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Site Layout Principles 1
Plant layout proceeds by iteration, but we must start the loop somewhere. Let’s start with whole-site level layout. The purpose of a good site layout is to provide a safe and economical flow of materials and people around a plant which is socially acceptable to workers and neighbors to produce a specified product reliably and cost-effectively.
Potential worst-case safety and environmental scenarios must be foreseen, and plans made for control at source of any potential problems which cannot be eliminated. These plans must ensure that any fire or release of hazardous materials can be safely and rapidly contained and controlled.
From commencement of the design, especially for new development on new land (‘greenfield’ sites), the implications of the proposed location for the design should be considered. Latitude, proximity to highways and residential areas, access points, topography, geotechnical considerations, (including seismic concerns, noise, and vibration sources), and biodiversity may all be factors.
Consideration must be given not only to the initial development, but also to any near future uses. The layout of the chosen site therefore needs to consider the possibility of future development and expansion. In the pharmaceutical sector, plants are often designed to be multipurpose. Early use of a Site Masterplan to consider possible future products can be very important in such cases.
For an existing site, the layout characteristics are, for better or worse, already established. Nevertheless, care must be taken to see that the layout of an extension does not violate the separation standards of the original layout and the flexibility of adjacent pre-existing plant or buildings. This is a common failing of many plant and building retrofits and extensions, so it may be worthwhile to consider whether a Masterplan would significantly aid planned site development or redevelopment.
The initial layout is usually based on the processes being positioned relative to one another based on the process flow diagram. The Illustration above shows a simple drawing which sets out the position of plant items and processes relative to one another. It should be noted that this is a simple general arrangement drawing based on a PFD, rather than being a PFD. PFDs do not show scale or structure, but provide a widely-understood representation in a simplified format of a plant’s unit operations and process flows. Since a PFD does not indicate any constraints on layout, an initial layout based upon it will always need to be modified to fit site and process conditions.
To ensure that a site’s physical constraints are understood alongside those of the process, there is a need to accommodate the needs of plant construction and operation. Such constraints are illustrated and considered by layout designers using 2D general arrangement drawings or plot plans, as well as 3D models in some cases. The initial layout is reviewed, then altered on the plot plan to accommodate the various constraints.
The objective of site layout is to provide a safe, stable platform for production over the design life of plants on site (a period usually measured in decades). It is therefore ideal that both current and future site plans are defined and built in to the layout from the start. Engineering does not, however, always conform with the ideal.
Site layout aims to make the best use of all features of the site and its environs, such as site topography, ground characteristics, water-courses, drainage, and climate. External facilities such as water, gas, electricity supplies, effluent disposal services, and facilities for transport of people and goods will also need to be considered.
Where the development of the site impacts on both local resources in the built and natural environment, these impacts must be socially acceptable. Groundwater and air quality are often key concerns with respect to the natural environment. Information on initial conditions under these headings is therefore needed at the earliest stages of site layout.
Layout designers will also need process and production information both for any existing plant, the proposed plant and, ideally, for future site developments. Core processes and materials need to be defined, as well as requirements for utilities, effluent, and ancillaries. Transport methods and volumes for materials and manpower also need to be established.
If the site has already been selected, the required information about site topography and infrastructure will be available. If the site is still to be purchased, an “ideal site” model of the most desirable site features may be constructed. This can form a guide for the evaluation of alternative potential sites during the search and selection process.
Site layout is entirely concerned with assembly of the plots into their optimal arrangement on the site. Dividing the plant into process plots, identifying the ancillary plots and making a rough layout of each plot to establish its size, shape and connection points are essential preliminaries to site layout.
Sizes of process plots depend on the plot layout considerations outlined above. The plots can then be treated as super-item entities and their inter-plot relationships identified ready for forming the plots into groups for layout.
Inter-plot relationships arise mainly from process inter-connection, or common utility and ground loading requirements. Other relationships will arise through the flow of vehicles and people through the site and between plots; and from site major hazard assessment policies.
Normally, these relationships are considered on an informal intuitive basis by very experienced designers and the layout finalized. It is uncommon for formal analysis to be required. However, to help identify all these relationships, diagrams of the connectivities between plots from flows of utilities, vehicles, and people can be made and the connection strengths estimated for the most important cases:
- Material and utility flows (under routine, startup, shutdown, and crisis conditions)
- Vehicles (under weekday, night, weekend, and weather disrupted conditions)
- Fire and emergency conditions
- People (under shift operation, daytime, shift change, day staff on/off, meal breaks, and evacuation conditions.)
This procedure may be particularly helpful for less experienced layout designers, or experienced designers dealing with very complex patterns of interconnection.
Material and utility flows will define process plots. Some process plots will also be strongly related to ancillary/utility plots by utility flows. These relationships can be used to help formulate the pattern of piperacks on the site.
Vehicle and people movements will form weaker relationships between all plots. Strong vehicle relationships will be created by loading bays, tank farms and access to and from a site gate house.
Fire stations, ambulance and medical centers have important relationships to all plots which must be checked. Similarly, strong people relationships will arise between all plots and canteens, or areas for bus stops, and between main offices and the site entrance. All these relationships will give a picture of site road patterns which can be superimposed on to the plot groups to yield a first layout concept.
Once the conceptual site layout design is agreed, plots can be spaced. An accepted initial value in traditional chemical industries is 15 m between main process plots or between main process and ancillary plots. Spacing between local sub-plots within a main plot is less critical and is often set by road or piperack widths. Spacing between ancillary plots is less critical and often the width of roads or piperacks is sufficient.
This is however just the first stage of site level layout. Next we must consider the process safety implications of our initial layout.
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 “Site Layout Principles.”
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!
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