May 23, 2013 at 6:01 am #1657
Good morning to everyone, I work for a company which has recently started operating a 2 drum coker unit. We have had 5 online spallings since we started operating the unit (6 passes heater).
The result of the different online spallings has been quite good in an overall view according to STK temperatures (up to 70-80ºC in some cases), but not that good in the coil pressure, which has not gone down after the spalling and in some cases it has even gone slightly higher after the spalling, when we have put feed in again.
This pressure, after a few days (couple of weeks), has been going down but not as much as desired.
My question is whether some of the coker operators have suffered this problem and if you know how to avoid/improve this effect in the pressure after the spalling.
Up to now it is not a problem because we are not limited in coil pressure nor in control valve feed OP, but if it continues as we make more and more online spallings will become a problem in the future for sure.
Any suggestions/recomendations will be appreciated.
June 3, 2013 at 1:56 am #4463
Pressure drop through the heater coil and the transfer line is very sensitive to coke laydown as that reduces the inside diameter for transporting the fluid. Pressure drop is proportional to the ratio of the diameters of the pipe to the fifth power. As an example, a reduction in the inside diameter of the heater coil to 80% (coke laydown of 10% of the tube diameter around the circumference (20% reduction in diameter overall) will result in the pressure drop increasing by a factor of three.
Do you know if the pressure drop is in the heater transfer line (from the heater outlet flange to the switch valve) rather than in the heater coil? Have you done a thermal scan of the heater coils after the on-line spall? Most spalls are evaluated by the decrease in the tube metal temperature. Tube metal temperatures should be able to return to initial start of run temperatures, unless the foulant is inorganic salts or clays that may be more difficult to spall.
The two ideas that come to mind are that 1) not all the coke is being removed from the heater coil and only the areas of the tube metal temperature indicators is being spalled enough to lower the readings; 2) the transfer line has had significant coke laydown between the heater and the switch valve and that has resulted in higher heater inlet pressures. Infrared thermography will determine if all the coke has been removed during the on-line spalling procedure. A pressure survey is needed to determine if the transfer line is fouling.
June 6, 2013 at 11:18 am #4460
Good afternoon Mrkimbrell. Thank you very much for your response.
Our only pressure transmitter is in the entrance of the heater, downstream the flow control valve. We have no other pressure indication between this point and the switch valve, so we can not determine whether the possible plugin is in the heater coils or in the pipe from the heater outlet flange to the switch valve.
We have not done a thermal scan of the heater coils after the spalling, as we are starting to know how our heater behaves when we do the on-line spalling.
Our heater has 18 F.G. burners and quite a lot of skin points from coil 4 at radiation zone to coil 30 at radiation ceiling. All of them show a big decrease in temperature, up to 80ºC before and after spalling, so we feel quite happy and confident at this point that the spalling is being very effective, but the bad point is that pressure increases when we put feed in again.
I will have in mind the possibility of an infrared thermography if the problem remains with more spallings.
When we close the feed valve at the beginning of the spalling we also close the flushing to the pressure transmitter (cold LCGO), and it remains closed until we put feed again once the online spalling is finished.
I know there are coker units where the flushing is not used for instruments to minimize throughput ratio with no measurement problems.
Does someone know if pressure drop mentioned effect after online spalling could be related with instrument flushing?
Kind Regards to everyone.
June 11, 2013 at 9:17 am #4457
It is a good practice to do before and after (on line spalling) thermal scans of the new heater, to learn how it behaves.
Did you do a scan right after the unit startup? This is a good reference to compare.
Too much flushing and/or steam will affect the pressure drop through the heater coil.
Flushing can also come from heater charge pump seal (if you have a Plan 32).
In a coker I worked we had pressure transmitters without flushing, that stopped working after a while.
We also had about 1.5 inches of coke formation inside the transfer line, but that took a few years.
Hope this helps
July 1, 2013 at 10:14 am #4448
Good afternoon Mr. Claus, my apologises for the delay in my response.
A couple of weeks before we have done another online spalling to other of the coils of the heater. We have made small changes in our spalling procedure, but one difference with previous spallings has been the ammount of BFW we put in the coil while we are doing the shocks.
Appart from other minor changes we have increased slightly the BFW we put into the coil when we are decreasing the coil outlet temperature in order to try to flush better the coil at that point, which was our suspect of the high pressure when we put feed in again.
The result has been quite good, with a decrease in skin points temperature of around 100ºC and between 2-3 kg/cm2 less than before spalling and then the rest of the passes of the heater.
This week we have another spalling, we will follow the last operating procedure and I will let you know if the result has been the same or not.
Thank you very much for your response.
August 2, 2013 at 5:36 pm #4433
Most of my experience for pressure transmitters at the heaters has had them be steam purged. I would leave the steam on during the spall so that the instrument connection would not coke up.
I think it is possible that the instrument connection is restricted and the indicated pressure is slightly higher than actual.
I would always purge instruments connected to the process around the heater and the coke drums with steam to prevent them from becoming plugged up.
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