Lecture 3. Plant Location and Layout<br />Selection of Site/Plant Location<br />The selection of plant site is very important to ensure that it has all the support required to make the venture a feasible and profitable. There are many factors that must be considered when selecting a suitable site. The principal factors to consider are:<br />Location, with respect to the marketing area.<br />Raw material supply.<br />Transport facilities.<br />Availability of labour.<br />Availability of utilities: water, fuel, power.<br />Availability of suitable land.<br />Environmental impact, and effluent disposal.<br />Local community considerations.<br />Climate.<br />Political and strategic considerations.<br />
1. Location, with respect to the marketing area<br />The selection of location with respect to the above criteria depends on the cost of production versus the cost of transportation. <br />Most chemical/petrochemical process plant has significantly higher production cost compared to the cost for bulk transportation if the distance is within certain range. Therefore, most of the time the location selected is near to the transportation hub particularly sea port. This will enable the delivery of the finished products to customer as quickly and as cheaply possible.<br />However, there are products that are produced in bulk quantities; such as cement, mineral acids, and fertilisers, where the cost of the product per tonne is relatively low compared to the cost of transportation which forms a significant fraction of the sales price. In such cases, the plant should be located close to the primary market. <br />All the above consideration may not apply or less important for low volume production, high-priced products; such as pharmaceuticals.<br />
2. Raw material supply.<br />The supply of raw materials is one of the most important factors especially when large quantities are involved. It lead to savings in the storage facilities as well as transports.<br />Thus the availability and price of suitable raw materials will often determine the site location.<br />eg . Proximity of steelworks to the major coalfields in the UK, major petrochemical complexes around Kertih where supply of natural gas from offshore Terengganu. <br />Plants producing bulk chemicals are best located close to the source of the major raw material; where this is also close to the marketing area.<br />
3. Transport Facilities<br />The transport of materials and products to and from the plant will be an overriding consideration in site selection.If practicable, a site should be selected that is close to at least two major forms of transport i.e., road, rail, waterway (canal or river), or a sea port. <br />Land transport such as road transport is being increasingly used, and is suitable for local distribution from a central warehouse.Rail transport will be cheaper for the long-distance transport of bulk chemicals. <br />Air transport is convenient and efficient for the movement of personnel and essential equipment and supplies, and the proximity of the site to a major airport should be considered.<br />
4. Availability of labour.<br />Although the general trend is for increased automation, many processes would still require a reasonably large labour force.<br />Labour will be needed for construction of the plant and its operation. Skilled construction workers will usually be brought in from outside the site area, but there should be an adequate pool of unskilled labour available locally; and labour suitable for training to operate the plant. <br />Skilled tradesmen will be needed for plant maintenance. <br />Local trade union customs and restrictive practices will have to be considered when assessing the availability and suitability of the local labour for recruitment and training.<br />In addition, the local pay rates, competing industries and turnover rates need to be also considered.<br />
5. Availability of utilities: water, fuel, power<br />Chemical processes invariably require large quantities of water for cooling and general process use, and the plant must be located near a source of water of suitable quality.Process water may be drawn from a river, from wells, or purchased from a local authority.At some sites, the cooling water required can be taken from a river or lake, or from the sea; at other locations cooling towers will be needed.<br />Electrical power will be needed at all sites. Electrochemical processes is an example of a process which require large quantities of power.<br />A competitively priced fuel must be available on site for steam and power generation.<br />
6. Availability of suitable land.<br />Suitable land area with reasonable land rates (for hire or purchase) has to be identified and selected.<br />May require approaching the local council office to explore the possibilities.<br />7. Environmental impact, and effluent disposal.<br />In a building chemical process plant, the environmental impact has to be assessed.<br />The assessment should be done to ascertain the impact of the surrounding as a result of building the plant. <br />Normally, an environmental impact assessment study will be conducted prior to project approval by the local council.<br />
8. Local community considerations.<br />The local community in terms of facilities and support that could be provided.<br />9. Climate.<br />Climate can have an important bearing on the economic operation of the process. Either a hot or cold severe climate, the cost of protective buildings, facilities for personnel and utilities must be considered especially when performing the economics. <br />10 Political and strategic considerations.<br />Stable country and political situation where there are not much possible public disturbance <br /> Financial incentives provided by the government and the tax policy<br /> Financial facilities provided by the local businesses.<br />
Site/Plant Layout<br />As with so many aspects of design, the layout of a process plant is not an exact science but rather an art, as it embraces a high degree of experience coupled with the need to anticipate the human elements in both operation and maintenance. It is an important factor in that a carefully planned, functional arrangement of equipment, buildings and pipeworks is the key to economical construction and efficient operation. <br />Some of the principles to consider;<br /><ul><li>The process units and ancillary buildings should be laid out to give the most economical flow of materials and personnel around the site.
