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Our overhead wash water which is demineralised water addition is not continuous at CDU plant. Water is added to 2 bundles in 4 hrs. Then water is added to the next 2 bundles and so on. This implies that the 1st bundle in which water is added receives wash water after a gap of about 1.5 days. We use neutralising amine and keep ph between 5.5-6.5 while having almost no corrosion on overhead lines (monel cladded). Caustic is also added at the downstream of the desalter. The contractor who provides services and chemicals is claiming that addition of more wash water to have continuous wash will decrease the consumption of neutralising amine. In our opinion this will not work since the amount of wash water will have no impact on the mass of chlorides available in overhead stream. Would you please comment.
 
Answers
12/04/2011 A: Berthold Otzisk, Kurita Europe GmbH, Berthold.Otzisk@kurita.eu
In general caustic addition after the desalter, a monel cladded overhead line and a pH control of 5.5-6.5 of the sour water are very good tools to keep the corrosion potential low. A continuous water wash is favored instead of periodical water injection to avoid salt fouling and corrosion. The design of the wash water injection system is very important. It can be the scale pan of a perfect salt removal or inadequate washing procedure.
The main questions are:
- How much water is injected during the periodical washing procedure ?
- Is an increase of conductivity, chlorides or corrosion byproducts reported after starting the periodical water wash ?
- Where is the wash water injected ?
- Are spaying nozzles used to disperse the water ?
05/04/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Continuous water wash in your case would reduce neutralizer usage marginally in your case if all the water comes from demineralizer make up. The fresh water dilutes out the acids so that less neutralizer is needed to control pH in a slightly acid range. The savings in neutralizer would likely be more than offset by the cost of the additional water, however. Most units recycle water from the accumulator so continuous water wash has no effect on neutralizer demand in that case.
The intermittent water wash that you are using is likely not related to your lack of overhead line corrosion. Water wash is really mainly trying to protect your overhead coolers. If you are experiencing short life on your overhead bundles, then going to a continuous water wash is likely to help improve bundle life. When water wash is not being injected salts and/or acids can condense on the tubes leading to corrosion. Your intermittent water wash may stop salt related fouling but won't necessarily stop corrosion.
05/04/2011 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Adding more wash water will reduce the CONCENTRATION of chloride in the total wash water, rendering it less corrosive, thereby requiring lower additions of corrosion inhibitor. Continuous wash water use is highly preferred over intermittent for this service because it lessens the development of corrosion cells. Far better to keep corrosion inducing deposit from ever forming than trying to ensure they are periodically removed.
05/04/2011 A: Sam Lordo, Nalco Company, salordo@comcast.net
if the wash water is truly demin water (which is not normal) the actual conc. of cl will be reduced as the demin will dilute the corrosive and it is the conc of Cl that is the enemy, the other aspect is demin water typically has a higher pH and hence some pH control will come with using water wash
Overall using water wash continuously is much better than ono-continuously because the salts that form and reside in the overhead bundles go thru a wet/dry cycle potentially creating very active corrosion cells