By the MECHFAB engineering team, Guwahati | Updated October 2026 | Reading time: about 8 minutes
An oil storage tank looks simple from the road: a steel cylinder with a roof. Yet every tank holding diesel, furnace oil or crude is engineered around one hard job. Keep a flammable liquid in, keep rain and air out, and stay standing for decades through heat, monsoon and ground movement.
| Quick answer: what is an oil storage tank? An oil storage tank is a welded steel container, usually vertical and cylindrical, that holds crude oil, diesel, furnace oil or other petroleum products at or near atmospheric pressure. Its type, plate thickness, roof and safety fittings depend on the product, the volume needed, the governing codes and site conditions such as soil and seismic risk. |
This guide covers tank parts, types, crude oil tanks, design codes, safety features and capacity sizing, drawing on the experience behind the oil storage tanks in MECHFAB’s steel fabrication product range.
The subject has deep roots in our region. India’s oil story began in Upper Assam, where the Digboi refinery was commissioned in 1901 and still runs today. Storage tanks remain the quiet backbone of every fuel supply chain in the region.
What Is an Oil Storage Tank and How Does It Work?
An oil storage tank holds liquid at roughly atmospheric pressure, so the steel mainly resists the outward push of the liquid itself. That push grows with depth, which is why the lowest ring of the wall is always the thickest. Under India’s Petroleum Rules, 2002, any petroleum receptacle larger than 1,000 litres counts as a “tank”, so even a modest factory diesel tank falls under formal rules.
Every vertical tank shares the same basic parts:
- Foundation: a compacted pad or concrete ring wall that spreads the load into the ground.
- Bottom plates: welded steel floor, sloped slightly towards a drain point.
- Shell courses: stacked steel rings, each thinner than the one below.
- Roof: fixed (cone or dome) or floating, depending on the product.
- Nozzles, manholes and vents: openings for filling, drawing, cleaning, inspection and safe breathing.
Types of Oil Storage Tanks
There is no single best tank. The right one depends on what you store, how much and how often the level changes.
Fixed Cone Roof Tanks
The most common industrial tank, with a conical roof welded to the shell. It suits low vapour pressure products such as diesel (HSD), furnace oil, lubricants and bitumen, and it is simple and economical to maintain.
Dome Roof Tanks
A curved dome handles slightly higher internal pressure and spans larger diameters without internal columns, keeping the inside clear for cleaning.
External Floating Roof Tanks
A steel deck floats directly on the liquid and rises and falls with it. With almost no air space above the product, evaporation and emissions drop sharply. This is the standard choice for large crude oil and petrol tanks.
Internal Floating Roof Tanks
A light floating deck sits inside a tank that also has a fixed roof. The roof keeps out rain, while the deck controls vapour loss.
Horizontal Cylindrical Tanks
Smaller tanks laid on saddles or buried underground, common at fuel stations, DG set yards, hospitals and construction sites. They are usually shop built and delivered complete.
| Tank type | Best for | Main advantage | Watch out for |
| Fixed cone roof | Diesel, furnace oil, bitumen | Low cost, simple upkeep | Vapour loss with volatile products |
| Dome roof | Larger fixed roof tanks | Clear span, stronger roof | More fabrication effort |
| External floating roof | Crude oil, petrol | Very low evaporation | Roof drains in heavy rain |
| Internal floating roof | Volatile products needing cover | Emission control plus rain cover | Two roof systems to inspect |
| Horizontal cylindrical | Small fuel stocks, DG sets | Quick to install | Limited capacity |
Crude Oil Tanks: Why They Need Extra Attention
Crude oil tanks are the most demanding on any site. Crude gives off vapour like petrol while also carrying water and sludge that settle at the bottom. A good design handles all of it:
- Vapour control: floating roofs minimise the space above the liquid.
- Water and sludge removal: sloped bottoms, sumps and draw-off nozzles drain settled water before it corrodes the floor.
- Mixers: side-entry mixers stop sediment from eating into working capacity.
- Heating for waxy crude: Upper Assam crude has a high wax content, which is why Digboi produces paraffin wax. Waxy crude thickens as it cools, so tanks often need heating coils and insulation.
Oil Storage Tank Design: The Engineering That Matters
Good oil storage tank design puts the right thickness, details and checks in the right places.
Which Codes Govern Tank Design?
In India the core reference is IS 803, the BIS code for vertical mild steel welded oil storage tanks, covering fixed roof and open-top tanks from materials to testing. Internationally, API Standard 650, Welded Tanks for Oil Storage sets minimum requirements for atmospheric tanks up to a 93°C design temperature. Many Indian specifications use IS 803 with API 650 filling the gaps, while API 653 guides inspection of tanks already in service.
How Is Shell Thickness Calculated?
Thickness depends on diameter, liquid height, product density and allowable steel stress, plus a corrosion allowance. Because pressure falls with height, each shell course is designed separately and the plates step down towards the roof.
Wind, Seismic and Foundation Loads
An empty tank is surprisingly vulnerable to wind, which is why large tanks carry stiffening rings called wind girders. Northeast India also lies in Seismic Zone V, the highest hazard zone under IS 1893. In an earthquake the liquid sloshes and adds large forces to the shell, so designers check anchorage, freeboard and base buckling, the same thinking behind earthquake-resistant steel construction in Assam.
Much of the Brahmaputra valley sits on soft alluvial soil with a high water table. A soil investigation, a well-compacted foundation and settlement checks during the water fill test prevent uneven sinking that distorts the floor.
