Search Results for Leakage

An engine cylinder head failed after operating just 3.2 km (2 mi) because of coolant leakage through the exhaust port. Visual examination of the exhaust ports revealed a casting defect on the No. 7 exhaust-port wall. A 0.9x examination of an unpolished, unetched longitudinal section through the defect indicated shrinkage porosity. This defect was found to interconnect the water jacket and the exhaust gas flow chamber. No cracks were found by magnetic-particle inspection. The gray iron cylinder head had a hardness of 229 HRB on the surface of the bottom deck. The microstructure consisted of type A size 4 flake graphite in a matrix of pearlite with small amounts of ferrite. this evidence supported the conclusion that the cylinder-head failure resulted from the presence of a casting defect (shrinkage) on the No. 7 cylinder exhaust-port wall interconnecting the water jacket with the exhaust-gas flow chamber. No recommendations were made.

Soft-soldered copper pipe joints used in refrigerating plants failed. The solder had not adhered uniformly to the pipe surface. In addition, there were some longitudinal grooves on the pipe surfaces, parts of which were not filled with solder. The unsoldered areas formed cavities within the joints, some of which had been in direct communication with the outsides via the grooves or interconnected cavities. On cooling, moisture condensed on the external surfaces. Some of this was drawn by capillary action into the cavities in open communication with the external surface. On continued cooling to below freezing-point, water that entered the cavities solidified. This was accompanied by a slight increase in volume, which collapsed the pipe walls. In the examples, the pipe ends had not been properly tinned. The solder used was found to be of the tin-antimony type, containing about 5% antimony, which is more difficult to use than the usual tin-lead alloys. The use of this particular type of solder was a contributory factor in the production of unsound joints in the samples examined.

Caustic cracking is the term used to describe one of the forms in which stress-corrosion cracking manifests itself in carbon steels. In the present study, persistent leakage occurred after ten weeks of service from tube expansions in the steam and mud drum of a two-drum D type boiler, which failed to respond to repeated expansion. The leakage was traced to circumferential cracking in the portion of Fe-0.11C-0.46Mn-0.018S-0.011P tubes within the expanded region. Microscopic examination indicated that all cracks started from the outer surface of the tubes in the expanded portion. The form of cracking which was mostly intergranular. Examination at higher magnification disclosed that a selective attack had taken place on the carbide constituents of the pearlite grains. An alkaline deposit on the fireside surface of the tube resulted from the evaporation of boiler water which had found its way past the tube expansions. This indicated that this operation had not resulted in a satisfactorily tight joint.

An unusual type of defect was discovered during hydraulic testing of a water-tube boiler after repairs to the superheater tubes following erosion from soot-blowers. When the pressure reached 700 psi, slight leakage was found to be taking place from one of the superheater tubes in a region where there appeared to be a split, approximately 8 in. long. What was thought to be a split was actually a pronounced fold. Microscopic examination showed that a corrosion-fatigue fissure had developed from one of the inside corners of the fold, presumably as a result of the fluctuating bending stresses to which this portion of the tube would be subjected because of the discontinuity in the tube wall. It was from this fissure that the leakage occurred. It was evident that the defect developed during the manufacture of the tube, probably in the course of a drawing or rolling operation without an internal plug. The diam of this portion of the tube was reduced by local collapse and folding of the section rather than by longitudinal extension of the tube itself.

In a main range in a power station, steam was conveyed at a pressure of 645 psi, and a temperature of 454 deg C (850 deg F). Pipe diameter was 9 in. and the joints were of the bolted type in which a thin steel ring, serrated on both sides, was inserted between plain flanges. Thin jointing material was interposed between the serrated faces and the flanges. The first intimation of trouble was the onset of a high pitched noise audible over a radius of a quarter of a mile. The noise arose from violent lateral vibration of the serrated ring, which attained an amplitude and persisted for a sufficient number of cycles to produce an extensive system of fatigue cracks that resulted in partial disintegration of the ring. Microscopic examination of the material showed it to be a mild steel of satisfactory quality. The trouble was started by slight leakage, possibly resulting from a relaxation of the interfacial pressure on the joint faces, which eroded away the joint material locally at one face of the serrated ring. This reduced interfacial pressure at the opposite face of the ring, with resultant leakage and erosion of the joint material on this side.

