While process plant layout is a critical aspect of chemical process industries (CPI) operations, the majority of the seminal works in this area have been published in trade journals [ 1–10] or classic texts such as Perry's Handbook [ 11]. Too often, process plant layout is covered in only a cursory fashion in the engineering curricula; as a result, much of the knowledge of how to lay out process plants resides in the heads of engineers who are nearing the end of their careers. This article was developed from a recently updated book on process plant layout by the author [ 12].
Good plant layout is as important as ever today. A recent study by Kidam and Herme [ 13] showed that 79% of process plant accidents involved a design error, and the most common type of design error leading to accidents was poor layout, as shown in Figure 1.
This article reviews the common terms and discusses the basic methodology for sound plant layout.
On a new "greenfield" site, the layout design will need to reflect the known needs of the process plant or process units to be constructed. Alternatively, a plant may be placed on a number of plots on an existing "brownfield" site.
In the latter case, it is a common scenario that requirements of the newer plant may not have been foreseen at the time of the original site layout. As a result, at least some of the access arrangements that would normally be provided on a new site will have to be provided post hoc by the layout designers. Existing access arrangements may need to be reconsidered to suit the evolving inter-relationships between the existing site and the new plant or equipment.
There is widespread disagreement about what the terms site, plot and equipment mean. This article attempts to standardize the description and use of these terms to avoid confusion, following the naming convention set forth in Ref. 12.
This article, and the book on which it is based [ 12], do not use the term "plant layout" for any of these activities, as it is loosely used to describe several of them individually and several combinations of them. Rather, we define a "plant" as being the whole collection of equipment necessary to make a product.
In traditional chemical process plants, an ideal site would be split up into individual plots by its principal road system, with additional access roads provided for the larger plots. However, in many sectors, plants may not be big enough to have such a road system. A complete set of individual process units (known as a plant) may fit onto a single plot, although larger plants may need two or more plots, and a site may contain a number of plots.
Piping layout. Within the discipline of layout design, a distinction is commonly made between piping layout (defined as "the layout of piping and associated support systems…") and equipment layout (defined as "layout at the level of a single process unit and associated ancillaries"). Both of these disciplines are often referred to colloquially as plant layout.
Initially, plot layout involves mainly equipment layout, and piping layout comes in later, at the detailed design stage. Plot layout often occurs in the context of a "masterplan," that aims to define the site's overall design intent, especially if architects are involved. In such cases, it may be important to follow the architect's approach to site and operational layout through the use of masterplanning (an approach outlined in Ref. 12, Appendix E).
For any given project, several of these approaches are often combined in various ways in different sectors, and such combinations vary among layout designers of different disciplines.
A formal technique is any logical method that provides definitive information on relationships between items or numerical data on spacing distances. It must be based on a procedure that is adequately defined and recorded and can be examined and criticized.
Before starting a layout, the relevant information should ideally be assembled. Such information typically includes process and site data, regulatory and contract requirements, company and other recognized codes of practice. Often, not all such data are available at the start of a project. To avoid delays and to provide a starting point, it is useful to have information on typical spacings. However, it is emphasized that such spacings are rough and must be confirmed or replaced later by the proper project data or design.
A first layout is almost always based on process flow. Intuition drawn from experience indicates that such a layout is basically a good one and can be altered successfully to accommodate the specific requirements of operation, maintenance and safety. The intuition (and experience) of the engineering design team usually indicates immediately what the principal alterations to this default case should be. Thereafter, the formalized methods shown in the bulleted list above should be used to finetune and improve the preliminary design that was put together based on the initial intuitive approach.
Historically, formal layout methods, mainly developed within the CPI, have tended toward optimization for minimum capital cost. However, the impetus for developing formal layout methods was generated by the changing attitude of society toward the CPI, and to the consequences of accidents in CPI operations.
Although safety has always been a major constraint in plant layout, its most visible effect on the layout was typically related to relatively simple rules for spacing and electrical zoning in accordance with codes of practice. The adoption of more dangerous processes, the increasing scale of plants and associated chemical storage, and the shortage of skilled staff, coupled with greater public concern, have required companies to be able to justify the reasons for selection of a given layout to a far greater extent than was necessary in the past. Today, engineering teams are required to develop and maintain records of potential problems, alternatives that were considered (with supporting data) to justify design decisions to satisfy prevailing legislation and to support a legal defense in the event that problems arise later.
There are still very few formal layout techniques available to the designer, and none can completely replace the designer's abilities either to conceive new solutions or to evaluate alternatives. As with all engineering, plants built without experience and intuition are "bad plants." The intuitive approach is tested and corrected as the result of using other formal techniques along with more experience. Computer-aided techniques can supplement but not replace engineering by experts.
