Modern day VLCCs are like floating mini-cities, with their own thermal power plants for generating high pressure steam to drive the turbines and for a variety of other uses. The boiler feedwater is high purity treated water operating in a closed loop; that is, the spent steam is condensed in onboard heat exchangers and fed back into the boiler feedwater stream. In conventional, land-based thermal power plants, cooling water for the heat exchangers is typically derived from city water supplies, with additional treatment for corrosion and biological controls and is utilized in a closed loop with cooling towers serving as atmospheric heat sinks. VLCC power plants, in contrast, use filtered seawater from the open seas as a once-through medium in the heat exchangers for condensing and recovering the spent steam. Makes sense doesn't it, considering how plentiful seawater is in the ocean? The only drawback, of course, is that seawater tends to be quite corrosive.

Copper-nickel alloys are the materials of choice for condenser tubes in seawater service because of their proven corrosion resistance in seawater. Their corrosion resistance is due to the formation of a thin, adherent, protective oxide film that starts forming naturally as soon as the tubes are exposed to clean seawater. The film is complex, predominantly cuprous oxide with copper hydroxy chlorides at the outer layer. The protective layer thickens and gradually stabilizes over several months, eventually resulting in a corrosion rate of less than 1 mil per year.

The integrity of the protective layer is critical to the service life of the condenser tubes. Factors that can cause damage to the film include high fluid velocities (greater than 8 ft/sec), internal tube blockage or defects that can cause local turbulence, and microbial-induced corrosion (MIC).

The following two case histories are a couple of examples of problems we've encountered with VLCC condenser tubes.

Defective Condenser Tubes

A VLCC was on its first voyage after its heat-exchanger had been completely retubed with brand new condenser tubes. The heat exchanger was a rectangular box configuration, two-pass system, with several thousand, 15-foot long, 3/4-inch diameter, 49-mil wall thickness, 90%-Copper, 10%-Nickel tubes. One week into the voyage, the high quality boiler feedwater was found to be contaminated with seawater. Two leaking condenser tubes were identified and plugged. A fluke perhaps? No such luck--in the upcoming weeks more leaking tubes were found, forcing the owners of the vessel to conclude that their brand new heat-exchanger had serious problems and they would need to take the ship out of service again and send it back to dry dock for more inspection and repair.

Once there, a comprehensive eddy current inspection revealed over a dozen additional leakers and several hundred tubes with significant internal and external defects in the form of laps, folds, cracks, and embedded foreign material. Corrosion pits had preferentially formed at the internal defects, leading to leaks in several of the tubes.

So why had the pits formed, you ask? That's pretty straightforward. Formation of the normal protective oxide layer was prevented at the locations of the defects because of local turbulence and the defect morphology. Without the protective film at the localized spots, the metal corroded, forming a pit. Once formed, pits can progress relatively rapidly, in a matter of days and weeks rather than years, because of highly localized anodic conditions at the defects/pits compared to the cathodic conditions over the remainder of the internal tube surface. This, in effect, sets up a natural battery, which results in high rates of metal dissolution at the defect sites. Elsewhere, away from the pits and other defects, the surfaces exhibited a uniform, protective oxide layer representative of normal seawater service.

The defects were attributed to a lack of quality control during tube manufacture and the absence of a suitable final inspection. Given the critical function of the heat exchanger, the owners of the VLCC decided to replace the entire set of two-month old condenser tubes with tubes from a different supplier. Our comprehensive assessment helped the owner to recover a substantial portion of his losses.

Microbial-induced Corrosion

In this project, the copper-nickel heat exchanger tubes in a VLCC had developed leaks after 1 1/2 years of service. Eddy current inspection revealed corrosion pits of varying sizes in about 200 condenser tubes out of a total of about 5,000 tubes. The affected tubes contained overlapping corrosion pits of varying sizes that were covered with corrosion tubercles (i.e., mounds of corrosion products) composed primarily of chloride- and sulfur-bearing compounds of copper.

The corrosion tubercles exhibited a layered structure, with high concentrations of sulfur compounds at the bottom and lesser concentrations at the top. The bottoms of the pits had an acidic pH of about 4.5, compared to a near neutral pH of 6.5 in the tube oxides away from the pits/tubercles. Suspecting that microbial-induced corrosion might be the cause of the problem, we ran an immunoassay test for the detection of sulfate-reducing bacteria. Just as we suspected, high concentrations of sulfate-reducing bacteria were present within the pits!



