Managing risk of SCC of fin fan air cooled heat exchangers

Abstract

The threshold temperature for stress corrosion cracking (SCC) of type 304 & 316 stainless steel has traditionally been considered to be 60°C. Recently however laboratory based testing has demonstrated that SCC of these alloys can occur at temperatures as low as 30°C when exposed to magnesium chloride solutions. The critical parameter controlling the corrosivity is the relative humidity at which magnesium chloride will deliquesce. Fin-fan air cooled heat exchangers fabricated from these alloys and operating in a temperature range from 40 to 90°C could be expected to suffer SCC if exposed to a magnesium chloride solution formed as a result of the deposition of windblown sea salt. A possible location for cracking is the highly stressed crevice formed where tubes are rolled into the tube plate. This paper reports the results of salt deposition measurements using ISO 9225 'salt candles" positioned in the air intake to the fin-fan coolers and at a location remote from the coolers. Salt deposition amounts have been determined at monthly intervals for a 1 year period. In addition local weather conditions are reviewed to ascertain the number of rain days expected and associated with a temperature which would allow a saturated magnesium chloride solution to form at the operating temperature of the tubes in the fin-fan coolers. Using this information, the relevance of laboratory derived SCC results to operating 304L fin fan cooler tubes is discussed. The context in which this work is considered is to demonstrate that risk of loss of integrity of the fin fan coolers is reduced to as low as reasonably practical (ALARP) which is required for the operation of plant within a Major Hazard Facility in Victoria, Australia. Show more