Changes in Blast Scenarios Due to Plant Expansions – Impact on Buildings

Abstract

Plant buildings that housing critical equipment and/or manned are generally sited at safe distances from potential explosion sources of process facilities or designed considering anticipated blast pressures during initial construction. However, addition of new process facilities during plant expansion generates new potential explosion sources, blast overpressure range of which exposes some existing facilities to new risk. This paper discusses the approaches followed to assess risks and mitigation measures employed to ensure safety of occupants and integrity of assets. Quantitative Risk Assessment forms the basis for blast considerations on buildings, wherein potential Vapor Cloud Explosion scenarios are identified, and blast overpressure are quantified in the form of contours on plot. Buildings located within blast contours are identified for blast-resistant design to ensure occupant's safety and facilitate safe shutdown of process units during explosion. Whenever Plant expansion is envisaged, a fresh QRA is carried out with potential explosion scenarios. For the explosion analysis, a detailed computer model of plant is generated to closely simulate the congested vulnerable sites and accurately predict blast overpressure, thus optimizing the impact on existing buildings. Based on outcome of the analysis, existing buildings requiring additional blast protection are recognized. In new set-up, process related buildings such as control rooms, OMS, IES etc. are located closer to process area and non-process buildings are sited away from process area. Buildings are designed to resist normal loads along with/or without estimated blast loads based on its location, occupancy and function. During plant expansions, due to space/operational constraints, some new process facilities get closer to the existing buildings. This imposes increased/additional blast load and causes risks to occupants, equipment and building integrity. Hence a comprehensive assessment needs to be carried out and mitigation measures explored. A study was done to assess the impact of such a scenario in one of the gas plants. The study found that some buildings which were originally designed for normal loads are now being subjected to certain intensity of blast loads. The criticality of the building was assessed considering the occupancy level and functional requirement of buildings. The inherent capability of buildings to withstand these additional loads were also evaluated. Based on the study outcome, different mitigation measures such as reducing occupancy, relocating critical items, retrofitting of structure, etc., were explored and feasible options recommended. Assessment of existing buildings for blast overpressure has gained importance in recent years due to steady expansion of old plants. The paper presents the approach followed in the study for such situations and effective measures that can be taken without compromising the safety of the personnel. These requirements are common for similar expansion projects and can be adopted across the industry.