Immediately after installation, leakage was observed at the mounting surface of several rebuilt hydraulic actuators that had been in storage for up to three years. At each joint, there was an aluminum alloy spacer and a vellum gasket. The mounting flanges of the steel actuators had been nickel plated. During assembly of the actuators a lubricant containing molybdenum disulfide had been applied to the gaskets as a sealant. The vellum gasket was found to be electrically conductive, and analysis (visual inspection, 500x unetched micrographs, galvanic action testing, and x-ray diffraction) supported the conclusions that leakage was the result of galvanic corrosion of the aluminum alloy spacers while in storage. The molybdenum disulfide was apparently suspended in a volatile water-containing vehicle that acted as an electrolyte between the aluminum alloy spacer and the nickel-plated steel actuator housing. Initially, the vellum gasket acted as an insulator, but the water-containing lubricant gradually impregnated the vellum gasket, establishing a galvanic couple. Recommendations included discontinuing use of molybdenum disulfide lubricant as a gasket sealer, and assembling the actuators using dry vellum gaskets.

A process vessel heating coil, consisting of several 3 ft diam turns, was supplied with steam at 400 psi and a temperature of 343 deg C (650 deg F). At bi-weekly intervals well water was introduced to effect rapid cooling of the contents. After about eight months, leakage developed from a circumferential crack on the underside of the uppermost turn. Shorter cracks were found at a similar location on the bottom turn, and further leakage occurred at pinhole perforations adjacent to the crack in the top turn and near to a butt-weld in the coil. Microscopic examination revealed that the cracks were predominantly of the intergranular variety. In addition, transgranular cracks were present. Material was an austenitic stainless steel of the type specified but the absence of columbium and titanium in significant amounts showed that it was not stabilized against intergranular carbide precipitation. The transgranular cracks indicated that failure was due partly to stress-corrosion. It was concluded that the chlorides provided the main corrodent for both the stress and intercrystalline-corrosion cracking.

Copper tubes from the cooler assemblies of a large air-conditioning unit exhibited leakage upon installation of the unit. Sections from two leaking tubes and one nonleaking tube were subjected to pressure testing and microscopic examination. The cause of leaking was determined to be pitting corrosion. Extensive pitting was found on the insides of all sections examined, with deep and numerous pits in leaking areas. Circumstantial evidence indicated that antifreeze solution left in the tubes from the manufacturing operation was the most likely cause of the pitting.

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.

Various aluminum bronze valves and fittings on the essential cooling water system at a nuclear plant were found to be leaking. The leakage was limited to small-bore socket-welded components. Four specimens were examined: three castings (an ASME SB-148 CA 952 elbow from a small-bore fitting and two ASME SB-148 CA 954 valve bodies) and an entire valve assembly. The leaks were found to be in the socket-weld crevice area and had resulted from dealloying. It was recommended that the weld joint geometry be modified.

Austenitic stainless steel (X 10 Cr-Ni-Mo-Ti 18 10, Material No. 1.4571) cooling coils were found leaking in 15 spots after eight weeks of service in an apparatus in which ammonium sulfide solution was converted into ammonium sulfate. The external temperature of the coil was approximately 175 deg C and it was cooled by water at 3 atm. Examination of two sections of the coil showed pinhead size pitting cavities at the exterior surface and partially parallel and partially angled array of fine cracks on external as well as the internal surfaces of the bend. Metallographic examination conducted on longitudinal and transverse sections showed predominantly transcrystalline cracks, originated from the pits at the external surfaces of the pipe. Their appearance suggested they were stress corrosion cracks that occur in austenitic steels under the combined effect of stresses and certain corrosion agents, especially chlorides. If chlorides were absent, hydrogen sulfide which causes similar pitting and is capable of causing cracks could be suspected. Favorable state of stresses, which could be residual or due to heat treating, bending or straightening operations, would be recommended for better behavior of the container.