Yankee dryer failures - An overview to improve tissue machine safety

Bill Corboy

The Yankee dryer has long been considered the heart of a tissue machine. This massive pressure vessel serves a vital transport function, carrying the lightweight tissue sheet through the drying process with no open draws. It is the place at which most of the water in the web is evaporated while at the same time the crepe and structure are added to the sheet as it is doctored off the Yankee.

The numerous process variables and operations taking place at the Yankee put it under considerable stress. Among the factors subjecting the Yankee to stress are: steam, with internal pressures up to 160 psi (10.9 bar); rotational velocity; temperature differential, through and between structural parts; line load from the press roll(s): as well as typically unaccounted for sources of stress, including corrosion-jacking and notches.

This combination of pressure, mechanical and thermal stresses, and their accompanying strains has unfortunately led to a number of Yankee dryer failures in tissue machines over the years, some with catastrophic consequences. Considering the primary importance of the Yankee to a tissue machine, we will in this article take a look at its characteristics, the environment in which it is called operate and the demands placed upon it. We will also discuss the types of failures that can occur. In a subsequent article we will suggest methods for developing inspection guidelines and repair methodologies, in order to maintain the Yankee in a safe, cost-effective, and productive state.

In recent years, as companies have sought to make ever greater quantities of tissue at ever higher quality from a given machine, increasing demands have been placed on the Yankee dryer. The largest Yankees used in tissue production are now 18 feet in diameter (5.5 m), with face widths over 300 inches (7.6 m) and speeds in excess of 7,000 feet per minute (2,100 m/min). Thus production demands have risen considerably while, concurrently, quality demands have increased as market developments have pushed companies to offer products with higher softness and bulk.

CAST IRON IS STILL THE BEST. Throughout the years, the unrivaled material of choice for Yankee dryer shells has been cast iron, which offers a unique combination of advantages that other materials have been unable to match. As a starter, cast iron is relatively inexpensive and easy to cast. It is comparatively strong, has good thermal stability and machinability, as well as adequate wear and corrosion resistance. Another major advantage of cast iron is that it offers a range of thermal conductivity which is about the practical upper limit for adhering a wet sheet.

While cast iron exhibits many positive attributes, there are of course some drawbacks. Due to the composite structure of the material, it is quite brittle, which means that failures can often be sudden and extensive. Thus instead of developing a slow leak, which might be a signal of impending troubles cast iron may fracture rapidly, with dangerous consequences.

In spite of these drawbacks the positive attributes of cast iron continue to outweigh the negatives and it remains the optimum material for Yankee dryers. There have been some attempts to employ steel or beryllium-copper for the Yankee material but these have not proven successful for a variety of reasons, including lower thermal stability and excessive thermal conductivity, respectively. Even though cast iron has remained the shell material of choice since Me Yankee's inception, significant design and casting improvements have greatly enhanced its performance over the past 40 years.

A short list of these improvements would include:

1) the Introduction of shell ribs and Class 60 iron, which created wronger shells and permitted dryer size to increase;

2) improved alloying and foundry practices, which reduced the amount of porosity and favorably located it away from the shell's outside surface;

3) utilization of finite element analysis techniques, which led particularly to a safer, more reliable head/shell joint design;

4) heightened heat transfer, through ribbing and the installation of turbulence generators (spoiler clips and bars);

5) improved shell chemistry.

So, although the gray cast iron is still with us, the Yankee continues its evolution toward higher productivity and quality output.

TAPPI TAKES AN ACTIVE ROLE. In the late 1970s, the Technical Association of the Pulp and Paper Industry (TAPPI) in the USA began to take a more active role in the study of Yankee dryer failures.

The TAPPI Yankee Dryer Safety Subcommittee (YDSSC) was formed at that time with the purpose of promoting safe operation of Yankee dryers. It met for the first time in April 1978 with eight attendees from four tissue manufacturers and three equipment suppliers. Since that time the group has steadily grown in number of attendees, especially since 1985 when the first head/shell interface corrosion problems came to light.

Documentation of Yankee failures has been crucial in accumulating data and experience; solving problems; improving designs; generating knowledge and helping to instruct professionals in ways to avoid future losses. As part of this documentation process, the Yankee Dryer Safety Subcommittee has catalogued 69 Yankee failures between the years 1934 and 2001. The modern database was begun in 1985, so naturally there is more information available regarding more recent failures. Between 1985 and the present there have been 42 identified failures. For our purposes, a failure is defined as: A dryer that has been temporarily removed from service for repairs or replacement of casting components, or permanently removed from service before the end of its normal/useful life due to damage, deterioration or failure of casting components.

