This study examines the role of calcium-precipitating bacteria (CPB) in heat exchanger tube failures. Several types of bacteria, including Serratia sp. (FJ973548), Enterobacter sp. (FJ973549, FJ973550), and Enterococcus sp. (FJ973551), were found in scale collected from heat exchanger tubes taken out of service at a gas turbine power station. The corrosive effect of each type of bacteria on mild steel was investigated using electrochemical (polarization and impedance) techniques, and the biogenic calcium scale formations analyzed by XRD. It was shown that the bacteria contribute directly to the formation of calcium carbonate, a critical factor in the buildup of scale and pitting corrosion on heat exchanger tubes.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

A straight-tube cooler type heat exchanger had been in service for about ten years serving a coal pulverizer in Georgia. Non-potable cooling water from a local lake passed through the inner surfaces of the copper tubing and was cooling the hot oil that surrounded the outer diametral surfaces. Several of the heat exchangers used in the same application at the plant had experienced a severe reduction in efficiency in the past few years. One heat exchanger reportedly experienced some form of leakage following discovery of oil contaminating the cooling water. This heat exchanger was the subject of a failure investigation to determine the cause and location of the leaks. Corrosion products primarily contained copper oxide, as would be expected from a copper tubing. The product also exhibited the presence of a significant amount of iron oxides. Metallographic cross sectioning of the tubes and microscopic analysis revealed several large and small well rounded corrosion pits present at the inner diametral surfaces. The cause of corrosion was attributed to corrosive waters that were not only corroding the copper, but were corroding steel pipes upstream from the tubing.

Several of the aluminum alloy 6061-T6 drawn seamless tubes (ASTM B 234, 2.5 cm (1.0 in.) OD with wall thickness of 1.7 mm (0.065 in.)) connecting an array of headers to a system of water-cooling pipes failed. The tubes were supplied in the O temper. They were bent to the desired curvature, preheated, then solution treated, water quenched, and then aged for 8 to 10 h. Analysis (visual inspection, slow-bend testing, 65x macrographic analysis, macroetching, spectrographic analysis, hardness tests, microhardness tests, tension tests, and microscopic examination) supported the conclusions that bending of the connector tubes in the annealed condition induced critical strain near the neutral axis of the tube, which resulted in excessive growth of individual grains during the subsequent solution treatment. Recommendations included bending the connector tubes in the T4 temper as early as possible after being quenched from the solution temperature. The tubes should be stored in dry ice after the quench until bending can be done. The tubes should be aged immediately after being formed. Flattening and slow-bend tests should be specified to ensure that the connector tubes had satisfactory ductility.

Cracks formed on cylinder inserts from a water-cooled locomotive diesel engine, on the water side in the neck between the cylindrical part and the collar. Cracks were revealed by magnetic-particle inspection. As a rule, several parallel cracks had appeared, some of which were very fine. The part played by corrosion in the formation of the cracks was demonstrated with the help of metallographic techniques. The surface regions of the cracks widened into funnel form, which is a result of the corrosive influence of the cooling water. Actual corrosion pits could not be found indicating that the vibrational stresses had a greater share in the damage than the corrosive influence. Cracks appeared initially only in those engines in which no corrosion inhibitor had been added to the cooling water. The cracking was caused by corrosion fatigue. The combined presence of a corrosive medium and cyclical operating stress was needed to cause cracks. No cracks appeared when corrosion inhibitor was added to the cooling water.

A spiral heat exchanger made from 316L stainless steel developed a leak after eight years of service as a condenser on a distillation tower. Examination identified the leak as being located on the cooling water side in the heat affected zone (HAZ) of a weld joining two plates. Cooling water deposits were observed in a V-shaped corner formed by the weld. A metallurgical examination identified the presence of transgranular cracks in the HAZ on the cooling water side. Analysis of the cooling water revealed the presence of chlorides. Based on the metallurgical analysis and other findings, it was determined that the cracks and associated leak were the result of chloride stress-corrosion cracking.

A cast iron cylinder liner from a diesel engine suffered localized damage on the cooling water side leading to serration of the edges and heavy pitting. This heavy damage was cavitation damage, frequently observed in diesel motor cylinders. To combat such damage the following measures are recommended in the specialist literature: reduction in piston play; reduction in the amplitude by thicker-walled linings; hard chromizing of the cooling water side; and, addition of a protective oil to the cooling water. The effect of the protective oil is presumably based on a film of oil which forms on the cylinder surface and which is not so easily scoured off during vibration. The effect of the imploding vacuum bubbles is reduced by the oil film which can renew itself from the emulsion.

