning of the project so it would have gone unrecorded to the extent that you have provided should the construction been continuous.

Brian

[Geoff (Carlisle)]

Hi Steve, you've done a lot of hard work and finished a beautiful loco. I remembered many years ago I had the laser cut Britannia frames in the vice with a hacksaw in my hand,is it going to be a Brit or a Clan and being a Kingmoor man I had to make it a Clan. If I had chosen a Brit , it would have been running around by now.

Geoff (Carlisle).

[Jim]

Jul 5, 2021 21:08:35 GMT Geoff (Carlisle) said:

Hi Steve, you've done a lot of hard work and finished a beautiful loco. I remembered many years ago I had the laser cut Britannia frames in the vice with a hacksaw in my hand,is it going to be a Brit or a Clan and being a Kingmoor man I had to make it a Clan. If I had chosen a Brit , it would have been running around by now.

Geoff (Carlisle).Hi Geoff,You've been very quiet for far too long and we're missing your Clan updates. It's a lovely loco that you're building and it's a great journey you're on as you head to its completion.Take care and stay safe,Jim

[springcrocus]

Thank you, gentlemen, for your continued interest and kind words. I'm just a reasonably competent machinist, nothing more, and I hope my descriptions encourage the less experinced to have a go for themselves.

I've also had some tiny weeps from the 3-cock gauges where the tapered valve stems pass through the bodies, mostly because the tapers are not exactly equal. This probably happened because the pressure from the tapered "D" bit reamer distorted the flimsy bodies and there is also a tendency for the reamer to deflect slightly. There is a great deal of force applied when reaming a taper, far greater than for a parallel hole. With this in mind, I completely redesigned my 3-cock gauges and made some new ones with not a single tapered spindle in sight.

The four bodies were remade first, this time using sections from 13mm bronze plate (pumphouse bronze, for those who remember this material from earlier) but this time the crosshole for the valve stem was reamed 9/32". The size was determined by what "O" rings I had available and a quick look in the cheap-and-cheerful selection box revealed some 9/32" x 5/32" rings.



Because I need flat shoulders for the valve spindle to bear against, I have not bothered with any shaping at this stage. They can be prettied-up once proof-of-principle has been established. All other machining was much the same as the originals which I covered HERE with the bodies finishing at 0.375" wide



The spindles were made from 3/8" dia Colphos and the first operation was to turn the stem to a full 9/32" diameter by 0.380" long and with an 8BA thread on the outer end. Then I machined a pair of grooves with a 1/16" grooving tool, touching on and plunging in fifty eight thou (0.116" off the diameter) to give four to five thou per side of squeeze on the "O" ring. Finally, the 9/32" diameter was polished down till it just fit in the reamed hole and the "O" rings fitted.



After parting off with a bit extra for the handles, the opposite end was machined to include a spacing boss, the O/D for the 3mm square drive and another 8BA thread to keep the handle in place. Then it was over to the mill to cut the square. One big advantage of the parallel valve stem is the ability to hold it in a collet!



The rest of the machining was as previous apart from four slim washers to fit on the outboard ends. The drain cocks at the bottom were also made with this form of valve spindle but using 3mm x 1mm "O" rings on a 5mm diameter valve stem. The tiny "O" rings were a pain to assemble so a small tool was made to assist, a taper at one end and roll the ring on, a bit like a spring compressor in reverse.



However, the drain cocks wept slightly under pressure and something even better was needed so I converted them to miniature versions of globe valves by turning up a bung for the cross-hole and soldering it into place. I started with some 1/4" diameter Colphos, turning the O/D to 5mm dia for a length of 3/8", pilot-drilling 2.3mm dia to the same depth, then drilling 3.3mm dia to a depth of 1/4" and tapping M4 for 3/16"



After soldering into position, the drain cock was screwed into a mandrel and the angled hole made, starting with a 2mm slot drill and finishing with a 1.6mm drill. The angle of the collet block was set by eye, offering the drill to the front of the workpiece. There was not much room for error here.