So, how does the presence of the bacteria cause a problem with the tubes? They're metal after all--it's not like they're alive and can get sick, right? Well, you're right about the not getting sick part, but that doesn't mean the tubes are unaffected by these particular bacteria. How so? Anaerobic sulfate-reducing bacteria generate local acidity, which causes the breakdown of the protective copper oxide layer that's preventing corrosion. Without that layer, the affected area becomes anodic and preferentially corrodes, with the surrounding area acting as a large cathode.

The presence of this condition in over 200 condenser tubes was attributed to exposure to polluted seawater, followed by stagnant conditions that promoted the retention and growth of the anaerobic sulfate-reducing bacteria.

Replacement of the affected tubes was the cure in this case. To prevent a recurrence, one solution would be to use an inhibitor such as ferrous sulfate during exposure to polluted seawater (the ferrous sulfate strengthens the protective film). Another solution would be to flush the system with clean, fresh seawater after exposure to polluted seawater, or to simply maintain continuous circulation of the polluted seawater, thereby avoiding the stagnant conditions that led to the growth of the bacteria.

Cigarette Fire Safety Testing

In June of this year, a new state law went into effect in New York requiring all cigarette brand labels sold in that state (some 1,300 in total, according to a recent article in the Wall Street Journal) to pass a test of their ability to self-extinguish. The intent of the new law is to reduce the incidence of cigarette-caused fire, which, according to the National Fire Protection Association, is the leading cause of fire-related fatalities in the United States.

While at present, New York is the only state with such a law, similar legislation has been introduced in several other states and there's a bill in Congress to establish a national requirement for self-extinguishing cigarettes. Beating us stateside folks to the punch, in March, the Canadian Parliament passed a bill that requires all cigarettes sold in Canada to be self-extinguishing.

Now, being in the testing business, we're never surprised to learn about the presence of a test procedure we've not previously encountered, and so it was in early June when we were contacted by a cigarette manufacturer and asked if we could test several of their brand labels.

It turns out there's an American Society of Testing and Materials (ASTM) test method for evaluating the capability of a cigarette if left untended to generate sufficient heat to continue burning and thus potentially cause ignition of bedding or upholstered furniture. The test method evaluates the ignition potential of a cigarette by placing it on a special grade of filter paper in a designated test chamber. If the cigarette self-extinguishes before burning completely down, it passes; if not, it fails.

Interestingly, while 40 cigarettes must be tested for each brand label, only 30 need to pass the test in order to certify the brand label.

The law doesn't specify how the manufacturers are to comply with the self-extinguishing requirements, but the approach that most seem to be taking is to use a modified paper containing bands of starch-like material that restrict the flow of oxygen, causing the cigarette to go out unless it's actively "puffed on" when the burn-front reaches the band.

Seems simple enough, doesn't it? Actually, it's probably quite an interesting engineering problem; that is, finding a reasonable balance between continued burning and self-extinguishing so that a smoker doesn't have to continually relight the cigarette, yet it goes out reasonably quickly if left undisturbed.

The test method calls for a specially constructed, draft-free chamber in which the cigarette is placed after lighting it in a specific manner and allowing it to pre-burn a prescribed amount. Then, with the cigarette setting on a specified grade of filter paper, the tester (that would be us) simply stands by to see if the cigarette goes out or continues to burn until reaching the filter (or its equivalent position on a nonfilter cigarette.) To avoid filling our smoke-free workplace with cigarette smoke, we placed the whole setup under one of the fume hoods in our chemistry laboratory.

So what did we find? The cigarettes we tested passed the test with ease. Of the 160 cigarettes we ran through the test (including those we tested while qualifying our procedure), only one failed to self-extinguish.

All in all it was quite an interesting assignment, just the kind we like to run; new and unusual, lots of specific detailed requirements, etc. Oh, and one last thing... the part we liked best about the ASTM test description? Well, that would be the part under the heading "Hazards," where it said that "personnel shall take proper precautions to avoid inhaling combustion products."Hmmmmm... no kidding. Never let it be said specification writers don't have a sense of humor or understand irony.

MEI-CHARLTON, INC. IS A CONSULTING ENGINEERING FIRM WHICH SPECIALIZES IN QUALITY ASSURANCE, FITNESS-FOR-PURPOSE EVALUATIONS, CORROSION, METALLURGY, WELDING, AND ENVIRONMENTAL AND ANALYTICAL CHEMISTRY
©1999 MEI-Charlton, Inc.