How To Classify Oil Tanks?

Oil tanks are ideal containers used to store crude oil or other kind of oil, they are widely used in refinery, oil field, oil tank farm, or other industrial production. Oil tanks play a very important role in the process of oil and gas storage and transportation. What’s more, oil tanks come in various of sizes and types according to different classification standards so that there are some differences in their installation to some extent.

Divided by structure of oil tanks

According to the structure, oil tanks can be divided into crude oil tanks, fuel tanks, lubricating oil tanks, edible oil tanks, etc. According to the characteristics of the stored oil, they can be divided into heavy oil tank and light oil tank. According to the depth of burying, they can be divided into aboveground oil tanks, underground oil tanks, and semi underground oil tanks.

Divided by the material of oil tanks

According to the material, oil tanks can be mainly divided into steel oil tanks, concrete oil tanks, and plastic oil tanks.

steel oil tanks concrete oil tanks plastic oil tanks

Steel oil tanks can be divided into vertical oil tank (including fixed roof tank and floating roof tank), spherical oil tanksand horizontal oil tank (cylinder tank).

Vertical fixed roof oil tanks, which are composed of fixed tank roof and vertical cylindrical tank wall, are mainly used for storage of non volatile oil, such as diesel oil and the similar oil. The most commonly used volume of fixed roof oil tanks right from 1,000m³ to 10,000m³. Floating roof oil tanks, which are composed of a floating roof that floats on the oil surface and a vertical cylindrical tank wall, are suitable to store volatile oil, like gasoline and the similar medium, and the volume of floating roof oil tanks are generally larger. The floating roof of oil tanks can increase or decrease following with the increase and decrease of the stored oil production in the oil tanks, a ring seal is installed between the outer edge of floating roof and tank wall. What’s more, the medium in the oil tanks is always covered by the internal floating roof directly, in order to reduce the evaporation of medium.

Steel oil tanks Vertical cylinder fixed roof oil tanks

The capacity of this kind of oil tanks is generally less than 10,000m³, the tank wall adopts sleeve connection method (fillet weld). Jacking method is the most used erection method for vertical cylinder fixed roof oil tanks, that is erecting the tank start from the tank top and install the tank wall from top to bottom layer by layer, with the help of jacking system to raise the tank. Compared with the traditional method, which is erecting the tank start from the tank bottom shell, then erecting the tank wall from top to bottom layer by layer, jacking method is safer as it reduces the risk of aerial work.

Vertical cylinder floating roof steel oil tanks

Floating roof steel oil tanks are equipped with double floating deck roofs or singe deck floating roof which can float up and down, the double deck floating roof can reduce the heat radiation effect, therefore, the oil evaporation loss is small. But when the capacity is larger (more than 10,000 m³), it is generally preferred a single deck floating roof in order to reduce the cost. This kind of oil tanks should be paid more attention on choosing of reasonable sealing device, which required excellent sealing performance, convenient installation and maintenance.

Vertical cylinder internal floating roof steel oil tanks

This kind of oil tanks consist of both doom roof and internal floating roof, internal floating roof can float up and down upon the liquid level. In addition to the characteristics of external floating roof tank, internal floating roof tank can also ensure the cleanliness of the stored oil.

Spherical steel oil tanks

This kind of oil tanks can withstand the working pressure ranging from 0.45MPa to 3MPa, the capacity is generally between 50m³ and 2,000m³.Spherical steel oil tanks are generally used to store liquid petroleum gas.

Horizontal steel oil tanks

The capacity of horizontal steel oil tanks is generally less than 50m³, it can store gasoline and volatile oil products.

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Sewage Treatment Plant on a Ship Explained

Discarding sewage produced onboard on a ship is one of the few tasks on a ship which should be taken utmost care of if one wants to same him and his shipping company from heavy fine. The sewage generated on the ship cannot be stored on the ship for a very long time and it for this reason it has to be discharged into the sea.

Though sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some regulations regarding discharging of sewage that needs to be followed. Sewage on sea is generally the waste produced from toilets, urinals and WC scuppers. The rules say that the sewage can be discharged into the sea water only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.

But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also the discharged sewage should not produce any visible floating solids nor should it cause any discoloration of surrounding water.

Generally, ships prefer treating sewage before discharging to save themselves from any type of embarrassment. There are different methods of treating sewage available in the market, but the most common of them is the biological type for it occupies less space for holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco friendly. It is to not that each sewage treatment system installed onboard has to be certified by classification society and should perform as per their requirement and regulations. 

Working of a Biological Sewage Plant

The basic principle of the working of a biological treatment plant is decomposition of the raw sewage. This process is done by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be disposed off in the sea. Air is a very important criterion in the functioning of the biological sewage plant because if air is not present, it will lead to growth of anaerobic bacteria, which produces toxic gases that are hazardous to health.Also, after decomposition of the sewage with anaerobic bacteria, a dark black liquid causes discoloration of water which is not accepted for discharging. Thus in a biological sewage treatment plant the main aim is to maintain the flow of fresh air.

Division of Processes

The biological sewage plant is divides into three chambers:- 

Aeration chamber

This chamber is fed with raw sewage which has been grinded to form small particles. The advantage of breaking sewage in small particles is that it increases the area and high number of bacteria can attack simultaneously to decompose the sewage. The sewage is decomposed into carbon dioxide, water and inorganic sewage. The air is forced through diffuser into the air chamber. The pressure of air flow also plays an important role in decomposition of the sewage. If pressure is kept high then the mixture of air and sewage will not take place properly and it will escape without doing any work required for decomposition. It is for this reason; controlled pressure is important inside the sewage treatment plant as this will help in proper mixing and decomposition by the agitation caused by air bubbles. Generally the pressure is kept around 0.3-0.4 bars. 

Settling tank

The mixture of liquid and sludge is passed to settling tank from the aeration chamber. In the settling tank the sludge settles at the bottom and clear liquid on the top. The sludge present at the bottom is not allowed to be kept inside the settling tank as this will lead to growth of anaerobic bacteria and foul gases will be produced.The sludge formed is recycled with the incoming sludge where it will mixes with the later and assist in the breakdown of sewage. 

Chlorination and Collection

In this chamber the clear liquid produced from the settling tank is over flown and the liquid is disinfected with the help of chlorine. This is done because of the presence of the e-coli bacteria present in the liquid. To reduce these bacteria to acceptable level chlorination is done. Moreover, to reduce the e-coli, the treated liquid is kept for a period of at least 60 minutes. In some plants disinfection is also done with the help of ultra violet radiation. The collected liquid is discharged to overboard or settling tank depending on the geological position of the ship. If the ship is in restricted or near coastline then the sewage will be discharged into the holding tank; otherwise, the sewage is discharged directly into the sea when high level is reached and is disposed automatically until low level switch activates.

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