What a storage tank does
Almost every process plant needs somewhere to hold liquid. Raw materials arrive in bulk and wait to be used. Finished product collects before it is shipped. Water, fuel, chemicals and wastewater all need to be held somewhere between the point where they arrive and the point where they are needed. A storage tank is the simple answer, but “simple” hides a lot of choices about shape, material, pressure rating and how the tank breathes.
Tanks also act as buffers. A pump or process that runs continuously can feed from a tank that is filled in batches, and the reverse is true as well. That buffering role is why a tank is often found right next to centrifugal pumps, which move liquid in and out of it.
Main tank types
The type of tank is usually grouped by its roof and its orientation.
- Vertical fixed-roof tanks. A cylinder standing on its base with a cone or dome roof. This is the common choice for water, many chemicals and a wide range of liquids at or near atmospheric pressure.
- Floating-roof tanks. The roof rests on the liquid surface and rises and falls with the level. This leaves very little vapor space above the liquid, which is why the design is often used for volatile liquids stored in large volumes. Some floating roofs are open to the weather; others sit inside a fixed roof.
- Horizontal tanks. A cylinder lying on its side on saddles or cradles. These are common for smaller volumes, for day tanks near equipment, and for underground fuel storage.
- Cone-bottom and dished-bottom tanks. The sloped bottom helps liquids with solids, slurries or viscous contents drain completely.
- Open-top tanks. Used for water, some wastewater processes and process steps where the surface needs to stay accessible. They give no protection from weather or contamination.
- Pressurized vessels. When the contents must be held above atmospheric pressure, the equipment is no longer an ordinary tank. These are built and inspected as pressure vessels, with their own design rules and protective devices.
Very large tanks are often built on site from steel plates. Smaller ones are usually shop-fabricated and delivered whole.
Materials
The material depends mostly on what the tank holds. Carbon steel is widely used for water, fuels and many non-corrosive liquids, sometimes with a protective coating or lining inside. Stainless steel is common where cleanliness matters, such as food, beverage and pharmaceutical work, or where the contents attack carbon steel. Molded polyethylene and fiberglass-reinforced plastic tanks are often used for corrosive liquids and for moderate volumes. Concrete is sometimes used for very large water or wastewater volumes. The choice is a matter of compatibility with the liquid, temperature, cost and how long the tank is expected to last, so the supplier’s compatibility information and the applicable codes should always drive it.
Venting: how a tank breathes
A tank is not a sealed box. When liquid is pumped in, the air or vapor above it has to go somewhere. When liquid is pumped out, air has to come in, or the tank is left with a partial vacuum. Temperature changes cause the vapor space to expand and contract as well.
For this reason most atmospheric tanks have a vent. Common arrangements include:
- A free vent, an open pipe, for tanks holding harmless liquids.
- A pressure/vacuum vent, which stays closed until pressure or vacuum reaches a set limit and then opens. This reduces vapor loss and keeps outside air and moisture out.
- Flame arresters, sometimes fitted for flammable liquids, to help stop a flame from traveling back into the tank.
- Emergency relief, an extra opening designed to handle abnormal conditions such as external fire, where vapor generation can far exceed normal venting.
- Blanketing with an inert gas such as nitrogen, used where oxygen or moisture in the vapor space is a problem.
A blocked or undersized vent is a well-known way to damage a tank. Thin-walled atmospheric tanks are not built to withstand much vacuum, and pumping out against a blocked vent can pull the shell inward. Overpressure from a blocked vent during filling is equally damaging. Vents therefore belong in inspection routines and should never be capped or painted shut.
Typical uses
- Water and process water, including fire water reserves, make-up water and treated water.
- Fuels and oils, from day tanks beside a generator to bulk fuel farms.
- Chemicals, which may need special materials, secondary containment and labeling.
- Intermediate and batch tanks, where product waits between process steps.
- Wastewater and equalization tanks, which smooth out variations in flow and composition ahead of treatment.
- Compressed-air buffering, where air receiver tanks are a form of pressure storage that smooths demand on the compressors.
Fittings and support equipment
A tank is only useful with the right connections: inlet and outlet nozzles, drains, manways for access, and instrument connections. Piping to and from the tank needs flexible connections or expansion allowance where the tank may settle or move, and pipe materials must suit the liquid. Heated or outdoor tanks often need pipe insulation and trace heating on connected lines. Level and overfill instruments are central to safe operation, as are valves that isolate the tank in an emergency.
Containment, grounding and hazardous areas
Tanks holding hazardous liquids are usually surrounded by a dike, bund or other secondary containment designed to hold a leak. Static electricity can build up when flammable liquids are filled or splash, so grounding systems are part of the design. Where flammable vapors may be present around vents and fittings, hazardous area equipment rules can apply to the electrical items nearby. All of this is governed by local regulations and by the engineering standards that apply to the liquid and the site.
Working around tanks
Tanks are confined spaces. Entering one for cleaning or repair calls for permit-based procedures, atmosphere testing, isolation of all connected lines and rescue planning, covered in general terms by confined space gear and entry rules. Maintenance on the tank’s pumps, mixers and valves should follow lockout tagout procedures. Nobody should enter a tank, climb its roof or work on its vents without training and without following the site’s own procedures and the manufacturer’s instructions.
This guide is general information and not engineering advice. Tank design, selection and inspection should be handled by qualified engineers working to the codes that apply where the tank is installed.