How would a flash steam boiler be approved?

[ripslider]

Hello all.

Without wishing to start WWIII, I was wondering if I could ask those in the know about boiler certification (or the signing off process by the club boiler inspector) and how one would go about having a flash steam boiler certified?

I am reading "experimental flash steam" currently - a very good read indeed - and it discusses in the general rather than the specific about there being no reason that a model train couldn't be built using flash steam as a motive force - the book suggests that actually it might turn out very well indeed and has some GA plans.

Something about making a flash steam engine appeals to me - it would seem to be *much* simpler ( not a stay or foundation ring in sight) , and a lot safer as well. And it's got that "different" factor which I like a lot - a good talking point.


However, the question of certification raises it's head.

Many of the boilers in the book are pretty much "Model train size" - probably best suited to 3 1/2 inch (although they look like they would scale well) but we designed for model hydroplanes and the racing engines that go with these, and so are producing working pressures of 2,000-3,000 PSI. A standard hydrostatic test would therefore need to test up to 6,000 PSI which seems impractical.

My thinking suggests that a "tuned down" version could be created, maybe using some older designs. I'm not sure a train could cope with 3,000 psi to be honest.

That however brings me to the other issue. The book, and another flash steam book I have been reading, are very clear that Stainless Steel is by far and away the most preferable material. Small, low rated boilers from copper tube are discussed, but all the "larger" stuff is stainless throughout. It also avocates welding over silver soldering due to the pressures involved.

I'm not looking to start another war of words about steel in boilers, but I would be interested what people think about a stainless steel flash steam boiler in a model train.

In theory, they are *MUCH* safer at 2,000 Psi than a water tube + super heater system at 150 psi, but I suspect that in reality they would be a swine to have certified?


Thanks in advance

Steve

[metalworker]

I have built a couple of flash steam boilers and they did not need certification because they were below the volume limit.
Flash boilers generally have very small water capacity and also do not have any reservoir of steam, so once the heat is removed they stop and also they do not have things like safety valves.
They also immediately stop if there are any leaks, so you do not have the same potential for danger as with other more normal boilers.

[havoc]

I see more problem in flash boilers used for trains than in certification. Certification says something about material sizes for rated pressures. Also about water levels (might be a point of contention), water feed and safety valves. If you comply with that and pass hydrostatic test and steam test it should be fine.

But a flash boiler in an engine that does start-stop service looks like a different proposition.

[russell]

The problems with start stop operation were overcome many years ago in steam cars such as the Locomobile and, the Rolls Royce of steam cars, the Doble.

Russell.

[ripslider]

Apr 8, 2009 19:41:47 GMT @havoc said:

I see more problem in flash boilers used for trains than in certification. Certification says something about material sizes for rated pressures. Also about water levels (might be a point of contention), water feed and safety valves. If you comply with that and pass hydrostatic test and steam test it should be fine.

But a flash boiler in an engine that does start-stop service looks like a different proposition.

Havoc,

Do you have anything more solid that you can point me at, or is this guesswork?

One of my specific questions above was regarding the hydrostatic test, if you read it again carefully. A 2x test of a fairly basic flash steam boiler will need to be working at 275 bar ( 4000 psi ) and highly engineered version would be working at 420 bar. That would mean that the hydrostatic test itself is far more dangerous than the boiler at full steam.


Therefore, I was assuming that there would be some other standardised set of tests for flash boilers? Something more practical?

Steve

[havoc]

Do you have anything more solid that you can point me at, or is this guesswork?

Part extrapollation more like it. I had a chat with our boilerinspector some time ago relating to boilers other than the locomotive boiler. This because our standard only talks about those boilers. He explained that whatever boiler comes up, they check according to the rules. So like metalworker says: if its volume is below a certain one they don't test because the rules say so. Otherwise the rules concerning materials and constructional details etc apply.

But as those rules change from location to location I think you better have a chat with your inspector.

Russell, you have some info on how they did that? Just out of interest.

[russell]

Hi Havoc,

I saw a Locomobile running at the Shuttleworth Collection some years back and it had no problems stopping and starting. I don't know how it worked though.

The Doble used a patented control system which was vaery advanced for the time (early 1930s). The burner was electrically operated rather like a modern oil fired domestic boiler to give instant on/off control. The water feed pump (steam driven) was solenoid controlled, whether in the feed or a bypass I don't know. The whole lot was controlled by a system of thermostats and pressure switches to maintain a constant steam temperature.

I'm sure the control system could be achieved much more easily and efficiently now with a microprocessor.

Russell.