This simple sketch shows what I wanted to achieve. A 1/8" diameter silicon nitride ball was used to seal the port. The lower hole was drilled through freehand on the pillar drill to complete the drain path.



And because I kept the plate that was used to set the bushes in the backhead, I was able to fix up a test rig to check each set for leaks without having to keep screwing them into the boiler and risk damaging the threads in the bushes. As can be seen, the first set is currently sitting at about 130 psi with no weeps from the drain but a quarter-turn of the screw and away it goes. I shall now modify an M4 stainless steel screw and fit the original brass handles.



Although the parallel-stem valves leak very slightly from input to output under high pressure, they don't leak to the outside. This doesn't matter too much for the shut-off cocks because they will only ever be used in a broken-glass scenario and they will work well enough to protect the driver from a faceful of steam. The drain is different and has to be able to hold full pressure at all times.

One advantage of this style of 3-cock gauge is that it doesn't require any special tools to be made, just some accurate machining, and if the "O" rings deteriorate, they can be replaced without removing the bodies from the boiler.

Steve

[springcrocus]

This regulator block has still been giving me grief, this time failing to seal at the valves. I have remade the valves with slightly longer stems for a better fit in the guides, polished and reseated the angular contact faces on numerous occasions, changed the springs, removed the springs, removed the actuating mechanism and recut the seats. Even a damn good thump with a hammer has failed to achieve a seal so, after wasting the best part of twenty hours, I have now abandoned this block completely. I have also deliberately destroyed it so that I am not tempted to have "just one more go" as I know it will end in failure. High blood pressure I can do without!

Plan "B" is to replace them with stainless steel ball valves similar to what Jim did on his Britannia. However, I have had to make various adaptors to allow them to fit and still keep my superheater assembly. I started with a choice of two styles to see what I could adapt the easiest.



The larger one looked promising but it was extremely stiff and a little to bulky for the available space. Using the small one, I trimmed it down in length and turned a spigot on one end. I had the copper cover plate that I used to test the superheaters but it had 4BA clear fixing holes. I made a new one (more pumphouse bronze) with 4BA tapped holes to fix to the steam chest because, with the limited space, getting nuts round the back would be extremely difficult.



With it all fitted together, this is how it looks. I've had it under pressure for the last quarter-hour and also used the valve to test the ease of movement. I'm very pleased with the outcome and now the race is on. If I can get the boiler back on and the plumbing reconnected before 10.00am tomorrow, it can go to Broadfields with me for it's hydraulic test.



Steve

[stevep]

Is that PTFE tape on the joint to the steam pipe?

As it is in the smokebox, I though PTFE is a complete no-no - I seem to remember it burns to produce phosgene (a poisonous gas).

[Jim]

Jul 20, 2021 12:05:30 GMT stevep said:

Is that PTFE tape on the joint to the steam pipe?

As it is in the smokebox, I though PTFE is a complete no-no - I seem to remember it burns to produce phosgene (a poisonous gas).

I've used Plumber's thread sealing tape to seal the threads on the washout plugs in the smoke box of my traction engine with out any problems at all, nor is there any sign of the tape burning where an edge is exposed.

Jim

[oldnorton]

Jul 20, 2021 12:05:30 GMT stevep said:

Is that PTFE tape on the joint to the steam pipe?

As it is in the smokebox, I though PTFE is a complete no-no - I seem to remember it burns to produce phosgene (a poisonous gas).

Well, PTFE survives on a few million frying pans around the world.

I would suggest it is fine in the smokebox especially when it is wrapped around something connected to the boiler. Technically it starts to break down around 300 degC.

[springcrocus]

Actually, Gentlemen, it's residue PTFE tape that I hadn't cleaned off yet. The original regulator block had a 7/8" x 32 tpi female thread in it and the tape was completely encased by the block when it was tightened up. This new arrangement used a double male adaptor to take the ball valve and I have tapped the steam pipe accordingly. It was already fitted when I took the second photo so didn't get included. The one in the photo joins the plate to the ball valve. All the adapters have red fibre washers at each mating face. The tape has since been scraped off.

