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.

An air compressor was installed at a chemical plant in which nitric acid was produced by burning ammonia with air. It was a 5000 hp, 5-stage centrifugal machine running at 6000 rpm, compressing air to 5 atm. Failure of the first stage impeller occurred due to a segment from the back plate becoming detached. On the remaining portion, cracks were visible running between the holes for rivets by which the vanes were attached. Metallographic examination of selected sections from the backplate revealed the material to be in the hardened and tempered condition, and the cracking to have initiated on the internal surface of the plate at the crevice between the plate and the vane. It was evident that the impeller failed by stress-corrosion cracking, which initiated in the crevice between the vanes and back plate and propagated through the plate along the line of the rivets where working stresses would be greatest. The compressor intake was situated in the vicinity of nitric acid pumps which had a history of leakage troubles, and which had evidently given rise to the nitrates found on the impeller.

A fireball engulfed half of a drill rig while in the process of drilling a shot hole. Subsequent investigation revealed the cause of the fire was the failure of the oil return hose to the separator/receiver in the air compressor. The failed hose was a 50.8 mm 100R1 type hose, as specified in AS 3791-1991 Hydraulic Hoses. This type of hose consisted of an inner tube of oil-resistant synthetic rubber, a single medium-carbon steel wire braid reinforcement, and an oil-and-weather resistant synthetic rubber cover. The wire braiding was found to be severely corroded in the area of the failure zone. The physical cause of the hose failure was by severe localized corrosion of the layer of reinforcing braid wire at the transition between the coupling and the hose at the end of the ferrule. This caused a reduction of the wire cross-sectional area to the extent that the wires broke. Once the majority of the braid wires were broken there was not enough intrinsic strength in the rubber inner hose to resist the normal operating pressures.

Macrofractographs of the fracture surface from a multibladed fan showed that cracks started at the corner where bending stress was concentrated and propagated through the blade by fatigue. Peak stress at the monitoring position was less than 10 MPa. To simulate crack growth, the rotor was repeatedly deformed by a hydraulic fatigue tester. Comparison of striations of the failed blade with that of the tested one revealed the failed blade was loaded with more than 30 MPa of stress. These tests confirmed that the rotor and blades had sufficient strength to withstand up to 3x the stress of normal operation. The casing of the fan was vibrated at 10 to 60 Hz. Peak stress easily overcame 30 MPa, which was enough to initiate cracking. The fracture surfaces and starting position were the same as those on the failed fan. It was concluded that the exciting force from an air compressor caused blade failure.

Tube sheets (found to be copper alloy C46400, or naval brass, and 5 cm (2 in.) thick) of an air compressor aftercooler were found to be cracked and leaking approximately 12 to 14 months after they had been retubed. Most of the tube sheets had been retubed several times previously because of unrelated tube failures. Sanitary (chlorinated) well water was generally used in the system, although filtered process make-up water (river water) containing ammonia was occasionally used. Investigation (visual inspection, chemical analysis, mercurous nitrate testing, unetched 5X micrographs, and 250X micrographs etched in 10% ammonium persulfate solution) supported the conclusion that the tube sheets failed by SCC as a result of the combined action of internal stresses and a corrosive environment. The internal stresses had been induced by retubing operations, and the environment had become corrosive when ammonia was introduced into the system by the occasional use of process make-up water. Recommendations included making a standard procedure to stress relieve tube sheets before each retubing operation. The stress relieving should be done by heating at 275 deg C (525 deg F) for 30 min and slowly cooling for 3 h to room temperature.

One 49-in. impeller of a two-stage centrifugal air compressor disrupted without warning, causing extensive damage to the casings, the second impeller, and the driving gear box. Prior to the mishap, the machine had run normally, with no indications of abnormal vibration, temperature, or pressure. Initial failure had taken place in the floating dished inlet plate (eye plate) of the first-stage impeller. Failure occurred predominantly by tearing along the lines of rivet holes for the longer blades, these extended for practically the full radial width of the dished plate. Examination of the fractured surfaces showed that failure had been preceded by fatigue cracking. The material from which the dish plate was forged was a Ni-Cr-Mo steel in the oil hardened and tempered condition. Fractographic examination of the surface of the cracks showed striation markings indicative of the progress of fatigue cracks. Failure of the one impeller and the cracking of the others were attributed to “low-cycle high-strain fatigue” due to fluctuating circumferential (hoop) stresses.

Numerous cracks observed on the surface of a forged A470 Class 4 alloy steel steam turbine rotor disc from an air compressor in a nitric acid plant were found to be the result of caustic induced stress-corrosion cracking (SCC). No material defects or anomalies were observed in the disc sample that could have contributed to crack initiation or propagation or secondary crack propagation. Chlorides detected in the fracture surface deposits were likely the primary cause for the pitting observed on the disc surfaces and within the turbine blade attachment area. It was recommended that the potential for water carryover or feedwater induction into the turbine be addressed via an engineering evaluation of the plant's water treatment procedures, steam separation equipment, and start-up procedures.

