The key to any successful steam locomotive rehabilitation is quality work done to exacting standards, particularly in the realm of the pressure vessel (boiler) and running gear. No locomotive will operate properly or safely if these areas are not completely and adequately examined, rebuilt, and maintained in service.
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.
Brandon, Gary (ret.),
and Zack check out
After this step, the next task was to bring in the local FRA inspectors to review the work and inspect the interior of the boiler, which must be done prior to the installation of fire tubes. To get ready for this step, the interior of the barrel was cleaned to bare metal, tube sheets straightened and dye penetrant tested to check for cracking, and tube holes polished. Our work passed the FRA’s stringent standards with flying colors and was approved for completion of the tube job.
Let it be reiterated that boiler work such as this is not to be undertaken lightly, and the importance of the public’s safety is always first priority when undertaking pressure vessel work. Stay tuned for the next installment in the continuing rehabilitation of Northern Pacific Railway steam locomotive 924: tubing!