TYPES OF STORAGE TANK

TYPES OF STORAGE TANK

According to the material, storage tanks can be divided into metal tank and nonmetallic tank. Steel container for storing liquid or gas is called steel storage tank. Steel storage tank is indispensable and important equipment for petroleum, chemical, grain and oil, food, firefighting, transportation, metallurgy, national defense and other industries.

Types of storage tanks can be divided in accordance with the position, shape, and structure of storage tank.

According to the position: aboveground storage tank, underground storage tank.
According to the oil type: crude oil storage tank, diesel tank, fuel oil tank, lubricating oil tank, edible oil tank, fire water tank, etc.
According to the application: industrial storage tank, commercial storage tank.
According to the shape: vertical storage tank, horizontal storage tank.
According to the structure: fixed roof tank, floating roof tank, spherical tank.
According to the capacity: tank with capacity more than 100m³ are large storage tanks, most of which are vertical storage tanks; the others are small storage tank, mostly are horizontal storage tank.

Aboveground storage tank

Refer to the tank bottom is located in the design elevation and above, the bottom of the tank below the design elevation but not more than the height of the tank 1/2 also known as the ground tank.

Underground storage tank

Underground tank refers to the storage tank whose liquid level is below 0.2m of the tank design elevation.

Vertical storage tank

Generally, small scale storage tank adopts horizontal tank, large storage tank adopts vertical storage tank. Liquid level in most of vertical storage tank are higher, in many cases, the static pressure of liquid column can not be neglected. Compared with horizontal storage tank, vertical storage tank covers smaller areas, but it also needs to take into account the wind load and static pressure in the design process.

Horizontal storage tank

Capacity of horizontal storage tank is generally less than 100m³, usually used in the production process or gas station.

Fixed roof storage tank

It refers to the storage tank for storing liquid with high flash points (eg. fuel oil, water, diesel, kerosene, bitumen, etc.) Fixed roof tank can also be divided into cone roof storage tank, dome roof storage tank and umbrella roof storage tank. All of these roofs are insulated in order to prevent the clogging of some materials, wherein the heat is provided steam coils in within the storage tank.

Dome roof storage tank

Dome roof tank is a kind of steel container with a spherical roof and a cylinder tank body. Dome roof storage tank has the advantages of simple manufacturing and low cost, thus it is widely used in various of fields. The common capacity is 1,000-10,000m³, the maximum manufacture ability of TICO for dome roof storage tank can up to 30,000m³.

Floating roof tank

Floating roof tank can be divided into external floating roof tank (usually called floating roof tank) and internal floating roof tank.

External floating roof tank

The floating roof of floating roof storage tank is a floating cover on the surface of the stored liquid, floating up and down with the input and output of the liquid. There is an annular space between the floating roof and the tank wall, the annular space has a sealing device, so that the liquid in the storage tank can be isolated from the atmosphere when the top roof is floating up and down, thereby greatly reducing the evaporation loss of the stored liquid in the storage process. When the floating roof tank is used to store oil, it can reduce the loss of oil by about 80%, compared with the fixed roof tank.

Internal floating roof tank

The internal floating roof tank is a floating roof tank with a fixed tank roof, is a new type of storage tank which is combined the dome storage tank with the floating roof storage tank. The top of internal floating roof tank is the combination of dome roof and floating roof, the exterior is floating roof, and the interior is fixed roof. Internal floating roof tank has the unique advantages, on the one hand, compared with external floating roof tank, internal floating roof tank can prevent the wind, sand, rain, snow and dust from invading, guarantee the quality of stored liquid absolutely. What’s more, the internal floating roof floats upon the liquid making the liquid has no vapor space, reducing evaporation loss 85%-96%, reducing the risk of air pollution, fire and explosion, it can easily ensure the quality of stored liquid, especially is suitable for the storage of high-grade gasoline, jet fuel and toxic petrochemical products. As there is no gas space on the liquid level, the corrosion of the tank wall and tank roof is reduced, thereby prolonging the service life of the storage tank. On the other hand, under the same seal condition, compared with external floating roof tank, internal floating roof tank can further reduce the evaporation loss. The internal floating roof tank also has disadvantages: compared with dome roof tank, internal floating roof tank consumes more steel plates, and the construction requirements are higher. Compared with external floating roof tank, the maintenance is inconvenience, it is a sealed structure, and the tank is not easy to be large scale, at present, capacity of internal floating roof tank is generally not more than 10,000m³.

Types of Industrial Gas Storage Tanks

There are different types of storage tanks in industries available. The type of industrial gas storage tank required by a plant depends on several factors including the holding capacity, measurement, and shape of the container.

The most common types of industrial tanks are as follows:

Internal Floating Roof

Internal floating roof storage tank, popularly known as IFR, is employed for liquids having lower melting points like Gasoline and Ethanol. These tanks have a cone shape with a floating roof within, hence its name is floating roof tank. The roof suspended within the storage tank moves along the level of the fluid, i.e., with the increase of the liquid, the roof moves up and with the lowering of the liquid, the roof goes down. If the tank has no liquid, the roof of the tank is held by the legs on which it is placed.

