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During the TCU shutdown, I observed the existing operational and maintenance activities. The shutdown planner assisted me in this process.

We examined all the major activities, to understand why they were being done, the sequence followed, and the results achieved. Since de-coking and cleaning were the primary activities, we kept a record of the volume of debris removed and the duration of each cleaning activity. If work was held up on a planned or unplanned basis, we recorded the reasons. An analysis of this data showed that there were some opportunities to reduce the work volume, duration, or both. We will discuss some of these in the following sections.

Soaker Vessel

This vessel provided a short residence time for the hot residue coming from the heater. The liquid pressure dropped as it entered the vessel, and this initiated cracking of the long chain hydrocarbon molecules. The Soaker is a vertical vessel about 10' diameter and 60' high, mounted on a cylindrical steel skirt about 10' high. It had a 6" thick insulation layer to minimize heat loss by radiation. There were 7 simple trays in it, each with a number of 6" holes. As the hot liquid flowed up the vessel, the long-chain molecules were broken down into shorter chain molecules. During this process, free carbon was liberated. At the end of a run, the coke buildup was nearly 5" thick on the trays and the holes were covered up so they were just 1-2" in diameter. There was

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coke on the vessel walls and dished ends. This coke had to be removed by high pressure water jet cleaning (hydro-jetting). Prior to vessel entry, it was steamed out with low pressure (LP) steam for eight hours to remove hydrocarbon vapors. Then it was aerated for four hours to make it safe for entry. The arrangement is illustrated in Figure 37.2.

Soaker Vessel

Figure 37.2 Soaker Vessel

The vessel remained hot for about three days after the steaming was completed. There was a 16" nozzle at the bottom, through which the oil from the heater entered the vessel. This opening provided access into the vessel once the flange was opened, and the pipe removed. A small scaffold had to be erected in the vessel skirt, to reach and unbolt the flange. This could only be done about 2 days after operations handed over the unit, because of the intense heat inside the vessel skirt.

On entering this vessel, I observed that it was very oily and slippery. Workers had to be extra careful, as the reaction forces from the hydro-jet nozzle could make them slip and fall down. The coke was not brittle, so it had to be gouged out in chunks. This meant that just to clear the coke it took nearly twelve days, working 24 hours a day.

I had three concerns:

  • 1) safety hazards for the workers in the oily environment,
  • 2) delay in the opening the vessel for entry, and
  • 3) the time it took to remove the coke.

After the shutdown, I discussed the steam-out procedure with the unit manager. LP steam can be slightly wet and we explored the use of high pressure (HP) steam, suitably throttled to low pressure so it would be very hot and

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well superheated. I asked that the steaming duration be extended to 36 hours to improve the dryness of the coke. This meant that operational activities would take longer and cost more, but there were clear benefits in terms of safety. I promised him that the additional steaming time would be recovered as overall duration would be reduced.

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