Pyrophoric Iron Sulfide in Oil and Natural Gas Wastewater Tanks
The danger of pyrophoric iron sulfide (PIS) as a source of ignition in refinery operations is widely recognized. Care is taken to remove accumulated PIS from refinery process equipment before the equipment is opened for maintenance. A number of chemical and mechanical methods are available. The potential for PIS accumulation in natural gas processing lines and equipment is also recognized. PIS is known to be one of the potential components of the “black powder” that causes plugging issues from natural gas wellheads to burners. Black powder is handled with care when exposed to air. It has been known to give off the telltale “white smoke” of PIS oxidation and filter elements loaded with black powder have been known to undergo pyrophoric ignition.
Less well documented is the potential for a PIS-initiated fire in oil and natural gas wastewater tanks. Flammable atmospheres can exist in the head spaces of these tanks due to the presence of dissolved and carried-over hydrocarbons. Fires have occurred in these tanks during operation and maintenance. Despite PIS being identified as the probable source of ignition in a number of these cases, there is little discussion in the literature of the potential for PIS formation or accumulation in sour water tanks. The recent industry expansion of new sources of oil and gas, involving different production techniques with potentially different impurity profiles, makes such a discussion particularly important at this time.
PIS can be formed by chemical reaction or by microbial activity. The formation can occur anywhere in the process from the wellhead to the water storage tank. To evaluate risk mitigation options, an operator must first identify the location and mechanism of formation. BakerRisk consultants have assisted clients in understanding and mitigating the risk of PIS-induced fire in their wastewater tanks. This assistance has included presentation of background information on iron sulfide formation, oxidation and hazards; development and management of analytical testing programs; incident investigation; facilitation of Process Hazards Analysis of wastewater collection and storage facilities: and development and evaluation of risk mitigation options.