Steve

[keith1500]

I tend to use liquid PTFE instead of tape. This stems from plumbing; it’s a pet hate of mine to see PTFE tape visible on joints such as radiator valves hence the change to liquid form. And so this has been use numerous times on my Speedy. So, would it as a PTFE product have the same issue?

Fingers crossed for the hydraulic test.

[jma1009]

I hope those are not steel bolts on the wet header? Or just temporary until replaced?

[Gary L]

Jul 20, 2021 21:25:50 GMT keith1500 said:

I tend to use liquid PTFE instead of tape. This stems from plumbing; it’s a pet hate of mine to see PTFE tape visible on joints such as radiator valves hence the change to liquid form. And so this has been use numerous times on my Speedy. So, would it as a PTFE product have the same issue?

Fingers crossed for the hydraulic test.

I believe it is true about burning PTFE; (or at least, I'm not prepared to be the guy who puts it to the test!) But to burn it, you have to raise its temperature past the flash point, and this is very difficult indeed when it is being used to seal copper alloys that are in contact with water. The smokebox gets hot, yes, but I've never seen flames in one, so it seems pretty unlikely to me that you would have any trouble, even with PTFE tape, and even less so with the liquid sealants where the PTFE is in suspension. (Have you checked the Data Sheet for the product you are using? They are always downloadable these days)

I don't like PTFE tape either, but sometimes it seems right for the job, especially in places where a lot of making and breaking the joint is expected. Maybe not so many of these in a smokebox?

-Gary

[racinjason]

I use Loctite GS1 on all the steam fitting at work and on models.
 cheers Jason

[Deleted]

+1 for Loctite although I would have to check which number I have. IIRC there are two recommended grades?

Pete

[gwrfan]

Jul 21, 2021 8:43:31 GMT @greenglade said:

+1 for Loctite although I would have to check which number I have. IIRC there are two recommended grades?

Pete

Following the advice of a well-known You-Tuber, from 'up North', I use Loctite 542, which is a thread sealer, but allows you to unscrew fittings without first having to break the bond by applying heat. Works for me anyway.

Geoff

[Deleted]

Hi Geoff, I chose mine after receiving advice on what others have found successful, looking at what's available it seems that there are a good number of suitable sealants for our needs. I chose 567, I can't recall why but when checking the tech sheets of the types suggested this seemed my best bet. It's a very low bond sealant and so no issue with removing the parts when required. The operating temp is over 200c and its sealing strength can handle 10,000 PSI.
I have no experience with this under steam yet although do recall steam being covered in the tech sheet, we shall see, I have used it recently to seal the threads for the cylinder relief valves.

regards

Pete

[springcrocus]

Jul 20, 2021 21:25:50 GMT keith1500 said:

Fingers crossed for the hydraulic test.

Thank you, Keith. Defeated by the hot weather, unfortunately, so next Wednesday instead.

Regards, Steve

[Jim]

Jul 21, 2021 18:22:04 GMT springcrocus said:

Jul 20, 2021 21:25:50 GMT keith1500 said:

Fingers crossed for the hydraulic test.

Thank you, Keith. Defeated by the hot weather, unfortunately, so next Wednesday instead.

Regards, Steve

Given the very high standard of your work so far I can't imagine you having any problems with the steam test. The Britannia will pass with flying colours I'm sure.

Jim

[runner42]

Excellent work Steve and a novel design of the regulator and wet header configuration.

Brian 

[Midland]

Steve
Pardon butting in. I have been looking at Britannia grate issues and found a thread with only two pages and yet you have about 60! Do you have another thread? Or better still may I put you and my friemd in touch. I think he wants to omit the two side drop doors and wonders of he can. Judging from your thread, the small one, the answer may be no!!!
Cheer sDavid