Plant layout refers to the arrangement of physical facilities such as machines, equipment, tools, utilities, furnaces, and control rooms etc. in such a manner so as to have quickest flow of material at the lowest cost and with the least quantity of handling in the production of the product from the receipt of the input materials to the dispatch of the finished products. The word ‘plant’ in the plant layout can refer to a production unit of a plant or a complete plant consisting of several production units.
Plant layout refers to arrangement of equipments and facilities or plant design. It is a blue print of internal structure and arrangement in a plant. It is a plan for proper and effective utilization of equipments and facilities for the production of the products. It provides smooth flow of materials, and facilitates smooth running of the technological processes in the plant.
Layout design normally involves getting a suitable arrangement of equipments or units and their connections within a pre-defined area based on pre-specified criteria. These items can be process-equipment, furnaces, electric control rooms, instruments and control rooms, product inspection area, and quality control laboratory, stores, workshops, offices, canteen and change rooms, and fire station etc. with connections by pipes, conveyors, vehicular transport or any other suitable material handling equipment. Plant layout includes plant roads and rail tracks.
Plant layout constitutes an important part of production planning. It is the most effective physical arrangement of the processing equipment for achieving a high level of co-ordination and efficiency of 4Ms (men, materials, machines, and methods) in the plant. It focuses its attention on the appropriate allocation of available space and effective arrangement of physical resources and facilities so that an efficient flow of materials takes place during the production, inspection, and storage of the products before their dispatch to the customers.
The plant layout planning is an important process during the plant engineering. Taking decisions regarding different aspects of the layout is very complex and challenging in nature. It needs the knowledge, experience, understanding, and expertise of different branches of engineering. Earlier, approaches to the plant layout design were from a practical point of view, when decisions were made based on engineering judgement, but especially from the late 1990s, more scientific approaches in terms of mathematical modelling have been adopted to solve the layout issues so as to have an optimized layout of a plant.
Plant layout designs have a tremendous impact on the costs and productivity of the processes involved. Hence, engineers specializing in layout designs have constantly sought out methods to find the best solution to such design problems, which are normally referred to as the layout problem.
Little adjustments in the position of machines and equipment in a plant can significantly alter the easy flow of materials. This can also affect the production costs and efficiency of the entire production process. The inability to get production processes right leads to delays, in-flexibility, in-efficiency, excess inventory, high costs, low product quality, and unhappy customers. Modification of an ineffective layout is quite expensive, hence there is the need to design a functional plant layout right from the beginning.
Plant layout can be defined as a technique of locating machines, processes and plant services within the plant so as to achieve the highest possible output of high quality at the lowest possible total cost of production. Over the past few decades, the layout of equipment in the plant has gained increasing relevance, from an economic, safety, operational, and maintenance-related point of view.
As per one of the definitions of plant layout ‘plant layout refers to the arrangement of physical facilities such as machinery, equipment, furniture etc. within the factory building in such a manner so as to have quickest flow of material at the lowest cost and with the least amount of handling in processing the product from the receipt of material to the shipment of the finished product’. Some of the other definitions for the plant layout are given below.
As per More, ‘plant lay out is a plan of optimum arrangement of facilities including personal, operating equipment, storage space, material handling equipment, and all other supporting services along with design of best structure to contain all these facilities’.
As per FG Moore, ‘any arrangement of machines and facilities are layout’.
As per George R Terry, ‘plant layout is the arrangement of machines, work areas, and service areas within a factory’.
As per Morris E. Hurley, ‘plant layout involves the development of physical relationships among building, equipment and production operations, which will enable the manufacturing process to be carried on efficiently’.
As per Spriegel and Lansburgh, ‘plant layout can be defined as a technique of locating machines, processes and plant services within the factory so as to achieve the greatest possible output of high quality at the lowest possible total cost of manufacturing’.
As per Riggs, ‘the overall objective of plant layout is to design a physical arrangement that most economically meets the required output – quantity and quality’.
As per JL Zundi, ‘plant layout ideally involves allocation of space and arrangement of equipment in such a manner that overall operating costs are minimized’. The cost takes into account the losses or gains, if any, because of the social and psychological needs, health and morale of people.
Objectives and basic principles of good plant layout – A well designed plant layout is one which can be beneficial in achieving its objectives consisting of (i) streamlining the flow of materials through the plant, (ii) facilitates the process of production, (iii) maintain high turn-over of process inventory, (iv) proper and efficient utilization of men, equipment and cubic (the three dimensional) space, (v) transportation of materials from one point to another point without any delay, (vi) proper utilization of production capacity, (vii) ensuring reduction in the material handling costs, (viii) ensuring maximum utilization of man-power efficiency, (ix) aiming to achieve ‘zero’ incidents and accidents by incorporating all the necessary safety features, (x) providing for volume and product flexibility, (xi) ensuring for ease of supervision and control, (xii) allowing for easy maintenance of machines and plant, (xiii) improving the plant productivity by reducing the bottleneck, and (xiv) providing for employee safety and health. There are six basic principles relating to plant layout. These principles are described below.
The first basic principle is the ‘integration principle’. This principle states that ‘all other things being equal, a good layout is one which integrates 4Ms i.e., men, materials, machines, and methods in the best possible manner’.
The second basic principle is the ‘flow principle’. This principle states that ‘all other things being equal, a good layout is one which provides for smooth and uninterrupted flow of men and materials’.
The third basic principle is the ‘space utilization principle’. This principle states that ‘all other things being equal, a good layout is one which make best use of available cubic space’.
The fourth basic principle is the ‘minimum movement principle’. This principle states that ‘all other things being equal, a good layout is one which results in minimum movement of men and material in the plant’.
The fifth basic principle is the ‘safety and comfort principle’. This principle states that ‘all other things being equal, a good layout is one which is safe and comfortable for employees working in the plant.
The sixth basic principle is the ‘flexibility principle’. This principle states that ‘all other things being equal, a good layout is one which provides flexibility’.
