Can a nearby unit restart during maintenance? The S-OIL Onsan explosion
An explosion and fire at S-OIL’s Onsan refinery on 19 May 2022 killed one person and injured nine. The case highlights isolation between operating and maintenance areas and the checks needed before restart.
What happened at Onsan
The explosion occurred at about 20:51 in the alkylate production process at S-OIL’s Onsan refinery in Ulsan, South Korea. Alkylate is a gasoline blending component used to improve octane quality.
One contractor worker died and nine people were injured. Early reporting associated the event with valve maintenance. Initial hypotheses should be distinguished from facts subsequently discussed in court.
Stopping a unit does not establish isolation
Reporting of the first-instance judgment described inadequate gas separation between the maintenance and operating areas and a butane release. It also described insufficient checking before a partial restart instruction.
A stopped unit may still be affected by connected operating equipment. Maintenance planning therefore needs to consider incoming material and changes beyond the immediate work point.
Check your workplace
- Which connected lines and alternative flow paths can affect the maintenance area?
- Who verified the isolation, and how?
- Who stops and reauthorises work when operating conditions change?
Make the permit match the equipment
Operations and maintenance personnel should identify the same equipment, work boundary and isolation points. Match equipment identifiers and the intended opening point against the plant and its drawings. A signature alone does not prove isolation.
The permit needs clear responsibilities, limits, stopping conditions and handover arrangements. If the work scope or operating state changes, reassess the authorisation before continuing.
Does a zero gauge reading prove safety?
HSE’s isolation guidance addresses identification, verification and continued control of isolation. A pressure gauge reading alone is not proof that all pressure has been removed.
Competent personnel must select and verify an isolation appropriate to the substance and equipment. Workers should not loosen a fitting to improvise a pressure test. Suspected leakage or pressure build-up requires stopping work and notifying the responsible person from a safe position.
Hand over completed work before restart
Even a partial restart can affect nearby work. Check for open equipment, people still working and conflicting permits. Maintenance completion and return to operation require a clear handover.
A useful handover identifies the equipment, outstanding work and reinstatement status. Isolation devices and tags remain under the established control procedure; workers do not remove them on their own initiative.
The first-instance court outcome
Reports of the 15 January 2026 first-instance ruling state that the production team leader received an 18-month custodial sentence. The plant manager received a ten-month sentence suspended for two years. Other defendants received convictions or acquittals.
S-OIL was fined KRW 120 million. Busan Ilbo reported that this included safety violations found after the accident. These are South Korean first-instance proceedings, not a statement that the judgment is final.
Putting it into practice
Suppose the control room proposes restarting an adjacent unit while a valve is being maintained. Pause the intrusive work.
The work supervisor and operations representative assess the effect on the maintenance area, recheck isolation and communicate renewed authorisation before work resumes under the site procedure.
Technical insight
Overlay restart and maintenance boundaries
Where restarting plant connects to unfinished maintenance, separate departmental completion decisions can leave an interface unmanaged. Examine routes by which material or energy could reach the work.
Compare the open-work list with the isolation drawing at restart review. This application question does not require assuming an unconfirmed action in the reported incident.
Korean safety resources
- KOSHA GUIDE C-C-2-2025 · Chemical plant maintenance planning
Korean-language PDF on maintenance planning and preparation; technical recommendations.
Sources
- 뉴스핌 · 2022-05-20 사고 및 현장 사진대한민국 / South Korea
- 서울신문 · 2026-01-15 1심 선고 보도대한민국 / South Korea
- 부산일보 · 에쓰오일 온산공장 사고 1심대한민국 / South Korea
- HSE · HSG253 Safe isolation of plant and equipment영국 기술지침 / UK guidance
- HSE · HSG250 Guidance on permit-to-work systems영국 기술지침 / UK guidance