When is a boiler fitting not a boiler fitting?....

[springcrocus]

...when it's an inner dome, apparently. Or maybe not.

An interesting point was raised by one of our esteemed members in my Calbourne blog concerning the choice of material for the inner dome. My understanding is that boiler fittings can be made from brass but the basic shell must not contain brass. The inner dome is, by definition, a fitting BUT because of it's position and size some may consider it to be an extension to the boiler.

Although I invited opinions in the Calbourne thread, it might be more appropriate to discuss it here rather than bloat the build log. This could be an interesting exchange of ideas or it may become a dead duck, who knows? The boiler code versus the science versus tradition - anyone like to contribute?

Regards, Steve

[uuu]

I have a boiler for Jessie from Western Steam. It came, as you'd expect, with all the relevant paperwork including the 2X shell test evidence.

But didn't include the dome top, which I've made myself. So - if this is part of the boiler shell, then I'll need to have another 2X test with it mounted. Or, if it's a fitting, it goes with all the other attachments into the 1.5x process. I'm imagining my local inspectors will take the latter approach - we'll see.

The UK boiler test code anticipates the use of brass in fittings. It's silent about brass in the boiler shell, as it is about many design matters.

Wilf

[stevep]

Just looked at Doug Hewson's drawings for Pansy, and the inner dome is quoted as "copper tube with brass or gunmetal flanges".

[Cro]

I think the sensible way to look at it is things like dome covers or inner domes are an extension of the boiler and therefore should be treated as such but are only detachable for the ease of the build/assembly.

That said if it was just a blank plate I know so many that are just brass (the 9f's is) - so where do you draw the line? Is the risk of De-zinc more prominent below the water line or just in general on any part of the boiler? I know first hand of this on Grandads Single with both clacks failing when I tried to remove them for maintenance this year and they are at the lowest point on the backhead.

Personally in your case Steve I would have made it from copper with at least a bronze cap but bronze flange as well if I could.

Clearly this needs more defining in the code.

Adam

[uuu]

I was led to believe that de-zinc is indeed more pronounced below the water line - and the Pumphouse has also serviced locos with failed lower backhead clacks.

Wilf

[keith1500]

Look at it from a boiler examiners point of view. If that item fails it’s likely to be catastrophic as opposed to a fitting which might just release a jet of steam.
The object is safety. It might not fail in the near future but the risk is there. So, eliminate the risk. No argument.

Keith

[springcrocus]

Nov 9, 2022 11:12:17 GMT Cro said:

Personally in your case Steve I would have made it from copper with at least a bronze cap but bronze flange as well if I could.
Clearly this needs more defining in the code.

Adam

Adam, I would have made it from copper if I had some but I didn't. However, that's not the point of this thread. Tell me why I can't use brass.

Nov 9, 2022 13:19:02 GMT keith1500 said:

Look at it from a boiler examiners point of view. If that item fails it’s likely to be catastrophic as opposed to a fitting which might just release a jet of steam.
The object is safety. It might not fail in the near future but the risk is there. So, eliminate the risk. No argument.

Keith

Keith, I think there is an argument because I don't agree with your assessment. Explain to me why a steam dome failing would be any more catastrophic (or any more likely, if it comes to it) than a snapped-off safety valve or clack. In my opinion, there is no difference in the risk.

At the moment, we seem to be in the realms of "we've always done it that way so it must be right". Has anybody got any science to add to the discussion?

Regards, Steve

[uuu]

Perhaps a question to be asked about the failure of any steam vessel is "will a small failure escalate into bigger one"? In the case of a small fitting - the size of the hole is unlikely to be bigger than the bush into which it fits. That's still going to be quite exciting! But if a small hole develops in a de-zincified brass sheet, will it tear into a bigger hole - would the whole dome top blow off? Like making a hole in a balloon - it goes pop. Alas it's years since I investigated the structure of alloys under an electron microscope.

Also to be asked: would the regular 1.5x hydraulic test expose the weakening of a brass component before it failed in steam?

Wilf

[Jo]

I always put a steel plate over the lower inner dome flange for my 2x pressure test. I did make the top half of the dome out of bronze same as all boiler fittings having seen a brass clack fail due to de-zincification at Guildford. The boiler inspectors post failure report was that clack was not part of the boiler test other than participating in the steam test.



Have you considered tinning the inside of the top of the inner dome with soft solder?


Jo

[keith1500]

Steve,

In my experience brass seems to de-zincify uniformly. A broken fitting continues to crumble having lost all strength.

Should that happen to the brass tube I think give the force over the area of the dome could result in a catastrophic failure. If your hoping it might just fail by springing a leak then your gambling on a calculated risk. You might be right. It may never fail given it’s exposed to pure water alias steam. Who knows? Perhaps some one does.

“You want to play thermonuclear war or a nice game of chess.” ( Wargames 1983)

Keith

Ps enjoying the build thread. Great work.