A failed laser mirror and another complete mirror of the same construction were analyzed. The laser mirror consisted of three layers of material brazed together to form channels through which the cooling water flows. Samples were analyzed with light optical and scanning electron microscopy. The corrosion product contained molybdenum and copper with a trace of gold. The base material was analyzed as molybdenum with negligible alloying additions. The primary mode of corrosion attack on the base material appeared to be intergranular, although uniform corrosion was evident also. It was concluded that corrosion attack sufficiently weakened the base material and the brazed joints, allowing catastrophic failure of the mirror due to the pressure of the cooling water. It was recommended that the mirrors be cleaned of all corrosion products present as a result of past service conditions and proof tested. It was recommended that the water system consisting of deionized water and formaldehyde be replaced with water having a low oxygen content and a cathodic inhibitor (oxygen scavenger).

After about 17 years in service, copper alloy C27000 (yellow brass, 65% Cu) innercooler tubes in an air compressor began leaking cooling water, causing failure and requiring replacement. The tubes were 19 mm in diam and had a wall thickness of 1.3 mm (0.050 in.). The cooling water that flowed through the tubes was generally sanitary (chlorinated) well water; however, treated recirculating water was sometimes used. Analysis (visual inspection, 9x and 75x unetched micrographs, and spectrochemical analysis) showed a thick uniform layer of porous, brittle copper on the inner surface of the tube, extending to a depth of about 0.25 mm (0.010 in.) into the metal, plug-type dezincification extending somewhat deeper into the metal. This supported the conclusion that failure of the tubes was the result of the use of an uninhibited brass that has a high zinc content and therefore is readily susceptible to dezincification. Recommendations included replacing the material with copper alloy C68700 (arsenical aluminum brass), which contains 0.02 to 0.06% As and is highly resistant to dezincification. Copper alloy C44300 (inhibited admiralty metal) could be an alternative selection for this application; however, this alloy is not as resistant to impingement attack as copper alloy C68700.

An aftercooler was of conventional design and fitted with brass tubes through which cooling-water circulated. Air at 100 psi pressure was passed over the outsides of the tubes, entering the vessel near to the upper tubeplate on one side and leaving it by a branch adjacent to the lower tubeplate on the opposite side. After a mishap, the paint had been burned off the upper half of the shell. Internally, most of the tubes were found to be twisted or bent. The casing of the pump used to circulate the cooling water was also found to be cracked after the mishap. All the evidence pointed to the probability that a fire had occurred within the vessel. Some months before the failure, one of the tubes situated towards the center of the nest developed a leak. Owing to the difficulty of inserting a replacement tube, the defective one was scaled by means of a length of screwed rod fitted with nuts and washers at each end. This assembly became loose, thereby allowing air under pressure to enter the waterside of the cooler and expel the water, leading to overheating and ultimately to the damage described.

Equipment in which an assembly of in-line cylindrical components rotated in water at 1040 rpm displayed excessive vibration after less than one hour of operation. The malfunction was traced to an aluminum alloy 6061-T6 combustion chamber that was part of the rotating assembly. Analysis (visual inspection, 100x/500x/800x micrographic examination, spectrographic analysis, and hardness testing) supported the conclusions that, as a result of improper heat treatment, the combustion-chamber material was too soft for successful use in this application. Misalignment of the combustion chamber and one or both of the mating parts resulted in eccentric rotation and the excessive vibration that caused malfunction of the assembly. Irregularities in the housing around the combustion chamber and temperature variation relating to the combustion pattern in the chamber were considered to be possible contributing factors to localization of the cavitation erosion. Recommendations included adopting inspection procedures to ensure that the specified properties of aluminum alloy 6061-T6 were obtained and that the combustion chamber and adjacent components were aligned within specified tolerances. In a similar situation, consideration should also be given to raising the pressure in the coolant in order to suppress the formation of cavitation bubbles.

