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The amount of the clean-up contract is usually determined by the volume of sludge in the oil storage tank, and the tariff is calculated per m³.
The size of the tank — the surface area — also matters as a secondary factor.
Washing heights are generally defined as "one / two sheets", i.e. 1.5 — 2.5 m; however, there are orders for cleaning up to the roof.

Atmospheric electricity.
Self-ignition of pyrophoric sediments.
Continuous tank operation without cleaning.

*Review of Atmospheric Storage Tank Fire Scenarios: Costs and Causes

During the operation of tanks for oil and dark petroleum products, deposits build up in their bottoms, the size of which depends on the length of tank operation, the quality of petroleum products and storage technology. The presence of deposits exceeding the bottom heel level reduces the usable tank capacity and increases wear and contamination of the equipment.

Due to certain difficulties in cleaning and disposing of the waste removed, the tanks are cleaned of bottom sediments as required. Deposits of dark petroleum products (fuel oil) are a product of the natural "ageing" of fuel. Due to the increased density, "ageing" products (asphaltenes, carbenes, carboids) are deposited at the tank bottom. The mixing of fuel oil with other petroleum products (cracking residues, diesel fuel, bitumen, liquid additives) during transport or storage accelerates the build-up of deposits.

Diesel fuel is used as a substitute for fuel oil in thermal power plants and may be mixed with fuel oil during storage. Intense sedimentation is facilitated by the fuel heating with inside steam heaters.

According to the regulations, the following frequency is set for periodic cleaning:

• at least twice a year for jet fuel, aviation gasoline;
• at least once a year for lubricating oil additives and compounded oils;
• at least once every two years — for other oils, motor gasoline, diesel fuels, paraffin and similar petroleum products.

Metal and reinforced concrete tanks for petroleum, fuel oils, motor fuels and similar petroleum products should be cleaned as required by the operating conditions. Water and impurities from the tanks should be removed at least once a year.