Senior Consultant for ABB CE Services
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
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
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
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
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 and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.