{"id":2708,"date":"2026-09-27T21:23:36","date_gmt":"2026-09-27T21:23:36","guid":{"rendered":"https:\/\/petrostreet.com\/main\/?p=2708"},"modified":"2026-09-27T21:23:42","modified_gmt":"2026-09-27T21:23:42","slug":"incident-investigation-report-pilot-operated-pressure-safety-valve-premature-opening-on-atmospheric-storage-tank","status":"publish","type":"post","link":"https:\/\/petrostreet.com\/main\/incident-investigation-report-pilot-operated-pressure-safety-valve-premature-opening-on-atmospheric-storage-tank\/","title":{"rendered":"Incident Investigation Report: Pilot-Operated Pressure Safety Valve Premature Opening on Atmospheric Storage Tank"},"content":{"rendered":"\n<h4 class=\"wp-block-heading\">Executive Summary<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">An unexpected process safety event occurred at an industrial facility located in a coastal marine environment, involving an atmospheric storage tank equipped with a pilot-operated pressure safety valve (POPSV). The safety valve experienced an uncommanded and premature opening during normal plant operations while the actual system operating pressure was maintained normally in the process. However, subsequent workshop testing and investigation revealed a critical nuance: once the valve was removed and brought to the workshop, it was found to be popping open at virtually 0 pressure.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A multidisciplinary root cause analysis (RCA) and technical investigation were immediately initiated. The investigation revealed that the primary failure mechanism was not within the primary pressure-sensing line or the main valve seat itself, but rather a catastrophic mechanical failure of a Non-Return Valve (NRV) positioned within the pneumatic control circuit of the pilot assembly. The NRV, constructed from aluminum alloy, suffered severe galvanic and atmospheric pitting corrosion due to continuous exposure to a high-chloride, marine saltwater-laden atmosphere. The resulting internal blockage entirely choked the control media flow, preventing the transmission of reference pressure into the dome area of the POPSV. Because the dome pressure required to create a net downward seating force was completely lost due to this zero flow\/pressure condition in the dome chamber, the main valve lost its mechanical seating bias and lifted prematurely. This technical report details the sequence of events, metallurgical findings, root causes, and definitive corrective and preventive actions (CAPA) implemented to prevent recurrence.<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">Plant Overview and Equipment Description<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">The incident occurred within a tank farm facility situated directly adjacent to a marine shoreline, subjecting all external instrumentation, control tubing, and safety accessories to severe atmospheric corrosion stressors. The equipment involved is a large vertical fixed-roof storage tank containing volatile hydrocarbon products. Due to the volatility of the stored media and environmental regulations, the tank is operated under low-pressure containing finished product hydrocarbon gas and protected against overpressure and excessive vacuum by a dedicated Pilot-Operated Pressure Safety Valve (POPSV) assembly.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Unlike direct-acting spring-loaded relief valves, the POPSV relies on a sophisticated control circuit. It consists of:<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>The Main Valve:<\/strong>\u00a0Featuring a large sliding piston or diaphragm held tightly against the nozzle seat by system pressure acting on top of the dome area.<\/li>\n\n\n\n<li><strong>The Pilot Valve:<\/strong>\u00a0A smaller, highly sensitive spring-loaded pressure-sensing valve that regulates the loading pressure inside the main valve dome. Under normal operating conditions, system pressure is directed via the control tubing into the dome, creating a net downward seating force that keeps the valve tightly closed, even at operating pressures close to the set point.<\/li>\n\n\n\n<li><strong>Control Tubing and Accessories:<\/strong>\u00a0Stainless steel or auxiliary tubing lines equipped with inline filters, restrictors, and Non-Return Valves (NRVs) designed to maintain directional control signals, prevent reverse flow, and damp pressure pulsations.<\/li>\n<\/ol>\n\n\n\n<p class=\"wp-block-paragraph\">In this specific installation, an aluminum-bodied Non-Return Valve had been historically integrated into the auxiliary sensing\/loading loop to prevent backflow during transient pressure fluctuations.<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">Incident Description and Chronology of Events<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">The incident unfolded during routine operational monitoring when field hydrocarbon alarms indicated an abrupt, unprompted opening cycle of the storage tank&#8217;s POPSV.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>T-00:03 (Normal Operations):<\/strong>\u00a0The storage tank was operating within its normal operating range. The POPSV remained securely seated, and all supervisory control and data acquisition (SCADA) parameters were within nominal ranges. System pressure was actively present in the process header.<\/li>\n\n\n\n<li><strong>T-00:08 (The Event):<\/strong>\u00a0Without any operational upset, liquid transfer, or thermal surge\u2014and while active system operating pressure was being maintained\u2014the POPSV main valve suddenly snapped open with an audible discharge of blanket gas.