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Description of Construction and Inspection Procedure for Steam Locomotive and Fire Tube Boilers

Print Date: 7/22/2024 10:41:50 PM

Richard Stone
Senior Consultant for ABB CE Services

Summer 1994  

Category: Design/Fabrication 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (5 printed pages)



Although steam locomotives are no longer used by the major railroad companies for transport, many remain in use on museum railroads for the public, while others are undergoing restoration. The following brief description of the steam locomotive boiler will acquaint inspectors with its construction and summarize the inspection procedure. For repair information and additional construction illustrations, the new NBIC Appendix H, Steam Locomotive Fire Tube Boiler Repair is recommended.

The common steam locomotive boiler is a horizontal straight fire tube boiler with riveted cylindrical boiler courses, a stayed internal firebox at the rear, and a smokebox at the front. Operating pressure ranges up to 300 psi.

The boiler shell is made up of cylindrical sections or courses which may be straight or tapered. Usually the longitudinal seam of each course is riveted together using a longitudinal butt joint with inner and outer butt straps, though older boilers may have longitudinal lap seams. The courses are joined to each other by riveted circumferential seams. Shell openings are reinforced with riveted reinforcement liners. The riveted seams are sealed by caulking (one plate edge is hammered down tight against the other plate using an impact tool such as a pneumatic chipping hammer). A steam dome is placed on the boiler shell top to collect steam for entry into the throttle pipe (dry pipe). The throttle valve may be located either in the dome or the smokebox. Operation is from the cab using a manual throttle control rod.

The boiler tubes are placed in the cylindrical boiler shell and enter into tube sheets at the firebox end and smokebox; they are surrounded by water and the combustion gases pass through them. Tube attachment is by rolling or prossering; the firebox tube ends are beaded and seal-welded, or just seal-welded. On the smokebox tube sheet the tubes are only rolled or rolled and beaded. Copper ferrules are sometimes used in the firebox tube holes.

The internal firebox generally uses flanged construction and is assembled using full penetration welds or riveted seams. Patch bolts are used in places where rivets cannot be applied. A riveted mud ring extends around the firebox, forming the waterleg bottom with the internal firebox and wrapper sheet (the external boiler sheet over the firebox). Staybolts support the internal firebox sheets to the wrapper sheet, and diagonal braces or gusset braces support the areas of the tube sheets and backhead that are not supported by staybolts or tubes. All sides of the internal firebox, except the bottom where the grate is, are covered by water.

Combustion chambers, arch tubes, thermic syphons or circulators are used in some fireboxes to improve water circulation and increase the firebox heating surface. Brick arches are used in coal-burning fireboxes to improve combustion.

Staybolts usually are threaded 12 threads per inch using threads of UNF, Whitworth, or V-form. The usual installation is to screw the staybolt through tapped holes in the sheets, then hammer (drive) each end using a pneumatic rivet hammer while supporting the opposite end with a bucking bar. This both expands the staybolt end radially against the sheet threads to prevent leakage, and forms the head. In addition, the staybolt heads often are seal-welded either before or after hammering. Telltale holes extending through the staybolt or at least 1-1/4" deep into the staybolt heads are frequently used to simplify broken staybolt detection. Rigid staybolts are used extensively, and flexible staybolts of the ballsocket type often are used in the firebox "breaking zone" to allow for firebox sheet expansion and movement. The crown sheet often is stayed with rigid, flexible, or expansion staybolts having a taper head on the fireside to provide additional support; however, some older locomotive fireboxes use crown bars or girder stays.

Many boilers are equipped with superheaters. The superheater elements are placed in large diameter tubes (up to 5-1/2" diameter) and have removable bolted joint connections to the superheater header. The superheater header is a casting divided into a superheated side and a saturated side. Located in the smokebox, it has bolted joint connections with the throttle pipe and cylinder steam delivery pipes.

Threaded boiler fittings and studs for the attachment of brackets and other equipment are attached to the boiler by screwing into tapered threaded holes drilled through the boiler or firebox plates. The thread series most often used for locomotive boilers is 12 threads per inch. Tapered threads are used frequently; the amount of taper ranges up to 2" per foot, although 3/4" and 1-1/2"-per-foot tapers are the most common. Washout plugs are located throughout the boiler and firebox. Most are threaded plugs using the tapered 12-thread-per-inch thread series, or straight Acme or square-thread.

Feed water is supplied by live steam injectors or feed water heater pumps that use exhaust steam for heating. All feed water devices and water level controls are manual; automatic controls are not used. Boiler water level is shown by gage cocks and either a water glass or a water column, all located in the cab. Fusible plugs are used in some fireboxes, and are installed by screwing into the firebox crown sheet. As with all boilers, maintaining water level control during operation is very important. If the water level falls below the height of the firebox crown sheet (the highest sheet of the internal firebox), the sheet will overheat, resulting in leaking staybolts and the crown sheet bulging. If the overheating is severe, the sheet may fail by rupture, resulting in a boiler explosion, for the fire tube boiler contains much water under saturation temperature and pressure. It should be noted that thick scale or mud deposits on the firebox sheets will cause the same sheet overheating problems.

Fuel is burned in the firebox. A grate is used if coal or wood is in the fuel, while fuel oil is burned in a refractory-lined fire pan. Draft is created during operation by the discharge of the exhaust steam from the locomotive's cylinders through the exhaust nozzle and into the smokestack located in the smokebox. The exhaust steam creates a vacuum in the smokebox, entraining the firebox combustion gases through the tubes and discharging them out the stack. Air is drawn up through the firebox grate by the vacuum and mixes with the fuel, continuing the combustion process.

