Inspectioneering Journal

99 Diseases of Pressure Equipment: Cooling Water Corrosion

By John Reynolds, Principal Consultant at Intertek. This article appears in the May/June 2005 issue of Inspectioneering Journal.

Cooling water (CW) corrosion may be the oldest form of corrosion in the petrochemical industry, yet the industry still struggles with it for two primary reasons:

  • One, the change from chromate treatments to phosphates was problematic for many operating sites, and two,
  • Cooling water corrosion control sometimes slips to a lower priority operating quality issue until it starts to noticeably impact process reliability and turnaround costs.

And one of the oldest maintenance problems in the industry is deciding when to clean and/or retube CW exchanger bundles. Too late, and you risk leaks, process economics and reliability impacts. Too soon and you waste money removing tubes that would last another run or cleaning bundles that don’t really need it. It takes dedicated, continuous efforts to track bundle service lives and heat exchanger performance in order to know the right time to clean or retube CW bundles, but because of the expense of not doing it right, it’s worth it.

Cooling water corrosion control is simply a matter of proper design, continuous maintenance of high quality water treatment, and proper operating practices. Definitely not rocket science. If one or more of these management systems breaks down, then you are likely to suffer the consequences associated with higher corrosion and/or fouling rates.

Cooling water corrosion and fouling are closely related and should be considered together. Critical factors in controlling both include: process and CW temperatures, heat flux, water velocity, type and quality of water (salt, brackish, fresh) and type of cooling system. Increasing process side temperatures or cooling water outlet temperatures typically will increase corrosion and fouling rates. Generally if the process side temperature is above 140F (60C), then there’s a potential for scaling on the water side. With very few exceptions, the CW needs to be on the tube side to minimize corrosion and fouling. Fluid velocity in the tubes needs to be high enough to avoid dropping out deposits (typically ~3+fps) and low enough to avoid erosion-corrosion problems. Corrosion in CW exchangers can manifest itself as general thinning, pitting, stress corrosion cracking and microbiologically induced corrosion (MIC) (covered separately). Electric resistance welded (ERW) tubes will sometimes suffer localized weld attack, especially if the CW quality is less than desirable.

Metallurgical upgrades may be necessary (and the economic choice) under some circumstances, especially where fluid velocities cannot be adequately controlled, process or CW temperatures are too high, or water chemistry is not good. Depending upon circumstances, those metallurgical upgrades may be to copper alloys (Admiralty brass or cupro-nickel alloys), duplex stainless steels, or austenitic stainless steels where chloride cracking or pitting is not a compounding problem. Corrosion in exchanger channels and floating heads can often be adequately controlled with a high quality coating system and/or installed sacrificial anodes, but both tend to be high maintenance corrosion prevention systems and need to be carefully planned and controlled.

Do you have a good understanding of the CW bundle service lives in each of your cooling water exchangers, as well as bundle performance (U-factor calculations over time), such that you truly understand the economics and reliability of your cooling water exchangers?

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