YANKEE FAILURE CATEGORIES (WORLDWIDE)

Explosions (between 1947-1999): 15

Axial Edge Cracking: 25

Head Cracking: 23

Shell Flange Extension (Overhang): 10

Warmup/Startup: 6

Excessive or indiscriminate Water Usage: 6

Shell Porosity and Repairs (Thermal Spray and Plugging): 6

A central problem in preventing Yankee failures variety of sources from which they originate. A partial list, drawn from the TAPPI database, would include:

-Rapid warm-up

-Excessive edge cooling

-Steam leaks

-Inadequate maintenance

-Lubrication

-Bearings

-Failed/compromised interlocks

-Failed insulating sleeves

-Infrared heaters

-Welding/brazing cast iron

-Head/shell interface corrosion

-Overhung shell flanges

-Undersized SRV

-Fire-fighting

-Impact

-Stationary warm-up

-Excessive superheat

-Management

These might be otherwise catalogued as design, repair and maintenance, operations and human error, ancillary system, and management. We can take a look at some of the contributors to failure in each of these categories.

DESIGN FAILURES. Head/shell joint corrosion, which was first recognized in 1985 as the cause of numerous failures has since been identified on more than 55 Yankees, of which 21 have been removed from service and one has exploded. The problem arose, apparently, due to the increasing use of chemical showers to improve creeping characteristics. When the operating environment was thus changed, and where a gap existed between the head and shell flanges at the Yankee's outside radius due to an insufficiency in clamping force, these chemicals caused corrosion to form in the crevice. The problem went unresolved for a full decade, before the design and environmental influences were fully identified, but along the way many new systems and methods of inspection were developed within the industry. At the same time, a community of professionals dedicated to improved dryer safety came together. Immediately we see the pitfall created by any attempt to catalogue failures, because head/shell interface corrosion results from the combination of design and operational (process environment) causes. Figures 1 & 2 illustrate the corrosion-jacking mechanism and its manifestation.

Head/shell joint corrosion is certainly not the first time that a design feature has been associated with a large number of Yankee failures. In the middle part of the 1900s, failures associated with cap-screw, head-to-shell connections came to light. In fact, eight of the initial 12 failures reported in the TAPPI database as occurring between 1934 and 1948 involved Yankees with the cap-screw, head-to-shell design. This led to the virtual banning of that design from US mills over the last 50, years. It should be stated, however, that this failure category seemed generally associated with US-built cap-screw dryers, in contrast to those manufactured In Europe. The cap-screw and trough-bolted configurations are shown in figure 3.

A third design feature, identified with a significant share of failures, is the overhung shell flange. This design arose from an attempt to protect the head-to-shell joint from corrosion. However, it too created a crevice where corrosion could form, and once-formed, the jacking forces tending to rotate the joint upward can become large enough to crack the flange (figure 4). This process resulted in the one explosive interface corrosion failure. It should be noted however, that even absent corrosion jacking, this rather innocuous-looking appendage can develop quite high stresses when it is over-cooled, say from excessive water applied to the shell edge. This possibility was not well understood at the design's inception, before the advent of finite element methods. However, the overhung shell flange now seems to have been designed-out, remaining an issue in older dryers.

MAINTENANCE AND REPAIR FAILURES. In this category, one has to include inadequate maintenance, such as around condensate systems, joints, bolting, and leakage through and between parts. A failure in the United States, which prompted the formation of TAPPI's, Yankee Dryer Safety Subcommittee, occurred from unattended steam leaks. These eventually eroded a large path through the flange, with the steam cutting the bolts, and a crack, developing in the shell, that ran across the entire dryer face. Also included in the category would be improper plugging and thermal spray procedures. Both have led to several explosive failures, yet proper procedures for both plugging and thermal spray spot repairs have been in place for many years and are detailed in available publications. In particular, specifications for plug interference with the prepared hole should be followed, the plugs should be neither interlocked, nor clustered, with the proper distance provided between plugs, and plugs should be limited to a given number within a given area. Welding and brazing of cast iron in pressure vessels is prohibited by safety codes and regulations, because the heat involved alters the microstructure. Thermal spraying, by contrast, does not alter the microstructure of the base metal, when properly performed.

OPERATIONAL FAILURES AND HUMAN ERROR.

