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In a Crude distillation unit using hot reflux and with a two stage desalter, repeated corrosion has been found in the Crude / Overhead vapor exchanger leading to tube failure. The crude /Overhead vapor exchanger is the first exchanger in the crude preheat train which heats the crude received from storage (at ambient temperature) using hot vapors from crude column. The MOC of subject exchanger is carbon steel and continuous dosing of filming amine, neutralising amine and wash water (stripped sour water used as wash water) is done. The exchanger is a horizontal floating head shell and tube with vapor on shell side and crude on tube side. The crude processed is Middle East. What are the possible causes and remedies to overcome repeated tube failure?
 
Answers
27/10/2011 A: Fabrizio Palmeri, Alfa Laval, fabrizio.palmeri@alfalaval.com
This position (first stage of the crude preheat from the tanks with the OVHDs from the column) is solved within several refineries using Compabloc Condensers (vertical if the pressure drop available allows it, in order to be able to reduce the T approach), taking special care of the metallurgy. A case by case analysis should be performed on the streams composition, but most likely Hastelloy C276 or 254SMO Compabloc heat exchangers could represent the good solution.
24/11/2007 A: gopalsankar ramanujam, chennai petroleum corporation limited, gopalsankar@cpcl.co.in
In general particularly routing Crude OVHD vapors at shell side in Shell and tube exchanger not a wise idea provided there is no corrosion anticipated. However, in CDU OVHD, and I presume in your case, the first droplet is occurring at the shell side of the condenser. Due to very low pH and in absence of neutralisation at the exchanger you will have severe HCL attack on the Carbon Steel tubes. The only possible solution in this case would be:-
1) See the possibility of routing vapors in the tube side
2) Injecting the neutraliser very close to the shell side of exchanger, and the last resort,
3)Change the tube metallurgy to duplex grade or better one to combat corrosion from shell side.
09/11/2007 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
In addition to the previous response, you need to look at how well designed the water wash injection system and chemical injection systems are. Is the water wash injected via a spray nozzle with a reasonable DP to get good distribution? Is there adequate contact time to get adequate absorption of HCl? Are your filmer and neutralizers added through separate injection quills? Is there enough diluent to help disperse the chemicals to get good coverage in the system? Sounds like you may not have good distribution of your wash water and/or corrosion control chemicals? If you don't get wash water well distributed, you can get localized water condensation, called shock condensation, on a cold tube surface. If neutralizer is not present when this is occurring you will get a very low pH and get corrosion. Crude vs. overhead exchangers are prone to this problem, especially in cold climates when the exchanger is the first in the preheat train. Crude in tankage can get pretty cold in winter and make it difficult to prevent shock condensation from occurring. Good luck.
11/09/2007 A: Amarjit Bakshi, Refining Hydrocarbon Technologies LLC, abakshi@rhtgulfcoast.com
The problem might not be from the vapor condensing side/shell side stream, but rather the desalting not being optimum. It could be one is having HCL in the tube side (desalting problems) and also skin temperatures might be hot on the tube side to enhance the corrosion due to HCL.
One needs to check the desalter operation and condensate boot liquid PH. You might be look at the composition of this liquid for a complete answer. ......
05/09/2007 A: Andrew Vieler, Simulation Engineering NL bv, avieler@simeng.nl
Assuming the corrosion is occurring from the vapour side of the tubes, is the distribution of the wash water and amines good? Is there enough wash water? Maybe a dumb question but are the desalters really desalting?
The pH of the first droplets of condensation of the vapour will likely be low, certainly much lower than carbon steel can tolerate. Assuming the cooled vapours (or mixed phase stream) are going to a reflux drum, do you have an ionic analysis of the aqueous phase decanted from this drum? One may be able to use this information to back-calculate the aqueous phase chemistry profile. Sorry to raise lots of questions to suggest answers to your question. ......