Tubing the 924

It has been a busy two weeks at the Conservation and Restoration Center with great progress shown in the rehabilitation of Northern Pacific Railway steam locomotive 924.

The last Blog article highlighted the work required to prepare the locomotive for its Federal Railroad Administration internal inspection and approval.  Now, the time had come for tubing the boiler.  Some may ask if this process is premature as the FRA mandates that the 15 year boiler clock begins one year after the first tube is placed or first fire, whichever comes first.  A great deal of work remains on NP 924, including a full running gear rebuild, and by committing to a tube job at this time, it means that completion must be reached within the next year otherwise federal boiler time is wasted.  Although this concern is valid, the rebuild of NP 924 is on an expedited timeline, with rehabilitation to service if not complete in one year, will be mostly so.

In order expedite the project, it was time to protect the internal surfaces of the pressure vessel from deterioration in service by the application of a product called Apexior, produced by Dampney.  Often referred to as a boiler paint, Apexior is actually a Bitumen coating that prevents rust, scale, and other unwanted byproducts from boiling water from forming or adhering to the interior sheets of the boiler.  An interesting fact about this product is that this same product has been continuously produced by the same company since the steam era on American railroads.  Therefore this product not only makes sense from an artifact care and investment standpoint, but also as a period-appropriate material and technique.  

At the same time as Apexior was being applied, the boiler tubes were being prepared for installation.  This process involved the removal of mill scale from the exterior and interior ends of the tubes.  By removing this oxide coating, the bare steel of the tube can be rolled into the hole in the tube sheets, assuring a tight seal with no contamination.  Following this step, the tubes were cut to length and ready to install.  Although the tube sheets in 924 were straightened as much as possible, any locomotive will exhibit variation in the lengths of various tubes.  The 238 tubes in locomotive 924 were no exception, which required every tube hole to be measured for length so that every single tube could be custom cut to fit.  

With tubes sitting at the ready, it was time to start inserting the tubes into the boiler.  This seems an easy task until one realizes the steam delivery pipes leading to the cylinders cover approximately 1/3rd of the front tube sheet, making simple insertion of many of the tubes impossible.  To complete this job, tubes had to be inserted through a hole near its final location in the front tube sheet, cross several holes over into the rear sheet, be pushed inside the front sheet, then manipulated with a bar to line up with the appropriate hole in the front sheet, then pushed forward into the hole.  Sometimes this had to be done several times to reach the final placement of a tube.

This task being completed, it was time to roll the tubes into the sheets. A tube roller, consists of three hardened steel rollers in a cage, driven by a central tapered pin.  This taper pin is rotated by an air motor which serves to expand the tube into the hole in the sheet, crushing it into the hole and forming a pressure tight seal.  The rolling process requires a great deal of judgment as it is possible to over-roll a tube and thin it to the point it would fail prematurely.  Many specialists in the steam locomotive rehabilitation field say the best rolled tube is the one that weeps just barely on its first hydrostatic test.

Although the seal formed by rolling is critical, it is not by any means the end of tubing a locomotive.  Structurally, the most important step is the next process known as beading.  Beading uses a special tool and small air hammer to roll the edge of the tube sticking out of the tube sheets over against the sheet.  This process turns every tube into a hollow rivet by forming a head that prevents the sheet from pushing off the rolled end of the tube.  This work is intensely physical, but makes for a much stronger product, and is historic.  Another important result of the beading process is that it makes the tubes much less prone to fire cracking and burning away of the steel.

Related to this prevention of fire cracking and burning, seal welding of the firebox end of the tubes is also critical.  The term "seal welding" is actually a bit of a misstatement as the weld is not intended to prevent leaking as the roll should provide adequate sealing.  Instead, a small weld is made around the circumference of the bead on the rear sheet in order to pull heat away from the thin tube, and allow it to flow into the much thicker tube sheet.  This prevents the tube ends from overheating and burning away or cracking to an even greater degree than just a bead alone.  In addition, this process was also historically used on steam locomotives, particularly on locomotives that burned oil rather than coal. Following seal welding, the tubes are lightly rerolled front and back to assure the seal was not disturbed during the beading and welding processes, and then the job was complete!

Substantially completing the boiler work on locomotive 924 involved many facets.  First and foremost, funding was secured.  King County 4Culture awarded a Landmarks Capital grant to fund purchase of the new 2 inch boiler tubes, seal welding rod, and new staybolt material.  Second, an awesome team of volunteers performed much of the cleaning, cutting, and tube installation.  Third, Curator Pappas' pressure vessel skills allowed quick and efficient completion of the work, including the rolling, beading and welding.

The next task is to install the blast nozzle in the smoke box, and reapply the smoke box front. After this, the running gear rebuild will begin.  So stay tuned for another update!

--Photos in this post by Dave Honan and Spike.  Used with permission.