Factors affecting plant layout – For achieving optimum layout effectiveness when designing a plant layout, several factors of operation need to be seriously considered. The design layout of equipment is an important factor to consider for both new construction as well as expansion. For existing operations, this is also important, but the review approach can be different since the equipment is already fixed in location.
A plant layout is required to act as a pace-maker, not a speed-breaker for the operation of the plant. Hence, decisions involved in plant layout are really difficult and complicated. The success or failure of a plant having an efficient production rest on the quality and the efficiency of the plant layout since while an appropriate plant layout results in a lot of benefits, a mis-fit plant layout brings home disastrous results. The important factors which affect the plant layout are described below.
The first is the policies and decisions of the plant management. A plant layout is considerably influenced by managerial policies and decisions. The nature and type of plant layout depend on managerial decisions with respect to the size of production, nature and type of products to be produced, possibility of future expansion, plan concerning integration of different facilities within plant, quantity of inventory of input materials and finished products in stock, sales policies, and integration of production processes etc.
The second is the location of the production units in the plant. The unit location affects the decisions related to plant layout. The shape and size, climate, soil, general environment and topography of a plant site influence the layout. The location factors affect the building, the civil foundations, and the arrangement of equipment and normal facilities in the building needed for the smooth operations in the production units.
The third is the technology and the type of production processes. The plant layout is considerably influenced by the technology and the nature and type of the production process. There are different types of production processes. These are chemical process, metallurgical process, metal forming process, and metal conditioning process. Further, processes can involve very high or very low temperatures as well as very high or very low pressures. All these processes and the process parameters have distinct features and requirements for the layout which, in turn affect the layout.
The fourth is the type of process. A process can be broadly categorized in two types namely (i) continuous process, and (ii) semi-continuous process. Continuous process is one where the input materials are fed to the process at one end of the plant and from where through its successive processing, they are turned into the product which comes out at the other end of the process. On the other hand, in the semi-continuous process, the production is carried out in several steps with each step producing an intermediate product which becomes input material for the next step. This is continued till the production of final product takes place.
The fifth is the system of production. The system of production also influences the decision concerning different aspects of the plant layout. There are three system of production namely (i) job production, (ii) mass production, and (iii) batch production. Job production is a specific type of production system. It involves production of special type of products which vary in nature and size. In this type of production system, products are produced as per the order and specifications of the customers. Mass production, is a system of production where products are produced at large scale on continuous basis. Production is done in anticipation of market demand, not on the basis of specific order of the customer. Batch production means a system of production wherein products are produced in batches or in group or in lots. It combines the qualities of both job and mass production since production, under this system is done either in anticipation of demand or on the basis of the specific order of the customer.
The sixth is the availability of floor space. Availability of floor space has a considerable effect on the plant layout. The internal arrangement of machines, equipments, materials, personnel, and facilities etc. depends mainly on the space available. Hence, while taking the decisions on the plant layout, availability of space is to be kept in mind.
The seventh is related to the type of products to be produced. There are different types of products. Some products can be small in size while other can be big, some products can be light in weight while other can be heavy, and some products can be in liquid state, some are gaseous products, while some other can be solids. The product type affects the nature and type of the plant layout.
The eighth is the flow of work. Maintenance of the flow of work is an important consideration for the plant layout. Its purpose is to ensure smooth flow of operations. The plant layout of a plant is can be designed in a manner where material moves forward (i) in straight line, (ii) in a ‘to and fro’ movements, (iii) in perpendicular direction, (iv) in curve of a circle, or (v) in U shaped movements. The plant layout while ensuring flow of work is to restrict the movement of the operators, the tools, and the materials to the minimum.
The ninth is the consideration regarding safety and health aspects. Plant process can involve risk of accidents or concerns about environment. Hence, while taking decisions on the plant layout provisions for safety requirements and environment protection is to be kept in mind. A good plant layout is one which eliminates or minimizes the possibility of accidents and hazards and provide good ventilation of the work area so as to have a work place which is safe and healthy.
Plant layout is a strategic decision since it represents long-term commitment. Once the layout has been frozen and implemented, changing it at a later stage has huge implications. The costs of such a change are substantial. Hence, before freezing of the layout, it is to be ensured that the layout provides (i) the optimum relationship among output, floor area, and the process, (ii) facilitates the production process, minimizes material handling, time, and cost, , (iii) allows flexibility of operations and smooth production flow, (iv) makes economic use of the building and space area, (vi) promotes effective utilization of manpower so as to have high efficiency, and proper supervision and control of the process, and (vii) provides for safety, comfort at work, environment protection, and maximum exposure to natural light and ventilation. Further, the layout is to make provisions for future expansion possibilities.
The plant layout substantially varies as per the organization-specific economics, process needs, operation philosophy, and maintenance method. Hence, the specification, which reflects the intent of the organization, is intended to help eliminate layout design difference among individual engineers and also to help minimize recycle work at subsequent stages. However, application of the specification is not simple.
Plant layout improvement, can be one of the tools to increase the process productivity. Plant layout design has become a fundamental basis of today’s production shops which can influence parts of work efficiency. It is needed to suitably plan, and position employees, materials, equipments, and other production support facilities to create the most effective plant layout. A good plant layout is designed to offer competitive advantage to the organization by improving the flow processes of inventory and information, hence leading to reduction in production cost and improvement in the productivity.
Different aspects relevant to design of plant layout which need consideration include proximity to populated buildings, other operations, and surrounding area. Any occupied temporary facilities such as trailers used during construction, maintenance activities, and office space during the project stage are to be considered. Placement means conducting a review of the location of equipment and piping with regard to (i) possible impact on human or environment, or (ii) where the plant operations can have dangerous impact on the equipment. In the first case, for example, design layout is required to consider such items as predominant wind direction and populated areas down wind, while in the second case, the considerations are to include items such as any flammable or potential explosive processes, which, if an incident occurs, can have dangerous impact on the equipment.
