This topic contains 4 replies, has 2 voices, and was last updated by Anonymous 10 years, 3 months ago.
July 4, 2012 at 8:55 am #1844
We process Kuwait and Arabian Heavy in our CDU.
The vacuum residue is then sent to DCU.
We find if the ratio of Kuwait and AH is too high(e.g. 60/40), the pressure of the coke drum will increase suddenly during water quench(fast cooling step).
Also it is more difficult to drain the water from the coke drum following the quench step.
We have an accident during vertical cutting when a large amount of hot water pouring down from the coke drum.
Anyone can help?
July 4, 2012 at 9:40 am #4615
Regarding 60/40 Kuwait/AH blend – Coke bed problems could be related to incompatibility of the resids and/or inadequate coking temperature. Can you explain how you overflowed the coke drum during the vertical (I assume bore hole) cut? The drill stem became stuck in the coke bed? The response should be to go to bypass and not fill the coke drum with water. What temperature is your cutting water? Was someone hurt?
July 5, 2012 at 9:41 am #4614
The coke handling system in our DCU is sluice way system with dewater bin and sedimentation tank.
The accident happened when coke cutting operator just finished the vertical cutting(bore hole), a large amount of hot water(90 deg C) from bottom head suddenly poured down to the sluice way and one operator worked there was hurt.
This mean the drainage step is not complete. But it is difficult to know whether there remain some water in the coke drum.
The cutting water temperature is about 50 deg C.
We operate the coke drum pressure at 0.8 barg during water quench, is it too low?
We do not add MCB from RFCC unit.
We also have problem that drill stem became stuck in the coke bed sometime.
July 6, 2012 at 12:12 pm #4612
Thanks for the information. Where is your coker located? capacity? age?
In regard to draining:
The first key point is to not expose personnel to the open top head during the initial stages of draining (coke beds can shift as the water drains past, exposing 800F coke to the water, resulting in an explosion of 180 – 200F water out the drum top). Automatic, remote top deheading can allow this operation during the early stage of draining.
The second key point is to insure that sufficient water has drained so that it is safe to remove the bottom head from the coke drum prior to removing the bottom head.
=> Leaving residual water in the coke drum (a short or inadequate drain) is the primary cause of coke fallouts during deheading.
Methods to insure a proper drain:
(1) Verify that a proper drain has occurred using three independent methods
+ Visually verify drain rate several times during drain
+ Verify level probes are dropping and the rate at which they are dropping (see Rick Enlow Calculation below)
+ Measure drain water recovery in tank and compare to expected amount for a “good” drain (this is only possible if water is not being drawn down for quench/cutting and if make-up water is not being added)
+ Verify drain time is normal
+ Add top water at end to visually verify water drains through the coke bed to exit point (“proof the drain”)
(2) Maintain good steam purge during switch to avoid leaving resid in feed line, which would then crystallize and restrict the path to the drain line.
(3) Avoid a high initial quench water rate when switching from steam to water. The high water rate can break coke, which then falls into the drain line and obstructs the drain. It is recommended to stay < 200 gpm at the start.
If drain was slow or abnormal in any way, the following procedures can be considered after reviewing and addressing all safety risk issues:
+ Blowback the drain line with a high rate of steam to possibly break-up or remove obstructions that may be acting like a check valve in the drain path (using this blowback method as part of the normal procedure is NOT recommended since it can pulverize the coke, causing it to partially collapse and worsen the drain)
+ Do a “Pressure Drain”, closing the top head and vent and carefully allowing pressurization of the top head to push the water with more force.
One of our sites blows the bottom utility header after warm-up to insure clear lines. In addition, if the drain is slow, they close the drain line blocks to the tank and maze and use a pressure gauge on the drain manifold to see how much water head there is. This pressure also reads out in the TDC for the console. The gauge is used to help verify that steam blowing worked to free the drain.
Another site has a TDC alarm that sounds if the time from start of drain until the water level drops below the highest level detector takes too long and also a TDC alarm that sounds if the time for the water level to drop between the highest and next highest level detectors (both above the normal coke outage level) is too long.
They also have a vacuum gauge on 1 coker to indicate if there is vent pluggage or closure, preventing excessive vacuum from damaging the coke drum.
Best regards – Mitch Moloney
July 8, 2012 at 7:49 pm #4611
Our DCU is located in Eastern Asia with capacity of 36,000 bpsd(4-drums).
We operate this plant for 9 years.
We mainly process Mid-East Crude.
As feed crude change very often,it is not easy to operate this plant.
Also the coke handling system in this plant is slurry , not pit type.
Anyway thanks for your helpful information.
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