Friday, October 16, 2015

Engineering the 924 boiler

In September 2015 another major milestone was passed in the rehabilitation to service of Northern Pacific Railway steam locomotive 924. 

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.

1 comment:

Xiaolei Duan said...

There have been a vast number of designs of steam boiler,particularly towards the end of the 19th century when the technology was evolving rapidly.