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Process and Plant Design Layout
Plant Layout has more or less disappeared from university courses in chemical engineering, but it has not disappeared from professional practice. An Applied Guide to Process and Plant Design aims to plug the gap.
“We need to consider layout in sufficient detail from the very start of the design process. Even at the earliest ages of design, attempting to lay plant out will throw up practical difficulties.
Layout is not just a question of making the plant look pretty from the air (or more usefully from public vantage points). The relative positions of items and access routes are crucial for plant operability and maintainability, and there are more detailed site specific considerations.
The three key elements which have to be balanced in plant layout are cost, safety and robustness. Thus, wide plant spacings for safety increase cost and may interfere with process robustness. Minor process changes may have major layout or cost consequences. Cost restrictions may compromise safety and good layout.
The layout must enable the process to function well (e.g., gravity flow, multi-phase flow, NPSH). Equipment locations should not allow a hazard in one area to impinge on others, and all equipment must be safely accessible for operations and maintenance.
As far as is practical, high cost structures should be minimized, high cost connections kept short and all connection routing planned to minimize all connection lengths.
Layout issues at all design stages are always related to the allocation of space between conflicting requirements. In general, the object most important to process function must have first claim on the space. Other objects must fit in the remaining free space, again with the next most important object being allocated first claim on the remaining space. The constraints always conflict, and the art of layout lies in balancing the constraints to achieve an operable, safe and economic layout.
Layout most obviously affects capital cost, since the land and civil works can account for 70% of the capital cost. Operating costs are also affected, most obviously through the influence of pumping or material transfer cost and heat losses, but more subtly in increased operator workload caused by poor layout.
The layout must minimize the consequences of a process accident and must also ensure safe access is provided for operation and maintenance. Things further apart are less likely to allow domino effects from explosion, fire or toxic hazards, but it is likely that lack of space means that complete mitigation of fire, explosion and toxic risk will be achievable by separation alone.
Layout starts by considering the process design issue of how the equipment items function as a unit and how individual items relate to each other.
For example, the individual items in a pumped reflux distillation unit should be close together for effective fluid flow and minimal heat loss. The condensers and drums should be near ground level to reduce the cost of associated structures, but the drums must be elevated to provide NPSH for the pumps. Such relationships may sometimes be identified by a study of the PFD or P&ID, but not all will be as obvious as this example. This is where a GA drawing, experience and judgment become vital to find and balance the physical relationships in the layout.
There are many other factors to consider. Equipment needs to be separated in such a way that it can be safely accessed for maintenance, for other safety reasons such as zoning potentially explosive areas, and to avoid unhelpful interactions.
Exposure of staff to process materials needs to be minimized. Access to areas handling corrosive or toxic fluids may need to be restricted. This may require the use of remotely operated valves and instruments located outside the restricted area.
In a real world scenario we will have a site or sites to fit our plant on to. Different technologies will have a different “footprint”. They will have a required range of workable heights and overall area. They may lend themselves better to long thin layouts or more compact plants.
There may be a choice to be made between, for example, permanently installed lifting beams, davits and so on, more temporary provision, or leaving the eventual owner to make their own arrangements.
There are no right answers to these question, but professional designers will need to have given these issues sufficient consideration as be able to argue that they have exercised due diligence. In the UK, the minimum extent of this due diligence is specified in the Construction Design and Management (CDM) regulations.
Layout does not require complex chemical engineering calculations but it does require an intuitive understanding of what makes a plant work, commonly known as professional judgment. If it were more common, we might call it “engineering common sense”.
These qualities cannot be taught formally, but must be acquired through practice. It is, however, possible to start the learning process in an academic setting by design practice, though judgment will mostly be developed in professional practice.
Mastery comes only from learning from experienced practitioners and by testing one’s own ideas rigorously. The best way to do this is by listening to those who build, operate and maintain the plants you design for fifteen years or so, and adopting their good ideas (no matter how embarrassing the learning process might be during layout reviews).
Generally speaking the most economical (and easiest to understand/explain to operators) way to lay out a plant is for the process train to proceed on the ground as it proceeds on the P&ID, with feedstock coming in on one side, and product out of the far side of the site or plot. There may, however, be arguments for grouping certain unit operations together in a way which does not correspond with the P&ID order if the site is on multiple plots with any degree of separation.”
The 3D modelling software now commonly used by those laying out process plant does not remove the need for design skill, it merely allows for input into the design process by people who cannot read 2D drawings. The understanding of complex interrelationships in three or four dimensions of items by process plant designers is still required. I merely outline the basis of plant layout in An Applied Guide, but it is the sole subject of my next book, which I am presently writing.
An Applied Guide to Process and Plant Design is available for purchase on the Elsevier Store.
Use discount code “STC215” at checkout and save up to 30% on your very own copy!
About the Author
Professor Moran is a Chartered Chemical Engineer with over twenty years’ experience in process design, commissioning and troubleshooting. 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.
In his role as Associate Professor at the University of Nottingham, he co-ordinates the design teaching program for chemical engineering students. Professor Moran’s university work focuses on increasing industrial relevance in teaching, with a particular emphasis on process design, safety and employability.
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