The alternative methodology in such a case is to develop spacing distances for the specific plant layout and process parameters through fire, toxic, and explosion consequence modelling. Because of the large numbers of equipment are involved in a plant layout, this can be a time-consuming exercise. Computer programmes are available to facilitate the calculations. The basic steps when taking this approach are (i) to identify the hazards inherent in the process unit, (ii) to identify the consequences which can result from incidents involving the hazards, (iii) to calculate the fire, explosion, and / or toxic impacts on exposed process or offsite equipment, populations, facilities and adjacent areas, (iv) based on the calculations, to estimate the spacing distance needed to minimize the consequences of these impacts on the exposed equipment, (v) this distance provides the minimum separation needed, (vi) to identify opportunities to prevent the incidents, (vii) to identify the opportunities to mitigate the consequences of incidents, and (viii) again, to evaluate the spacing distances.
Plot plan in case of plant layout is classified into two categories. The first category is a general plot plan and the second is a unit plot plan (equipment layout).
Process of preparation of plant layout – Plant layout is an activity which starts at the beginning of the project. Initially a general plot plan is prepared. This plot plan is continuously developed and improved upon as the engineering work on the project progresses. Decisions regarding every aspect of the layout are to be taken after considering all aspects of the plant processes since they can have repercussions on the plant layout. The layout is more or less frozen as the engineering activities come to an end. After this, no major changes in the layout can be made, except some feasible minor modifications during the project implementation stage. Once the construction activities have been completed as per the frozen plant layout, any amendment needed in the layout after this is almost impossible to implement, and if it is required to be done, is a very costly exercise needing a lot of time and funds.
Hence, before freezing a plant layout, the layout engineer has an important responsibility. It is since several of the decisions regarding physical location of different facilities and associated equipment need knowledge of what is taking place in the operation as well as the hazardous factors of explosion, fire, toxicity, and so on. The layout engineer is normally called upon to describe the layout requirements and limitation to the other engineers like civil, structural, mechanical, electrical, and instrument engineers.
By progressive discussion of the process, others can note the requirements, which can affect the normal or routine design approach to each phase of the project. This review is not to be limited to the design aspects of the engineering but rather is to include how to operate the plant and how product is to be stored, dispatched, and so on. After the project begins to take shape and preliminary layouts of the overall as well as sections of the plant are partially completed, design work related to the other phases of engineering need consideration and the evaluation of details of a particular phase as they are related to the process performance. A general checklist of factors, which normally needs reviewing for the proper layout considerations of the plant is described below.
The first is items related to plant site. These items are (i) ground contour and its relation to general orientation of buildings and equipment, (ii) drainage and waste disposal, and details to prevent erosion, (iii) setting of plant elevations which include floor elevations of buildings and bottom of the footings for equipment and large storage tanks, (iv) location of any existing or probable new locations for railroads, roads, power lines and power sources, telephone lines, water supply, office, and / or industrial buildings or structures, and (v) statutory requirements and permits.
The second is items related to climate and environment. These items are (i) prevailing wind so that hazardous vents, burning flares, waste burning pits, waste settling ponds can be located downwind of plant, (ii) nature of climate, where seasonal and daily temperature variations, dust, fog, cyclones and earthquakes are considered, (iii) defining duration of conditions for design, (iv) corrosion since the plants located close to seas, oceans, bays, lakes encounter more severe corrosion, (v) pollution of air and water where allowable limits for atmospheric vent as well as liquid wastes are to be followed, and (vi) determination and compliance of statutory regulations.
The third is the utilities and input materials. This includes sources and methods of transportation and packaging. The utilities include water, steam, air (both compressed air and dry air for instruments), industrial gases, fuels, and power. Layout is to have provision for receiving, testing and storage of utilities and input materials.
The fourth is the packing, storage, and dispatch of the product. Pant layout needs to have provision for packing, storage and dispatch of the product. The dispatch can be by rail, by road, or by pipe line.
The plant layout calls for the use of models. It has (i) to consider maintenance activities associated with each building, process area, and equipment, (ii) to consider access for cranes and road vehicles, work space for local repairs, and operating conditions of adjacent parts of process to allow local repairs, (iii) initial construction sequence and issues associated with it, (iv) materials of construction for buildings, (v) access roads with their paving and width, (vi) working floors in operating and adjacent areas, (vi) fencing, and (viii) plant guard or security system.
The plant layout is to consider electrical and fire hazards. It is to (i) define plant areas handling hazardous and toxic materials and set rules for design considerations, such as ventilation explosion, and walls etc. (flammable storage materials can need enclosed dikes, foam systems, and the like), (ii) define plant areas needing explosion-proof, drip-proof and open motor and associated electrical components referring to national or international standards and codes, (iii) define areas and building to use wet and dry sprinklers systems, foam systems, location to hand and hose fire extinguishers, fire carts, and fire tenders, (iv) define location of fire walls, and fire hydrants, (v) review layout for fire equipment access, and secondary and emergency exit roads from each area, and (vi) review entire fire and other hazards programme and evaluate the associated issues.
The plant layout is to consider the safety requirements such as (i) special design issues for emergency handling of dangerous or toxic materials, (ii) safety as it is reflected in factors of safety in design of pressure vessels, pressure testing of piping and vessels, use of different codes, and code stamps on the equipment, (iii) areas needing safety showers and eye wash stations, (iv) design and selection philosophy for use of safety devices for pressure relief and alarm, (v) safety control valves, and (vi) emergency power and other facilities to control safe operation or shut-down.
The plant layout is to have provision for future growth for which it is to (i) define areas of future growth and associated space requirements, (ii) correlate future expansion plans to required utilities and raw materials as related to economics of required installation, and (iii) consider spare equipments, present and future.
In the plant layout, the process units and ancillary buildings are to be laid out to give the most economical flow of materials and personnel around the plant. Hazardous processes are to be located at a safe distance from other buildings. Consideration is also to be given to the future expansion of the plant. The basic objective of the layout decision is to ensure a smooth flow of work, material, people, and information through the system.
