Share this article:
Site Layout Principles 2
Whatever method is used to generate the initial plot plans, once they are generated, major hazard assessments should be carried out on all main, local, and ancillary plots to calculate the plot spacings more accurately. The aim is to prevent a “domino effect”, in which a fire, explosion, or toxic release on one plot impacts upon an adjacent plot or outside the site boundary.
Several computer programs for Quantitative Hazard Analysis (QHA) are available (most notably DNV’s “Phast”) which enable the consequences of a release to be evaluated. Blast overpressure and thermal radiation levels can be calculated and presented as intensity contours, to identify the distances at which, for example, the severity of personal injury is considered “acceptable” and not imposing excessive risk on people. When overlaid on the site plan, these contours show regions where action must be taken to reduce the impact of releases either to on-site staff or facilities or to the public outside the site boundary.
The best method for developing the site layout depends upon the type of project. Three cases are possible:
Case 1: Greenfield – Site Unknown
In this case, the first layout reflects the ‘ideal’ solution if unlimited flat land is available. Extreme (plan view) aspect ratios should be avoided – a value between 1.2 and 1.6:1 should be aimed for.
However, the ‘ideal’ site characteristics will seldom be provided by the real site subsequently selected and modifications will therefore need to be made to the ‘ideal’ layout.
Simple repositioning of some plots may be enough, but more serious changes involving piperacks and roads may be needed. When making major changes, care must be taken to maintain the relationships identified for the ‘ideal’ case, to maintain the functional logic of the layout.
Case 2: Greenfield – Site Known
The ‘ideal’ case does not arise here, so the layout proceeds within the factors and relationships which arise from the conditions imposed by features of the site.
Sometimes, a new site is available but is irregularly shaped with canal, river, and road bridge crossings and even road highway crossings. Sometimes, the site is crossed by a natural watercourse which may require diversion (if allowed by authorities) to accommodate the plant.
Case 3: Brownfield- Existing Site
The new plant must conform to the conditions imposed by the existing site and its constituent plants. Definitions of new process and ancillary plots must be made, taking account of use of facilities provided by existing ancillary plots. Relationships between new and existing plots will be as valid as all other relationships.
Layout proceeds as described above, using existing facilities such as piperacks, roads and utilities in accordance with site development policies, which may direct new plant to specific site areas. Attention is needed to the assessment of hazardous interactions between existing and new plots. The potential for interaction between construction activities and operating plots must also be considered. In all three cases, temporary ‘plots’ needed for construction activities must be considered.
Site layout must consider the major hazards which can cause both inter-plot consequences and off-site effects. The location and nature of sensitive off-site targets, such as housing, schools, hospitals, and leisure facilities have a significant impact on the location of hazardous plots. Equally, the possibility of on-site effects from off-site causes such as gas leaks, vehicle collision, or neighboring hazardous plant must also be investigated.
In either case, remedial action will need to be taken to reduce consequences to acceptable levels. This may include repositioning of plots to increase spacing, or reorienting plots to shield hazard sources or sensitive targets. Layout designs can only reduce major hazard effects by repositioning plots. If satisfactory reduction cannot be achieved in this way, major process changes will be required.
Changes to process plot shapes can make marginal reductions, but this should be done only if no other course of action is open, since this requires a new plot layout to be produced. Ancillary plots are often more tolerant to changes of shape though some, such as boiler plant, are less flexible. If repositioning plots cannot reduce hazards to acceptable levels, more fundamental action must be taken to amend the process conditions or even locate the plant elsewhere on grounds of public safety and acceptability.
If you found this article interesting, please click here to access the entire book, Process Plant Layout, on ScienceDirect. We are pleased to offer you a free chapter from the book called “Plot Layout Principles,” below:
About the book
The second edition of Process Plant Layout explains the methodologies used by professional designers to layout process equipment and pipework, plots, plants, sites, and their corresponding environmental features in a safe, economical way. It is supported with tables of separation distances, rules of thumb, and codes of practice and standards. The book includes more than seventy-five case studies on what can go wrong when layout is not properly considered. Sean Moran has thoroughly rewritten and re-illustrated this book to reflect advances in technology and best practices, for example, changes in how designers balance layout density with cost, operability, and safety considerations
About the author
Professor Sean Moran is a Chartered Chemical Engineer with over twenty years’ experience in process design, commissioning and troubleshooting and is regarded as the ‘voice of chemical engineering’. He started his career with international process engineering contractors and worked worldwide on water treatment projects before setting up his own consultancy in 1996, specializing in process and hydraulic design, commissioning and troubleshooting of industrial effluent and water treatment plants.
Whilst Associate Professor at the University of Nottingham, he co-ordinated the design teaching program for chemical engineering students. Professor Moran’s university work focused on increasing industrial relevance in teaching, with a particular emphasis on process design, safety and employability.
Sean’s latest books are available to order on the Elsevier Store. Use discount code STC317 at checkout and save up to 30% on your very own copy!
Most of the major scientific challenges of the 21st century — including sustainable energy resources, water quality issues, and process efficiency in the biotechnology and pharmaceutical industries — revolve around chemical engineering. Elsevier’s broad content in this area examines topics such as bioprocessing, polymer nano-composites, biomass gasification and pyrolysis, computational fluid dynamics, industrial proteins, catalysis, and many others with great significance and applicability to researchers today. Our books, eBooks, and online tools provide foundational information to students, and cutting-edge coverage to advance corporate research and development. Learn more about our Chemical Engineering books here.