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Conference Article: Gas erosion of impeller housing in the operation of a high-temperature, high-pressure helium circulator

Sanders, J.P.; Heestand, R.L.; Young, H.C. (Oak Ridge National Lab., TN (USA))

Abstract

Three gas-bearing circulators are installed in series in a high-pressure, high-temperature loop to provide helium flow up to 0.47 m3/s at a total head of 78 kJ/kg. The design pressure is 10.7 MPa, and temperatures of 1000 deg. C can be obtained in the test section. The inlet temperature to the circulators is limited to 450 deg. C. The 200-kW motor for each circulator is enclosed in the pressure boundary, and the motor is cooled by circulating the gas within the cavity over a water-cooled coil. The full operating speed is 23,500 rpm. A full-flow filter, absolute for particulate above 10 mum, is installed upstream of the circulator to protect the gas bearing surfaces. The minimum clearances between these surfaces during operation are in the range of 15 to 30 mum. During a routine examination of the circulator, deep V-shaped grooves were found in the stationary surface of this cavity. At the same time, a very fine, dark particulate was observed in crevices of the housing. At first it was assumed that the grooves were formed by particulate erosion. However, examination of the grooves and discussions with persons experienced with large circulator operation changed this opinion. Erosion caused by particulate is characteristically rounded on the bottom and has a greater width to depth aspect than the V-shaped grooves, which were observed. Analysis of the particulate indicated that it was essentially the material of the housing that had undergone reactions with impurities in the circulating gas. It was subsequently concluded that the impeller housing had not been heat treated in a sufficiently oxidizing atmosphere after machining to form an adherent oxide coating. This suboxide coating was eroded by the shear forces in the gas. The exposed layer of metal was then further oxidized by the impurities in the gas, and these layers of oxide were successively eroded to produce the grooves. This erosion problem was eliminated by machining a ring of the same material, heat treating it to form an adherent stable oxide, and bolting it in place in the cavity.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Specialists' meeting on gas-cooled reactor coolant circulator and blower technology. San Diego, CA (USA). 30 Nov - 2 Dec 1987
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--17, pp:156-163