David Parrish, P.E.
FM Global, Standards Division
Summary: The following article is a part of the
National Board Technical Series. This article was originally published in the
Winter 2001 National Board BULLETIN. (3 printed
Yankee dryers are very large drying rolls predominantly used
in the production of tissue-type paper products.
Considered as parts of papermaking machines, they are
critical to an industry which produces about 300 million metric tons of paper
A typical Yankee dryer is a cast-iron cylinder of about 6 m
(19.5 ft) outside diameter, 7.75 m (25.5 ft) width, and weighing about 180 t
(200 short ton). It is typically heated by 1.1 Mpa (160 psi) steam and operated
at 1900 m/min (6200 ft/min) while one or more pressing rolls apply pressure to
the paper sheet and dryer felt on the Yankee face. Fuel-fired air impingement
drying is typically provided over the Yankee to increase the drying rate and thus
the production rate.
A Yankee dryer is an assembly of several large gray
cast-iron parts (shell, heads, multi-piece center shaft, and journals). The
shell is commonly made to ASME Code Section VIII, Division 2, using SA-278,
Class 30, 40, 50, or 60 gray cast-iron material and internally may be of
smooth-bore or ribbed construction. Heads, center shafts, and journals are
sometimes ductile cast iron or may be fabricated from steel.
The owner/user must maintain control of specific operating
parameters of Yankee dryers to address adverse stresses. Control of heating is
critical during startup. Yankees have cracked from application of the heated
impingement air or from applying steam without first starting rotation. Adverse
stresses also develop from application of high impingement heat or full steam
pressure before the Yankee is at operating speed and the paper sheet has been
applied to the surface. When operating, immediate response is required to
reduce heat input if the paper sheet breaks. Controlled cooling is also
necessary on shutdown of the machine. Control of water sprays, as might be used
for control of temperature at the edge of the Yankee for cleanup or
firefighting, is critical to avoid thermal shock.
Yankee dryer steam leaks, vibration, or unusual noise during
operation should be investigated promptly and addressed. Observation of an
unusual condition requires prompt evaluation. The unusual condition may
indicate a critical safety issue or a noncritical production issue. Issues
determined to be critical must be corrected immediately. Production issues may
be resolved at the next scheduled shutdown. Typically, either deterioration in
quality of the paper product or visual steam leaks will indicate a need to repair
the shell face of the Yankee.
Yankee dryers should be shut down and inspected annually.
Both an internal and external visual inspection are needed. Other inspection
methods (dye penetrant, magnetic particle, ultrasonic, acoustic) may also be
used. If inspection finding requires repair by grinding, or if thinned areas
are found, the remaining thickness should be determined by additional
ultrasonic inspection or by another method of thickness measurement. The actual
thickness should then be used to determine the concurrent maximum allowable
operating steam pressure and maximum press roll load for the planned operating
Repairs and Alterations: Different Types
Repairs made to Yankee dryers are limited to grinding,
plastic metal (epoxy) filling, plugging, or thermal spray metal coating. Fusion
welding is not an appropriate or allowable repair for cast-iron materials, as
the heat input causes cracking.
Yankee dryer-specific repairs include:
or machining of the shell’s external surface to remove imperfections
caused during the operation.
spray coating of external shell to restore surface.
(epoxy) metal filling of external pits or imperfections.
of external pits or imperfections.
through-wall shell leaks by internal application of plastic filler metal
and external installation of plug.
or machining the heads or internal shell surface to remove cracks or other
of head-to-shell joint steam leaks with pumped sealant and replacement of
of internal steam distribution or condensate removal parts.
Yankee dryer-specific alterations may include:
Repair Guide for Yankee Dryers
of Yankee dryer pressure-containing parts (e.g., fasteners, heads, center
stay) with material other than conforming to that listed on the
Manufacturer’s Data Report.
or machining of the heads, head-to-shell joint, or shell to a degree that
modifies the design described on the Manufacturer’s Data Report.
the rotational velocity or press roll loading above limits established by
the manufacturer with no change in operating temperature or pressure.
Imperfections, indications, or damage may result from the
original casting process, normal operation, or an accident. Areas requiring
attention must be analyzed separately to determine cause and appropriate repair
or replacement. Examination of the imperfection, indication, or damaged area by
one or more nondestructive examination methods may be used to determine the
extent of the affected area or volume. Through-wall indications should be
examined internally and externally so all leakage paths may be addressed.
Procedures for Yankee Dryers
Machining of cast iron is a well-known process. Due to the
size of the Yankee, machining is typically done with the Yankee still in the
paper making machine.
or machining of the shell external surface to remove imperfections caused by
normal operation: Yankees are originally constructed with excess shell
thickness for resurfacing, to restore the finish necessary for production. The
Yankee manufacturer provides a chart showing the relationship of shell
thickness to operating pressure and press roll loading. When thickness is
ground to less than the minimum set by the manufacturer, “derating” the Yankee
may be necessary.
spray coating of external shell to restore surface: Thermal spray
coatings are sometimes used to achieve a particular surface finish or as a wear
surface when the shell thickness approaches the minimum specified by the
manufacturer. The coating does not contribute to the pressure-containing
strength of the shell.
metal (epoxy) filling of external pits or imperfections in the shell: Small imperfections in the shell surface may
be repaired by plastic metal filling; it cannot be used to restore a pressure
boundary (through-shell steam leak).
of external pits or imperfections in the shell: External shell surface
pits or imperfections may be repaired by driven plug. External plugging is a
surface treatment only. The limits for use are:
- cannot be used to restore a pressure boundary
(through-shell steam leak).
- drilled hole diameter, depth, and distance
between holes not to exceed limits in jurisdictional or construction code.
- plug is the same material as the shell.
- plug is driven-fit, not threaded.
- plugs should be installed to the maximum depth
permitted by the code or to at least 20% of the shell thickness.
- plug should have a finished diameter slightly
larger than the drilled hole to assure a tight, driven fit.
- plug should be slightly longer than the hole
- plug should be driven, swaged and carefully
ground to match the shell surface.
- the location of drilled holes should be recorded
on a map of the shell surface.
through-wall shell leaks by internal application of plastic filler metal and
external installation of plug: Internal pressure boundary repair may be
by application of a plastic metal filling material or by threaded plug.
Reference the construction or jurisdictional code for plug requirements.
or machining the internal shell surface or the heads (internal and external) to
remove cracks or other imperfections: Since welding is not an
acceptable repair method for cast iron, repair may be accomplished by grinding
to remove indications. Structural analysis may be used to confirm that the
Yankee has sufficient remaining strength for continued operation at the
manufacturer’s allowable limits after the grinding repair is completed.
of head-to-shell joint steam leaks: A steam leak at a head bolt may be
corrected by first confirming that the bolt is not cracked, then removing and
refurbishing the bolt and nut. If the bolt is cracked, replace the nut and
bolt. Torque the bolt as specified by the manufacturer.
A steam leak at the head-to-shell
joint which cannot be corrected by replacing head bolts may be addressed by
pumping a sealant into the joint. Tapped holes in replacement bolts are fitted
with grease fittings and a sealant is pumped into the joint or into a
sealant-pumping groove provided in the joint.
of internal steam distribution or condensate removal parts: Internal parts may loosen or break, causing
internal erosion of the shell. These must be replaced or repaired and conform
to the Yankee manufacturer’s specifications. Shell thickness should be measured
to determine minimum required thickness remaining for planned operating
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.