Centrifugal Impeller Failure via a Thermite Reaction

Abstract

A 2500 HP integral gear compressor in an air separation plant experienced an impeller failure in the first stage of compression. The failure was extensive and took the dry air compressor off-line. Plant production was curtailed for several weeks. Initial investigation focused on the 1st stage centrifugal compressor impeller and upstream dust filter. Most of the aluminum impeller was missing and portions of the air filter element had melted. With the sequence and cause unclear, a formal root cause analysis (RCA) was undertaken. The RCA process ultimately revealed that an unexpected thermite reaction occurred and consumed most of the aluminum impeller. During compressor shutdown, products of the intensely exothermic thermite reaction backflowed into the upstream dust filter causing the element to partially melt. The thermite reaction initiated in the impeller back disk seal area where metal oxides and fuel (debris) had collected. Viscous heating and temperatures greater than expected allowed the rotating labyrinth seal to creep. Contact with the babbitted stationary seal generated additional heat and further accelerated the creep process. Frictional heating in the impeller back disk seal initiated the reaction. Corrective actions to prevent reoccurrence included a creep resistant impeller back disk seal and a passive debris removal system to prevent debris particles from collecting in the impeller back disk seal area. This compressor and a twin have operating reliably since 2003. Recent inspection revealed neither back disk seal creep no the collection of debris particles. These facts have verified the effectiveness of the solution.