Locomotive boilers use a special type of safety valve known as the locomotive type safety valve. This type has extremely rugged construction and high accuracy, giving it the ability to discharge repeatedly without losing its calibration or requiring frequent overhaul; one identifying feature of the valve is it does not have an external operating lever. The safety valves are installed directly onto the boiler top using threaded adapters. It is common for the safety valves to discharge frequently during operation because the locomotive has a variable steam demand and the deep fuel bed gives the boiler a significant time delay for responding to load changes.

Locomotive boilers were originally designed to ASME Code Sections I and III (originally titled Boilers of Locomotives ) and to Interstate Commerce Commission (ICC) and Federal Railroad Administration (FRA) regulations (Federal Railroad Administration Act, 49CFR230). The original ASME and ICC/FRA maximum allowable stresses for staybolts and braces are lower than the present values allowed by ASME for stationary service. Maximum staybolt stress is limited to 7,500 psi by both ASME Section III and ICC/FRA. Maximum brace stress is limited to 9,500 psi by ASME Section III while ICC/FRA design hydrostatic test pressure requirement is 1.25 x MAWP (not 1.5 x MAWP as used in the present Section I) and it should not be exceeded. The minimum required factor of safety for locomotives operating under FRA regulations is 4.

The calculation used to determine the maximum allowable working pressure on the cylindrical boiler course is:

P = TS x t x E
         R x FS

(Ref. Section III, Boilers of Locomotives , Part L-21), where:

TS = tensile strength of boiler course plate. If this is not known, NBIC assigns the value of 55,000 psi, while the ICC/FRA assigns the value of 50,000 psi.

t = minimum thickness of boiler course plate, inches.

E = lowest efficiency of boiler course longitudinal joint rivet seam. The 1971 and earlier editions of the ASME Boiler & Pressure Vessel Code , Section I Appendix contain formulas for calculating the boiler rivet seam efficiency for several joint configurations.

R = inside radius of boiler course, inches.

FS = factor of safety.

P = maximum working pressure, psi.

The factor can be rewritten to solve for the Factor of Safety as:

FS = TS x t x E
             R x P

As is apparent, Section III was retitled to "Nuclear Components" after commercial steam locomotive manufacture stopped.

Boilers were constructed from materials meeting ASME Section II as well as AAR (Association of American Railroads) specifications. Low-carbon steel of flange and firebox quality was the standard material. High-strength alloy steels were often used for large locomotives, while wrought iron sometimes was used on small ones. Staybolts were primarily made from wrought iron or steel. Steel rivets became standard in the early 1920s, replacing wrought iron rivets almost entirely.

Boiler inspection follows standard procedure. Riveted seams should be checked for grooving, cracks, pitting, separation of plates, excessive or deep caulking of plate seams and rivets, and cracks at rivet holes. Broken rivet heads or cracked plates may result from sodium hydroxide cracking. Riveted longitudinal lap seams should be given careful examination, possibly using X-ray NDE, for this type of construction is prone to cracking.

The flanged section of all flanged sheets should be checked for pitting and cracks, especially the firebox tube sheet, and the bottom section of the smokebox tube sheet. The flanges should have smooth, uniform curvature, not one formed by a series of bends, and should make a smooth transition to the flat sheets.

Stayed sheets should be examined for scale and mud deposits, grooving around the staybolt holes, pitting, fireside corrosion (especially behind refractory or grate bars), overheating (if the overheating occurred recently, the sheet will have less mud or scale on the waterside than adjacent sections), fire cracks at lap seams, cracks in tube sheet ligaments and between staybolts, and bulges from mud burns or broken staybolts. The waterside surfaces at the mud ring should be checked for grooving.

Staybolts should be inspected for cracks or breakage, erosion of the driven head, thinning of the driven head from repeated redriving, plugged telltale holes, and waterside corrosion at or near the sheets. Broken staybolts can be detected by leakage through telltale holes and by hammer testing -- both methods are most effective when the boiler is under hydrostatic pressure of at least 50 psi. Flexible staybolt sleeves should be checked for corrosion, cracks, damaged threads, and scale/mud accumulations inside the sleeve that could restrict bolt movement. Also, when the sleeve is open after the threaded cap is removed, the flexible staybolt's ball head should be checked for breakage by hitting or twisting it.

The dry pipe of boilers having dome-mounted (internal) throttle valves should be checked for erosion and cracks; a steam leak into the pipe will send an unregulated flow of steam to the cylinders. The water glass and gage cock boiler connections should be checked for mud and scale blockage. Threaded attachment studs should be checked for corrosion, cracks, and damaged threads. The throttle handle and its mechanism should be inspected for ease of operation and security. Diagonal braces should be taut (a simple test of their tightness is to hit each brace with a hammer -- the brace will ring with a bell-like sound if it is taut). The brace pins should fit the brace clevis and eye securely and be retained from coming out by some form of retainer. All boiler and superheater tubes should be checked for fire cracks, pitting and erosion, with close attention paid to the firebox tube ends. Washout plugs and their holes or sleeves, especially those having square or Acme threads, should be checked for damaged or cracked threads. The fire door and its locking mechanism should be checked for easy, safe operation.

Locomotives operating under FRA jurisdiction are subject to FRA rules for inspection, maintenance and repair. In the past, the FRA has prohibited welded repairs to unstayed sections of the boiler, but now allows them on a case-by-case basis. Before this type of repair is undertaken, it should be reviewed with the FRA regional inspector and the authorized inspector involved in the repair.



Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.