Bullet Tanks

Bullet tanks are sphere and flat-shaped industrial storage tanks used mostly in large plants. These types of tanks are usually preferred over the other types to store liquefied gases like Butane and Ammonia for which the melting point is very low.
Its name derived from the tank’s long cylindrical form, the tank stores these liquids at a temperature below -148 degrees Fahrenheit. These types of tanks mostly have the ability to hold huge amounts of gases that range from 5,000 to 30,000 gallons. Additionally, they can be installed horizontally as well as vertically. A unique characteristic of bullet tanks is that they have the ability to hold fuel as well as other additives required for the operation of the plant.

LNG Storage Tank

LNG storage tank or liquefied natural gas storage tank is specially used to store Liquefied Natural Gas. These types of storage tanks in industries are available in LNG carriers, in ground, or above ground. LNG storage tanks have the special characteristic of being able to hold LNG at the extremely low temperature -162 °C.

These tanks consist of double containers – the inner container holds LNG whereas the outer one holds insulation materials. The full containment storage tank makes the most common type of tank. Roughly, the diameter of such a tank is 75 m and the height is 55 m (180 ft.).

If the LNG vapors aren’t released in such tanks, the temperature and pressure inside the tank would keep on rising. Being a cryogen, Liquefied Natural Gas is stored in its liquid form at extremely low temperatures. The temperature inside these tanks would stay constant in case the pressure is maintained as constant by releasing the boil off gas from the tank. Such phenomenon is referred to as auto-refrigeration.

Difference between Compressed Natural Gas & Liquefied Natural Gas

Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG) are often confused with each other. Both are natural gas in stored form. The chief difference between the two is that Compressed Natural Gas is stored at high pressure and ambient temperature, whereas Liquefied Natural Gas is stored at ambient pressure and low temperature. Under their respective conditions of storage, CNG is supercritical fluid and LNG is liquid.

The cost of storage and production is lower for CNG than LNG since it doesn’t need cryogenic tanks and a costly cooling process. However, for CNG, much larger volume is required for storing an energy equivalent of petrol or gasoline as well as extremely high pressures (205 to 275 bar, or 3000 to 4000 psi). Due to this, Liquefied Natural Gas is often employed for the transportation of natural gas over long distances, in pipelines, trains, or ships, where conversion of the gas into CNG is done before distributing to end user.

SPHERICAL STORAGE TANK :

Spheres are typically used to store below ambient temperature liquids and pressurized gases such as ammonia, propylene, LPG, butadiene, etc. Most (or many) spheres operate at low temperatures with -50°C (-58°F) as a lower limit. Gases are stored under pressure at a temperature lower than their liquefaction temperature.

The main advantage of the spherical construction is that the stress concentration in a spherical shape will be minimal while storing pressurized gases as the stress resistance will be uniform over the total surface. Another advantage is the reduced exterior surface for a given storage volume as compared to all other possible shapes.

Because of the cold storage temperatures and the often, high ambient temperatures and humidity outside the sphere, it is a key requirement to have a well designed and installed thermal insulation system in place to keep the temperature inside the sphere consistent.

STORAGE TANK NECESSARY ITEMS ACCORDING TO THEIR DESIGN REQUIREMENT

• Sampling and gauging hatch – Sometimes referred to as a thief hatch, this is necessary to retrieve samples of contents for analysis. It is also used as an access point for a dip-stick to gauge the level in the tank and to check level measuring instruments.

• Vapor vent – The vapor vent is an essential feature of a tank and is often called a breather valve. Without a properly functioning vapor vent, tanks are subject to damage from the buildup of pressure when filling. They also are at risk of imploding from the vacuum created during emptying.

• Relief valve – Relief valves are designed to relieve excess pressure should it rise above the safe operating limit.

• Inert gas blanket – Inert gas blankets are used to displace air that may form an explosive or flammable mixture with vapors from volatile liquids in the tank. Nitrogen is often used for this purpose.

• Drain– Drains are used to remove any water that separates from the product. They also are used to empty the tank during cleaning operations.

• Fire protection – Tanks are often fitted with a ring of nozzles that spray foam into and over the surface of the tank in the event of a fire.

• Earth connection – Static electricity tends to form during filling and emptying of the tank. This constitutes a potential explosion or fire hazard, especially when handling flammable liquids. An earth connection prevents the buildup of static electricity.

• Manhole – Manholes can be located on the roof of the vessel or at ground level to facilitate access to the inside of the tank for inspection, cleaning and maintenance.

Floating Roof Storage Tanks

Many tanks are designed with a roof that floats on top of the liquid. The floating roof moves up and down as the level of liquid in the tank changes during filling, emptying or with ambient temperature variation.

A floating roof minimizes the amount of vapor between it and the liquid, leaving little space for evaporation and reducing vapor losses. In many older tanks, the floating roof is merely a large flat disc covering the surface of the liquid. However, in recent years, the design of floating roofs has substantially improved to minimize vapor loss.