Corrosion failure occurred in a titanium clad tubesheet because of a corrosive tube-side gas-liquid mixture leaking through fatigue cracks in the seal welds at tube-to-tubesheet joints. The tubesheet was a carbon steel plate clad with titanium on the tube side face. The seal weld cracks were initiated by cyclic stress imposed by exchanger tubes. The gas-liquid mixture passed through cracks under tube-side pressure, resulting in severe corrosion of the steel backing plate. The failure started with the loosening of the expanded tube-to-tubesheet joints. Loose joints allowed the exchanger tubes to impose load on seal welds and the shell side cooling water entered the crevice between the tubesheet and the tubes. The cooling water in the crevice caused galvanic reaction and embrittlement of seal welds. Brittle crack opening and crack propagation in seal welds occurred due to the cyclic stress imposed by the tubes. The cyclic stress arised from the thermal cycling of the heat exchanger. The possible effects of material properties on the failure of the tubesheet are discussed.

Admiralty brass (Alloy C44300) cooling tubes which were part of a heat exchanger in a turbogenerator that provided electricity to a manufacturing plant failed. A mixture of non-recirculating city and “spring pit” water flowed through bundles of tubes to cool the oil in which they are immersed. However, a problem developed when several of the brass tubes cracked transversely, allowing cooling water to mix with the oil. The presence of a tensile stress, intergranular cracks, and a corrosion product suggested the tube failures resulted from stress-corrosion cracking. The main corrosion product was cupric hydroxychloride. In addition to switching to a more corrosion-resistant alloy, extreme care should be taken in the manufacturing of the replacement tube bundles to avoid imparting any residual tensile stresses in the tubing. Analyses of city and spring-pit water were recommended also, to determine which contained the least-harmful corrosive chemicals.

Leakage was detected in a malleable iron elbow (ASTM A 47, grade 35018) after only three months in service. Life expectancy for the elbow was 12 to 24 months. The piping alternately supplied steam and cooling water to a tire-curing press. The supply line and elbow were subjected to 14 heating and cooling cycles per hour for at least 16 h/day, or a minimum of 224 cycles/day. Steam and water pressure were 1035 kPa (150 psi) and 895 kPa (130 psi) respectively, and water-flow rate was estimated to be 1325 L/min (350 gal/min) based on pump capacity. Water-inlet temperature was 10 to 15 deg C (50 to 60 deg F) and outlet temperature was 50 to 60 deg C (120 to 140 deg F). The pH of the water was 6.9. Investigation (visual inspection, chemical analysis, and 67x nital etched micrographs) supported the conclusion that the elbows had been given the usual annealing and normalizing treatment for ferritizing malleable iron. This resulted in lower resistance to erosion and corrosion than pearlitic malleable iron. Recommendations included replacing the elbows with heat-treated fittings with a pearlitic malleable microstructure.

Several failures occurred in 64-mm schedule 80 type 304 stainless steel (ASME SA-312, grade TP304) piping in a steam-plant heat-exchanger system near tee fittings at which cool water returning from the heat exchanger was combined with hot water from a bypass. Various portions of the piping were subjected to temperatures ranging from 29 to 288 deg C. Each of the failures were revealed to consist of transgranular cracking in and/or close to the circumferential butt weld joining the tee fitting to the downstream pipe leg, where the hot bypass water mixed with the cool return water. The transgranular cracks suggested that thermal fatigue was a more likely cause of failure than SCC. It was concluded by temperature measurements that circumferential temperature gradients, in combination with inadequate flexibility in the piping system as a whole, had caused the failures. The tee fitting was redesigned to alleviate the thermal stress pattern.

The horizontal heat-exchanger tubes made of copper alloy C70600, in one of two hydraulic-oil coolers in an electric power plant, leaked after 18 months of service. River water was used as the coolant in the heat-exchanger tubes. Several nodules on the inner surface and holes through the tube wall, which appeared to have formed by pitting under the nodules, were revealed by visual examination. Steep sidewalls, which indicated a high rate of attack, were revealed by microscopic examination of a section through the pit which had penetrated the tube wall. The major constituent of reddish deposit on the inner surfaces of the tubes was revealed to be iron oxide and slight manganese dioxide. Effluent from steel mills upstream was indicated by the presence of these and other constituents to be the source of most of the solids found in the tubes. It was concluded that the tubing failed by crevice corrosion. The tubing in the cooler was replaced, and cooling-water supply was changed from river to city water, which contained no dirt to deposit on the tube surfaces. An alternate solution of installing replacement tubes in the vertical position to make deposition of solids from river water less likely was suggested.