The majority of Process Plants require a plot plan with somewhat less stringent and less complicated arrangements than a refinery. Equipment spacing requirements will vary with the type of plant and location. Plot plans are considered key documents to projects and are normally initiated in the pre-contract, conceptual and development stages of a proposal. After the contract is awarded for engineering, plot plans are developed at a rather rapid pace with very limited information. This early stage plot plan usually is very limited in detail, containing only enough dimensional data to define the outer limits of the available property selected for plant development. Located within the boundaries of the available property, rough equipment sizes and shapes are pictorially positioned, along with anticipated pipe rack configurations, structure shape and rough sizes. The plot plan at this level of detail is then used for constructability evaluation and is normally submitted to the client for approval.
The piping engineer has to understand the concept of equipment layout and consideration. While preparing layout, the piping engineer should design a steady process, non-hazardous utility and facility. In general the various process plants we come across are petroleum refineries, petrochemical complexes,
fertilizer plants, and chemical and pharmaceutical plants which need special attention due to the nature of materials and products handled.
The major variables affecting the final layout are interconnected pipe sizes, insulation thickness, steel work spacing, matters of operational convenience, safety, ease of erection and maintenance which calls for critical judgment on the experience and the study of existing and known limitation.
Economic: Basically economic consideration means installing the unit in the smallest possible space, consistent with the operability ease of piping material, structural sheets and concrete with the proper layout considering further economies which can be achieved in way of pumping and utilities.
Safety: Where toxic or hazardous materials are handled, layout may be needed to isolate a section of the plan equipment, which could be a possible source of hazard. It should be grouped together and where possible located separately from other areas of the plant. i.e. furnaces, flare stacks or other equipment containing naked flares. Mechanical equipment handling flammable or volatile liquids which could easily leak or spill out of the equipment thus causing flammable conditions. However such consideration should not override considerations of cost. For instance, the process heater must necessarily be located close to other equipment to conserve expensive alloy piping. Furnaces using gas as a feedstock do not normally constitute hazards. Due consideration shall be given for fire hazardous areas and isolated by providing firewall, fire door, etc. The BOCA (Building official and code administration) published by the National Building Code shall be followed. The equipment area handling acids or other toxic material, which cause damage or endanger personnel by their spillage should in general be grouped together and isolated. To isolate hazardous areas it may be necessary to build walls with self-closing doors.
Process: Process considerations may require some items be elevated to provide gravity flow of materials to accommodate pump suction requirement for NPSH. The other process consideration could be the limitation of pressure or temperature drop in the transfer lines deciding the proximity of the furnace, reactors and columns.
Operational: Thought should be given to the location of equipment requiring frequent attendance by operation personnel and the relative position of the control room to obtain the shortest and most direct route for operator. When on routine operation however the control room should be placed so that they are easily accessible and the indicators are easily readable. Generally a batch or semi batch process needs more attention by the operator and therefore more consideration has to be paid to the ergonomics of the layout.
Maintenance: The need to remove for servicing, retuning or replacements, the heavy, servicing equipment. The indivisible plant will dictate their location when access for cranes is called for regular or rotating and other machinery calling for dismantling. This often makes their grouping within the machine house desirable. The position of items needing replacement of internal spent catalyst etc. or frequent internal cleaning has to be carefully considered.
Constructional: The plot should be so designed that adequate access is available to lift the large items of equipment or columns into place. Such equipment is positioned close to the boundary limits so that erection must take place from outside these limits. A careful check must be undertaken to ascertain whether space will be available at the time of erection for positioning the crane or lifting the delivery equipment which is known, may well arrive fairly late in the construction program and therefore have to be fitted into place after most of the surrounding equipment has already been installed. It is important that the insulation requirement be considered during the layout of the plant.
Appearance / Aesthetic: An attractively laid out plant with the equipment in rows also economically laid out gives an aesthetic appearance. The building structure and groups of equipment should form a neat, symmetrical balanced layout consistent with keeping the pipe run to a minimum and allowing proper access for maintenance. The tower and large vertical vessels should be arranged in rows with a common centreline. They should be of similar size but if the diameter varies, due note must be taken of the building line manholes on the adjacent tower should be at a similar elevation and orientation to streams. These as far as possible should be made identical. Such arrangement for parallel streams or similar groups of process equipment require much the design work and also for construction and subsequently operation but help in reducing the amount of standby equipment.
Future Expansion: Thought should be given to the likely expansion of both equipment and pipe work, so that additions can be erected and tested with the minimum interference to plant operations. On main pipe runs, it is desirable to leave 30% space. At least 30 m distance from flame proof plant area is needed for safe welding where no special precaution are needed.