There are two main types of floating roof storage tanks in use today. They are:

• The pontoon roof – Pontoons are closed compartments that are fitted to the outer edge of a central plate under the roof, which allow the roof to float on top of the liquid.

• The double-deck roof – As the name suggests, this design incorporates two decks of steel plates separated by compartments covering the entire surface area of the decks. This provides added stability and is advantageous because the lower deck in closest proximity to the liquid is not exposed to the sun’s rays, which considerably reduces vapor losses.

Floating roof storage tanks share many of the features as fixed roof tanks, with some notable differences:

• Access ladder – The access ladder extends over the top and down the inside of the tank. As the floating roof moves up and down, the internal ladder slides back and forth on a track.

• Seal – A strip of flexible, rubberized material is used to provide a seal between the rim of the floating roof and the tank shell to reduce vapor leakage.

• Water drain – The roof slopes slightly to a well in its center. Rainwater runs into this water trap and is drained to the outside of the tank via a flexible pipe.

• Roof supports – Support legs are fitted underneath the roof to leave a space when the tank is empty. This enables access for cleaning and maintenance.

Following table lists the API standard which are relevant to design, operation and maintenance activities for petroleum and petroleum product storage tanks.

API Standard	Title	Description
API-620	Design and Construction of Large, Welded, Low Pressure Storage Tanks	Guide for construction of tanks with internal pressures up to 15 psig. For large tanks which are assembled in field and are used for storage of petroleum intermediates and petroleum products.
API 650	Welded Steel Tanks for Oil Storage	This document governs the construction of tanks storing hydrocarbon products at low operating pressure (up to 2.5 psig).
API-651	Cathodic Protection for Above Ground Petroleum Storage Tanks	Discussion on corrosion problems in above ground petroleum storage tanks and associated piping. Plus description of two commonly used methods of providing cathodic protection against corrosion.
API-652	Lining of Above Ground Petroleum Storage Tanks	Guide effectively limiting corrosion by lining the tank bottom, for hydrocarbon storage tank made of steel. Guidelines are given regarding lining materials, their applications, inspection of tank bottom lining etc.
API-653	Tank Inspection, Repair, Alteration, and Reconstruction	Guideline for inspection, repair, alteration, and reconstruction of above ground hydrocarbon and chemical storage tanks, made of steel. Minimum requirements for maintaining the integrity of welded or riveted, non-refrigerated, atmospheric, above ground storage tanks, are discussed.
API-2000	Venting Atmospheric and Low-Pressure Storage Tanks	Guide for normal and emergency vapor venting requirements for above ground petroleum and petroleum product storage tanks, above ground and underground refrigerated storage tanks. The tanks discussed in the document are designed for low pressures ranging from full vacuum through 15 psig.
API-2015	Cleaning Petroleum Storage Tanks	Guide for preparing, emptying, isolating,ventilating, atmospheric testing, cleaning, entering into, hotwork and recommissioning activities in, on and around atmospheric and low-pressure (up to 15 psig) above ground storage tanks containing flammable,combustible or toxic materials.
API-2517	Evaporating Losses from External Floating Roof Tanks	Guide for estimating the total evaporation losses or atmospheric hydrocarbon emissions from external floating roof tanks containing multicomponent hydrocarbon mixtures or single component storage stocks.
API-2519	Evaporating Losses from Internal Floating Roof Tanks	Summary of performances of internal floating covers to prevent evaporation losses. Effects of internal floating covers on tank operation as well as the provisions for gaging, sampling, and temperature reading are described in the document.
API-2350	Overfill Protection for Petroleum Storage Tanks	Covers overfill protection for all aboveground storage tanks in petroleum facilities, including refineries, terminals, bulk plants and pipeline terminals that receive Class I (flammable) or Class II (combustible)
API-2550	Measurements and Calibration of Petroleum Storage Tanks	Procedures for calibrating upright cylindrical tanks which are larger than a drum or a barrel. Procedures for making measurements are discussed in Part I of the document and methods of calculating the tank volume are given in Part II.
API-2517	Evaporating Losses from External Floating Roof Tanks	Guide for estimating the total evaporation losses or atmospheric hydrocarbon emissions from external floating roof tanks containing multicomponent hydrocarbon mixtures or single component storage stocks.
API-2519	Evaporating Losses from Internal Floating Roof Tanks	Summary of performances of internal floating covers to prevent evaporation losses. Effects of internal floating covers on tank operation as well as the provisions for gaging, sampling, and temperature reading are described in the document.
API-2350	Overfill Protection for Petroleum Storage Tanks	Covers overfill protection for all aboveground storage tanks in petroleum facilities, including refineries, terminals, bulk plants and pipeline terminals that receive Class I (flammable) or Class II (combustible)
API-2550	Measurements and Calibration of Petroleum Storage Tanks	Procedures for calibrating upright cylindrical tanks which are larger than a drum or a barrel. Procedures for making measurements are discussed in Part I of the document and methods of calculating the tank volume are given in Part II.