Steam & Excursion > Why two steam domes

At some  point in the future I may get to build a stylized steam engine model for an artistic project.  The drawing I was given show a locomotive with a tapered boiler and two steam domes.  It has the whistle on the front one and what might be safeties on the rear.  When and why would a locomotive have two steam domes?  I thought it was a stylistic choice but one reference photo shows the Minnetonka and the William Crooks.  "Bill" has the arrangement I just described but it was originally built with a straight boiler and two domes.  Later it was rebuilt with a tapered boiler and one dome then "restored" with two.  What is the difference between a straight and tapered boiler (besides looks) in operation?  When would you build a loco with two steam domes and why?  Thank you for sharing your knowlege.

Speculating here....

The purpose of the domes is to physically raise & separate the steam-only piping from the sloshing, frothing water in the boiler.
In the old days, the throttles, when opened could suck water into the pipes leading to the cylinders...

Some unscrupulous builders, also, could copy another builder's boiler, virtually identically,  but could avoid a patent infringement lawsuit by putting the safeties and whistles on a separate dome.  Most early practices, like this, ----- did many things that, later, became dropped, as being too costly, and frivolous.

'Conical'  boilers are bigger at the back end, where the firebox ( furnace) is, and where water boiling and sloshing are most violent...the forward part is 'calmer', and not very much heating/ boiling, going on.  You want to reduce the total amount of water you're trying to heat, so the reduced diameter of the forward sections allows fewer gallons of water to have to brought up to boiling temperature.

 Modern boilers tended to be very skinny ahead of the combustion chamber.  The designers tried to get away from boilers being big steam tanks, and become more "steam generators", with lots of capacity to produce steam, and reduced storage capacity, to 'store' steam...

At the end of steam in the 1940s, new construction boilers were fitted with a 'slotted drypipes',  equipped with suitable baffles, located totally inside the dome-less boilers.

These locos were fitted with [ flat] man-hole covers, on which the safeties are sometimes mounted.  
Additionally, there are commonly,  smaller dry pipes at the rear of these boilers' interiors ---- feeding the 'steam turrets', that are supplying steam to accessories and appliances operated from the cab.

Also, it is common that front end throttles, on these modern engines,  to have some appliances' supply pipes mounted at the inlet side of the superheater header ( forward end of the boiler) --- for supplying non superheated steam to forward mounted air compressors and whistles, etc.  Non-superheated piping cand be identified since it will be located near the front tube sheet and it's rivets, at the rear of the smoke box.

Some locos, like N&W's newer Y-class engines, are fitted with an external appliances-feeding drypipe extending out the rear of the steam dome, proper, ( splitting the rear sand box) -----towards the cab and the "steam fountain(s)" [ sometimes called the turret] with their myriad of globe valves supplying the pipes leading to the appliances, -----like stokers, steam-draft blowers, dynamos, etc.

​W.

The Uintah articlateds that later went to the Summpter Valley Ry were built with 2 steam domes due to the intense crades encountered on the Uintal line.

Here is SVRy #250 on her last run in April 1947.  Her 2 steam domes are quite evident.

Martin



Edited 2 time(s). Last edit at 09/24/16 04:49 by LoggerHogger.

---

During the Santa Fe's breif love affair with the Mallet articulated, they had Baldwin Separable Boilers with up to 5 domes (counting the manhole access hatches).

-John

---

There are some information about two domes in this book, stating the main reason bad water (?)

https://books.google.no/books?id=1A4iiGAz628C&lpg=PA96&ots=ps-wDpyWRx&dq=%22two+steam+domes%22&pg=PA96&redir_esc=y&hl=no#v=onepage&q=%22two%20steam%20domes%22&f=false

The primary purposes of having a dome are to (a) provide an access hatch into the boiler, and (b) to raise the throttle high above the water line to discourage foaming or priming water from getting into the dry pipe, which is what can happen with bad water as noted above.

Yesterday I looked at both the Minnetonka and Wm. Crooks at the Duluth museum.
The Minnetonka has one dome.

The Crooks, as you said, has a dome over the firebox for the throttle and safeties and a second dome forward of that where the whistle is located. I suspect the forward dome provides easier access, without having to deal with the sling stays of the crown bar firebox. In fact, I wonder why more engines with crown bar fireboxes didn't have a second dome.

John