Effective plant layouts also (i) minimize material handling costs by minimizing and controlling the material handling and internal transportation from one operation to the next, (ii) utilize space efficiently and most effectively (it can be cubical utilization), (iii) utilize man-power efficiently by providing employees’ convenience, promoting job satisfaction and safety for them, (iv) eliminating bottle-necks so that the production bottle-necks and points of congestions are eliminated and input materials and semi-finished parts move fast from one production area to another, (v) facilitate communication and interaction between operators, between operators and their supervisors, or between operators and equipments, (vi) reduce the production cycle time, (vii) eliminate wasted or redundant movement and avoid unnecessary investment of capital, (viii) facilitate the entry, exit, and placement of material, products, and people, (ix) incorporate safety and security measures, (x) promote product and service quality, (xi) encourage proper maintenance activities, (xii) provide a visual control of operations or activities, (xiii) provide flexibility to adapt to changing conditions, (xiv) provide overall satisfaction to all concerned, (xv) provide high work in process turnover, (xvi) help in effective utilization of man-power, (xvii) lead to increased productivity and better quality of the product with reduced capital cost. (xviii) provide space for future expansion of the plant, and (xix) provide proper lighting and ventilation of the areas of work.
The ancillary buildings and services needed at the production shop, in addition to the main processing units (buildings), include (i) storage for input materials and products including tank farms and store-houses, (ii) maintenance workshops, (iii) store for maintenance and operating supplies, (iv) laboratories for process quality control, (v) fire stations and other emergency services, (vi) utilities such as process steam, compressed air, utility gases, ventilation and air-conditioning, electric sub-station and transformer etc. (vii) effluent disposal unit for waste water treatment, and solid and or liquid waste collection etc. (viii) offices for general administration, (ix) canteens and other welfare buildings, such as first aid and occupational health centres, and (x) parking lots.
Whatever be the type of layout being contemplated, certain factors are to be considered since these factors have got considerable influence on the design of the layout. These factors are described below.
The first is the human factor which consists of safety and working conditions, man-power requirements including the skill levels, number needed, and their training needs, man-power utilization in the plant, and human relations.
The second is the material factor which includes the different input materials such as raw materials, semi-finished products, and materials in process, scrap, finished products, packing materials, tools and other services, design and specifications of the product to be produced, quantity and variety of products and materials, physical and chemical characteristics of different inputs materials, and spare parts or material and their sequence of operations i.e. how they go together to generate the final product.
The third is the equipment factor. The operating equipment is one of the most important factors hence all the information regarding equipment and the tools are necessary for inspection, processing and maintenance etc. The processes and methods are to be standardized first. Equipments and tools selections depend upon the type of process and method, so proper equipment and other supporting equipment are to be selected on the basis of volume of production. Equipment utilization depends on the variation in production, requirements and operating balance. Equipments are to be used to their optimum levels of speed. Equipment requirement is mostly based on the process / method. Maintenance of equipments and replacement of parts is also important.
The fourth is the movement factor. It mainly deals with the movement of men and materials. A good layout is to ensure short moves and always tends towards completion of product. It also includes inter-departmental movements and material handling equipment. This includes the flow pattern reduction of unnecessary handling, space for movement, and analysis of handling methods.
The fifth is the waiting factor. Whenever material or men is stopped, waiting occurs which costs money. Waiting cost includes handling cost in waiting area, and money tied up with idle material etc. Waiting can occur at the receiving point, materials in process, and between the operations etc. The important considerations in this case are (i) location of storage or delay points, (ii) method of storing, (iii) space for waiting, and (iv) safeguard equipment for storing and avoiding delay.
The sixth is the service factor. It includes the activities and facilities for personnel such as fire protection, lighting, ventilation and air-conditioning etc. Services for material include quality control, production control, services for equipments such as repair and maintenance and utilities like power, fuel / gas and water supply etc.
The seventh is the building factor. It includes outside and inside building features, shape of building, and type of building (single or multi-story) etc.
The eighth is the flexibility factor. It includes consideration because of changes in material, equipment, process, man, supporting activities, and installation limitations etc. It means easy changing to new arrangements or it includes flexibility and expendability of layouts.
When the preliminary plant layout is made, the process units are normally placed first and arranged to give a smooth flow of materials through the different processing steps, from input material to final product storage. Process units are normally spaced at least 30 m (metres) apart. Higher spacing can be needed for hazardous processes. The location of the principal ancillary buildings is then to be decided. They are to be arranged so as to minimize the time spent by the personnel in travelling between the buildings. Administration offices, stores, and laboratories, in which a relatively large number of people are working, are to be located well away from potentially hazardous processes. Control rooms are normally located adjacent to the processing units, but those with potentially hazardous processes are to be placed at a safer distance. Fig 1 shows a simple typical plant layout.
Fig 1 A simple typical plant layout
The location of the main process units determines the layout of the plant roads, pipe trestles, and drains. Access roads to each building are needed for construction and for operation and maintenance. Utility buildings are to be placed to give the most economical run of pipes to and from the process units.
The final plant layout combines the several engineering considerations such as for soil conditions, drainage, rail track, mobile equipment and services access, receiving of input materials, waste materials disposal, effect of climate on outdoor against indoor operations and on types of structures, prevailing wind direction for vent as well as climate moisture, corrosion, plant expansion and growth, access to people, and several other general evaluation points. From these broad considerations the details are developed to suit the particular plant process and the combined effects of the location.
The economic construction and efficient operation of a process unit depends on how well the plant and equipments specified on the process flow-sheet are laid out. The principal factors to be considered are (i) economic considerations consisting of construction and operating costs, (ii) the process needs, (iii) convenience of operation (iv) convenience of maintenance, (v) safety, (v) future expansion, and (vii) modular construction.