While an attempt was being made to start the diesel engine of a dragline type excavator, a severe explosion took place, the operator unfortunately being killed. The engine was normally started by compressed air from an air bottle or receiver charged to a pressure of 350 psi by a small auxiliary engine driven compressor fitted on the excavators. On an occasion when this had broken down, the air bottle was replenished by the engineer-in-charge from a cylinder of compressed air. The compressor engine remained unserviced and when the starting bottle again became depleted, the excavator operator re-charged it himself from another cylinder without the knowledge of the engineer. The engine was started satisfactorily, and the bottle charged again to 350 psi in readiness for the start on the following day. It was while an attempt was being made to start the engine on this occasion that the explosion took place. The cylinder used by the operator for re-charging the receiver contained not air but oxygen.

Two oil-pump gears broke after four months of service in a gas compressor that operated at 1000 rpm and provided a discharge pressure of 7240 kPa (1050 psi). The compressor ran intermittently with sudden starts and stops. The large gear was sand cast from class 40 gray iron with a tensile strength of 290 MPa (42 ksi) at 207 HRB. The smaller gear was sand cast from ASTM A536, grade 100-70-03, ductile iron with a tensile strength of 696 MPa (101 ksi) at 241 HRB. Analysis (metallographic examination) supported the conclusion that excessive beam loading and a lack of ductility in the gray iron gear teeth were the primary causes of fracture. During subsequent rotation, fragments of gray iron damaged the mating ductile iron gear. Recommendations included replacing the large gear material with ASTM A536, grade 100-70-03, ductile iron normalized at 925 deg C (1700 deg F), air cooled, reheated to 870 deg C (1600 deg F), and oil quenched. The larger gear should be tempered to 200 to 240 HRB, and the smaller gear to 240 to 280 HRB.

Initially, two vertical double-acting two-stage compressors delivering chlorine gas at a pressure of 100 psi appeared to be running satisfactorily. About six months later the LP piston-rod of the No. 2 compressor failed due to burning, the compressor being worked double-acting at the time. About five months later, the HP piston rod of the No. 1 compressor failed in a similar manner. Specimens for microscopic examination were cut from the rod in the region of the failure and from the extreme end that had been situated above the piston and hence not subjected to an appreciable rise in temperature. The material was a steel in the normalized condition with a 0.35% C content. It appears probable that deficient lubrication of the gland resulted in overheating of the rod due to friction. The presence of a sprayed-metal coating was probably an additional factor in promoting failure, as it would present to the gas a surface area considerably greater than that of a homogeneous material.

Postflight inspection of a gas-turbine aircraft engine that had experienced compressor stall revealed that the engine air-intake bullet assembly had dislodged and was seated against the engine-inlet guide vanes at the 3 o'clock position. The bullet assembly consisted of an outer aerodynamic shell and an inner stiffener shell, both of 1.3 mm (0.050 in.) thick aluminum alloy 6061-T6, and four attachment clips of 1 mm (0.040 in.) thick alclad aluminum alloy 2024-T42. Each clip was joined to the outer shell by 12 spot welds and was also joined to the stiffener. Analysis (visual inspection, dye-penetrant inspection, and 10x/150x micrographs of sections etched with Keller's reagent) supports the conclusion that the outer shell of the bullet assembly separated from the stiffener because the four attachment clips fractured through the shell-to-clip spot welds. Fracture occurred by fatigue that initiated at the notch created by the intersection of the faying surfaces of the clip and shell with the spot weld nuggets. The 6061 aluminum alloy shell and stiffener were in the annealed (O) temper rather than T6, as specified. Recommendations included heat treating the shell and stiffener to the T6 temper after forming.

Several compressor diaphragms from five gas turbines cracked after a short time in service. The vanes were constructed of type 403 stainless steel, and welding was performed using type 309L austenitic stainless steel filler metal. The fractures originated in the weld heat-affected zones of inner and outer shrouds. A complete metallurgical analysis was conducted to determine the cause of failure. It was concluded that the diaphragms had failed by fatigue. Analysis suggests that the welds contained high residual stresses and had not been properly stress relieved. Improper welding techniques may have also contributed to the failures. Use of proper welding techniques, including appropriate prewelding and postwelding heat treatments, was recommended.

Rotor blades in the compressor section of a J79 engine had failed. Optical, stereoscopic, microhardness testing, and SEM examinations were conducted to determine the cause. The blades were made of STS403 and were used uncoated. They were damaged over an extensive area, from the 15th through the 17th compressor stages, as were stator vanes and casing sections. The fractured surface of the 17th blade showed multiple origins along with secondary cracking and extensive propagation that preceded separation. The metallographic analysis of the microstructure suggested work hardening. Based on the results, the cause of the fractured blade was high-amplitude fatigue due to severe stall. After normal engine usage of five months, the blade fractured sending fragments throughout the combustion and turbine sections.