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In our DHDS plant (Axens licensor, revamped in January 2018 ) with both a hot high pressure separator and cold high pressure separator, we are facing several tube leaks(A179-CS tubes ) in our stripper feed/stripper bottom exchangers (life three years).Corrosion is mainly on the stripper feed side and corrosion is due to localised under-deposit corrosion on the OD side of tubes near the floating head tubesheet, probably due to carry over of water and salts from upstream separators. Our hot separator is operating at 40 ksc and 90 degC operating temperature against the design 100 degC. In the same plant we are facing severe choking issues in our stripper overhead fin fan coolers where a complete header box was found choked with deposits. Around 76 % of the foulant collected is iron, and ammonia is also present. Has anyone faced such issues? Is operating the hot seperator at lower temperature the cause ? Has anyone used Alloy 825 tubes for stripper feed /stripper bottom exchangers?

 
Answers
22/12/2020 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita-water.com

The corrosion problems described have also been observed in other DHDS or NHDS. In most cases, the cause of an increase in corrosion was chloride deposits, which were not initially considered critical in the presence of significantly higher sulphide concentrations. The chlorides have a very high corrosion potential and can lead relatively quickly to underdeposit corrosion, especially if no or only little wash water is used for the reactor effluent exchangers.
The lower operating temperature of 90°C certainly also has an influence on stronger formation of under-deposit corrosion. It is the ammonium chloride that precipitates first when the dew point temperature is reached, before ammonium bisulphides can also precipitate later.
In practice, the use of Kurita's ACF technology has proven successful. This is a strong organic base that directly reacts with the chlorides to form a pH-neutral liquid salt with very low corrosion potential. The ACF salt is highly hygroscopic and remains in the sour water (sometimes only a few liters water), which is collected and discharged in the low-pressure separator. In this way, no ammonium salts can be deposited, which could later lead to underdepoisit corrosion. When dosed into the reactor effluent stream, the ACF product usually contains an additional film-forming component to ensure direct corrosion protection on the metal surfaces at the same time.
If a differential pressure increase is observed in the stripper due to ammonium salt deposits or corrosion, dosing of the pure ACF component is recommended. The dosing then takes place in the overhead reflux from the stripper. The dissolved ammonium chlorides then leave the stripper as water-soluble ACF chlorides via the overhead system.
The use of higher quality steels is definitely a way to protect this area. However, this often does not prevent salt precipitation in the stripper if the salts are carried further.
15/12/2020 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
This sounds like you have a significant problem with oil/water disengagement in the high pressure separators. Does the HHPS have a water boot? Normally they do not, but at 90°C this is not "hot" and liquid water is highly likely to form (along with ammonium salts). The design/operation of these vessels is critical to separating water to avoid exactly the problem you describe.

The stripper feed/bottoms exchanger you are likely boiling water that contains salts, which would deposit on the tubes. Have you checked if the deposits are chloride or sulfide salts (or a combination)?
15/12/2020 A: Mike Watson, Tube Tech International Ltd, mike.watson@tubetech.com
It often boils down to “how clean is clean?”. To reduce under scale corrosion back to bare metal, cleanliness is essential. Over 30 years of removing under scale corrosion deposits, we have found HP water jetting alone rarely removes this tenacious iron oxide scale, leaving tramlines of scale behind. We have therefore developed the world's first Tube Polishing technology.
The only system to guarantee back to metal IRIS and Eddy Current Standard cleanliness for asset integrity inspection. The added benefits of tube polishing: extended run time, reduced energy consumption, improved asset life and logically CO2 reduction.
15/12/2020 A: Mainak Sarkar, Indianoil Corporation Ltd, sarkarm@indianoil.in
The issue may be related to formation of ammonium bisulfide. Check the wash water injection rate. Another aspect is slippage to sour water from HPS if your HPS is a 3-phase separator and LPS is 2-phase.