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  Method for Cleaning Tanks  
 
Tank Farm
Example of a Tank Farm in Taiwan

Equipment Requirements:

1) Sludge Reduction

  • Software package for preparation of Topographic Survey
  • Sampling equipment for obtaining samples of sludge
  • Plumb system for determining levels of sludge in various parts of the tank
  • Actuating nozzle
  • SRJ submerged rotary jet mixing machine
  • Manway cover with two pipes fitted c/w gate valves and connectors for connection to the SRJ Shell mounted unit or Actuating Nozzle
  • 8" cargo hose 2 off 8 metres long
  • Inline filter basket with 0.25" mesh
  • 1 x mobile pump unit diesel driven with all safety features for zone 1 operation to specification to provide up to *800 tonnes per hour of fresh crude oil at 10 bar pressure to the actuating nozzle or the nozzle on the shell mounted SRJ machine

2) Residual sludge removal

  • Mini Dozer hydraulically driven
  • Sludge pump
  • Twin diesel driven hydraulic power pack to provide motive power to the pump and mini dozer
  • Tank for receiving the sludge
  • Breathing apparatus c/w up to 90 metres of air hose on reels (Optional)

3) Sludge treatment equipment

  • Hydrocyclone unit for liquid/solids separation
  • Hydrocyclone unit for liquid/liquid separation
  • Buffer tanks as required for feedstock to each unit and receiving tank for recovered oil
  • Conveyor unit for treated solids to be removed to skips
Oil Storage Tank

The Concept

SRJ Submerged Rotary Jet Mixers use fresh crude oil in the tank to be treated and by pumping the crude oil through the nozzles at 10 bar pressure (or less as the case may be) at a rate of up to 800 tonnes per hour to blast through the sludge with such force that it resuspends all the materials that constitute the sludge into the overhead oil. The soluble hydrocarbons go back into solution in the overhead oil, so being recovered. The other components of the sludge (BS&W's) remain in suspension. When the operation of the SRJ Submerged Rotary Jet Mixer is stopped, the suspended material returns to the bottom of the tank.

The original quantity of sludge is reduced by the amount of soluble hydrocarbon, which has gone back into solution. Typically, this can be as much as 90% of the original volume of sludge. So, if the original quantity of sludge in the tank was 5,000 tonnes, then as much as 4,500 tonnes is converted back into saleable material in the crude oil, with as little as 500 tonnes of residual sludge left in the tank bottom for further treatment.

The Operation

Once the Topographic survey is complete and the assessment is made of quantity, type and position of the sludge in the tank bottom, the decision can be made as to whether or not, manway access covers can be removed safely. If not then consideration must be made for cold cutting and inserting actuating nozzles in the first instance. This is connected to the mobile pump and connected to an external source of crude oil, to pump through the actuating nozzle and reduce the overall height of the sludge to a level so that the manways can be removed, ready to install the SRJ machine(s).

Actuating Nozzle

In tanks where the diameter is below 45 metres, then only one shell mounted SRJ machine is required. This is mounted on a ready-made manway cover, to replace the one, which is removed. If the tank is more than 45 metres in diameter, but less than 90 metres in diameter, then two SRJ machines and pump units are required, preferably operating directly opposite one another in the tank.

SRJ  Shell Mounted 'Swan Neck'

The refinery (or storage facility) pump out the crude oil and the floating roof is allowed to land in its maintenance position. The selected manway(s) is/are then removed and the SRJ ('s) is/are installed. The gate valves remain closed. The mobile pump is connected to the pipework of the specially prepared manway cover with the SRJ . During this time the facility refills the crude oil storage tank with fresh crude oil. The valves are opened and the pumping operation is started. The crude oil in the tank is drawn through the pump inlet and pumped through the pump outlet through the SRJ nozzle.

There is only one nozzle at a time, fully operational on a shell mounted SRJ , to ensure there is no damage to the tank wall. The jet of crude oil passing through the nozzle directed at the tank wall slowly closes as it rotates. There is a special plate insert in the shell mounted SRJ which controls this flow. Centre mounted SRJ 's (which are usually permanent installation) allows crude oil to be pumped through both of the two nozzles at the same time.

SRJ  and Pump

This pumping operation lasts for at least 48 hours, continuous operation. The time is decided upon, dependent upon the results of the topographic survey. This gives the contents time to be thoroughly mixed and blended. With the energy provided by this pumping action and the rotating mixing action of the SRJ (one complete rotation can be set to between 3.5 hours to approximately 6 hours) all components of the sludge are put into the overhead oil. The soluble hydrocarbons go back into solution and the suspended inorganic materials remain in suspension.

The pumping operation is then stopped. The contents are then allowed to settle for a period of 48 hours. This gives the BS&W's time to settle back to the bottom of the tank.

The tank can then be pumped out and the SRJ machines and ancillary equipment can be removed. The first phase of the operation is complete. The reuspension and recovery of the soluble hydrocarbons is now complete.

The situation is now dependent upon the overall plan of action. If the tank needs to be cleaned and residual sludge removed, then the next phase of the operation then starts at this point.

In some cases, there is only need for the Phase 1 (resuspension) operation required, since the residual sludge content may only be nominal.

Assuming that the tank needs to be cleaned for inspection and repair to be handed over in gas free condition, then proceed as follows:

Sludge Removal and Sludge Treatment

The manway with all the fittings - the SRJ , the pipework, etc, is removed and the mini dozer is installed in the tank. There are two types of minidozer to choose from, each hydraulically operated.

One is controlled by remote controls and one is controlled by a person sitting on the minidozer. This person is fully kitted out with breathing apparatus as well as a person positioned at the open manway access during the whole of the time that the minidozer operator is inside the tank.

Minidozer

The sludge pump is set up and all connections made to the twinned hydraulic power pack. The minidozer then pushes all the residual sludge to the inlet suction pipe of the sludge pump. The sludge is pumped to a buffer tank and is then transferred to the first of the two hydrocyclones. The solids and liquids are separated. The solids are conveyed to a waste skip for onward safe disposal. The liquid is sent to the second hydrocyclone, which separates the oil and water. The water is sent to the site API separator for final discharge and the oil is pumped to a storage tank for testing prior to being pumped into a crude oil storage tank, by agreement with the facility.

Hydrocyclone

All other items equipment normally used in tank cleaning, lights, fans reflectors etc can be deployed as deemed necessary during this phase of operation.

The tank washing procedure can then be instigated and final gas free certification can be given on completion of all safety procedures deemed necessary by the site management team.

Please note, there is no need for any sludge lagoons to be created by using this system of sludge treatment.

Phase 1 of this operation need last no more than 10 days.

Phase 2 depends upon the type of oily sludge being treated and ambient temperatures etc, but 20 tonnes per hour should be achievable, so allow 4 days to treat 500 tonnes, (plus setting up and demounting time).

Environmental and Economical Benefits:

  • Resuspension and soluble hydrocarbon recovery is much more advantageous than other systems.
  • It is much quicker, but by far the biggest advantage is cost recovery.
  • The valuation of the crude oil in the tank is increased dramatically. Consider 5,000 tonnes of sludge, with 90 % recovery of soluble hydrocarbon, let's even say 80% hydrocarbon recovery, then we have 4,000 tonnes of soluble hydrocarbon recovered at $20 per bbl.
  • Assume 6 bbls = 1 tonne so we have 6 x 4000 x $20 = $480,000.00 increased value of crude oil in the tank.
  • The environmental advantages, no more sludge lagoons and very costly clean up. This is all done at (and in) the tank as illustrated above.

 

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