Plant location and plant layout

Why it matters

Where a plant is built fixes its freight bill, its utility costs, its environmental clearance and its access to people for its whole life; a poor site cannot be corrected later. How the equipment is arranged on that site decides piping cost, safety distances, ease of maintenance and room to expand. Both decisions are made early, with incomplete data, so engineers use structured methods rather than instinct.

Key ideas

Plant location (site selection) is usually done in two steps: choosing a region, then a specific site within it. The main factors are:

• Raw materials – distance, reliability and price of supply. Processes that lose much weight (mass of raw material per tonne of product well above 1, as in ore processing or cement) are pulled towards raw materials.
• Markets – distance to customers; products that are bulky, hazardous or perishable, or whose freight rate is high, pull the plant towards the market.
• Energy and utilities – cheap reliable power, fuel and steam; large volumes of cooling and process water; for electrolytic or energy-intensive plants this can dominate.
• Transport – rail, road, port and pipeline access; jetty facilities for bulk liquids.
• Water supply and effluent disposal – permission to draw water and to discharge treated effluent; a receiving body that can take it.
• Climate and site conditions – flooding, earthquake zone, soil bearing capacity (affects foundation cost), humidity and temperature (affect cooling-tower performance).
• Labour and community – availability of skilled people, housing, schools, hospitals, attitude of the local community.
• Regulation and incentives – environmental clearance, pollution-control board consent, zoning, state tax incentives, special economic or chemical zones.
• Land – cost, area for future expansion, buffer distance from habitation.

Comparing sites. Two common methods:

1. Cost comparison – for factors that can be costed (freight, power, water, land, taxes), compute the total annual cost at each site.
2. Weighted factor rating – for factors that cannot be costed directly, give each factor a weight (weights sum to 1), score each site on each factor (say out of 10), and compare the weighted totals. If totals are close, test how sensitive the ranking is to the weights.

Plant layout is the arrangement of process units, storage, utilities, buildings and roads on the site, and of equipment within each unit. Principles:

• Follow the process sequence so that materials move short distances and piping is short (piping can be a large share of fixed capital).
• Use gravity flow where it saves pumps.
• Keep safe separation between hazardous units, storage tanks, fired heaters, control room and the site boundary; locate flammable storage downwind of ignition sources, with bunds (dykes) around tanks. Separation distances come from codes and insurer guidelines — take them from your code book.
• Provide access for cranes, fire tenders and maintenance; leave space for tube-bundle pulling and catalyst change.
• Allow for expansion.
• Group utilities, and keep offices and the control room away from hazardous areas.

Types of layout. Chemical plants are usually laid out on a unit-area or flow-line (product) basis, with equipment grouped by process section. A process (functional) layout groups similar equipment (all mills together, for example), suits multipurpose or batch plants and gives flexibility at the cost of longer material movement. A fixed-position layout, where the product stays put and resources come to it, suits one-off construction projects. Layout studies use plot plans, 3-D models and elevation drawings.

Formulas

Freight cost = Σ (mass moved × distance × freight rate) Total annual cost of a site = Σ (costable annual items) Weighted score of a site = Σ wᵢ·sᵢ, with Σ wᵢ = 1 Mass of raw material per unit product: r = m_raw / m_product

Symbols: mass in tonnes (t), distance in km, freight rate in ₹/(t·km), wᵢ weight of factor i (–), sᵢ score of the site on factor i (same scale for all factors), r raw-material ratio (–). Weighted scoring compares sites only on the scale chosen; it is not a cost.

Worked examples

Example 1 (standard). Two sites are scored out of 10. Weights: raw materials 0.35, market 0.25, utilities 0.20, labour 0.10, climate 0.10. Site X scores 8, 6, 7, 5, 9; site Y scores 6, 9, 8, 7, 5. Which site ranks higher?

1. Score = Σ wᵢ·sᵢ.
2. X: 0.35×8 + 0.25×6 + 0.20×7 + 0.10×5 + 0.10×9 = 2.80 + 1.50 + 1.40 + 0.50 + 0.90 = 7.10.
3. Y: 0.35×6 + 0.25×9 + 0.20×8 + 0.10×7 + 0.10×5 = 2.10 + 2.25 + 1.60 + 0.70 + 0.50 = 7.15.
4. Site Y ranks higher, 7.15 against 7.10 — but the margin is tiny, so the weights should be checked before deciding.

Example 2 (GATE level). A plant makes 50 000 t/yr of product using 1.4 t of raw material per tonne of product. Freight rates: raw material ₹2.5 per t·km, product ₹3.0 per t·km. Site A is 40 km from the raw-material source and 300 km from the market; site B is 250 km and 60 km. Compare annual freight.

1. Raw material moved = 1.4 × 50 000 = 70 000 t/yr.
2. Site A: 70 000 × 40 × 2.5 + 50 000 × 300 × 3.0 = 70 00 000 + 4 50 00 000 = ₹5.20 crore/yr.
3. Site B: 70 000 × 250 × 2.5 + 50 000 × 60 × 3.0 = 4 37 50 000 + 90 00 000 = ₹5.275 crore/yr.
4. Site A is cheaper by ₹7.5 lakh per year. Even though the process loses weight, the higher product freight rate almost offsets it; a small change in either rate could reverse the choice.

Common mistakes

• Comparing sites on freight alone and ignoring power, water, effluent consent and land cost.
• Forgetting that the raw-material tonnage is r times the product tonnage.
• Weights that do not add to 1, or scores on different scales for different factors.
• Treating a narrow weighted-score win as decisive without a sensitivity check.
• Laying out hazardous storage close to the control room or upwind of fired heaters.
• Leaving no room for maintenance access or future expansion.

For GATE CH

This topic produces mostly conceptual questions: which factor dominates the location of a given type of plant, which layout suits a continuous versus a multipurpose plant, and layout safety principles. Simple numericals on freight or weighted scores are possible. Practise reasoning about weight-losing versus market-oriented industries.

Quick check

1. Why are cement and ore-processing plants usually located near their raw materials?
2. Name the layout type best suited to a multipurpose batch plant.
3. Weights 0.5, 0.3, 0.2; scores 6, 8, 10. Weighted score?
4. Give two reasons tank farms are surrounded by bunds and kept away from the control room.

Answers: 1. They lose much weight in processing, so moving raw material is costlier than moving product. 2. Process (functional) layout. 3. 0.5×6 + 0.3×8 + 0.2×10 = 7.4. 4. Bunds contain spills and limit the spread of a pool fire; distance protects people from fire, explosion and toxic release.