PLANT LAYOUT

INTRODUCTION

• After the process flow diagrams are completed and before detailed piping, structural, and electrical design can begin, the layout of process units in a plant and the equipment within these process units must be planned.

• This layout can play an important part in determining construction and manufacturing costs, and thus must be planned carefully with attention being given to future problems that may arise. Since each plant differs in many ways and no two plant sites are exactly alike, there is no one ideal plant layout.

SOME IMPORTANT CONSIDERATIONS FOR PLANT LAYOUT

• Operational convenience and accessibility• Economic distribution of utilities and services• Type of process and product control• Type and quantity of products to be produced• Type of buildings and building-code requirements• Health and safety considerations• Waste-disposal requirements• Space available and space required• Roads and railroads• Possible future expansion

Plant layout contributes to safety and loss prevention

• Efficient and safe construction• Efficient and safe operation• Efficient and safe maintenance• Safe control room design• Emergency control facilities• Segregation of different risks• Minimization of vulnerable pipework• Containment of accidents• Limitation of exposure• Fire fighting facilities• Access for emergency services• security

Layout Generation

• Flow principle• Correlation and compatibility• Process plant layout• Segregation

Flow principle

• For process plants, the most appropriate method is generally to lay the plant out so that the material flow follows the process flow diagram. This is the process flow principle.

• This arrangement minimizes the transfer of materials, which is desirable both for economics and safety.

• Long runs of pipe work with vulnerable features are an undesirable addition to the hazards of the plant.

They relate particularly to: • Requirements for gravity flow• Equipment needing specially strong foundations• Access for construction, commissioning, operation

and maintenance• Future extension• Operator protection• Escape and fire fighting• Containment of accidents• Environmental impact

Correlation and compatibility

Correlation and compatibility techniques are used for:

• Elimination of layout arrangements which are incompatible or impossible

• For the preliminary formulation of compatible arrangements

Process plant layout

• The design of a process plant layout involves first synthesis and then analysis. A structured approach to the generation of the layout has four stages:

• Three-dimensional (3D) model• Flow• Relationships • Groups

Segregation

• The minimization of the risk to the target is effected by segregating the hazard from the target.

Layout Techniques and Aids

Layout Techniques• Classification, rating and ranking• Critical examination• Hazard assessment• Economic optimizationLayout Aids• Visualization aids• Computer aids

Critical examination

The technique starts with and involves analysis of a proposed layout, but insofar as other possible solutions are suggested, it may be regarded also as a method for the generation of alternatives which can then be evaluated.

Economic optimization

• Economic optimization is a principal method of selection from among the alternative layout generated due to process layout generation.

• Some factors which are of importance for the cost of a plant layout include :

foundations, structures, piping and pipe tracks, and pumps and power consumption.

Visualization aids

• There are various methods of representing the plant to assist in layout design. These include drawings, cutouts, block models and piping models.

Site Layout Features

Site constraints and standards Once a site has been selected the next step is to

establish the site constraints and standards. These include:

• Topography and geology• weather• environment• transport• services• Legal constraints

• Topographical and geological features are those such as the lie of the land and its load-bearing capabilities.

• Weather includes temperatures, wind conditions, solar radiation, and thunderstorms.

• Environment covers people, activities and buildings in the vicinity.

• Services are power, water and effluents.• Legal constraints include planning and building,

effluent and pollution, traffic, fire and other safety laws, by laws and regulations.

Site standards should also be established covering

• Separation distances• Building lines• Building construction • Road dimensions• Service corridors• Pipe bridges

Site services

• The site central services such as the boiler house, power station, switch station, pumping stations, etc., should be placed in suitable locations.

• They should not constitute sources of ignition for flammables.

• Electrical substations, pumping stations, etc., should be located in areas where non-flame proof equipment can be used.

Use of buildings

• Building is used where the process, the plant, the materials processed and the associated activities are sensitive to exposure.

• Since ventilation in a building is generally less than that outdoors, a leak of flammable or toxic material tends to disperse more slowly and a hazardous concentration is more likely to buildup.

Location of buildings

• Analytical laboratories should be in a safe area.

• Administration buildings should be situated in a safe area on the public side of the security point. The main office block should always be near the main entrance and other administration buildings.

