Under the Dome and more

The New Year is off to a blazing start at the Northwest
Railway Museum!  Already, significant progress
has been achieved on Northern Pacific Railway steam locomotive 924 with
substantial completion of some major boiler work and, thanks to new funding from the Emery Rail Heritage Trust, has included pretty much
everything under the dome… 

The pressure vessel on NP 924 was by no means in good
condition for an easy return to service, and as previous blog postings have
noted, new firebox side sheets had to be installed, as well as a new steam
dome.  In particular, the new steam dome
construction involved an alteration of the original design in order to assure
safety in operation.  The original design
employed a longitudinal lap seam where the flat plat the dome was rolled from
overlapped slightly and was held via rivets. 
This design of joint induces nonlinear stresses that degrade the sheet
material adjacent to the joint, and if not properly monitored can lead to
potentially catastrophic failure. 

The original dome exhibited signs of degradation such as
cracking and rivet shearing, indicating the original design was not ideal.  In order to rectify this situation, a new
dome was built employing riveted buttstrap construction of the longitudinal
joint.  A buttstrap joint has the rolled
round of the barrel butt together for the longitudinal seam, and employs plates
inside and out with rivets through all three on either side of the seam.  This design is far superior and does not
impart any non linear forces in the plates other than hoop tension.  In order to accomplish this repair, the joint
efficiency needed to be calculated so as to assure a safety factor of at least
four times.  In the final analysis, the
safety factor was found to be over seven times, assuring a long life of service
ahead.  After the design process was
completed, the Federal Railroad Administration (FRA) in Washington DC had to
approve the alteration and engineering so as to assure public safety.  This step completed, and using funding awarded by the Emery Rail Heritage Trust, construction of the dome
was able to progress unfettered. 

At least until appropriate rivet material could not be
sourced.  Fabricating the dome and
readying it for installation on the boiler was accomplished with appropriate
boiler rivets from a local source. 
However, the smaller ¾” rivets that would secure the dome to the flange
on the back of the barrel could not be acquired.  The steel used for pressure vessel riveting
is made to a specification called ASTM A31 Grade B (or A31b for brevity), which is an extremely ductile, low carbon steel.  In searching for a cost reasonable source, it
became clear that proper materials for steam locomotive construction continue
to get harder to find.  Luckily, the
Museum’s curator Pappas had a small stash of A31b bar, and through the
construction of a rivet jig, new rivets could be formed in house at the Museum using
material of the correct specification. 
Once this task was completed, the dome was riveted on and ready for
service. 

Following this task, part of the longitudinal seam on the
first barrel course was in need of work. 
The sand dome had collected water beneath it and extensively corroded
the outer welt strip of the buttstrap joint. 
This welt strip needed partial replacement, as well and replacement of
22 rivets, and a small pad weld on each side of the seam.  Pad welding is compliant with the regulations,
but must be conducted to a national standard. 
The areas in question were small enough that they did not require prior
approval, but the process was still communicated to the regulatory authorities prior
to undertaking the work. 

Pad welding was
conducted by curator Pappas, who holds 6g welding certification, and following
appropriate National Board Inspection Code (NBIC) procedures so as to eliminate internal stresses from the
welding process.  The work was then
radiographed (x-rayed) and proven to be clean and free from all defects.  Following this, a new welt strip patch was
welded into the seam, and new rivets installed. 

As noted above, the large rivets needed for the seam were difficult to
source so these were also manufactured in house of A31b.  The installation of these rivets was doubly
difficult in that they had to be inserted from the inside of the boiler, backed
with an air buck, and the formed head driven on the outside with an air
hammer.  This may not seem overly
difficult, until one realizes the delivery pipe from the throttle runs directly
under this seam, only three inches from where the final rivet heads would be!  Through the creation of specialized tooling,
and a large influx of enthusiastic volunteer labor, the Museum’s team was up to
the task, and the riveting was completed in one day. 

FRA inspectors Brandon, Gary (ret.), and Zack check out the 924.