The cost of construction can be minimized by adopting a layout which gives the shortest run of connecting pipe between equipment and the least quantity of structural steel work. However, this is not necessarily be the best arrangement for operation and maintenance. For the minimizing of the cost, the things to be considered are (i) design of pipe rack structure for cost saving and to minimize pipe rack lengths, (ii) to minimize common duct lengths, (iii) to minimize piping lengths, and (iv) to minimize cable lengths.
Process needs which are to take into account the process considerations, is the need to elevate the base of columns to provide the necessary net positive suction head to a pump or the operating head for a thermo-siphon reboiler. The process needs to be considered are (i) relation between PFD (process flow diagram) and the equipment layout, (ii) relation between the gravity flow lines and elevation of related equipment, and (iii) spaces between equipment.
Equipments which need to have frequent operator attention are to be located convenient to the control room. Valves, sample points, and instruments are to be located at convenient locations and heights. Sufficient working space and head-room are to be provided for allowing easy access to equipment. If it is anticipated that equipment has limited life (e.g., change parts) and needs frequent replacement, then sufficient space is to be allowed to permit access for lifting equipment.
The points to be considered in operation are (i) access ways for operator’s access around each individual item of equipment, (ii) location of entrances to structure ladders and the control room, and (iii) considerations for inspection round routes.
Plant layout is required to facilitate maintenance. For example, heat exchangers need to be located in such a place so that the tube bundles can be easily withdrawn for cleaning and tube replacement. Vessels which need frequent replacement of catalyst or packing are to be located on the outside of buildings. Equipment which needs dismantling for maintenance, such as compressors and large pumps, are to be placed under cover.
Plant layout is to take care of safety and environment aspects. Plant layout is frequently a compromise between a number of factors, including safety aspects such as blast walls is needed to isolate potentially hazardous equipment and confine the effects of an explosion. At least two escape routes for operators are to be provided from each level in process buildings.
The important considerations with regards to safety and environment are (i) geographical features of the plant location, (ii) location of high temperature furnaces, (iii) control rooms and their access points, (iv) plant roads, (iv) locations of high pressure equipments, (v) location of large-capacity storage tanks containing hazardous and / or flammable or explosive material, (vi) location of pumps intended to handle flammable materials, (vii) location of furnace stacks, (viii) escape way for emergency, (ix) where spacing is reduced considerably, it is necessary to compensate for the increased degree of risk by providing additional safety facilities such as fire-fighting equipment, water sprays, fire proofing, and emergency shut-down facilities etc., (x) familiarization with pertinent environmental statutory regulations, (xi) attention is to be given to the pertinent safety regulations, including health and welfare needs, (xii) hazardous and flammable materials need special handling, which can take up layout space, (xiii) If the process fluids are especially toxic, layout is affected by the need for close chemical sewers and other protection measures and hence, security requirements need special layout design when the plant produces a high-value product, (xiv) governing standards and statutory regulation at the plant location affects the layout concept, (xv) distances for transfer of materials between plant and storage units to reduce costs and risks, (xvi) interaction with existing or planned facilities on site such as existing roads, drainage and utilities routings, (xvii) spaces for plant operability and maintainability, (xviii) preventing and / or mitigating the escalation of adjacent events (domino effect), and (xix) ensuring that safety within on-site and off-site occupied buildings is maintained.
Plant layout design techniques applicable for the reduction of the risks from release of flammable or toxic materials include (i) locating the storage of flammable / toxic material outside process areas, (ii) locating hazardous units away from main roads through the site, (iii) fitting remote-actuated isolation valves where high inventories of hazardous materials can be released into vulnerable areas,(iv) allowing for the provision of dykes and sloping terrain to contain releases, so as to increase the safety, and reduce environmental effects, (v) locating of units within buildings as secondary containment, (vi) locating of units in an open air environment for ensuring rapid dispersion of minor releases of flammable gases and vapours and hence prevent concentration build-up which can lead to flash fires and explosions, and (vii) hazardous area classification for flammable gases, vapours and dusts to designate areas where ignition sources are to be eliminated.
Equipments are to be located in such a way so that they can be conveniently tied up with any future expansion of the process. Space is to be left on pipe racks for future needs, and service pipes are to be oversized to allow for future needs.
These days modular construction practice for equipments is frequently being used. In recent years there has been a move to assemble sections of a plant at the plant manufacturer’s place. These modules include the equipment, structures, piping, and instrumentation. The modules are then transported to the plant site.
The advantages of modular construction are (i) improved quality control, (ii) reduced construction cost, and (iii) lesser need for skilled personnel at the plant site. Some of the disadvantages of modular construction are (i) higher design costs, (ii) more structural steel work, (iii) more flanged connections, and (v) possible issues with assembly on site.
Plant layout design is required to provide maximum flexibility. A good plant layout is to be capable for easy modification so that it can meet the ever-changing demands created by the changes taking place because of technology advancement and the market. Further, plant layout is to be designed to assist in attaining its rated production output at the shortest possible time.
Efficient utilization of space needs the provision of sufficient space around the equipments and path ways, as well as ensuring that adequate spaces are made available for storage points within the units. Also, plant layout is to be designed to improve communication and easy flow of information among the different departments / units, as well as between man and machine.
A well-designed plant layout has promotional value. It improves the image and reputation of the organization.
Safety of men and machines is an important aspect in a plant layout. Since the importance of safety in all human endeavours is not to be over-emphasized, a good plant layout is designed to function efficiently and ensure that accidents and its causes are reduced to the barest minimum. Further, the plant layout is to ensure maximum accessibility. This facilitates the repairs and maintenance of equipments since they have been made readily accessible. This implies that equipments are not to be placed against the walls in order to ensure that maintenance and servicing operations can be easily undertaken.
Some key issues related to the plant layout
Terrain – In the first place, considerations are to be given to the physical setting. It is not to be automatically assumed that it is necessary to level the site. Instead, there can be ways that the process can take the advantage of whatever slopes are present. With respect to terrain, assess is to be made, whether, there is adequate space in general. If not, ingenuity is needed to meet such requirements as those for flares. Available space can help to decide whether the plant can be located on one floor or instead occupy several floors. The physical setting is also to be considered in light of the transportation requirements for raw materials, products, wastes, and supplies.