When used to supplement or verify the designer's experience, the formal techniques available today are able to improve the layout and provide rational justification of layout decisions.
Ref. 12 discusses ten formal approaches to plant layout that are widely followed by chemical engineers, piping designers and process architects in different CPI sectors. The approach discussed below is a modified Mecklenburgh method [ 15], adapted from that given by Mecklenburgh in the first edition Ref. 14.
However, existing brownfield sites (with their pre-existing conditions) will impose particular constraints, so some of the following site-layout steps may not be needed.
The layout methods described below suggest a highly formalized, structured, rigorous and frankly, expensive process. The version of the layout process suggested here is typically only appropriate to the largest process plants. Professional judgment is required to understand how much of this approach is appropriate, given the site-specific requirements and constraints of the project being considered.
Step 1. Plot data. The data needed at this step include preliminary process flow diagrams (PFDs) and piping and instrumentation diagrams (P&IDs), which must show the size of major pipework and suggested elevations of major equipment), process engineering design for the equipment (such as size and shape), the results of preliminary hazard assessments of the flowsheet, and the codes of practice to be followed in the plant design.
Step 2. Plot layout. The layout is made using the data gathered in Step 1, in the sequence of the process flow using the experience of the engineer to recognize constraints, such as major piping and cabling. Typical layout spacings (described in Ref. 12, Appendix C) are useful at this stage. Simple drawings and cutouts are typically employed.
Step 3. Elevation. The elevation assumptions in the flowsheet should be questioned. This enables the process objectives and constraints on elevation to be defined. Various alternative elevation arrangements are generated, possibly by using formal techniques, such as travel and correlation charts.
The cost of each potential elevation alternative is examined, primarily for differences between, for example, the number of plant items needed to achieve the objective, or differences in the material-transfer costs, such as piping, pumping required to elevate items and power consumption.
Simple elevation drawings can be prepared showing only heights and relative positions of items, but structure and floor levels are not introduced at this point.
Step 4. Plot plan. Plant items, buildings and principal pipe and cable runs are laid out in a plan, to ensure that the obvious layout constraints (operation, maintenance, construction, environmental, safety and drainage areas) are accommodated. Cutouts are helpful at this stage. A cost assessment is made of each competing arrangement being considered. The more promising arrangements may be optimized to produce even more economical layouts.
Step 5. Plot buildings. Housing CPI plants inside buildings is more expensive than having plants in the open, even for plants on elevated structures. The need for enclosed buildings specified in the process design should therefore be examined critically.
Step 6. Plot layout. The selected plan and elevation layouts are now combined with building studies to determine possible positions of support and access structures, and to study civil requirements (such as foundations). These may force relaxation of earlier constraints. The layout alternatives are usually presented as 2-D drawings, though 3-D computer models may also be used (Figure 3).
These models will help both the layout designer and other disciplines to visualize functional and safety aspects. Consequently, it is useful at this stage to have brief and mainly intuitive reviews of the layout be carried out by the various disciplines.
Cost evaluations are carried out again or finetuned for the acceptable layouts and a short list of particular layout arrangements is developed and recorded as plot plans (ideally just one selection is chosen, but in some cases, competing options are still on the short list).
Step 7. Hazard assessment of plot layout. Areas within the plot where loss of containment can occur must be identified, and the amount of materials that could potentially be lost must be quantified by analyzing various potential hazard scenarios. The consequences of each loss with respect to explosion, fire or toxicity should be calculated.
Within the plant, these calculations will indicate separation distances between potential sources of ignition and sources of leaks and will specify the various hazard zones for electrical equipment and fired equipment. The safe positioning and protection of control rooms will also be calculated. These calculations are also essential to assess and predict the potential losses that could occur at various distances outside the plot, with regard to the danger to people, equipment and buildings from fire, explosion and toxicity. The layout may well have to be adjusted.
The Mond Index method [ 15] may be used prior to (but not instead of) the above assessments.
Step 8. Layout of piping and other connections. The principal piping and pipe routes are confirmed during this step. Principal electrical mains routes are also checked. Various connecting arrangements are considered and the most promising ones are further optimized. Piping models can be used as aids and computerized versions can be used to support optimization efforts. The best layout arrangement should now be selected and recorded.
Step 9. Critical examination of plot layout. The proposed arrangement should satisfy all of the obvious requirements in light of all the information available. It should be examined formally by the various disciplines to make sure less obvious features (discussed below) have not been omitted.
3-D models can be good aids to the review process, though they are expensive to produce. Check lists to assist with this process can be found in Ref. 12.
The results of the critical examinations carried out by the various disciplines, and the results of the hazard assessment, must be reconciled. Carrying out such a multi-disciplinary consultation is essential.
