Example 1
Corrosion Under Mineral Wool Insulation. During a 2003 turnaround in a major Gulf Coast refinery, several units in a large crude processing block underwent a thorough CUI inspection. Mineral wool insulation originally installed when the unit was built in 1975 was removed for visual inspection.
Coatings were present in most cases, but they were not immersion grade coating. Of 18 pressure vessels inspected for CUI or CUF (corrosion under fireproofing), all were found to have suffered CUI, two were found to have CUF in addition to CUI, and 12 vessels required weld buildup to restore minimum wall thickness.
Example 2
Corrosion Under Mineral Wool Insulation. Heat exchanger in a major Gulf Coast refinery. A critical service shell and tube heat exchanger was found to have suffered severe CUI. Mineral wool insulation retained wetness in multiple areas along the bottom of the shell. The coating was from original construction and was not an immersion grade coating. It was typical practice to install mineral wool blanket encased between two layers of wire mesh to allow for better handling. Poultry netting and other square shaped wire mesh have been used. After removal of the insulation, a small seepage type leak began in two locations. The exchanger could not be removed from service without shutting down a large part of the refinery. Because the service was not flammable, a mitigation method involving rolled steel plates, each fitted with drain tubes and adhered to the vessel, then installation of a fiberglass wrap was utilized to protect vessel from further corrosion and route the drip type leakage to a safe location.
Example 3
CUI on Red Lead Primer Steel Surface. A pressure vessel built in 1975 and originally 7
coated with red lead primer was stripped of its mineral wool insulation for visual inspection in 2003. The vessel operates continuously at 125ºF (52ºC). Because the non-immersion grade coating remained continuously wetted under the water absorbent mineral wool insulation, the resultant corrosion was extreme. Technical analysis, based only on the depth of pits measured before complete removal of rust scale, resulted in a decision to replace the drum. Replacement of the drum required shutdown of a tower, installation of jumpover piping, and a unit slowdown of several days length. After removal from service, further examination revealed external metal loss in excess of 0.5 in (13 mm). It was determined that the new drum did not require insulation. It was noticed that the red lead primer was still intact in places. The light area in the lower right is the saddle. Looking up at the side of the vessel shell revealed horizontal grooves caused by corrosion at the locations where the wire mesh was in direct contact with the vessel. The authors believe the wire mesh was austenitic stainless steel. This area was directly beneath the location of water entry into the insulation.
Case Histories
Case 1
Wastewater Towers. In 1991, two wastewater towers in a major Gulf Coast refinery near Houston, Texas, were stripped of their old insulation to be recoated and reinsulated. The towers operate continuously and uniformly along their height at 250ºF (121ºC). At the time of application of two coats of epoxy phenolic, the steel temperature measured 190ºF. At this point in history, there were no specialty coatings formulated for application to hot steel surfaces. A "slow" thinner was used to help reduce dry spray. After the coating application, the towers were re-insulated with expanded perlite insulation. Recent inspection of the exposed manway covers and under the insulation through ports intended for ultrasonic thickness measurements revealed the coating is in very good condition. No cracking, flaking or other signs of degradation were observed.
Case 2
Debutanizer Tower. In June of 1990, a debutanizer tower in southern Louisiana was stripped of its thermal insulation for inspection. The tower operates at 180º-280ºF (82-138ºC), a critical region for CUI. The tower was insulated with calcium silicate and mineral wool insulation in different areas. It was found to be water saturated with a poorly maintained mastic and wire weather barrier. The steel substrate was severely pitted in some areas. The steel was abrasive blast cleaned to SSPC SP-10 and coated with a 2-coat solvent borne epoxy phenolic system. The vessel was reinsulated with expanded perlite block and aluminum jacketing. After 9 years of operation, the tower was recently stripped of the insulation and inspected as part of an ongoing CUI inspection program. The epoxy phenolic coating was found to be in excellent condition. Although rust staining was noted at support brackets for platforms and other appurtenances, this was found to be blast abrasive and debris which remained in contact with the intact coating. Apparently, debris from the scaffolding and other access structures fell onto and collected on some surfaces, after they were properly painted but was not removed prior to installation of the new thermal insulation.
Case 3
Coating Applied on Marginally Prepared Steel Surface. Some coating manufacturer claim their organic coatings are not only suitable for under insulation environments, but that they may also be applied to marginally prepared steel. Usually, this means that grit blasting is not required and that tightly adhered rust may be left on the steel. It is common practice for such coatings to be tested on steel coupons of similar condition at the service temperature. It is also important to know whether the steel to be coated is contaminated and the nature of the contaminants to assure that the test conditions accurately represent the actual steel to be protected.
A refinery in Asia experimented with an organic coating that was claimed to be applicable to non-blasted, rusted steel. After 2 years of operation under insulation, the coating exhibited complete failure due to lack of adhesion. The coating also exhibits signs of heat degradation,
CONCLUSIONS
Risk based inspection strategies and evolving NDE technologies are critical tools in an effective CUI inspection undertaking. In addition, an understanding of history is important to assure undesirable events are not repeated.
In the case of CUI, this is recognition that:
- Water will almost always get into insulation systems
- Absorbent insulation will exacerbate the corrosion
- Coatings selected for protection from CUI on hot or cold steel surfaces under insulation must be proven to resist the "hot and wet" or "cold and wet" conditions which exist for very long periods under insulation
- Surface preparation and application must be quality assured to assure the selected coating is capable of the desired long term corrosion protection
Source;http://wenku.baidu.com/view/9307622a915f804d2b16c1c2
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