Underground Bacteria Generates H2S and Trips Control Panels
Instrument equipment shelters (IESs) are important control buildings within operating plants, housing the critical instrumentation and control system (ICS) panels that are essential for plant operations.
At one such IES building in one of the plants operated by Abu Dhabi Gas Industries Limited (GASCO), frequent tripping of ICS panels was encountered, without pre-alarm indication, leading to plant upsets and posing risk to the safe operations of the plant. Occasionally, the odor of hydrogen sulfide (H2S) was observed. This paper addresses the measures undertaken to identify the source of generation and diffusion of corrosive agents into the IES and remedial measures to eliminate this chronic problem.
A detailed study was carried out that included soil- and water sample collection and specialized laboratory testing, collection and testing of electronic cards, discussion with control-system vendors, and desktop evaluation of data. Analysis of electronic cards identified corrosion of electronic components within the panels as the primary reason for the tripping of panels. Corrosion of electronic components was concluded to be caused mainly by diffusion of H2S into the building, also as confirmed by control-system vendors.
Ingress of corrosive agents through air inlets/doors was eliminated as a cause because of the absence of any signs of corrosion in heating, ventilation, and air-conditioning (HVAC) air intakes and because of maintenance of positive air pressure within the building. Extensive investigation and testing of soil indicated the presence of a high concentration of sulfate-reducing bacteria (SRB) in the vicinity of the building. A high groundwater table containing sulfates provided nutrition to the SRB. The bacteria reduced the sulfates present in the groundwater to H2S. The H2S, thus generated, diffused into the building through cable entry sleeves, reaching the area beneath the false flooring on which panels were mounted, and subsequently entering the panels. The presence of conducive temperature and humidity facilitated the reaction of H2S with electronic cards, thereby causing corrosion and tripping of panels. The key remedial measures that were recommended—namely, dewatering the surrounding area and lowering groundwater levels to starve the bacteria, thus rendering it dormant and preventing H2S generation, and sealing of cable entry sleeves to prevent any external gas ingress into the building—were implemented, resulting in resolution of the issue and maintaining the safety and integrity of plant operations.
Tripping of ICS panels because of H2S generation by bacteria present in soil is a rare phenomenon, not commonly encountered in oil and gas plants. In areas where the groundwater table is high, the construction of buildings needs to be planned carefully. Geotechnical investigation shall include testing for the presence of organic content, sulfates, and SRB in soil and groundwater, in addition to conventional tests.
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