<\/li>\n\n\n\n<li><strong>T-00:14 (Immediate Response):<\/strong>\u00a0Operations personnel responded to the field. Local pressure indicators confirmed that the system pressure was normal and healthy. Despite this healthy process pressure, the pilot exhaust port was actively venting, and the main valve remained in the open position.<\/li>\n\n\n\n<li><strong>T-00:41 (Isolation and Safe-Guarding):<\/strong>\u00a0Maintenance teams isolated the affected POPSV by closing the upstream block valve to prevent continuous loss of the hydrocarbon. A temporary replacement safety valve was installed, and the failed POPSV assembly was transported to the metallurgical and instrumentation workshop for rigorous forensic examination.<\/li>\n\n\n\n<li><strong>Workshop Bench Findings:<\/strong>\u00a0Upon setting up the removed POPSV on the test bench, technicians discovered a critical diagnostic anomaly: the valve was popping open at essentially 0 pressure, failing completely to retain any loading pressure in the dome chamber.<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">Technical Investigation and Forensic Findings<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">To uncover the underlying cause of the POPSV lifting while system pressure was normal\u2014and subsequently popping at 0 pressure on the test bench\u2014the safety valve assembly, pilot mechanism, and all associated pneumatic control components were systematically disassembled, cleaned, and examined under laboratory conditions.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">1. Visual and Dimensional Inspection of the POPSV Assembly<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">The main valve body, elastomer seals, and spring mechanisms were found to be free of mechanical wear, debris, or polymeric degradation. The main disc and seat showed no signs of wire-drawing or mechanical erosion. However, bench testing of the pilot valve assembly revealed that it failed to properly trap or maintain pressure inside the dome chamber, causing the valve to actuate at zero differential pressure.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">2. Discovery of the Control Circuit Blockage and Dome Pressure Loss<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Attention shifted to the auxiliary pneumatic control loop, specifically tracking the flow path from the main process tap through the filter and check valves into the pilot dome. Disassembly of the auxiliary line revealed a severe flow restriction across the inline Non-Return Valve (NRV). While the rest of the stainless steel tubing network was pristine, the NRV body exhibited heavy white powdery oxidation deposits and deep pitting across its outer casing and internal moving components.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">3. Metallurgical Failure Analysis of the NRV<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Further metallurgical evaluation of the failed NRV yielded critical insights into the root cause:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Material Mismatch and Selection Error:<\/strong>\u00a0The failed NRV housing and internal poppet were manufactured from an unalloyed or poorly treated aluminum grade, selected during a historical maintenance replacement due to immediate stock availability rather than engineered chemical compatibility.<\/li>\n\n\n\n<li><strong>Marine Saltwater Atmospheric Attack:<\/strong>\u00a0Due to the facility\u2019s immediate proximity to the coastline, marine air carrying high concentrations of airborne sodium chloride (salt spray) continuously enveloped the external surfaces of the exposed control accessories.<\/li>\n\n\n\n<li><strong>Galvanic and Pitting Corrosion:<\/strong>\u00a0Aluminum in a marine environment is highly susceptible to localized pitting and galvanic corrosion when exposed to moisture and salt deposits, especially if coupled with stainless steel fittings without adequate dielectric isolation or protective coating.<\/li>\n\n\n\n<li><strong>Internal Choking and Loss of Dome Pressure:<\/strong>\u00a0Corrosion byproducts (aluminum hydroxide and white rust) migrated into the internal cavity of the NRV. The volumetric expansion of these corrosion oxides jammed the internal check poppet in a stuck closed position. This completely blocked the flow of process gas through the auxiliary line. Consequently, <strong>no flow could reach the dome area of the POPSV, resulting in a total loss of pressure in the dome chamber<\/strong>. Without this essential pressure acting on top of the main piston to keep the valve closed, the valve lost its seating force and lifted prematurely despite healthy system pressure. When brought to the workshop, this lack of internal holding pressure manifested as the valve popping at 0 pressure.<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">Root Cause Analysis (RCA)<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">Using the Fishbone (Ishikawa) diagram and 5-Why analysis methodologies, the investigation team traced the failure path to a combination of material selection oversight, environmental degradation, and maintenance control gaps:<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Why did the POPSV lift during normal operations and pop at 0 pressure in the workshop?<\/strong>\u00a0Because the dome pressure required to hold the main valve closed was completely lost, removing the downward seating force and causing the valve to actuate prematurely.<\/li>\n\n\n\n<li><strong>Why was there a total loss of pressure in the dome area of the POPSV?<\/strong>\u00a0Because flow through the auxiliary control circuit was entirely blocked by a choked Non-Return Valve in the sensing\/loading line, preventing process media from entering the dome chamber.<\/li>\n\n\n\n<li><strong>Why did the Non-Return Valve become choked?