Q & A > Question Details
In our NHT unit of isomerization plant we are frequently facing issues of reactor effluent air fin coolers tube leak. It has happened twice in 6 months and this time around afc inlet line developed leak as well. Feed composition total Sulphur is 140ppm and chloride is 2ppm. Our wash water injection is continuous and injection rate has been increased to twice the design flow as recommended by licensor. In recent shutdown salt deposit was also observed before water injection point in last 2 effluent exchangers. Boot sample result is having pH :5.7, cl : 70ppm & iron content : 50 ppm. Our unit is designed for handling 17ppm chloride impurities in feed but still the corrosion rate is higher. Please suggest any solutions.

20/06/2018 A: Keegan Denny, Petrotrin, keegan77@hotmail.com
You should first verify that the metallurgy of the pipework is as per design, it has happened where substandard materials escaped the QA/QC process and were installed. Unscrupulous contractors and a weak QA/QC process during construction could lead to this.
19/06/2018 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Corrosion control can be complicated. Are the leaks experienced external (, air side) or internal corrosion. Were they caused by mechanical vibration erosion or fatigue fractures? Was the corrosion generalized or spot corrosion under deposits, or non deposit spot corrosion cells?
The boot water is very corrosive as indicated by the iron and chloride levels and acidic pH. Excessive liquid turbulence (from adding for instance a thermo-well) will cause erosion of the 'protective film' on alloy steels and rapid metal attack.
You will probably have to start injecting caustic or ammonia in the reactor effluent piping to control the pH and also begin using a 'filming amine' to prevent corrosion. You should also review the location of wash water injection and dispersion to prevent salt deposits. Effective spray nozzles and mandatory. Otherwise the water will behave as liquid running along the bottom of the pipe. You may have to use more than a single injection point for adequate distribution of the wash water.
19/06/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The air cooler corrosion is a common problem, even with continuous water injection. Some designers insist on multiple injection points at the cooler inlet subheaders to alleviate the problem. You may want to examine your design. Injection quill design and layout are considered special knowhow as well.
19/06/2018 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
I suspect the 2 ppm chloride you are reporting in NHT feed is inorganic chlorides rather than total chlorides. One possible explanation for the symptoms you describe is contamination of the feed with organic chlorides which thermally decompose at NHT operating conditions to form HCl which results in HCl acid corrosion at and downstream of the physical moisture dew point location. Organic chloride contamination can occur if the crude oil producer is using chlorinated solvents for degreasing producing equipment or is injecting chloride-containing cold flow improver chemicals into crude. There have also been incidents in which isomerate or reformate slops contaminated with PERC are reprocessed.
I would recommend 1) re-test the NHT feed for total chlorides, 2) identify and eliminate any sources of inorganic chloride contamination and 3) inject aqueous ammonia into the continuous washwater system to control the NHT water effluent pH. You should also consider using the intermittent wash water injection points in the NHT feed/effluent exchanger train (if applicable) to remove the salt deposits in the last 2 exchangers.