• All buildings should be upwind of plants.

Segregation

• In process plants it is usually necessary to provide some additional space and to practice a degree of segregation.

• The site layout should aim to contain an accident at source, to prevent escalation and avoid hazarding vulnerable targets.

• A block layout is appropriate with each plot containing similar and compatible types of hazard and with different types segregated in separate plots.

Fire containment

• The site layout should contribute to the containment of any fire which may occur and to combating the fire.

Compact block layout system in the process area of a petrochemical plant

• Fig 1 show a compact layout, which minimizes land usage and pipework.

• There are two main process areas and at right angles to these is an area with a row of fired heaters, and associated reactors, steam boilers and a stack.

• This layout has several weaknesses. The lack of firebreaks in the main process blocks would allow a fire to propagate right along these, particularly if the wind is blowing along them.

• There is entry to the plant area from the 6m roads from opposite corners. But the only access for vehicles to the process plots is the 4.5m roads.

• In the case of a major fire, appliances might well get trapped by an escalation of the fire.

• The layout is also likely to cause difficulties in maintenance work.

Compact block layout system in the process area of a petrochemical plant

• The alternative layout shown in Fig 2 avoids these problems.

• The process areas are divided by fire breaks. • There are more entry points on the site and

dead ends are eliminated. • The roads are 6m wide with an effective

clearance of 15m. • The crane access areas provide additional

clearances for the fired heaters.

Transport

• It is a prime aim of plant layout to minimize the distances travelled by materials.

• Access is required to plots for transport of materials and equipment, maintenance operations and emergencies.

• Road and rail traffic should not go through process areas except to its destination and even then should not violate hazardous area classifications.

Effluents

• The site layout must accommodate the systems for handling the effluents such as gaseous, liquid and solid and fire water.

Emergencies• The first step in emergency planning is to study the scenarios of

the potential hazards and of their development.• Emergency arrangements should include an emergency control

centre. This should be a specially designated and signed room in a safe area, accessible from the public roads and with space around it for emergency service vehicles.

• Assembly point should also be designated in safe areas at least 100 m from the plants.

• The site should have a road round the periphery with access to the public roads at two points atleast.

• Arrangements should be made to safeguard supplies of services such as water, electricity and steam to plants in an emergency.

Security

• The site should be provided with a boundary fence and all entrances should have a gate house. The number of entrances should be kept to a minimum.

Plot Layout Considerations

• Process considerations• Economic considerations• construction• operations• maintenance• hazards• Fire fighting• escape

Process considerations

• Gravity flow and availability of head for pump suctions, control valves and reflux returns

• pressure drop in pipes and heat exchangers and across control valves and temperature drop in pipes

• provision of straight runs for orificemeters, the length of instrument transmission lines and arrangements for manual operations such as dosing with additives, sampling

Economic considerations

• Some features which have strong influence on costs are foundations, structures, piping and electrical cabling.

Construction

• Construction work may require an area in which the construction materials and items can be laid out.

• The installation of large and heavy plant items require space and access for cranes.

Maintenance

• Plant items from which the internals need to be removed for maintenance should have the necessary space and lifting arrangements.

Ex- Tube bundles from heat exchangers, agitators from stirred vessels and spent catalyst from reactors.

Hazards

• The hazards on the plant should be identified and allowed for in the plot layout.

• Hazardous areas should be defined.• Plants which may leak flammables should generally

be built in the open or, if necessary, in a structure with a roof but no walls.

• Fire spread in buildings should be limited by design.• Ventilation is necessary for buildings housing plants

processing flammables or toxics.

Fire fighting

• Access is essential for firefighting. Fire water should be available from hydrants on a main between the road and the plant.

• Pipes for fire water supply should be protected against explosion damage.

• Fire extinguishers of the appropriate type and fire blankets should be placed at strategic points.

Escape

• A minimum of two escape routes should be provided for any work space, except where the fire risk is very small, and the two routes should be genuine.

• Escape routes should be signposted.• Escape routes across open mess area should have solid

flooring.• Escape stairways should be in straight flights and

preferably be on the outside of building.• Good lighting should be provided on escape routes and

arrangements made to ensure a power supply in an emergency.