Materials, Corrosion and Testing
Most tanks use structural carbon steel plate, protected by internal linings, external paint and, on larger tanks, cathodic protection under the floor. Before handover, welds go through radiography and other non-destructive tests, bottom seams are vacuum box tested, and the tank is filled with water to prove it is leak-tight.
Key Safety and Design Features Every Oil Storage Tank Needs
Most tank incidents trace back to a missing or neglected fitting, not a weak shell. Specify these from day one:
- Pressure and vacuum vents: let the tank breathe as it fills, empties and warms up.
- Flame arrestors: stop flames entering through vents on low flash point products.
- Level gauges and high-level alarms: prevent overfilling, a leading cause of spills.
- Dyke wall or bund: contains the product if the tank leaks.
- Earthing and lightning protection: carry static and lightning safely to ground.
- Firefighting systems: foam, cooling water and hydrants sized under the relevant OISD standards.
- Roof seals and drains: hold vapour in and carry monsoon rain off floating decks.
- Safe access: stairways, handrails and platforms for gauging and inspection.
Oil Storage Tank Capacity: How to Size Your Tank
| Capacity formula Volume (m³) = 3.1416 ÷ 4 × Diameter² × Height 1 m³ equals 1 kilolitre (KL), or 1,000 litres. |
This gives nominal capacity. Working capacity is lower, because space is reserved at the top for expansion and the liquid below the outlet cannot be pumped out.
| Diameter x height | Nominal capacity | Typical use |
| 3 m x 4 m | about 28 KL | DG set or small boiler |
| 6 m x 6 m | about 170 KL | Industrial plant fuel |
| 10 m x 10 m | about 785 KL | Depot or large plant |
| 20 m x 12 m | about 3,770 KL | Terminal tank farm |
Worked example: a plant burning 4 KL of furnace oil a day wants 15 days of stock, or 60 KL working capacity. If about 85% of gross volume is usable (an illustrative figure), you need roughly 71 KL nominal. A 4.5 m by 4.5 m tank gives about 72 KL.
In the Northeast, monsoon landslides and flooded highways can delay tankers, so many sites keep extra days of cover.
Licences and Compliance for Oil Storage in India
Petroleum storage is regulated by PESO under the Petroleum Rules, 2002. Products are grouped by flash point into Class A (such as petrol), Class B (such as kerosene) and Class C (such as diesel and furnace oil). Licence needs depend on class and quantity, as set out on PESO’s licensing page and in its FAQ on the Petroleum Rules.
Checklist: What to Decide Before You Order
- Which product will you store, and what is its flash point class?
- What is your daily use, and how many days of stock do you need?
- Do you have a soil report for the tank location?
- Should the design follow IS 803, API 650 or both?
- Which roof suits the product: fixed, floating or internal floating?
- Shop built or site erected? Large tanks are built in place, and hill roads limit transport.
- Which drawings, test reports and certificates come with handover?
Oil Storage Tanks by MECHFAB, Guwahati
MECHFAB Engineering Industries Pvt. Ltd. has fabricated steel in Northeast India since 1978, when Mr. Bhanwarlal Agarwal founded the company. Alongside pre-engineered buildings, bridges, water tanks and chimneys, MECHFAB builds oil storage tanks for local conditions: Seismic Zone V, heavy monsoons and soft valley soils. MECHFAB has delivered more than 600 projects across the region.
| Planning an oil storage tank? Let’s talk. Get a quote: share your product, daily use and site location for a suggested tank size and type. Talk to an engineer: call [phone number to be confirmed by client] or email [email to be confirmed by client]. See our work: browse completed projects at mechfab.org/project. Free download: ask for our Oil Storage Tank Planning Checklist. |
Key Takeaways
- Match the tank to the product: cone or dome roofs for diesel and furnace oil, floating roofs for crude and petrol.
- Design to IS 803 and API 650, and check wind, seismic and soil conditions for your site.
- Build in vents, alarms, dyke walls, earthing and firefighting from day one.
- Size on working volume and confirm PESO licensing before freezing the design.
Frequently Asked Questions About Oil Storage Tanks
1. What is an oil storage tank made of?
Most are made of welded carbon steel plate, protected by internal linings and external paint. Stainless steel is used for special products, depending on the product, temperature and service life.
2. Which type of oil storage tank is best for crude oil?
Large crude oil tanks usually use an external or internal floating roof, which reduces evaporation, product loss and fire risk. Waxy crude, like that from Upper Assam, may also need heating coils.
3. How do I calculate oil storage tank capacity?
Multiply 3.1416 by the diameter squared, divide by 4, then multiply by the height. The result in cubic metres equals kilolitres. Working capacity is lower than this nominal figure.
4. Which code is used for oil storage tank design in India?
IS 803 is the main Indian code for vertical welded oil storage tanks. Many projects also follow API 650, along with IS 875 for wind and IS 1893 for seismic loads.
5. Do I need a licence to store diesel in a tank in India?
It depends on the quantity. Diesel is a Class C petroleum product, and PESO sets the limits above which a storage licence is needed. Check the current thresholds on the PESO website before installing.
6. How long does a steel oil storage tank last?
A well-designed, well-maintained steel tank can serve for several decades. Coatings, corrosion protection, water drainage and regular inspection matter most.
7. Can an oil storage tank be built safely in an earthquake zone like Assam?
Yes. Tanks in Seismic Zone V are designed for sloshing forces, with proper anchorage, freeboard and buckling checks, on a foundation built over well-investigated soil.
