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API RP 945:2003(2008) pdf download

API RP 945:2003(2008) pdf download.Avoiding Environmental Crackingin Amine Units.
Limited laboratory data and field experience have indicated that in wet H,S services. removal of protective scales from the internal surfaces of equipment by surface preparation to facilitate internal inspection might increase the likelihood of cracking when the equipment is returned to operation. This phenomenon is cxpected to be dependent upon the severity of the environment, specific start-up conditions, and the crack. ing susceptibility of the base metal or weldment. Recent research conducted using a large-scale pressure vessel exposed to severe hydrogen charging conditions has con- finned that this is a viable concern liii, Removal of the nor- mall> protective films on the steel surfaces led to a short period of higher-than-normal hydrogen flux dunng simulated stan-up conditions and produced increased cracking that was conilmied by acoustic emission testing (A1’). UE and post- test metallographic sectioning of the test vessel. Use of certain inhibitors applied directly to the cleaned surfaces after inspection was found to minimize the levels of hydrogen flux during simulated stan-up conditions. Coatings, while not specifically addressed in this research work, may also be a suitable mitigation method. Notwithstanding the results of this research, industry experience has not indicated that surface preparation has subsequently led to signilicant additional cracking. especially in amine service.
6.5.1 General
The repair methods listed in 6.5.2 and 6.5.3 primarily apply to equipment and large diameter piping. Small diameter piping [50 mm (2 in.) and smallerl can usually be replaced with new PWHT componenis at a lower cost than in situ repair and heat treatment.
6.5.2 Crack Removal by Grinding and Gouging
For all repairs. amine residuals and contaminants should be removed 1roii equipment surfaces prior to grinding, gouging. welding, and PWHT. Flushing with copious amounts of water is usually effective; in some cases additional cleaning with an inhibited acid solution, followed by water flushing. is required. Caution needs to be exercised when acid cleaning sulfide scales because of potential l-12S release.
Careful grinding is the preferred method for removing cracks and other discontinuities. The procedure requires careful control to avoid defect growth. During the grinding procedure. the area in question should be periodically checked (preferably by WFMT) to assure that all defects are eliminated.
Flame gouging and arc gouging (if used) must be performed with care, since these procedures may also cause the defects to increase in size. These methods can be used effectively as the first stage of crack removal, This should be followed by grinding and periodic WFMT to check for defect removal as discussed above.
If the defect depth is less than the corrosion allowance, an acceptable repair could consist of removing the defect by grinding, and feathering, or contouring the edges of the grind- out area by removing sharp edges and providing a smooth transition to thc surrounding surface. Welding may not be necessary when this repair method is used.
lithe defect depth is greater than the corrosion allowance. the evaluation and fitness-for-service methods methods specified in API 510, API 570 and RP 579. should be used to determine whether the vessel or piping with the locally thinned area is fit for continued service.
6.5.3 Crack Repair by Welding
Prior to any welding, consideration should be given to the need to remove (oulgas) residual atomic hydrogen from the area to be welded. This is most likely for equipment in rich amine service that has been subjected to a significant level of corrosion and hydrogen charging. Outgassing should not be needed for equipment in lean amine service. An acceptable outgas procedure consists of heating the area to a metal temperature of 232°C — 316°C (450° — 0°F) and holding that temperature for 2 to 4 hours. Other similar procedures have also been used effectively.
The area to be weld repaired should be preheated as required (see API 510 and RP 582). When all repairs are completed, repaired areas should be examined using the same nondestructive test method that was initially selected (preferably WI-MT). Other methods may be used to supplement the examination of the repairs as desired.


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