A storage tank is a container designed to hold liquids, gases, or compressed materials in bulk quantities, either temporarily or for long-term use. These tanks range from small polyethylene vessels holding a few hundred gallons to massive steel structures spanning hundreds of feet in diameter. They’re found at gas stations, refineries, water treatment plants, farms, breweries, and chemical facilities, serving as the backbone of how industries store and move materials at scale.
How Storage Tanks Are Classified
Storage tanks are categorized in a few overlapping ways. The most basic distinction is placement: aboveground or underground. Underground storage tanks (USTs), like those buried several feet below gas station forecourts, benefit from natural insulation that keeps fuel at a relatively constant temperature. Aboveground storage tanks (ASTs) are easier to inspect and maintain but require more environmental safeguards since leaks are harder to contain passively.
Beyond placement, tanks are classified by shape and pressure rating. Cylindrical tanks are the most common, built either vertically (for large-volume storage on a small footprint) or horizontally (easier to transport and install). Spherical tanks are reserved for high-pressure applications because their shape distributes internal stress evenly across the entire surface, making them ideal for storing compressed gases or volatile liquids.
Rectangular tanks exist too, though they’re less common in heavy industry. You’ll see them more often in water treatment or aquaculture settings where floor space and stacking matter more than pressure tolerance. Tanks can also be single-wall or double-wall, with the second wall acting as built-in spill containment.
Common Materials and Their Strengths
The material a tank is made from depends almost entirely on what it will hold and where it will sit. Carbon steel is the workhorse of the oil and gas industry. It handles high-concentration sulfuric acid well (above 93%), but its corrosion resistance drops sharply at lower acid concentrations, which limits its use without protective linings.
Stainless steel offers better corrosion resistance and a polished appearance, which is why breweries and food-processing plants favor it. The tradeoff is cost. Stainless steel tanks are significantly more expensive to fabricate, and for many chemical storage applications, the investment isn’t justified when other materials perform equally well.
High-density crosslinked polyethylene (a type of engineered plastic) has become a serious contender for chemical storage. These tanks resist even harsh chemicals that would eat through metal, because the compounds can’t penetrate the tank wall. They’re also remarkably tough. In durability testing, semi-filled polyethylene tanks have been dropped from 45 feet without structural failure. Mining operations use them extensively for storing sulfuric acid at varying concentrations.
Fiberglass-reinforced plastic (FRP) is another option, particularly for underground installations. FRP tanks don’t corrode at all, which eliminates the need for additional corrosion protection systems that steel tanks require underground.
What Storage Tanks Hold
The list is long: heating oil, diesel fuel, gasoline, acids, bases, treated water, biofuels, liquefied petroleum gas, and compressed gases. In the food and beverage industry, tanks store milk, wine, beer, and cooking oils. Municipal water systems rely on large tanks for drinking water and wastewater. Chemical plants use them for raw materials and intermediate products at every stage of manufacturing.
The stored substance dictates nearly every design decision, from wall thickness and material to venting requirements and temperature controls. A tank storing liquefied gas at extremely low temperatures needs a fundamentally different engineering approach than one holding water at ambient temperature.
Design Standards and Pressure Ratings
Two widely referenced engineering standards govern most industrial tank construction. API 650 covers atmospheric storage tanks that operate at up to 2.5 PSI of internal pressure and design temperatures between negative 40°F and 500°F. These tanks have no size restriction and are the standard across oil, gas, and chemical storage. If you’ve seen a large cylindrical tank at a refinery or fuel terminal, it was almost certainly built to this specification.
API 620 applies to larger, pressurized tanks with contents up to 15 PSI, typically at lower temperatures (down to negative 325°F). These are designed for cryogenic and liquefied gas storage where both pressure containment and extreme cold resistance matter. Tanks built to this standard can exceed 300 feet in diameter.
Safety Components on a Typical Tank
Every storage tank needs a set of fittings that prevent dangerous pressure buildup and allow safe monitoring of contents. Conservation vents control normal pressure and vacuum fluctuations that occur as tanks heat, cool, fill, and empty throughout the day. Without them, even small temperature swings could stress the tank walls beyond their design limits.
Emergency relief vents serve a different purpose. They open rapidly during abnormal events like fires, equipment failures, or sudden pressure spikes, releasing pressure before the tank can rupture. Gauge hatches provide physical access points where operators can manually measure liquid level, temperature, and take product samples. Blanketing valves maintain a layer of inert gas (usually nitrogen) over the tank’s contents to prevent oxidation or the formation of explosive vapor mixtures.
Any fencing or enclosures around aboveground tanks must be ventilated. Trapping vapors in a confined space around a tank creates explosion and confined-space entry hazards for workers.
Corrosion Protection for Underground Tanks
Steel tanks buried underground face constant contact with moisture and soil chemistry, making corrosion the primary threat to their lifespan. Several protection strategies exist. Cathodic protection uses sacrificial metal anodes attached to the tank that corrode preferentially, drawing corrosive chemical reactions away from the tank walls. Any underground system using cathodic protection requires corrosion testing every three years.
Jacketed steel tanks and composite tanks wrap the steel structure in a corrosion-resistant external cladding, combining steel’s structural strength with an outer layer that resists soil and groundwater. Some designs, like the STI-P3 standard, combine a high-quality dielectric coating, cathodic protection anodes, and isolation devices that electrically separate the tank from other underground metal structures that could accelerate corrosion.
FRP tanks sidestep the issue entirely. Since fiberglass doesn’t corrode, these tanks need no additional protection systems, which simplifies installation and long-term maintenance.
Environmental and Spill Prevention Requirements
Because leaking tanks can contaminate soil and groundwater, storage facilities face strict regulatory oversight. Under EPA spill prevention rules, every bulk storage container installation must include secondary containment, a structure like a dike, berm, or vault that catches the contents if the primary tank fails. The secondary containment must hold the entire capacity of the largest single tank at the facility, plus enough additional space (called freeboard) to capture rainfall so the containment area doesn’t overflow during a storm.
State-level requirements add further layers. In Michigan, for example, any facility storing flammable or combustible liquids in a tank larger than 1,100 gallons (with a flash point under 200°F) must be reviewed and certified by the state. Liquefied petroleum gas facilities with a single tank over 2,000 gallons water capacity, or multiple tanks exceeding 4,000 gallons combined, face the same certification requirement. Every underground system storing hazardous substances requires state review regardless of size.
These regulations apply to heating oil distributors, gas stations, chemical plants, farms with large fuel tanks, and any other facility where a spill could reach waterways or drinking water sources. The costs of secondary containment and regulatory compliance are a significant part of any tank installation budget, but they’re far less than the cost of a major environmental cleanup.