Safety and environment – Familiarization with relevant environmental regulations, and how they can change is necessary during the preparation of the plant layout. Essential period is till the conclusion of pre-project studies. Also, attention is required to be given to the relevant safety regulations, including health and welfare needs. Hazardous and flammable materials need special handling, which can take up layout space. If the process fluids are especially toxic, layout is affected by the need for close chemical sewers and other protection measures.
Security needs can require special layout design when the plant produces a high-value product. If a plant location is governed by particular building, piping, electrical and other codes, then these can affect plant layout. Similar governing standards and regulation in plant location site affects the concept of the layout.
Throughput – It is important not only to know the initial capacity but also to have a good feel for how much the plant is going to be expanded in the future, as well as how likely the process technology is going to be up-graded. These factors indicate how much space is to be left for additional equipment. Multiple processing lines (trains), are frequently needed for the plant. Pairs of trains can either be identical or be mirror images. The former option is less expensive. But the mirror image approach is sometimes preferable for the layout reasons. Two such reasons are (i) for operator access through a central passage, and (ii) the need that the outlet sides of two lines of equipment (e.g., pumps) point toward each other so that they can be readily connected to one common line.
General – The plant layout is to be arranged to (i) maximize safety, (ii) prevent spread fire, (iii) facilitate easy operation and maintenance, (iv) consider future expansion, and (v) economize the plant.
Blocking – The plant site is to be blocked in consideration of hazards attendant to plant operation in the area. All blocked areas are to be formed as square as possible by divided access roads and / or boundary lines.
Location and weather – The plant layout is to be arranged in consideration of geographic location and weather in the region of the plant location. Where the prevailing wind is defined, the administration and service facilities, and furnaces etc., are not to be located in the windward direction of the process units and storage tanks etc.
Layout indication – The basic requirements to be met are to be made in the appropriate diagram when making a piping and equipment layout. These are to indicate (i) all equipment, ladders, structures, jib cranes, and trolley beams etc., (ii) location of all instruments, (iii) all valves and hand-wheel orientations, (iv) drip funnel locations for underground drains, (v) all electrical switch gear, and lighting panels, and (vi) all sample systems.
Classification of hazards – The plant layout is to be made in consideration of classified hazardous areas.
Plant layout features
Area arrangement – Classified blocked areas, such as process areas, storage areas, utilities areas, administration and service areas, and other areas are to be decided on the bases of several factors namely (i) soil characteristics (ii) main road, rail track access, (iii) location of pipelines to and from plant, (iv) direction of prevailing wind, (v) local statutory regulation which can affect the location of units and storage facilities, and (v) natural elevation for location of upstream / downstream units and equipment (such as feed and product storage tanks, waste-water plant, and oil / water separator etc.). which are arranged as described below.
The process area is to be located in the most convenient place for operating the process unit. The units are to be separated by roads. Major roads need to have minimum width of 10 m. The minor roads need to have minimum width of 6 m. Minor roads are not to be in an area classified as zone ‘0’ or zone ‘1’. The most common values used for zone ‘0’ is explosive atmosphere for more than 1,000 hours per year (h/yr) and for zone 1 is explosive atmosphere for more than 10 h/yr, but less than 1,000 h/yr.
A plant can contain one or several process units. Where any unit processes flammable fluids and can be operated independently (i.e., one unit can be shut down with others are in operation), the minimum spacing between equipment on the two adjacent units is to be at least 20 m. The storage area is to be located as far as possible from buildings occupied by personnel at the plant, but is to be located near the process area for ready operation of the feed stocks and product run downs.
The utilities area is to be located beside the process area for ready supply of utilities. Except where they are an integral part of a process unit, plant utility units are to be grouped together in an area classified as non-hazardous.
Loading and unloading area is to be located on a corner of the plant site with capable connection to public road directly, for inland traffics. Loading / unloading areas for road transport are to have adequate space to provide access for filling, parking and manoeuvring of the vehicles. A drive through rack arrangement is preferred. The loading and unloading facilities are to be downwind or crosswind from process units and sources of ignition, based on the direction of prevailing wind. For marine transportation, the area is to be located on the seaside or riverside in the plant site.
The administration and service areas are to be located at a safe place on the plant site in order to protect personnel from hazards. It is preferably be located near the main gate alongside the main road of the plant.
Flare and burn pit are to be located at the end of the plant site with sufficient distance to prevent personnel hazard. Waste water treating unit is to be located near at the lowest point of the plant site so as to collect all of effluent streams from the processing unit.
The process unit to which the input materials is charged first, is to be located on the pace near the input material storage area, to minimize the length of the conveyor and feed line. The process unit from which the final product(s) is (are) withdrawn, are to be located on the side near the products tanks to minimize the length of the product run-down line. Process units, in which large quantities of utilities are consumed, are preferably be located on the side near the utility centre.
All the plant sites (plants or complex) are to be within a security fence. The minimum space between security fence and units’ boundary is to be 20 m, and between security fence and equipment is to be 30 m. In case of special units such as flammable material storage with vapour release and toxic materials, minimum space is to be at least 60 m from site boundaries adjacent to the centres of population. Fire water pumps and equipment are to be sufficiently remote from processing, storage, and loading areas, where a major fire can occur.
Plant roads and access ways – These are to offer easy access for mobile equipment during construction and maintenance, fire-fighting and emergency escape in a fire situation. Access roads are to be at least 3 m from processing equipment between road edges to prevent vehicle collisions.
Pipe racks and sleepers – In general, pipe racks for process units and pipe sleepers for the off-site facilities are to be considered as the principal support of the pipe way. Run pipe lines overhead are to be grouped in pipe racks in a systematic manner. Pipe rack runs oriented in the same direction are to be at consistent elevations. Pipe rack runs oriented opposite to these runs are to be at other elevations to accommodate crossing of lines at pipe racks junctions and to accommodate branch line intersections. Single level pipe racks are preferred, if more than one level is needed, the distance between levels oriented in the same direction is to be adequate for maintenance but not less than 1.25 m.
