Battle of the Bulge

When someone speaks of the Battle of the Bulge, most people probably either think of their waistline or, if they're WWII history buffs, the battle in the Ardennes during the closing months of the war. We, on the other hand, conjure up our own image, which, as you might suspect, involves something a little different, namely, bulges in metal structures.

Over the years, of course, we've seen plenty of examples of bulged, bent, beat, broken, or otherwise abused metal structures. Interestingly, while the appearance of bulges are sometimes similar, we've found them to be caused by quite a range of problems. We thought our readers might find a few examples interesting.

Lime Kiln Bulges

Our first example involves bulges in a lime kiln at a pulp and paper mill. On the order of 200 feet long and 20 feet in diameter, a lime kiln is essentially an enormous rotating oven used to reclaim chemicals used in the pulping process.

With temperatures at the hot end of around 2,100°F, the carbon steel shell material is protected by a thick layer of refractory or fire brick on the inside surface. Unfortunately, the refractory material doesn't always hold up, and if it fails, the effects are dramatic, particularly at night.

When a section of refractory fails, the location of the failure is real easy to spot: it's the part of the shell glowing bright red. And even after the kiln has been shut off, the failure location is still easy to spot: it's the part of the shell where all the lime dust has flaked off and the shell is covered with a thick layer of grayish-black high-temperature oxides (as is the ground underneath the shell). Oh, and then of course, there's a large bulge or warped area in the shell where it used to be smooth and uniform.

As you might expect, this degree of overheating isn't particularly good for the shell material. Not only is the microstructure radically altered by the overheating, but more importantly, the strength is usually dramatically reduced, as is the hardness.

The saving grace in all this, is that the materials of construction are usually quite tolerant of abuse and because kilns are not pressurized, they don't carry the inherent risks associated with pressure vessels. Thus, they can often continue to operate even though the material has been degraded to some degree. Eventually, however, if the refractory failures continue and the areas of degradation spread, replacement of entire sections becomes advisable.

Pressure Pipe Bulging

Our next example involves a series of localized bulges in some 6-inch diameter, Schedule 10S, Type 304L stainless steel pressure piping at a pulp mill. In this application, the piping was feeding weak black liquor to a set of evaporators at a design pressure of 50 psi.

The system was only a couple of years old, when one night, a section of the pipe suddenly sprang a leak and started hosing down the nearby area with caustic cooking liquor. After shutting down the system and removing the insulation from the pipe at the location of the leak, the operators discovered a small, fingernail sized hole in the pipe, centered on a 3/4-inch high by 4-inch long bulge that went around approximately 45 degrees of the pipe circumference. Additional inspection of other nearby sections of the line revealed several other bulges. Several sections of the line were then sent to our lab for analysis.

Our analysis focused on the morphology of damage and the material properties of the line. We found that the bulges and hole were both due to mechanical overload. The pipe material surrounding the bulges and leak site exhibited significant necking down the pipe's cross section prior to final rupture, and the fracture was entirely ductile in nature.

The pipe material met all the requirements of the governing specification (ASME SA-312) for chemistry, tensile strength, yield strength, and elongation, and its microstructure and microhardness were both as expected. Additionally, the pipe met the thickness requirements for a Schedule 10S pipe, with no evidence of preincident thinning. (Of course, the pipe was thinned in the bulged areas, but the pattern of grain distortion and cold-working showed this thinning was a consequence of the bulge rather than a cause for it.)

So how did the pipes get bulged? Well, it turns out the system was undergoing a significant amount of hydraulic hammering under certain operating conditions. This hammering was sufficiently strong to cause the bulges.

The client was already in the process of replacing sections of the line with Schedule 40S material, which has a wall thickness about twice that of Schedule 10S. We suggested that while this may prevent additional bulging, it didn't really address the root cause for the problem, namely the hammering, which was undoubtedly inducing stresses beyond the designers expectations. Thus, we suggested that it might be advisable to install pressure sensors to measure the peak pressures under hammering conditions, not only so the stresses could be evaluated relative to the new Schedule 40S material, but also so the system operators could alter their operating procedures and observe exactly how much their changes reduced the stresses in the line.

Recovery Boiler Tube Bulges

Our next case history involves bulges in some ASME SA-178, Grade A recovery boiler waterwall tubes. In this project, the client reported several bulges had been detected in the wall tubes up above the tertiary air ports during a routine inspection. A more detailed look at the wall then revealed over 200 bulges.

The bulges were about 3 inches long by about 1/16 inch high and extended around the full circumference of the tubes (see photo at right). Our analysis showed the tube properties were exactly as expected for an SA-178, Grade A material; the tensile strength, yield strength, elongation, and chemistry all met the specification requirements; the hardness was about as expected; there was no evidence of preferential thinning at the bulges or elsewhere; there were no cracks; and the internal scale deposits were very light.