Under this subheading are failures that might occur due to negligence, and lack of training or procedures. Yankees and their attendant systems are designed with certain assumed loading conditions. Operational failures and human error can create problems which subject the pressure vessels to extraordinary loading, such as might occur during warm-up or cool-down (non-uniform heating/cooling, causing high thermal stresses), or use of excessive amounts of water over a local area of the dryer, or by-passing interlocks, or improperly removing of paper jams in the hood. The list of failures is filled with such actions, which have caused the loss of a Yankee. Figure 5 is of a cracked dryer head, the result of a high-temperature, high-velocity hood being operated while the dryer had no protective sheet on it. The shell would thus expand when heated by the hood, relative to the unaffected centerstay, placing tremendous forces on the head/shell joint. As Yankee design and manufacturing quality improve, a smaller proportion of failures is due to design deficiencies, so that increasing emphasis needs to be placed upon this category of failure.

ANCILLARY SYSTEM FAILURES. Many failures in the database are recognized to occur, not due to any fault of the Yankee or its maintenance and operation, but because of the failure of some system or component supporting the Yankee. An explosion in Sweden was reported to have occurred because a pressure control valve failed in the "open" position (valves on the steam supply side are to be designed to fail "closed") and the safety relief valve (SRV), even though it functioned properly, was only sized to exhaust a small fraction of the supply. Steam pressure built over a short period of time and the dryer exploded.

Likewise, bearing failures have the potential to cause such severe damage to the journals that the dryer is rendered inoperable.

This means that considerable attention must be given to bearing installation and lubrication. Since, almost invariably, bearings are designed with a B10 life in excess of 20 years, any failures short of that time should be investigated thoroughly. Bearing metallurgy, operating clearance, journal taper quality, lubrication viscosity, flow, temperature, cleanliness, and moisture are all important to achieving the expected bearing life - and attendant dryer safety. Interlock and safety device failures, described in the case of the SRV and the over-ridden hood interlock, as well as in numerous other cases, have contributed to damaged Yankees. A formal program for inspecting these critical components is fundamental to safe and long dryer life.

MANAGEMENT FAILURES. Even a cursory review of the accumulated literature surrounding Yankee dryer failures will suggest the predominant role management has played in those occurrences. In a very real sense, all failures are management failures. This understanding is perhaps conveyed best in an extract from H.W. Heinrich's classic work on industrial safety: Management selects, purchases, installs and makes use of equipment. It may in certain cases actually design and build the equipment.

Management is the owner of such equipment and may be said to be the sole authority in final decisions as to its handling, operation, maintenance, placing and guarding....

Persons charged with the task of building safety into mechanical equipment, and otherwise making and maintaining safe working conditions are directed by management.... In every sense of the word, therefore, management is logically and properly responsible for safe mechanical and physical conditions in the workplace. Management creates (employment) and then directs.

Management selects persons upon whom it depends.....Management may also train and instruct its employees, acquaint them with safe methods and provide competent supervision. Therefore, no other proper conclusion can be reached than that, because of its ability and its opportunity, management is responsible for safety.

Yankee dryers are often purchased solely upon the basis of lowest initial cost. Quite often procedures are unwritten and crews are untrained. The commitment to productivity frequently supercedes the demand for safety, while maintenance and inspection are given only brief treatment. As long as this continues to be the case, more Yankee dryer failures will unfortunately be added to the database. Certainly, by this time sufficient published information is available to guide any owner/operator in managing the safety of his Yankee drying equipment.

CONCLUSION. This brief overview of failures and failure categories is intended to impress upon the reader the need to understand the multiplicity of ways in which Yankee dryer losses have occurred, so that he can be better prepared to address similar possibilities at his facility. There is a wealth of information available to those charged with the protection of personnel and assets in the tissue mill, both with regard to the failures and how such failures can be prevented. Reference to some of those publications will be found below. With an enlarged understanding of the hazards attending these massive, rotating pressure vessels, it is incumbent upon owners and operators to institute the schedules and methods of inspection recognized to reduce the probability of their loss. The topic of Yankee dryer inspection will be addressed in a subsequent article.

Did you know?

The origin of the term 'Yankee' is not entirely clear. There are a variety of story about the source of this term. The one that seems to be the most credible suggests that a Dutchman named Yonke was development of the dryer. When the concept of a large drying cylinder for papermaking was imparted to the United States, the Americanized version of the name became, quite conveniently, Yankee. Does anyone out there have any other theories about the origin of the name Yankee dryer that they believe to be more correct?