The pipeline that carried 50% caustic solution needed cleaning. I reckoned this called for a flow of at least 30 gal/min in the 500-ft 2-in. line to maximize velocity — and, so, minimizing the ¾-in.-hose connections. Instead, operations opted for 150 ft of hose and water from the hot water system; this produced a paltry 9.4 gal/min, as I confirmed with a bucket test. After a few hours, the pH had dropped from 14 to 11. That, as I informed operations, was the easy part; the U.S. Environmental Protection Agency mandates a pH of 8 for caustic at landfalls. So, the flushing began again the next day. After a few more totes, operations claimed to have reached a pH of 9.4. I had my doubts, though, because I’d calculated a 1%-by-volume caustic residual in the pipeline and gotten field measurements of pH 11.
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Based on dilution alone, I estimated it would take 10 totes, at 275 gal/tote, to achieve a pH of 10.94 and 789 totes to reach a pH of 9. However, the actual amount of water required depends on its applied velocity and the way the contaminant bonds to the surface.
Disgruntled, operations blew out the pipe again to remove the residual. I captured a sample of the spray at the end of the hose — the pH was 10.96, as confirmed with two pH meters.
Operations staff had another trick up their sleeves. They filled up the pipe, let it soak for an hour, blew it out again and then flushed for a third time. Finally, after 16 totes (4,100 gals, as the totes weren’t full), we achieved a pH of 9.6. A final test of the water used for the hydro-pressure test that followed the construction showed a pH of 9.3.
My report concluded that blowing took the place of the velocity I was seeking but, without flushing, the caustic residual would have remained in the pipe.
Several factors can prompt the need to clean a line: 1) tie-points require clean welds; 2) the safety of the construction crew; 3) decommissioning or mothballing; or 4) to eliminate sources of food for biological contaminants.
I spent five years in industries — food, consumer product and pharmaceutical — where cleaning is an everyday affair. I’ve learned that salts, acids, bases and other ionic compounds are the most difficult to remove from pipes. Of course, scaling from hard water and material that sticks to a surface like caramelized molasses require harsh treatment, perhaps even abrasive cleaning that can endanger the texture of polished surfaces.
Measuring the efficacy of the cleaning of contaminants from acid and base lines is much easier than assessing the removal of bacteria — because pH meters provide instantaneous results that are easily and quickly duplicated. However, complications can arise in operations that use the same batch tanks in different campaigns.
Chemicals sometimes can improve cleaning but also may result in product contamination, a problem that can occur in any type of process plant. I remember a hydrotest on a chlorine system that took over a month to dry with nitrogen; opting instead for pneumatic testing would have made much more sense. In addition, a chemical that is highly toxic or has a pH greater than 10 or less than 4 will be more difficult to take to a landfill. Then, there’s the safety angle: acids and bases aren’t the only combinations that are exothermic — carefully consider venting in such situations. These are reasons why water often is the best choice. That said, a chemical that can be recycled or easily disposed of might work as a cleaning agent in some circumstances.
Looking at drainage and dead legs is important in all process and utility pipe designs. A good practice is what I call the “fit test.” It involves loosely assembling pipe components on a bench. Operators and engineers then can look at the spool and define the proper orientation and location of the components. For example, it’s best to place bleed and drain valves as close as possible to isolation valves.
Also, just once, I’d like to find a pipeline subdivided to make cleaning easier.