The microstructure showed the tubes had not been subjected to either long-term operation at temperatures over 1,000ºF or short-term operation at temperatures over 1,300ºF

Based on our analysis, we concluded the bulges were caused by internal pressure-induced stresses that exceeded the material yield strength at the incident temperature. Although we did not determine the combination of pressure and temperature that caused the bulges, our calculations indicated both overtemperature and overpressure excursions of relatively short duration were probably responsible for the bulges.

One interesting twist to the project was that while we were examining the tube bulges, we found severe incomplete fusion welding defects in the as-manufactured, electric-resistance longitudinal weld seams of the tubes. The defects were halfway through the tube wall thickness and were present in about half the tubes submitted for our analysis.

Our conclusions? We told the client the bulges were not serious and shouldn't be detrimental to continued service, but we recommended additional inspections be done to determine the full extent and severity of the welding defects, and further, repairs (via tube sectioning) might be necessary for some of the deeper defects.

Boiler Transfer Tube Bulges

Our fourth example again involves bulged tubes in a boiler, but this time, the bulging wasn't due to internal pressure or material deficiencies, but rather, an error in the method of installation.

The boiler had only been in service for a few months when a routine inspection revealed bulges in all nine of the 6-inch diameter, 4-foot long water transfer tubes between one of the drums and a header. Assuming the problem was indicative of a service-related deterioration of the tubes, the owner was in the process of having all the tubes replaced. And, since he believed the bulges were service-induced, the contractor was installing the new tubes in exactly the same manner as the original tubes, one at a time.

But no sooner would the contractor replace a tube, but it would be found to be bulged; the one next to it would then be replaced, and it too would be found bulged, and so on. It was at this stage that we were retained to sort things out.

In pretty short order, we had a fairly clear idea of what was happening. It turns out the welding procedure for installing the tubes included a post-weld heat treatment (stress relief) at 1,100°F. The contractor, aware that the strength of the tube metal at 1,100°F is significantly lower than it is at room temperature, and believing the tubes would be unable to support the weight of the lower header if they were all heated at once, was stress relieving only one tube at a time.

So what was happening? Well, at the stress relieving temperature of 1,100ºF, the tube metal was undergoing considerable thermal expansion. Unfortunately for the tube being stress relieved, the other eight tubes were all at room temperature and thus weren't subject to thermal expansion. So not only was it "eight against one," with the eight cold tubes fighting the one hot tube, but the eight cold tubes were all at their full room-temperature strength, compared to the significantly lower, 1,100ºF strength of the heated tube.

The consequence was a compression bulge in the heated tube, caused by its attempt to expand while restrained by the nonheated tubes.

To further evaluate the cause for the bulges, the client requested we install high-temperature strain gages on the tubes so we could monitor the service stresses as the boiler was returned to service. Additionally, John R. Rodgers, PE of Engineering Analysis Services was retained to develop a finite element model of the tubes to evaluate the cause for the bulges and to determine if there had been any degradation to the reliability or safety as a result of the bulges. (Regular readers of our newsletter may recall we've teamed with John on other projects were the combination of finite element modeling and empirical stress analysis has been particularly insightful into the root cause of a problem.) John's analysis, in combination with our strain gage data, confirmed the bulges were caused by the stress relieving operation. Additionally, his analysis showed the bulges should not be detrimental to the serviceability or safety of the tubes.

Oh yes, as for the original bulges which the client believed were service induced? They were undoubtedly there all along since the initial installation, but simply hadn't been discovered.

Dr. Richard Garber Joins MEI-C Staff

We are pleased to announce Dr. Richard I. Garber has joined our staff as a Senior Metallurgical Engineer. Dr. Garber has more than twenty years of industrial research and consulting engineering experience in the areas of metallurgy and materials science. His special areas of expertise include failure analysis, forensics, corrosion, manufacturing processes, and ferrous metallurgy and alloy-steel development.

Most recently, Dr. Garber was a Senior Metallurgical Engineering Consultant at the mid-western offices of an international metallurgical testing and materials engineering firm. Prior to that, he was a senior member of the technical staff in the metallurgy/material sciences division of a consulting engineering/scientific company in California. Additionally, Dr. Garber has over ten years of industrial research experience in the areas of ferrous metallurgy and alloy development of oil-field steels at the Research and Development Laboratories of Climax Molybdenum and Exxon Corporation. One of his research efforts, which involved the production of specialized steels for oil industry sucker rods, resulted in a patent award.

He is a graduate of the Carnegie-Mellon University, with bachelor's, master's, and doctorate degrees from the Department of Metallurgy and Material Sciences. He is a registered professional engineer in the state of Ohio and holds certification as a Material Selection/Design Specialist from the National Association of Corrosion Engineers.

He has more than fifteen technical publications to his credit, in the areas of sulfide stress cracking, corrosion, failure analysis, hydrogen embrittlement, and microstructure/property relationships of specialty steels. He has been the lead technical investigator for many projects encompassing a wide variety of client groups including manufacturers, end users, insurance companies, and attorneys. One of his special areas of interest is working with attorneys specializing in product liability issues, and he has served as an expert witness on several product failure cases.

Dr. Garber brings a wealth of experience and knowledge to MEI-C, which complements our services to our valued clients.

MEI-C People