Hydrogen gas in sulfuric acid plants “What we thought we knew we knew”
H2SO4+ Fe —> FeSO4+ H2
It is well known and understood by all in the sulfuric acid industry that when sulfuric acid is exposed to steel the resulting chemical reaction produces hydrogen gas, but how many are familiar with all of the potential risks to sulfuric acid plants resulting from this reaction? Most plant operators are aware of hydrogen grooving in storage tanks and have experienced the minor hydrogen “bangers” that can happen when welding on high points in gas system ductwork or equipment. Fewer are familiar with incidents involving larger hydrogen explosions that can occur in sulfuric acid plants or the magnitude of the equipment damage that can result. Even fewer are familiar with the frequency that these incidents have occurred within the sulfuric acid industry in recent times. Following the investigation of three such hydrogen related incidents, informal networking contacts throughout the industry have accumulated primary or secondary knowledge of a total of 13 hydrogen related incidents in sulfuric acid plants. Eleven of these 13 incidents have occurred within the past 10-years, equating to an alarming average rate of one per year! Three incidents have occurred to date in 2011 raising additional concern regarding what could potentially be further indication of an increase in the frequency of these serious incidents.
The incidents are typically initiated by steaming equipment or acid cooler leaks that result in severe weak acid excursions. The weak acid corrosion then generates enough hydrogen to accumulate explosive concentrations in various locations of the gas side process equipment, ultimately causing an incident.
The equipment location where hydrogen gas accumulation most commonly occurs appears to be at the top of the interpass acid tower, but accumulations in drying towers, converter beds, heat exchangers, and ductwork have also been reported. Boiler leaks, economizer leaks, acid cooler leaks and weak acid corrosion damage can all happen even at the best operated and maintained sulfuric acid plants.
Most experienced sulfuric acid plant operations and maintenance personnel have probably seen equipment failures similar in nature to the types shown in the photos above. Most, fortunately, have not had the misfortune to experience any of the types of equipment damages shown in the photos below that can potentially result from the subsequent hydrogen explosion. This article highlights the critical knowledge learned so far from the investigation of the three incidents depicted in these photographs. Incident investigations “What we learned” A Root Cause Failure Analysis (RCFA) was performed on each of these three incidents. Every known aspect of each plant’s design, operating conditions, failure sequence of events and the resulting equipment damages was analyzed in an attempt to determine the mechanism of failure and develop the appropriate preventative measures and safeguards to minimize or eliminate the risks to acid plant equipment and operating personnel. The first incident was initiated by a severe economizer tube failure that produced weak acid concentrations in the range of 85 percent. The resulting hydrogen explosion occurred in a high point of the ductwork for an internal hot pass heat exchanger of a radial flow converter design approximately 1-hour and 20-minutes after emergency plant shutdown. No ignition source was ever identified, but the operating temperature in the area of the converter (Catalyst Bed #2 Outlet) where the internal duct that the hydrogen is suspected to have accumulated is reasonably close to the auto-ignition temperature of hydrogen gas. The second and third incidents were also initiated by economizer leaks, but in these scenarios the explosion occurred in the tops of IPA towers, one a traditional acid tower design and the other an alloy heat recovery acid tower design. Process gas temperatures present in the tops of acid towers are well below the known auto ignition temperature of hydrogen so the ignition source in these cases is hypothesized to be a static charge buildup occurring in the non-conductive fiberglass bed of the acid mist eliminators.