Storage tank dewatering systems

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Storage tanks for flammable liquids and hydrocarbons inevitably take in water due to venting and leakages of floating roof tank seals or through en-trained water in the product itself. Over time this will result in a layer of water at the storage tank bottom. Between the two fluids a so called “rag layer” tends to form consisting of an oil-water mixture, creating an environment for bacterial growth. Eventually this will cause product quality degradation, corrosion of the storage tank interior and the forming of sludge.

Tank Dewatering systems

Tank dewatering systems

Regularly draining the water is therefore recommended. Manual or other traditional ways of controlling the water-draining however has its disadvantages. The presence of a rag layer increases the chance of spilling hydrocarbon products. It´s also likely an amount of water will remain in the storage tank. Furthermore, draining of tanks containing flammable liquids will cause safety hazards. To prevent these safety hazards, efficient and safe separation of the two liquids is desirable.

In addition to increased costs of waste-water treatment, man hours and environmental liabilities an automatic tank dewatering system will be a solution.

The Hydro-Carbon Automatic Tank Dewatering System is specially designed for automatic draining of free water from hydrocarbon storage tanks. The Hydro-Carbon Automatic Tank Dewatering System is a safe and environmental friendly way for dewatering storage tanks.

General principle

The Tank Dewatering System will consist of a pressure vessel, containing coalescing plate packs for oily water separation. The oily water mixture is pumped into the vessel with a positive displacement pump. The position of the inlet ensures a two-stage separation. The mixture first will flow through the pre-separation chamber allowing the larger droplets of hydrocarbon product to separate from the water. This chamber also allows possible solids to settle at the bottom of the vessel. Smaller oil-droplets will enter the coalescing plate pack. Due to the unique functioning plate packs, the hydrocarbon product is separated by gravity from the water and will be collected in the dome on top of the vessel. By means of level-control in the dome, either the treated water discharges or the product outlet line is opened. The separated retrieved product is pumped back to the product-storage tank. The treated water can be discharged to a water treatment facility.

Safety philosophy

Uncontrolled draining or discharging of hydrocarbon product needs to be avoided. In order to prevent any undesired effects, several safety measures will be applied, among which:

  • Oil-in-water monitoring at the water outlet side.
  • Flow and pressure control at the pumps pressure side.
  • Non return valves on every inlet and outlet connection.
  • A control unit which will ensure correct operation of pump and valves.

In case of an alarm or any other unforeseen event the system automatically shuts down. The pump will stop and valves will be closed.

Working mechanism

The high efficiency of the coalescing plate pack system is based on a limited rise distance for the oil droplets as well as enhanced hydrodynamic coalescence due to the configuration of the plates. According to ‘Stokes’ law the rising velocity of an oil droplet in water is a function of i.a. the square of the diameter of that droplet. The merging of very small oil droplets into larger ones within the plate pack will therefore create extremely enhanced coalescence. Sufficient retention time within the plate pack will cause the rising oil droplet to be captured by the next higher plate. The oliophilic material of the plates ensures that oil droplets will never reach the outlet, ensuring separation. Product concentrated at the top of the corrugated plates will rise through the weep holes to the developed oil layer above the plate packs.

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