Hazardous processes must be located at a safe distance from other buildings. Consideration must also be given to the future expansion of the site.
The ancillary buildings and services required on a site, in addition to the main processing units (buildings), will include; </li></ul>Storages for raw materials and products, tank farms and warehouses.<br /> Maintenance workshops.<br /> Stores, for maintenance and operating supplies.<br /> Laboratories for process control.<br />
<ul><li>The ancillary buildings and services required on a site, in addition to the main processing units (buildings), will include (cont); </li></ul>Fire stations and other emergency services.<br />Utilities: steam boilers,compressed air, power generation, refrigeration, transformer stations.<br />Effluent disposal plant.<br />Offices for general administration.<br />Canteens and other amenity buildings, such as medical centres.<br />Car parks.<br />There are two general methods which plant equipment could be positioned ;<br />i. Group Pattern – vessels, exchangers, columns, pumps etc., are grouped together in separate areas for ease of operation and maintenance<br />ii. Flowline Pattern - equipment is laid out as arranged on the process flowsheet.<br />In larger plants, the first method is always used due to large numbers of similar units being employed. However, in practise a compromise between the two methods is normally used.<br />
The following guidelines may be employed in designing the site/plant layout ;<br />i. Minimum Labour Demands<br /><ul><li>When labour cost is high, automation could result in significant reduction in labour demands. Therefore, a central control room is required. Nevertheless, some labour outside the control room is still required to perform certain manual operation but is kept to a low number.
For batch operation and start up/shut down operation of continuous plants where labour demands are high, considerable savings could be made by arranging the units in an integrated manner as much as possible. This will minimise movements required over all the unit and thus could reduce the labour required. This will also help in the case of maintenance provided proper spacing is allocated for the all the units.</li></ul>ii. Elevation of Equipment<br /><ul><li>Elevation of equipments is expensive and should only be kept to the absolutely necessary ones to ensure efficient operation eg. units employing gravity flow. There are cases where heavy and bulky items are elevated such as reactor but to arrange it’s elevation with the rest of the process according to the process flow will enable advantage to be taken on using the gravity as the flow ‘force’ this eliminating pumps/compressors/conveyor belt thus cheaper on maintenance.
In general, heavy and bulky unit should be placed on the ground with proper support even at the expense of using pressure to force the flow unnaturally eg. pressure in distillation unit pushes the vapour flow down to almost ground level where condenser is located.</li></li></ul><li>iii. Operating Convenience<br /><ul><li>Equipments requiring frequent attention should be grouped together to facilitate operation and maintenance. However, the safety clearance between the units have to be observed to ensure safest possible arrangement and the most hazard prone equipment is placed at the location most convenient for it to be removed.
A rectangular setup with a central over head pipe rack permits equipments to be installed along both sides of the pipe way with ease of access. </li></ul>iv. Lay out of Specific Plant Equipments<br /><ul><li>It is convenient to locate pumps in line along each side of an access way with the motors aligned outwards for easy access – maintenance.
Equipments requiring large cranes for services should be located at the perimeter of the rectangular set up, adjacent to a main roadway.
Compressors (expensive items) should be installed to allow for rapid dismantling and reassembly thus avoiding from the needs to have a stand by unit. Use compressors with bottom suction and discharge connections and supporting it on a platform above ground level (approx 2.5 m or so)</li></ul>v. Layout of Process Units<br /><ul><li>Large individual process units should be separated for efficient operation and maintenance and to avoid possible spread of fire and explosion.
