In November 2014, the boiler tubes were removed from the
locomotive and a preliminary engineering package was created based upon the
thicknesses and geometries of the existing vessel. During this time, a complete ultrasonic
thickness grid was laid out and measured from the water and fire side of the
boiler sheets, and the results were more than encouraging. However, for the locomotive pressure vessel
to be legal under the regulations of the Federal Railroad Administration (“FRA”),
the exterior jacketing and lagging had to be removed. This allowed for a visual inspection and for documentation
of the final ultrasonic thickness measurements of any critical areas that were
not possible to reach from the inside of the boiler. So following the abatement of the asbestos
lagging in late July and early August, it was time to finalize the form 4
engineering package, external visual inspection, and ultrasonic grid. 
Preparing the pressure vessel for an external inspection and
for ultrasonic measurements involved a variety of tasks. First, the boiler shell was stripped of rust
and scale, as well as any other appurtenances that obstructed the exterior of
the boiler. Then, a grid pattern was
laid out with data points polished to bright steel so as to allow the
ultrasonic transducer to couple with the material and give an accurate
reading. Any areas found to exhibit
additional pitting or deterioration were also tested to find the thinnest
points in each boiler component. 
Preliminary calculations indicated the pressure vessel to be
in good shape, especially following the installation of new firebox side sheets
in January. However, visual inspection
found questionable rivets and some cracking present in the steam dome barrel
near these rivet holes. The original design
of the dome also included a longitudinal lap seam, which although legal to
remain in service, would require additional inspection and care in
service. The dome ring at the top of the
steam dome was also found to be cast iron, which was not a good choice for
pressure vessel use due to its lack of ductility and low tensile strength. Curator Stathi Pappas made the decision to
replace the steam dome with a new fabrication using A-516-70 steel and butt
strap riveted construction to provide for both safety and longevity of service.
At this point, Curator Pappas brought in friend and
colleague Jon Brewster - who is an expert in boiler calculations - to run the
final numbers and determine the health of the pressure vessel with the
alterations and repairs as noted above. Although
locomotives may operate with severely derated boiler pressures, these
locomotives are often but shadows of their former selves in capability, power,
and economy. In order to be true living
history, philosophically, locomotives should be rebuilt to behave as they would
have in their period of significance.
This being the case, Northwest Railway Museum is pleased to announce
that NP 924 will once again be legal for her original full working pressure of
180 psi, just as Rogers intended!!! 
Following this achievement, no time was wasted in beginning
the construction of the new steam dome.
A new upper ring was machined on the Museum’s 48” Bullard Spiral Drive
vertical turret lathe from a 5.5” thick plate of A-516-70 steel to match the
original. This machine was able to bore
the center hole removing up to 1” of material in one pass. Now that is mid-century American machine
tools being used as intended! At the
same time a new dome barrel was rolled by Liberty Metal in Portland, Oregon out
of A-516-70 steel. By mid September, the
new ring and rolled round were fitted together and welded preparatory to
riveting. Although the rivets are more
than strong enough to support the joints in this new dome, Curator Pappas (who
is a certified 6G welder) welded the dome for additional strength, prevention of
leaking seams, and dimensional stability during the riveting process. By using appropriate riveted construction and
welding, this dome may even outlast the rest of the pressure vessel. During this time, the old dome was removed
and all rivets from the dome saddle removed preparatory to installation of the
new dome. 
Fall 2015 will see the completion and installation of this
new dome on the 924. Following this achievement, the stay bolt work on the new
side sheets can them be completed, tube sheets prepped, and the vessel will be
ready for tubes and a hydrostatic pressure test.
Stay tuned for continuing developments from the Northwest
Railway Museum Conservation and Restoration Center!
"Engineering the 924 boiler" was a guest post by Northwest Railway Museum Curator Stathi Pappas.
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