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.

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.

An investigation of the impeller and deposit samples from a centrifugal compressor revealed that an aluminum IR-12 refrigerant reaction had occurred, causing extensive damage to the second-stage impeller and contaminating the internal compressor components. The spherical surface morphology of the impeller fragments suggested that the aluminum had melted and resolidified. The deposits were similar in composition and were identified by XRD as consisting primarily of aluminum trifluoride. In addition, EDS analysis detected major amounts of chlorine and iron. Results of a combustion test indicated that the compressor deposit was comprised of a 9. 8 wt% carbon and that the condenser deposit contained 8.7 wt% carbon. It was concluded that the primary cause of failure was the rubbing of the impeller against the casting and that a self-sustaining Freon fire had occurred in the failed compressor