A master plan should be made for grouping these equipments together and for future expansion. </li></li></ul><li>Other useful factors to consider;<br /><ul><li>The cost of construction can be minimised by adopting a layout that gives the shortest run of connecting pipe between equipment, and the least amount of structural steel work.However, this will not necessarily be the best arrangement for operation and maintenance.
Valves, sample points, and instruments should be located at convenient positions and heights.
Heat exchangers need to be sited so that the tube bundles can be easily withdrawn for cleaning and tube replacement.
Vessels that require frequent replacement of catalyst or packing should be located on the outside of buildings.
Equipment that requires dismantling for maintenance, such as compressors and large pumps, should be placed under cover.
Blast walls may be needed to isolate potentially hazardous equipment, and confine the effects of an explosion.
Equipment should be located so that it can be conveniently tied in with any future expansion of the process.
Space should be left on pipe alleys for future needs, and service pipes over-sized to allow for future requirements.</li></li></ul><li>Other useful factors to consider (cont.) Services.;<br /><ul><li>Buildings of an ancilliary nature such as offices, workshops, canteen and power supply should be located so as to afford maximum convenience with minimum interference with operation of the plant.
Relief devices that can vent inflammable and noxious fumes in an emergency should be located down wind of the administrative facility.
Storage areas should be positioned for ease of access from public roads and railways and remote from hazardous areas.
Facilities for generation and distribution of services (power supply, steam & water supply) should be located in a completely safe area.
For road used by all types of vehicles at all times, they should be surfaced and main two-way road should be at least 20ft (6.1 m) width with 30 ft (9.3 m) minimum centre line radius to permit the turning of 3-4 axle vehicles.</li></ul>Other useful factors to consider (cont.) Piping;<br /><ul><li>Overhead pipe network / yard piping which carries process materials and main utilities are normally long and should be sited at or below ground level using racks. In most cases, the power supply and instruments line are carried on the same structure.
Large pipes carrying process materials through the main process equipments should be made as shortest possible but meeting all the clearance required. This could be achieved by grouping the common equipments together.</li></li></ul><li>Conclusions…<br />We have gone through a number of main factors governing ;<br /><ul><li>the selection of site location which could have significant impact on the construction and operation of the process plant.
the design of site layout which takes into account of operation and maintenance requirement</li></ul>Above all, there is no clear / systematic guidelines or procedures for designing site layout other than leveraging on ‘some common sense rules’ thus making it more of an art rather than scientific approach.<br />
Lecture 4. Plant Utility System<br />Many processes operate with a common utility system generated and located centrally.<br />Emission<br />BOILER FEEDWATER TREATMENT & DE-AERATOR<br />VHP<br />Fuel<br />BOILER /USING FUEL OR GAS TURBINE EXHAUST<br />POWER<br />Boiler Blowdown<br />BACK PRESSURE TURBINE<br />Condensate Return<br />HP<br />POWER<br />POWER<br />LET DOWN VALVE<br />BACK PRESSURE TURBINE<br />BACK PRESSURE TURBINE<br />MP<br />Steam Mains to Process<br />POWER<br />LET DOWN VALVE<br />CONDENSING<br />TURBINE<br />LP<br />CW<br />
The centralised utility system supplies steam, power and cooling water to the processes. It can also supply compressed air and refrigeration.<br />Cooling water cycle<br />Steam Supply<br />PROCESS 1<br />Condensate Return<br />Steam Supply<br />PROCESS 2<br />CENTRALISED UTILITY SYSTEM<br />Condensate Return<br />Steam Supply<br />PROCESS 3<br />Condensate Return<br />POWER<br />
Steam is extensively used in most chemical/petrochemical plants for ;<br /><ul><li> indirect heating in steam heaters
Does not require expensive materials of construction</li></li></ul><li>The main components or equipments in the central utility system normally comprise of;<br />1. Boiler feedwater treatment<br />To treat water for removing suspended solids, dissolved solids, dissolved salts and dissolved gases.<br />Filter<br />Ion Exchange<br />To remove particularly calcium and magnesium which could cause fouling in heat exchanger<br />To remove all inorganic salts by using strong acid cation and base anion resin.<br />Vent<br />To remove dissolved gases principally O2 and CO2 which could cause corrosion.<br />LP steam<br />Chemical Treatment<br />To further remove the remaining metal ions and dissolved gases left<br />Deaerator<br />To boiler<br />
2. Steam Boilers<br />Fire tube boiler.<br />A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.<br />
