| Conventional | One-step washing with diesel ｜ 2S-TC |
| A comprehensive system for tank cleaning which includes the removal of bottom sludge from the tank, (crude oil sludge), oil sludge treatment, and tank washing. To illustrate this, consider the need to clean a 100,000 KL tank with 4,000 KL of tank bottom sludge as an example for assessment. |
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 Conventional Method | TOP |
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Conventional method of tank cleaning technology is includes, sludge removal, oil sludge treatment and tank washing. These 3-steps are set out as follows: |
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1. Remove bottom sludge from the tank |
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It can take 3-4 months (90-1 2 0 days) even when using experienced operators with machines in the tank. This method is highly depen dan t on the number of operators available, so if the manpower can be increased, the operation period could be reduced to between 75 -90 days. |
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Utilizing remote control tank cleaning machines so eliminating the need for operators to be inside the tank, requires nearly double the time compared with the system described in (a) above - with operators in the tank. |
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2. Oil sludge treatment |
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Usually, oil sludge treatment requires a 3-phase separation process to separate oil sludge into O.S.W – recovered oil (hydrocarbon), solid, and waste water. This is normally carried out outside the tank. |
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3. Tank washing |
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a |
Utilizes high pressure cleaner(s) to clean internal wall and floor of the tank, and de-rust by means of sand-blasting. In this case, it takes 45-60 days by one operation team, but can be reduced to 2 5-30 days if 2 teams work at the same time. The sand blasting phase to clean the (in?)external tank wall, can take a further 30-60 days to complete depending how many teams are working on site. |
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b |
Alternatively, utilizing ultra-high pressure cleaning system to wash the tank is a much more efficient way, since it combines the tank washing, de-rusting, and de-painting in one operation, but capital cost is relative high. |
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| 1-step tank washing technology | TOP |
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This is different from the (so called) conventional way. Certain companies have developed a particular method for tank cleaning, which is (so called) One-Step Tank Washing Technology, which is similar to Crude Oil Tanker Washing technology (COW) using diesel. This technology relies on the installation of jet nozzles suspended from the tank roof and then diesel is pumped through these nozzles to create a diesel jet-flow to wash the internal tank wall. The oil sludge/diesel is then pumped out from tank bottom. Some technologies also use 3-phase separator to separate debris and oil sludge in the process.
The basic concept of this technology is good, but, the main problem is that most of oil sludge remaining in the tank on the tank bottom is still relatively solid, like cake, too hard to be blasted or melted by diesel initially. In time, this oil sludge cake will eventually dissolve, but the time it takes to act in a 100,000 KL tank could be up to 4 months.
Additional disadvantage for one-step tank washing technology is the high risk of explosion, due to high speed diesel jet flow is likely to explode due to the build up of static electricity. It is essential, therefore, when using this method using diesel to wash a crude oil tank, inert gas – such as Nitrogen should be blown into the tank to fill the complete internal void in the tank (Nitrogen Blanketing). In Aug, 2 00 2 , a contractor using this technology to carry out tank cleaning contract in Iran caused a crude oil tank to catch fire due to failure of Nitrogen Blanketing.(see picture).
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Also with this system of tank cleaning, consideration for the temporary storage of oil sludge and diesel should be made, so an additional tank is required when the tank washing is carrying out. This increases (in fact – doubles) the crude oil tank outage when this type of operation is used. |
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| Advanced 2-Step Tank Cleaning Technology | TOP |
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Thanks to many years of -experience, 2 S-TC, an advanced 2 -step Tank Cleaning Technology is now available. This is entirely different to other technologies. 2 S-TC saves working time required for tank cleaning and residual sludge treatment, from months to weeks. This system the 2 S-TC is described as follows: |
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●Step 1: |
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By using Submerged Rotary Jet Mixer submerged rotary jet mixing machine for dissolving the soluble hydrocarbon content of the sludge back into solution in the oil layer, which is so called re-suspension (hydrocarbon re-suspension), this process can recover more than 90% of hydrocarbon from the bottom sludge. This leaves just 10% residual bottom sludge is left on tank bottom. It takes about 10 working days for the complete re-suspension process, and only 3-day for the equipment operation. The other 7-days are needed for mounting the equipment, draining down and refilling the tank, etc. |
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●Step 2: |
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If, after step one is completed, the client decides that the tank does not require further treatment, the contractor can simply put the refinery manway cover back on to the crude oil tank, and leave. |
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If the client requires further removal and treatment of the residual sludge, then the operation can continue utilizing pumps, dozers, and so on to remove the residual oil sludge out from the tank. Because the residual oil sludge is now much less than would normally be found on the tank bottom (10%), and since it has been through a very violent mixing process for three days or so, the resultant sludge left in the tank is very soft and pumpable. It will take a very short time in to remove this sludge in comparison to the conventional method. |
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