Tank

Gas and oil tanks in a petroleum refinery

Tank on a ship

A tank is a container for the supply or storage or transport of liquids or gases and can have almost any dimensions.

Tank shapes

Most of the time, a tank has the shape of a cuboid or cylinder with or without segments. A rough distinction is made between fixed roof, floating roof and floating roof tanks, pools and closed containers. A basin is an open-topped tank. A closed container can also be used with positive and negative pressure. In Europe, the Pressure Equipment Directive 97/23 / EC, which contains the basic safety requirements for placing on the market, applies to a permissible overpressure of more than 0.5 bar .

Tanks constructed for transport purposes are subject to additional requirements. You are e.g. B. provided with additional fixtures ( baffles ).

Fixed roof tank

The shell of the fixed roof tank is cylindrical and firmly connected to the round roof. When filling , the gas phase saturated or partially saturated with stored goods is displaced; when emptying, gas / air is sucked in. Due to changes in temperature and pressure in the environment depending on the time of day, stored goods are released into the atmosphere ( tank breathing ). In special constructions even LNG (liquid natural gas, –165 ° C) can be stored.

Floating roof tank

With the floating roof tank, the roof, which is provided with floating chambers ( pontoons ), floats on the liquid so that there is no saturated or partially saturated gas phase above the liquid. The roof rises when filling or lowers when emptying. This has the advantage that there is no room in which z. B. can form an explosive atmosphere . In addition, floating-roof tanks are more favorable in terms of emissions than fixed-roof tanks , since tank breathing cannot take place and any remaining storage goods on the tank wall can only get into the atmosphere when they are emptied. Floating roof tanks are mainly used for products with high vapor pressures , such as B. gasoline or crude oil is used.

Fixed roof tank with floating cover

A fixed roof tank with a floating roof is comparable to the floating roof tank in terms of its properties, but also has a fixed roof. The existence of fixed roof tanks with floating roof is often due to the fact that a fixed roof is retrofitted in existing floating roof tanks to reduce emissions, but the floating roof remains. Fixed roof tanks with floating roofs are very similar in their emission behavior to floating roof tanks.

Fixed-roof tanks with floating roofs can also serve to comply with TA-Luft requirements by feeding the TA-Luft-relevant substance into the space between the floating and fixed roof for gas processing.

pressure vessel

For the storage of liquid gases such as propane / propene , butane / butenes , but also for DME and pentanes , spherical pressure vessels (see 1st picture above) or buried steel cylinders are used. Due to the high pressure of its own vapor, the product is mainly in liquid form. Thanks to strong insulation and permanent cooling, even ethene can be stored in liquid form in spheres.

Flexible tanks

Flexible tanks have a filling volume of up to 1,000 m³. They are portable, relatively light and can be used anywhere. Situations for flexible tanks are spontaneous environmental problems with liquids or gases, temporary storage needs, rough terrain, lack of infrastructure, flexible transport.

Use, materials and design

Tanks are required for simple substances such as water, for toxic substances, acids and alkalis and for flammable liquids such as fuels and oils of all kinds, as well as for liquid gases such as propane and butane. In addition, the industry uses tanks for chemicals of all kinds (raw materials, intermediate and end products). Simple tanks are made of plastic, fiberglass or steel as well as ferrous materials; they are single-walled or double-walled. Other metals as well as special linings and coatings are used to meet the requirements of the ingredient (e.g. corrosion resistance). Further requirements can be insulation, heating or cooling. There are site-made tanks that are sized to fit the storage room , and pre-built battery tanks that can be lined up as needed. The use of materials depends on the type of liquid and on whether the tank is stored underground or above ground.

Safety precautions

In a pipeline tank farm , each tank is surrounded by a dike that can hold back the entire contents of the tank

Tanks must be secured against floating in floodplains

Depending on the type of liquid to be stored, numerous storage regulations and safety-related aspects must be observed. There are special requirements for the underground storage with regard to the durability and leak monitoring of the tank. The above-ground storage allows a visual inspection of the tank and has, instead of a leak monitoring a safe collection of the entire tank contents in the event of leakage ensure, as indicated by a so-called pan is ensured, which will be able must receive the leaking fluid volume completely. With large tanks, this tub becomes a veritable embankment .

Leak monitoring systems

Leak detector with alarm lamp on an oil tank

Leak monitoring in underground storage tanks is ensured by leak detection devices that monitor a control room between the inner and outer tank walls. As a monitoring medium, a case drain (level control) or a signal generated from the leak detector can vacuo be used. If a leak is detected, the leak detector generates an optical and acoustic alarm. Optionally, above-ground, cellar-welded storage containers according to DIN 6625 can be equipped with a vacuum leak protection system, in which case the liquid-tight collecting area can be omitted.

Protection against overcrowding

An adjustable limit switch or an overfill protection system interrupts the liquid supply when the tank is filled to the maximum permissible level.

Furthermore, it must be ensured that no unacceptably high during filling or by heating pressures or when emptying or by cooling vacuums can occur which damage the tank.

equipment

Agitator at the bottom of a crude oil tank

Tank farm with loading station for road tankers

So that the tanks can be filled, they must have a vent line through which the air displaced during the filling process can escape. Conversely, air must flow in when emptying so that no impermissible negative pressure can occur. It may be necessary to ventilate the tank with an inert gas when emptying it , if air flowing in leads to an impairment of the contents. To reduce emissions, tanks are often connected to a gas exchange system or a VRU (VAPOR RECOVERY UNIT).

Some tanks are equipped with stirrers at the bottom, especially for crude oil. On the one hand, it should be kept more liquid, on the other hand, it should be mixed when two different types of oil have been introduced into this tank. Without mixing, the heavier type of oil would collect at the bottom and the lighter type at the top of the tank ( segregation , mixture separation ).

Hydraulically operated stirrers with a jet nozzle (Crude Oil Mixing System), mounted on the floating cover, can stir up deposited sludge for cleaning purposes, thus reducing the amount to be disposed of, which would be applied manually with regular large tank cleaning. (Crude oil tank cleaning)

Oil sludge

Despite the high quality of branded heating oils, it cannot be prevented that, after a certain period of time , harmful deposits form on the bottom of the tank as a result of sedimentation from paraffins , rust and sand ( inorganic components of crude oils). In addition to the components described above, this deposit, the so-called "slack", also contains components from the crude oil. The proportions of benzene and other volatile hydrocarbons are particularly problematic here . In the case of steel tanks, this can even lead to rust perforation. Timely tank cleaning is also advisable in private households. Especially if the tank is often run almost empty, sucked in oil sludge can cause malfunctions (clogged oil filter ), which lead to the failure of the heating system .

Expert examination

Due to water law provisions ( Section 62 of the Water Management Act , the Ordinance on Systems for Handling Substances Hazardous to Water (AwSV) and the system regulations of the federal states), fixed tank systems for substances hazardous to water in Germany from a certain size must be regularly checked for their condition by an expert organization. This applies to both the commercial and the private sector. Underground tanks that are not double-walled must be emptied and completely cleaned.