<\/strong>\u00a0Because severe atmospheric corrosion generated massive internal oxide deposits that jammed the check valve poppet in the closed position.<\/li>\n\n\n\n<li><strong>Why did the NRV suffer such severe atmospheric corrosion?<\/strong>\u00a0Because the component was fabricated from aluminum, which has extremely poor resistance to unmitigated coastal saltwater environments when unprotected.<\/li>\n\n\n\n<li><strong>Why was an aluminum NRV installed in a severe marine environment?<\/strong>\u00a0A historic maintenance replacement utilized an incorrect bill-of-materials (BOM) substitute part from general warehouse stock, bypassing stringent material verification protocols for corrosive coastal environments.<\/li>\n<\/ol>\n\n\n\n<h4 class=\"wp-block-heading\">Impact Assessment<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Safety and Environmental:<\/strong>\u00a0The uncommanded opening of the safety valve resulted in the heavy leak of hydrocarbon vapors. Although no ignition source was present and no fire or explosion occurred, the risk of ignition of a flammable mixture range was critically high.<\/li>\n\n\n\n<li><strong>Economic and Operational:<\/strong>\u00a0Loss of valuable finished product gas, unbudgeted downtime for emergency valve replacement, and the expenditure of engineering resources for the forensic investigation.<\/li>\n\n\n\n<li><strong>Asset Integrity:<\/strong>\u00a0Highlighted a latent vulnerability across other identical tank installations where non-metallic or corrosive-prone materials might have been inadvertently utilized in auxiliary instrument tubing lines.<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">Corrective and Preventive Actions (CAPA)<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">To ensure that a similar incident never recurs across the facility, the following comprehensive corrective and preventive measures have been formulated, approved, and enacted:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><tbody><tr><td><strong>No.<\/strong><strong><\/strong><\/td><td><strong>Corrective \/ Preventive Action<\/strong><strong><\/strong><\/td><td><strong>Target Implementation<\/strong><strong><\/strong><\/td><td><strong>Responsibility<\/strong><strong><\/strong><\/td><td><strong>Status<\/strong><strong><\/strong><\/td><\/tr><tr><td><strong>1<\/strong><\/td><td><strong>Immediate Replacement:<\/strong>&nbsp;Scrap the failed aluminum NRV and replace the entire POPSV control tubing assembly with marine-grade 316L stainless steel components.<\/td><td>Immediate<\/td><td>Maintenance \/ Workshop<\/td><td>Completed<\/td><\/tr><tr><td><strong>2<\/strong><\/td><td><strong>Material Specification Standardization:<\/strong>&nbsp;Update engineering standards to explicitly forbid the use of aluminum or non-alloyed components in all external instrument loops, tubing, and safety valve accessories within 5 kilometers of the coastline.<\/td><td>30 Days<\/td><td>Materials Engineering<\/td><td>In Progress<\/td><\/tr><tr><td><strong>3<\/strong><\/td><td><strong>Facility-Wide Audit:<\/strong>&nbsp;Conduct a comprehensive audit of all safety relief valves (PSVs and POPSVs) across all storage tanks to verify the material metallurgy of inline filters, check valves, and pilot tubing.<\/td><td>60 Days<\/td><td>Inspection \/ Integrity Team<\/td><td>Scheduled<\/td><\/tr><tr><td><strong>4<\/strong><\/td><td><strong>Warehouse Inventory Purge:<\/strong>&nbsp;Quarantine and remove all non-compliant aluminum check valves from local plant warehouses to prevent future erroneous issuances.<\/td><td>15 Days<\/td><td>Supply Chain \/ Warehouse<\/td><td>Completed<\/td><\/tr><tr><td><strong>5<\/strong><\/td><td><strong>Preventive Maintenance (PM) Enhancement:<\/strong>&nbsp;Incorporate external visual inspections for corrosion and functional proof-testing of POPSV pilot control loops into the annual statutory turnaround (TA) schedule.<\/td><td>45 Days<\/td><td>Reliability Engineering<\/td><td>In Progress<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h4 class=\"wp-block-heading\">Conclusion<\/h4>\n\n\n\n<p class=\"wp-block-paragraph\">The premature lifting of the pilot-operated pressure safety valve during active system pressure\u2014and its subsequent behavior of popping at 0 pressure in the workshop\u2014was a classic systemic failure originating from an improper material selection compounded by aggressive coastal atmospheric conditions. The use of an aluminum Non-Return Valve in a marine saltwater environment led to severe localized corrosion and internal flow choking. This restriction resulted in a complete loss of pressure in the dome area of the POPSV required to keep the valve closed, causing uncommanded actuation.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This incident underscores the critical importance of rigorous material traceability, management of change (MOC) for spare parts replacement, and specialized engineering design considerations for equipment operating in severe coastal microclimates. Through the rigorous implementation of the identified CAPA items, the facility has significantly enhanced its safety posture, equipment reliability, and operational integrity against similar corrosion-induced process upsets.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Executive Summary An unexpected process safety event occurred at an industrial facility located in a coastal marine environment, involving an [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":2709,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_uag_custom_page_level_css":"","_ppp_document_settings_meta":"{\"product_ids\":[{\"label\":\"Incident Investigation Report: Pilot-Operated Pressure Safety Valve Premature Opening on Atmospheric Storage Tank - 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