Help with New Build Specs...

[Casey Akin]

Hi all, I am new to this forum.

My brother and I are in the very early stages of planning a 7 1/2" gauge live steamer. This will be a large engine, and I have some basics questions that I would like to get right on the first try. I am an avid CAD user, and have about 20 years experience in drawing model trains, so I will be personally designing this model.

To begin, our engine will be rather large; total loco and tender length is about 14 feet. Weight will be 2,000+ lbs (if my math is correct, probably more like 2,500lbs). Boiler dia will be near 16" x 6' long. Driver dia is about 9 1/4" with a cylinder bore and stroke of 4" x 5" scale. While the loco dimensions are to be as exact scale as we can get, the internals are what puzzles me...

Should the scale cylinder bore and valve bore be used, or should we scale it down? If so, how do we determine the best sizes?

How do I determine what the appropriate or best suitable steam PSI should be?

How do I determine the proper dry pipe and steam to cylinder supply diameters?

Opinion: Superheated or not? The prototype was...

Most of the engines this size use a 3/8" steel boiler shell with brass or copper flues. What thickness flues, and how do I determine the proper quantity and spacing between them.

As this is a 100% custom build that I have never seen done, there are no exact examples to base our model from. Also, for stupid reasons, I am a little hesitant to reveal the exact prototype we wish to build until the project is underway.

Thank you for any help, and I'm sure more questions will follow...

Casey Akin

[Harold_V]

Before you make any decisions about boiler size, it would be prudent to investigate what is considered legal in your area. You may find that a 16" boiler is beyond a size that would be permitted for a model. Same goes for operating pressure. Some states are limited to 100 PSI.

Each state appears to set its own boiler code in regards to what is and what isn't acceptable.

H

[Casey Akin]

I am in Texas. From what a some local club members told me, the only restrictions I need to worry about is the firebox grate size. I don't remember the exact size, but it will fit my model choice but a little small. If anyone knows the exact rules, please correct me...

Casey

[Casey Akin]

Texas rules are 12" diameter or less; OR, 2 square foot fire grate area. This can be achieved in our model...

Casey

[Dick_Morris]

A key issue to boiler certification is who is going to build the boiler. If it is built in a code shop and follows the code the size limitations for hobby boilers don't apply.

Bore, stroke, driver diameter, pressure, and weight on drivers are all interdependent. The formulas have been around for a long time. I suggest you get a book like Modern Locomotive Construction by Meyer, available on the web at https://archive.org/details/modernlocomotive00meyerich. It's one of the most complete and easy to understand books on circa 1890 locomotive design.

Brass isn't used inside a boiler.

I don't know if it covers that large of a boiler, but the Australian Model Boiler Code has common sense information on boiler design.

[Trainman4602]

Sounds like you attempting to build and 3 foot gauge locomotive may be a EBT 2-8-2 with 44 inch drivers.

[Bill Shields]

Have you ever actually BUILT a live steam locomotive?

[Marty_Knox]

Texas rules are 12" diameter or less; OR, 2 square foot fire grate area. This can be achieved in our model...

Casey

It is 12" Inside diameter or less, AND 2 square feet of grate area or less. Both limits can not be exceeded, for non-commercial use.

[Bill Shields]

Marty:

Is there also not a PSI limit, irrespective of size?

I seem to remember that it is part and parcel of when ASME code takes over (been a long time since I looked at it for Delaware, let along TX).

[KenG]

Marty is the boiler expert and I would suggest you get someone like him to design and build the boiler. Boilers are not something you don't want to mess up.

For the rest of the locomotive I suggest you buy plans of a proven design and use those for a guide. For big narrow gauge locomotives, Allen models have the Sandy River #24 but it looks like the drawings are are based on the Maxi Lucky Seven 2-4-4 from Reeves in the UK. Richard Ulin just sold his D&RGW K27 drawings to Alcowest who should be able to provide copies.

In general as you scale down from the prototype the factor of adhesion (weight on the driving wheels divided by the tractive effort) falls so a model with scale cylinder bore even with reduced boiler pressure will spin the wheels. otherwise just scaling down original dimensions should work but most builders simplify things to make them more robust.

[Marty_Knox]

Marty:

Is there also not a PSI limit, irrespective of size?

I seem to remember that it is part and parcel of when ASME code takes over (been a long time since I looked at it for Delaware, let along TX).

Bill, the Hobby Boiler rule for Texas is an exemption. I was on the advisory committee to the Texas Board of Boiler Rules working on rules for hobby boilers when Fred Springer contacted a legislator. He was able to get a bill passed to amend the law and grant the exemption. The Law states that the following boilers are exempt - 3) manually fired miniature boilers that:

(A) are constructed or maintained for locomotives, boats, tractors, or stationary engines only as a hobby for exhibition, recreation, education, or historical purposes and not for commercial use;

(B) have an inside diameter of 12 inches or less or a grate area of two square feet or less; and

(C) are equipped with a safety valve of adequate size, a water level indicator, and a pressure gauge;
Pressure is not a factor.
ASME code for miniature boilers has a 100 PSI limit. The ASME is a construction code; each jurisdiction(which is usually a state) decides what rules they want to follow. Here in Michigan you can run at any pressure you can show your boiler is safe at. One of my locomotives was approved for 235 PSI, but I ran it at 125PSI.

[Marty_Knox]

Texas rules are 12" diameter or less; OR, 2 square foot fire grate area. This can be achieved in our model...

Casey

It is 12" Inside diameter or less, AND 2 square feet of grate area or less. Both limits can not be exceeded, for non-commercial use.

Casey, you were right - it does say OR.