What is a Storage Tank? Types, API 650 & API 620 Design, Materials & Capacity

What Is a Storage Tank?

A storage tank is a container designed to hold liquids (and sometimes gases or solids) until they are needed. In industry, tanks store raw materials, intermediates, finished products, fuels, water and wastes. Unlike a pressure vessel, which is built to contain significant pressure, most storage tanks operate at atmospheric pressure or just above, so their walls are thinner and their design codes are different.

Choosing the right tank affects safety, product quality and plant economics. A tank that is the wrong material can contaminate product or corrode; a tank that is the wrong size can limit production or tie up capital. Creative Engineering fabricates storage tanks from 1 KL to 200 KL and above, in vertical and horizontal configurations, in carbon steel, stainless steel and alloy materials.

Types of Storage Tanks

By orientation

Vertical tanks stand on a foundation or ring wall, make efficient use of land, and are used for larger volumes. Horizontal tanks lie on saddle supports, are easier to install and inspect, and suit smaller volumes, underground burial and tanker-sized installations.

By pressure

Atmospheric tanks operate at or very close to atmospheric pressure and are normally designed to API 650. Low-pressure tanks operate at a small internal pressure and follow API 620 or ASME Section VIII, and are protected by pressure and vacuum relief valves.

By roof type

Bottoms can be flat, dished or sloped; sloped or cone-down bottoms drain completely at a low-point nozzle, which helps when changing products or removing sludge.

Design Codes: API 650, API 620 and ASME Section VIII

• API 650: welded steel tanks for atmospheric storage; sets plate thickness, weld details, nozzle reinforcement and testing for vertical cylindrical tanks.
• API 620: large, welded, low-pressure tanks, typically up to a modest internal pressure above atmospheric.
• ASME Section VIII Div 1: for tanks that operate at higher pressure and for horizontal pressurised tanks.
• IS standards may also be referenced for Indian service, together with the customer's own specification.

Creative Engineering fabricates to all three code families and holds ASME U and R stamps, NB registration and IBR approval, which are valuable when tanks form part of a larger pressure-system.

Materials and Linings

For non-corrosive liquids such as water, diesel or many hydrocarbons, carbon steel or mild steel with an external coating is used. Chemical storage commonly calls for SS 304, 304L, 316 or 316L, and aggressive fluids need Duplex, Super Duplex, Inconel, Hastelloy or titanium. Where the liquid is corrosive but the cost of alloy is high, the tank can be lined with rubber, FRP, Triflex rubber, bitumen or other coatings. Carbon-steel tanks are shot-blasted and painted with epoxy or polyurethane systems for external protection.

Storage Tank Accessories and Fittings

• Manholes on roof and shell for access, cleaning and inspection.
• Nozzles and flanges for inlet, outlet, drain, vent, overflow, level and sample points.
• Level gauges and transmitters for inventory control.
• Vents and PV (pressure-vacuum) relief valves on low-pressure tanks.
• Ladders, handrails and platforms for safe access to roof fittings, see platforms and ladders.
• Heating coils or jackets and insulation for viscous or freezing liquids.
• Earthing lugs and lifting lugs.

Capacity, Sizing and Foundations

Tank capacity is expressed in kilolitres (KL) or cubic metres. 1 KL equals 1 cubic metre. The working capacity is lower than the nominal capacity because there must be freeboard at the top and an unusable layer at the bottom. When you specify a tank, state whether the volume is nominal or working volume.

Foundations are as important as the tank. A ring wall or concrete pad must carry the full weight of the liquid and spread it evenly, and it must be level; uneven settlement can distort the shell and stress the welds. Horizontal tanks need saddles on a rigid base, with provision for thermal expansion at one saddle.

Venting, Safety and Environmental Protection

Every tank breathes. When liquid is pumped in, vapour inside is pushed out; when liquid is pumped out or the tank cools at night, air is drawn in. Vents and PV valves must be sized for these flows and for emergency cases such as external fire, otherwise the tank can be over-pressurised or sucked in. For volatile or toxic liquids the vent is connected to a vapour recovery or scrubber system, and flammable liquids need earthing, bonding and in many cases nitrogen blanketing to keep air out.

Secondary containment, usually a bund (dyke) around the tank, holds a spill if the tank leaks. The bund is normally sized to contain the contents of the largest tank in it. Overfill protection, high-level alarms and leak detection add further layers of safety. For hazardous chemicals follow your site safety rules and local regulation when you select accessories.

Storage Tank vs Pressure Vessel vs Silo

Creative Engineering manufactures all three, so a plant can source tanks, vessels and silos from one fabricator and one quality system.

Typical Storage Tank Applications by Industry

• Chemical and specialty chemical plants: raw material, solvent and product tanks in stainless steel or lined steel.
• Pharma and API plants: polished SS 316L tanks for purified water, solvents and intermediates.
• Oil and gas and petrochemical: fuel, condensate and process liquid tanks, often with heating and insulation.
• Fertilizer and agro plants: liquid chemical and process water storage.
• Utilities: water, diesel and firefighting tanks for any industrial site.

Related equipment on our site includes the receiver vessel for process holdup and skid-mounted units that integrate tanks, pumps and piping.

Inspection and Maintenance of Storage Tanks

• Check the shell, roof and nozzle welds for leaks, cracks and corrosion at regular intervals.
• Measure plate thickness periodically, especially near the bottom and at the liquid line, where corrosion is faster.
• Inspect and test vents, PV valves and level instruments.
• Keep the external coating intact to protect carbon steel from rust.
• Clean the tank at product changeovers or when sludge builds up; sloped bottoms with a low-point drain make this easier.
• Inspect the foundation for settlement or cracking.

Good records of dimensions, thickness readings and repairs extend tank life and help plan replacement.

Fabrication, Testing and Quality

1. Material certification and traceability of plates and fittings.
2. Cutting, rolling and welding to qualified procedures.
3. NDT of critical welds (radiography, dye penetrant, vacuum box testing for floor seams as specified).
4. Water-fill or hydrostatic test to check leaks and distortion.
5. Blasting, coating and final inspection.
6. Documentation: drawings, calculations, MTCs, test certificates.

Our QC covers RT, UT, PT, MT and VT by certified personnel and calibrated instruments. Product ships only after required inspections and tests are completed.

How to Order a Storage Tank

1. Liquid stored, density and corrosivity.
2. Required capacity and whether it is working or nominal volume.
3. Design temperature and pressure (atmospheric, low pressure or vacuum).
4. Orientation, roof type and bottom type.
5. Material and coating or lining requirement.
6. Number, size and position of nozzles and manholes.
7. Code and inspection requirement, delivery site and schedule.

Customers in Bharuch, Vadodara, Ahmedabad and export markets can contact us through the contact page. For tanks of square or rectangular shape, see our guide to the square tank, and for dry bulk see silos.

Frequently Asked Questions