Maximum pipe rack widths are to be 10 m. If widths larger than 10 m are needed, the pipe rack is to be designed to be of two stages. Actual widths are to be 110 % of the needed widths or the needed widths plus 1 m. In cases where air fin coolers are to be placed on the pipe racks, the pipe rack widths are to be adjusted based on the length of the air coolers.
The flat turns are to be avoided. When changing directions, the elevation is to be changed. Ample space is to be allowed for routing instrument lines and electrical conduit. 25 % additional space is to be provided for future instrument lines and electrical conduit adjacent to which needed. 20 % additional space is to be provided on the pipe rack for future piping. This space is to be continued and clear on each level for the full length of the rack.
The width allocation can be split in two sections but not more than two. A continuous clear area of 4 m high by 4 m wide is to be provided below main racks in process units for maintenance access ways. Pipe racks outside process areas are to have the minimum overhead plant clearances consisting of (i) main road – 5 m, (ii) access roads – 4.5 m, and (iii) railway tracks – 6.7 m top of rail.
Typical layouts of pipe rack for process plant depend on the number of process units incorporated and the process complexities are given in Fig 2. There are four numbers of layouts as shown in Fig 2.
Fig 2 Typical layout for pipe rack for process plant
A ‘single rack type’ layout shown in Fig 2a is suitable for small scale process complex consisting of two-three process units. It is economical without needing any large area. A ‘comb type’ layout shown in Fig 2b is desired for use in process, complex consisting of three or more process units. ‘Single rack type’ in this case is not suitable since separate maintenance and utility administration in normal operation is difficult because of the utility and flare lines which are placed on the common rack.
A ’double comb type’ layout is an expansion of the ‘comb type’ which is desired for the use in large scale process complexes where five to ten process units are to be arranged. This layout as shown below in Fig 2c can be conveniently utilized. A ‘U type’ layout shown in Fig 2d is desired to be used in case of process units whose maintenance cannot be conducted separately, within the complex. This type can be regarded as an expansion of the ‘single rack type’. Even process complexes of this nature, can be regarded as one process unit in the planning of their layout.
Location of pipe racks is to be in general agreement with the plot plan. Overhead racks can contain more than one level. For steel pipe racks, the height of levels is to have one of these elevations namely (i) main pipe racks – 4.6 m, 6.2 m, and 7.8 m, and (ii) individual or secondary pipe racks – 3.8 m, 5.4 m, and 7 m. In special cases for large size pipes or concrete pipe racks, the distance between the different floors can be increased.
Except for special cases, minimum width of pipe rack is 6 m. The width of pipe rack is to be designed to accommodate all pipes involved plus 20 % space for future expansion or modification. Where the pipe rack supports air coolers, the preferred width is the width of air coolers. In multi-level pipe racks, pipe carrying corrosive fluids is to be on the lower level, and utility lines are to be at the upper floor. Large size or heavy weight pipes are to be located at the lower level and on extreme sides.
Layout of control room and electrical sub-station – The control room and sub-station are to be located as close as possible to the plant equipment, maintaining a minimum distance from the view-point of noise and safety requirements. The control rooms, and sub-station are to be spaced at least 15 m from the nearest process equipment surface. The control room and substation shall be located with consideration to convenience in daily operation.
The control room and sub-station are to be located from an economic standpoint so as to minimize the length of electrical and instrument cables entering and leaving the unit. The control room is to be positioned so that the operator can command a view of the whole system which is under control. Large buildings, or equipment are not to be placed in front of the control room.
Fire-fighting requirements – Each individual process unit is to be provided with sufficient open space around the unit, fire tender can be run and operated thereat. The width of access way there to, is to be 6 m minimum.
Process units consisting of large hazardous material storage tanks are to be located desirably in outer area in the complex plant site. High-pressure gas lines are not to pass through a process area or run within 30 m of important structures or equipment without shut-down valve for ensuring that the portions of piping within the process area can be isolated from the main gas line and depressurized in the event of an emergency. However, extensive use of shut-down valves is not to be done, since the increased complexity of the system needs a higher degree of preventive maintenance if un-warranted shut-downs are to be avoided. Shut-off valves, sometimes known as ‘isolation valves’, are to be provided on all gas and product pipelines into and out of the plant.
A by-pass line with a normally shut-off valve is normally needed between plant inlet and discharge lines. All isolation valves and by-pass valves, if any are to be located at a minimum distance of 75 m but not more than 150 m from any part of the plant operations. Care is to be taken in locating these valves so that they are not exposed to damage by plant equipment or vehicular traffic.
At least two remote emergency shut-down stations, located at a minimum distance of 75 m apart, are to be provided. Actuating points are to be located at least 30 m from compressor buildings and high-pressure gas lines. More than two shut-down stations are normally needed, depending on the size and complexity of a given plant. One of the actuating stations is to be located in the control room. It is to be distinctively marked and equipped with signs stating the proper method of actuation in the event of an emergency.
Waste-water separators handling hydro-carbons are to be spaced at least 30 m from process unit equipment handling flammable liquids and 60 m from heaters, furnaces, or other continuous sources of ignition. Preferably, waste-water separators are to be located down-grade of process equipment and tanks.
Fire training areas are ignition sources when in use. Because of the smoke produced, they can also create a nuisance for the plant and neighbouring facilities. Fire training areas are to be 60 m from process unit battery limits, main control rooms, fired steam generators, fire pumps, cooling towers, and all types of storage tanks. They are also to be 75 m from plant overall boundaries, administration, shops, and similar buildings and from the main sub-station.
Building requirements – Service buildings include offices, control rooms, laboratories, houses, shops, stores, garages, canteens and occupational health centre. These structures and areas need protection of personnel from possible fires and explosions of major plant equipment and can need additional spacing from high-risk facilities. The service buildings are to be located near the entrance of the plant and be readily accessible to a public road. Spacing at the plants for the buildings are to be in accordance with statutory norms.