Water tube boiler.<br />A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. They are used for high-pressure boilers. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes. In smaller boilers, additional generating tubes are separate in the furnace, while larger utility boilers rely on the water-filled tubes that make up the walls of the furnace to generate steam. The heated water then rises into the steam drum. Here, saturated steam is drawn off the top of the drum. In some services, the steam will reenter the furnace through a super heater to become superheated. <br />
3. Steam Turbines<br />A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.<br />HP<br />LP<br />
4. Gas Turbines<br />A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature and pressure of the exhaust gas.<br />GE H series power generation gas turbine: in combined cycle configuration, this 480-megawatt unit has a rated thermal efficiency of 60%.<br />
4. Steam System Configuration<br />The steam system configuration enables the use of steam to be systematically and efficiently exploited. The general policy adopted for steam usage is that for heating, lower pressure steam is always used in preference to the high pressure steam. This would enable ;<br /><ul><li>Higher pressure steam to be used for power generation
Higher latent heat content in the steam for the steam heater
Lower capital cost heat transfer equipment due to lower pressure.</li></ul>Steam system typically has 3 levels of steam pressure namely HP, MP and LP. For larger sites, a higher level namely very high pressure (VHP) steam is generated for power production. The let down steam from the turbine is then channeled into the steam system.<br />At times, the steam is expanded to lower pressure using expansion valve. <br />Flow control, or metering, of the expanded fluid is accomplished by use of a temperature sensing bulb filled with a similar gas as in the system that causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. <br />
Also, every steam heaters used for heating process stream will be equipped with steam traps.<br />A steam trap is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically. Others, like venturi traps, are based on turbulent 2-phase flows to obstruct the steam flow. The three important functions of steam traps are:<br /><ul><li>Discharge condensate as soon as it is formed.
Have the capability of discharging air and other non-condensable gases. </li></li></ul><li>5. Cooling Water<br /><ul><li>Cooling water circulation is used for process cooling such as in condenser or cooler. The cooling water used has to be treated where possible to minimise fouling and corrosion thus extending equipment life and maintaining efficient heat transfer.
Natural and forced-draft cooling towers are generally used to provide the cooling water required on a site; unless water can be drawn from a convenient river or lake in sufficient quantity.
Sea water, or brackish water, can be used at coastal sites, but if used directly will necessitate the use of more expensive materials of construction for heat exchangers.
Cooling tower constitutes the main component of the cooling water circulation system besides the pumps needed for the water circulation and the chemical treatment required.</li></li></ul><li>Cooling Tower<br />Industrial cooling towers can be used to remove heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and for other industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, etc. A typical large refinery processing 40,000 metric tonnes of crude oil per day (300,000 barrels (48,000 m3) per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.<br />There are 3 main types of heat transfer mechanism employed:<br /><ul><li>Wet cooling towers or simply open circuit cooling towers operate on the principle of evaporation. The working fluid and the evaporated fluid (usually H2O) are one and the same.
Dry Cooling Towers operate by heat transfer through a surface that separates the working fluid from ambient air, such as in a tube to air heat exchanger, utilizing convective heat transfer.
Fluid coolers or Closed Circuit Cooling Towers are hybrids that pass the working fluid through a tube bundle, upon which clean water is sprayed and a fan-induced draft applied. </li></ul>There are three types of cooling towers:<br />Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. <br />Mechanical draft, which uses power driven fan motors to force or draw air through the tower. <br /><ul><li>Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower.
Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. </li></li></ul><li>A forced draft cooling tower<br />
6. Refrigeration (Vapour Compression Cycle)<br />Vapor-compression refrigeration is one of the many refrigeration cycles available for use. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems.<br />
Conclusions…<br />We have gone through the main components and arrangement of a typical plant centralised utility system which consists of ;<br /><ul><li>Boiler feed water treatment