When it comes to large, welded steel storage tanks, two of the most widely used standards are API 650 and API 620, both published by the American Petroleum Institute. While these standards share a common goal — ensuring safe, reliable aboveground storage of liquids — they are designed for different service conditions. Understanding the differences between API 650 and API 620 tanks is essential for selecting the right solution for your storage needs.
API 650 Tanks: For Atmospheric and Low-Pressure Storage
API 650 covers the design and construction of welded steel tanks for oil storage and other liquids at near-atmospheric pressures. These tanks are typically vertical, cylindrical, and flat-bottomed, with an internal pressure limit of 2.5 pounds per square inch gauge (psig).
They are ideal for storing:
- Crude oil and refined petroleum products
- Potable and industrial water
- Chemicals and certain food products
API 650 tanks are the industry standard for refineries, fuel terminals, water treatment plants, and manufacturing facilities where large volumes of liquid must be stored at or near atmospheric pressure. They are engineered for durability and environmental conditions such as wind, seismic activity, and temperature changes.
API 620 Tanks: For Low-Pressure and Cryogenic Service
API 620 addresses the design and construction of large, welded, low-pressure storage tanks. These tanks can handle internal pressures up to 15 psig and are suitable for much lower operating temperatures — as low as −325°F — making them ideal for cryogenic storage.
API 620 tanks are often used for:
- Liquefied natural gas (LNG)
- Liquefied petroleum gas (LPG)
- Liquid nitrogen, oxygen, and other cryogenic liquids
- Petroleum products requiring slightly higher pressure storage
Many API 620 tanks feature double-wall construction with insulation to maintain extremely low temperatures. They can be vertical or horizontal and are often much larger in capacity than API 650 tanks, holding tens of millions of gallons.
Key Differences Between API 650 and API 620
The main differences come down to pressure, temperature, and application:
| Feature | API 650 | API 620 |
|---|---|---|
| Pressure Limit | Up to 2.5 psig | Up to 15 psig |
| Temperature Range | −40° F to 500° F | −325° F to 250° F |
| Typical Use | Atmospheric liquid storage | Low-pressure & cryogenic storage |
| Common Products | Oil, water, chemicals, food-grade liquids | LNG, LPG, cryogenic gases, pressurized liquids |
| Construction | Vertical, cylindrical, flat-bottom | Vertical or horizontal; can be double-walled; allows for different shapes and configurations as long as it has a single vertical axis of rotation such as spheres |
| Capacity | Thousands to tens of millions of gallons | Thousands to tens of millions of gallons |
Choosing the Right Tank Standard
Selecting between API 650 and API 620 depends on the product being stored, the required operating pressure, and the temperature range.
- If you need a standard atmospheric storage tank for oil, water, or chemicals, API 650 is the go-to choice.
- If your application involves low-pressure gases, cryogenic liquids, or products requiring higher internal pressure, API 620 is the better option.
Both standards ensure that tanks are engineered for safety, environmental protection, and long-term performance. By choosing the correct API specification, facility owners can maximize operational efficiency while ensuring compliance with industry best practices.