[springcrocus]

Nov 9, 2022 17:20:19 GMT Jo said:

Have you considered tinning the inside of the top of the inner dome with soft solder?


Jo

Jo, a colleague proposed using copper sulphate to copper-plate the dome and your option is also possible - IF it needs it!

However, I've yet to hear a convincing argument AGAINST using brass, which is the point of the thread.

Regards, Steve

[springcrocus]

By the way, before anyone accuses me of being argumentative, I will say that I discussed this with two of our boiler inspectors earlier today and a decision has been reached regarding the way forward. I'm not going to say what that decision is because it changes the discussion into one about our boiler inspectors which would not be fair, regardless of what has been decided.

This thread should be about the suitability of brass in this or similar situations and it would be nice to see some scientific reasons rather than traditional ones. If the regs need changing, that's a different thread for another time.

Regards, Steve

Edit: sorry, Keith, our posts crossed so I didn't see your reply until after both my posts. Thank you for your observations.

Regards, Steve

[ettingtonliam]

Some full size boilers had brass tubes, which were thought to be more corrosion rsistant than steel ones, even though they were below the waterline.
Just saying.

[jo479]

As a boiler inspector I wouldn't see any problem with a brass inner dome, I wouldn't class it as a fitting anyway. Brass fittings (clacks , water gauges etc ) are a different kettle of fish, I have had a few dezincify but only noticeable when you try to remove them after a number of years, during the lockdown I decide to overhaul my Simplex which I finished in 1984, had run satisfactorily ever since, I regretted it, all the fittings sheared off except the Blower valve and the Blowdown valve, much use of a file tang was called for. Most of the fittings you buy today are still made of brass, so unless you make your own it's a cleft stick situation.

[runner42]

If the fitting is permenantly silver soldered to the pressure vessel then brass should (shall) not be used. Other situations should be considered on it's merits such as thickness of brass used etc.

Brian

[mbrown]

Just throwing in my two-pennorth - but we keep mentioning "brass" without discussing the grade or composition, which varies significantly.

My understanding is that fittings made from "screw rod" with its high zinc content are a bad risk for obvious reasons. But I very much doubt if a piece of 2" tube is to that spec. The question, surely, is about the grade of the "brass" being used.

Afraid I am not enough of a metallurgist to know what the "right" answer is - but I do know that what is sold as brass comes in very different specs.

Malcolm

[kipford]

As Malcolm says it is about the grade of brass being used. Brass with a Zinc content below 15% is resistant to dezincification. Above 15% and you can get problems although this can be alleviated by the addition of a small amount of arsenic (about 0.04%). Factors which cause increased rates of dezincification in brass are high temperature, high chloride content of water, and low water speed. Dezincification is also likely to occur preferentially in crevices where there is a low degree of aeration. So our boiler is the perfect breeding ground for dezincification. The big unknown is how long before we get an issue? In practice we know this can be a number of years probably because the actual exposure times to the conditions required to damage the brass are very short say a couple of hours a month for six months a year for an average loco, hence the accumulated time required to reach a failure point is probably a long time. Should Steve use his tube. If his boiler inspector is happy then do so. By the way MACC models sell 2" diameter tube by the inch.
Dave

[keith1500]

Bounced this one around with a few beers with a boiler inspector. You are good to go.

Brass not immersed in water should be fine. Same as brass water gauges and other fittings.

Generally a boiler will be in use for a limited time, blown down and spend much of its life dry. Some fittings may retain water which explains their failure.
Example clacks and pumps.

Onwards
Keith

[springcrocus]

Nov 9, 2022 17:08:55 GMT uuu said:

Also to be asked: would the regular 1.5x hydraulic test expose the weakening of a brass component before it failed in steam?

Wilf

I think Wilf has a point here that may be worth further discussion. Surely this should be sufficient to give advance warning of impending doom?

One thing that I will share, that comes from the world of plumbing, concerns the brass stop cock that shuts off your house water supply. Plumbers have told me that, left alone, those stock cocks will happily sit there for a hundred years doing their job, as long as you operate them on a regular basis. Leave them to jam up, however, and they will probably break off if you try to force them. They become as brittle as glass. In this situation, they are constantly immersed in slow-flowing water so Dave's comments are relevant.

Regards, Steve

[flyingfox]

Greetings,
Whilst there is a lot of discussion about this, and I personally think it depends on the type of brass used, I am not sure that a piece that you "happen to have" will be of the correct specification.

Where there is a published design, the inspector needs to ensure that the boiler is made to that design. Thus if the designer specified "Brass" in the construction of the boiler then its use would be presumed to be OK.

If the Inspector decides that a change to the boiler design or spec is OK then he, the Inspector, is signing the certificate and presumably accepting the risk involved.

So, what was the original specification of the dome?

Regards

Brian B