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We have taken recently two shutdowns of DHDT unit due to HHPS overhead exchanger leak (CLPS HC to stripper vs HHPS overhead gases), leading to sudden increase in stripper overhead pressure. What can be possible reasons?

May-2022

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Nelmo Fernandes, PETROBRAS SA, nelmoff@petrobras.com.br

First you need to know the contaminants in the feed that you process. The amount o Cl is very important. The formation of NH3Cl is an important issue. If you have a good wash water system try to verify if you are injecting enough water and if the point of the injection is able to wash NH4Cl or not. Usually this wash water is focus on NH4SH formation. The quality of this water is crucial also, A good read of the API_RP-932-B-Design, Materials, Fabrication, Operation, and Inspection Guidelines for Corrosion Control in Hydroprocessing Reactor Effluent Air Cooler (REAC) and API_PUBL-932-A-A Study of Corrosion in Hydroprocess Reactor Effluent Air Cooler Systems.

May-2022
Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com

Michael is right that there are multiple possible causes of failure that are relevant in almost any exchanger. There are also a couple of specific things to bear in mind in this particular service:
1. Corrosion due to the formation of ammonium chloride salts in the HHPS overhead gas as it cools.
2. Water wash is often used to mitigate the ammonium chloride mentioned in the previous point. Poor water wash facilities or procedures can also cause failures.
3. Entrainment of water in the CLPS oil, which evaporates in the exchanger leading to localised acidic conditions and corrosion on the cold-side.
4. Various stress corrosion cracking mechanisms are possible if the tubes are made from austenitic stainless steel.

When you shut down to repair the exchanger, were you able to recover a piece of the failed tube? Knowing whether the failure started on the hot-side or cold-side of the tube would help narrow down the possibilities. A metallographic examination of the failure can also help diagnose the problem.

May-2022
Michael Olesky, Consultant, mike.olesky@verizon.net

With all due respect, you should consider getting a good heat exchanger troubleshooting guide and work through the various causes of heat exchanger tube leaks (there are many!). Good luck!

May-2022