Access ways within the plant are to be provided for maintenance, emergency case, and for fire-fighting from the road around the plant. Piping system is to be laid in such a manner so as to make possible passage of mobile equipment. Operating passage-way between equipment or piping and adjacent equipment normally is 1.05 m minimum except when otherwise shown on drawings.
Minimum widths of access way are to be (i) vehicular access ways within units – 4 m, (ii) pedestrian access ways and elevated walkway – 1.2 m, (iii) stairways and platforms – 0.8 m, (iv) footpaths in tanks areas – 0.6 m, (v) maintenance access around equipment – 1 m, and fire tender access way – 6 m.
Minimum headroom clearance for access is to be (i) over railway tracks or main road – 6.8 m, (ii) over access roads for heavy trucks – 6 m, (iii) for passage of vehicle – 4 m, (iv) for passage of personnel – 2.1 m, and (v) over fork-lift truck access – 2.7 m.
Preparation of plot plan drawing – The general plot plan drawing gives the layout of the whole plant(s). It is to be prepared as per one of these scales namely 1:500, 1:1,000, or 1:2,000. The items shown in plot plan drawing are (i) battery limits of complex (area boundary), (ii) plant geographical and conventional ‘North’, (iii) elevation, with regard to the nominal plant ‘0’ (zero) elevation, (iv) co-ordinates of main roads, process units, utility units, buildings, storage tanks, and main pipe rack, (v) location of flares and burn pit, and (vi) direction of prevailing wind. The plant coordinates can be started from point N = 0, and E = 0.
Layout in process units
General – In cases where process fluid is run by gravity head, elevated layouts are to be considered. Unless there are any such limitations as indoor arrangement and confined locations, equipments are to be placed at grade as a rule except in cases where gravity flow is specifically needed for any reason.
Since the directions of the incoming feed stocks, outgoing products and utility supply are determined on the overall plot plan, first the direction of the pipe rack in the unit is to be decided. Equipments are to be arranged to minimize piping runs (particularly, for large sized piping and alloy piping) as far as possible. Clear access ways having minimum width of 600 mm are to be provided for the operators’ access around equipment.
Large capacity storage tanks containing flammable and explosive fluids are to be located in outer areas as far as possible. Space is to be allowed for the provision of future spare equipment. Consideration is also to be given to future plant expansion. The drawing is to be prepared as per one of these scales namely (i) 1:1,500, or 1:1,200. The items to be shown in the drawing are (i) conventional ‘North’, (ii) co-ordinates of battery limits and roads, (iii) symbols for equipment and co-ordinates of their centre lines, (iv) finished floor elevation, and (v) equipment index list.
The area and the length of any unit depends on the technological process. For units processing flammable fluids, the central control building is to be adjacent to a road. It is not to be located in any area classified as zone ‘0’ to zone ‘1’.
Process requirements – Equipment are to be laid out along the flows on the process flow diagram. The fractionators and the reboiler, condensers and overhead receivers are specially to be collectively located. Gravity flow lines are to be laid out with consideration given to related elevations, so that their lengths become minimum. In the case of lines in which liquids flow near at their boiling points, related equipments are specially to be located close to each other so that the lines need not be elevated.
Pieces of equipment which are to be connected by large-size piping or alloy piping are to be located close to each other. Air coolers are to be laid out so that no heated air can be recirculated.
Safety requirements – All process equipments are to be kept at least 15 m from fired heaters, or furnaces. Exceptions are permitted for certain pieces of equipment where the heater in question is being used to heat the process flow of the equipment and where any leakage from the equipment probably ignites instantly, hence no additional fire hazard. Such an exception is permitted when locating reactors on a reforming unit, for an example. Each exception is to be individually examined as for its potential hazards and is not to violate any process practices in the similar plants and governing codes.
The fire heaters are to be located on the side of the process unit from which the prevailing wind blows. This is done to blow gas away from the heaters instead of towards them. Fired equipment are not to be located in ‘hazardous area classification’. Normally, fired equipment are to be located at a distance of more than 15 m from any sources of hazards (hot oil pumps, light end pumps, and compressors etc.).
Emergency showers, if needed in a certain process, are to be located as near the hazard as possible and indicated in the plot plan. Adequate and easy access and outlet is to be considered for not only safety, but also operation and maintenance needs.
Control rooms and their roadways are not to be located in hazardous area, classified in project specification. Normally, they are to be located at a distance of more than 15 m from the nearest equipment. High pressure gas compressors are to be located at protected locations. Large capacity hazardous material storage tanks are to be located in outer areas as far as possible.
Pumps intended to handle flammable materials (which fall under the control of the hazardous area classification), are to be located on the basis given in Tab 1.
|Tab 1 Basis for Location of pumps handling flammable materials|
|Use of pumps||Under pipe rack||Under air-cooled exchangers|
|Cold oil pumps||Acceptable||Acceptable|
|Hot oil pumps||Unacceptable||Unacceptable|
|Light end pumps||Acceptable||Unacceptable|
In Tab 1, hot oil means the oil whose operating temperature is higher than auto-ignition point. A space of minimum 3 m is to be provided from pipe rack. Hot oil pumps can be located under pipe rack, when the proper devices for fire protection such as fire detector, and water spray etc., are provided.
Light ends pumps are not to be located under air cooled exchangers except where concrete decks are provided under the air-cooled exchangers. Light ends pumps can be located under air coolers without concrete decks, when the proper devices for fire protection are provided. Light ends mean the fractional distillates with boiling point range between 110 deg C and 120 deg C. These are consisting of mixtures of benzenes, toluenes, xylenes, pyridine, phenol, and cresol etc.
Fired heater stacks are to be located at such points so that the performance of the air coolers and operators on tower tops is not be adversely affected by hot flue gas emitted through them. The directions of prevailing wind are to be taken into consideration in the determination of the stack locations.