In some cases, it may be found that the layout is impractical or even impossible. When this occurs, it will be necessary to rethink the process design or even undertake further laboratory and other development work. In most cases, though, the results of the critical examination will mean adjusting the layout by means of further iteration from Step 2 onward.
Step 10. Site data. Steps 1–9 will be carried out for each separate plant and storage area within the proposed site. This will provide the size and shape of each plot, will help to define access requirements (for vehicles and people during construction, operation, maintenance and emergencies), and will provide an approximate evaluation of the separation needed around each plant to ensure proper hazard containment.
Using the process data of the various plants, the layout engineer should compile information related to the site materials and utilities PFDs, pedestrian and vehicular traffic capacities (for both internal and external movements), size and shape of the plots, buildings, and required utilities, central services and amenities.
Step 11. Site layout. The PFD for the site allows the various processes to be positioned relative to one another. The flow pattern may be modified in order to isolate hazardous processes and to accommodate the proposed rail and road entry points or wharf positions.
Next, services, such as the boiler house and effluent plant, are added in the most convenient positions, subject to the provision that they are not likely to be put out of action by a disaster. The central buildings are placed so that the distances traveled by personnel who use them are minimized, providing that these buildings are in safe places.
After this, the road and rail systems are marked in greater detail. Try to keep the various types of traffic segregated as far as is possible and desirable. There should be access from at least two directions to all parts of the site, to allow for emergencies, as shown in in Figure 4.
The size of the site is determined from the area of individual plants, storage areas and central buildings plus the clearances between the plants. It is also necessary to allow ample space for things like parking, loading and unloading, stores and firefighting water storage. Typical clearances, size and areas are given in Ref. 12. During the development of this layout, allowance must be made for future plant expansion and for general construction and other access considerations.
It is essential to establish, within the overall layout, all the important positional relationships between elements of the layout that must be maintained. This consideration is crucial because available sites may not conform in shape or topography to the preliminary layout and some compromises and amendments to the layout are almost certain to be needed. In light of this, it must be known which relationships can be relaxed to fit the plant in the available space.
Simple drawings and paper cutouts are very useful for site-layout development. Physical and 3-D computer models may also be used especially to gauge the visual impact.
Step 12. Hazard assessment of site layout. The plots on the site where loss of containment can possibly occur are noted. Vulnerable parts of the site are listed, such as offices, central utilities, key commercial plants and the site boundary (representing the start of the public domain).
The consequences of each loss through fire, explosion or toxicity on the vulnerable items are calculated. The layout is adjusted so that the consequences become acceptable; in particular, the chance of escalation of an occurrence throughout the site (via the "domino effect") is made less likely. Ref. 12 goes into further detail on this topic.
Step 13. Site-layout optimization. When there are feasible alternative arrangements, cost estimations should be developed for each with respect to transport and piping connections between the various plots and so on. The most economical layout can then be subject to further optimization of the plot spacings, subject to the hazard constraints.
Additional points are included in the checklist in Ref. 12. In the worst case, it may become obvious during the review that one or more of the proposed plants is unacceptable, but in most cases, the review will result in adjustments to the site layout, iterating from Step 11 onward.
These factors, plus the others outlined in Ref. 12, will guide the selection of a suitable site.
It is likely that while the site is being selected and purchased, further process and engineering design and market research work has been undertaken on the individual plants and their products. The plot layouts could have been updated and the further information, relevant to the site layout, made available.
The hazard assessment can now take account of known vulnerable features outside the site boundary. The critical examination will, in addition to the items given in Step 14, also check that site constraints and standards have not been violated. Extensive consultation will be made with the various regulatory and emergency authorities during the detailed layout stage.
The final site plan will show the roads, railways, site pipe routes, sewers, central buildings and services. It will ideally be produced in the form of, and with the aid of, detailed drawings and possibly models, whether computer,nmanual or both.
Step 19. Plot layout. Most of the initial steps, particularly Step 4, and 6–9 are repeated in greater detail and subjected to the site constraints given in Step 18.
It is important that, in repeating Step 6, there is good coordination between the layout, process, operating, piping, civil, structural and mechanical disciplines. The piping arrangement studies (repeat of Step 8) done here are discussed in more detail in Ref. 12.
The hazard reassessment (Step 9) will be mainly concerned with internal plant spacings, such as area hazard classification zones and control room and other plant building positions. Inter-plot spacings are considered in hazard assessment of the site layout. The repeat of the critical examination should (as well as considering the aspects given in Step 9) also see that standards, regulations and site constraints on the plot have not been violated.
The approach discussed here is a formalized summary of the steps that are typically taken by chemical engineers on large or dangerous plants. Less formal approaches are more commonly used in practice, however, for reasons of economy and practicality. Critical examination in particular is a resource-hungry exercise.
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