Buildings: Plant buildings should be kept to a minimum on the basis that most of equipment including pumps, heat exchanger boilers, cyclones, etc. may be safely installed in the open. The philosophy should be that the plant is supported on an open steel structure unless there is good reason not to do so. Thus it is to be installed in the open with centralized control facilities housed in buildings.
1) Cut out to visualise plant layout: 2 dimensional scale templates or scale cutouts of unit areas and equipment layout are prepared.
2) Block models: These models are chiefly used to develop the plot plan, floor plan, elevation, major equipment and major pipe racks in the correct co-relation with each other.
3) Drawings: For the development and evaluation of the layout it co- relates the basic project information on the equipment and site condition and provides source of data for the final layout.
4) Piping models: For accurate detailed layout of the process piping utility and control facilities.
Concept of the Plant Layout: A process plant which consists of the various different sections mentioned earlier should be arranged so as to follow the general route of the raw material, to process, to intermediate/finished product stage to dispatch. The entire plot area will be divided into blocks, the size of the blocks depend on the facilities to be accommodated. While locating the blocks and further sub-blocks within them, the following points runs should also be minimum.
1) The layout planning shall be in the sequential order of the process flow so that the piping run is also minimum.
2) The block shall be arranged considering the prevalent wind direction such that flammable gases should not be carried by the wind onto a source of ignition.
3) The process unit block shall be centrally located with a straight approach from the main gate.
4) Storage tanks shall be grouped according to product classification. In undulating areas, storage tanks shall be located at lower elevations.
5) Utility blocks shall be located adjacent to the unit block.
6) The main power receiving station shall be close to the boundary line so that the minimum overhead power lines pass through the plant.
7) The electric sub- stations in the plant shall be located in such a way that cabling to major load centres are minimum.
8) The location of power plants shall be near the process/utility block and close to the main receiving station.
9) Flares shall be located upwind of the process units so that the inflammable gas from the plant are not carried towards the flares.
10) Truck loading facilities shall be located close to the product movement gate. Rail loading facilities are arranged generally at the periphery of the plant.
11) The effluent plant shall be located away from the other units on the down elevation. The preferred location is at the lower elevation than the other plant units in order to facilitate gravity flow.
12) The fire station and firewater pump house shall be at a safe place away from hazardous areas. The fire station shall be near the main gate with a straight approach to the process units and other critical areas.
13) Two adjacent process unit locations shall be decided based on the annual shut down philosophy for the maintenance of the units. If the shut downs are be at different times, it is preferable to increase the distance between the two units so that the risk for the operating units due to heat jobs in the units under shut down can be minimised. Also this will facilitate easy maintenance.
14) Process Units shall be located on the high ground.
15) The adjacent neighbourhood installations, if any, shall be taken into consideration before fixing the plant layout. The limitations imposed by the neighbourhood facilities cannot be ignored, e.g. an ignition source.
16) Flares, furnaces/heater, dusty operations and cooling towers shall be oriented depending on the prevailing wind direction. The first two shall be located upwind of the Process units and the last two on the downwind directions of the process units.
17) Due consideration to the construction and erection of the plant shall be given while deciding the plant layout, especially tall towers, reactors, furnaces, etc. shall not be in congested areas and sufficient open spaces shall be provided for erection at any stage.
18) Equipment requiring frequent maintenance shall have easy accessibility. So also equipment which need removal of parts and also for free access for hoisting equipment.
19) Provisions for future expansion shall be considered. It is preferable to have similar type of facilities with adjacent space for expansion adjacent. Care shall be taken to install the expansion facilities without any disturbance to the existing facility.
20) Roads in between for the purpose of access and safety should separate the blocks developed. Fire fighting facilities are to be provided on these roads. There shall always be an alternative approach for fire fighting and maintenance in case one route is blocked. The layout shall be so designed that truck traffic inside the plant is minimized.
21) Boiler house, air compressors, fuel oil facilities shall be grouped into one block adjoining the process unit blocks, so that spreading of various utilities can be avoided and also facilitate easy operation.
22) The layout shall be designed so as to minimise the capital cost for earthwork and pavement, special foundations and the extent of a vacant plot as wasteland.
Equipment Location Drawing: Equipment location drawings (plot plans) show the exact location of equipment in relation to the plantís physical boundaries. One of the most difficult concepts to explain to a new process technician is the scope and size of modern chemical processing. Most chemical plants and refineries closely resemble small cities; they have well-defined blocks and areas connected by a highway of piping and equipment. Equipment location drawings provide information about the neighborhood.