What to Do When You Have a NFPA Compliant Burner Management System Design but Have Identified Gaps during Your Risk Analysis

Stress corrosion cracking (SCC) occurs due to the combined influence of corrosive environment and tensile stress. SCC of stainless steel (type 304) has been widely studied in various aqueous solutions (such as acidic chloride solution, thiosulfate solution) at room temperature. However, it has not been studied in crude oil or ethanol environment, which is common in transportation of these chemicals in rail tank cars. There exist conditions where the stainless steel stem of the pressure relief valve under substantial tensile load is exposed to the corrosive environment. Pressure relief valve (PRV) is used a relief device from pressure build-up in the tank and the stem that retains a spring exerting a large static load (over a ton). Here, the chemical penetrates the crevice formed between the stem and spring retainer creating a latent corrosion site, where electro-chemical potential may initiate the corrosion cracking mechanism. In the present study, stainless steel corrosion containers with necessary support systems (e.g., pressure gage, temperature gage, purging system, drainage system) have been fabricated to test the stress corrosion cracking of stainless steel stem exposed to crude oil and ethanol environments. The full size PRVs are mounted on the top of the container and the lower part of the PRV is immersed into the liquid to simulate actual conditions for several months. Actuators have been setup to create waves and splashes inside the tank. The potential crack generation and growth are have been monitored by combination of liquid penetrant method and eddy current testing apparatus. The experiments will enhance our understanding of why the stainless steel stems (type 304) are failing while tank cars are carrying crude oil and ethanol.