1、 Standard Practice Prevention, Detection, and Correction of Deaerator Cracking This NACE International standard represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone, whether he or she has
2、 adopted the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not in conformance with this standard. Nothing contained in this NACE International standard is to be construed as granting any right, by implication or otherwise, to manufacture, sel
3、l, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against liability for infringement of Letters Patent. This standard represents minimum requirements and should in no way be interpreted as a restriction on the use of bet
4、ter procedures or materials. Neither is this standard intended to apply in all cases relating to the subject. Unpredictable circumstances may negate the usefulness of this standard in specific instances. NACE International assumes no responsibility for the interpretation or use of this standard by o
5、ther parties and accepts responsibility for only those official NACE International interpretations issued by NACE International in accordance with its governing procedures and policies which preclude the issuance of interpretations by individual volunteers. Users of this NACE International standard
6、are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determining their applicability in relation to this standard prior to its use. This NACE International standard may not necessarily address all potential health and safety problems or environmen
7、tal hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this standard. Users of this NACE International standard are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with approp
8、riate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this standard. CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be revised or withdrawn at any time in accordance with NACE
9、 technical committee procedures. NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial publication and subsequently from the date of each reaffirmation or revision. The user is cautioned to obtain the latest
10、edition. Purchasers of NACE International standards may receive current information on all standards and other NACE International publications by contacting the NACE International First Service Department, 1440 South Creek Dr., Houston, Texas 77084-4906 (telephone +1 281/228-6200). Revised 2007-03-1
11、0 Revised 1996-03-30 Approved April 1990 NACE International 1440 South Creek Dr. Houston, Texas 77084 +1 281/228-6200 ISBN 1-57590-0111-4 2007, NACE International NACE SP0590-2007 (formerly RP0590-96) Item No. 21046 SP0590-2007 NACE International i _ Foreword NACE Task Group T-7H-7 on Deaerator Crac
12、king (now Task Group TG 244 on Deaerators: Prevention, Detection, and Correction of CrackingRevision of NACE Standard RP0590) was formed in 1984 to conduct an organized study into the cause of the high incidence of serious deaerator cracking problems in steam generating plants. The task group had pr
13、eviously sponsored technical symposia in which several papers were published on deaerator cracking.1-8 This standard is intended to be the primary source of information on deaerator cracking and is directed toward operators and designers of deaerator equipment used in steam generation. Information p
14、resented in this standard reflects the work of the many individuals involved in documenting the deaerator cracking problem and is based on studies of carbon steel units. Similar cracking has been found in blowdown flash tanks, sedimentation tanks, hot water storage/disengaging vessels, and steam and
15、 feedwater piping. In developing this standard, the TG considered the case of a southeastern U.S. paper mill that had experienced loss of life as a result of a ruptured deaerator storage tank. The catastrophic failure resulted in an increase in deaerator inspections and widespread concern for vessel
16、 reliability and personnel safety. A “Deaerator Advisory,” published by the Engineering Division of TAPPI,(1)reported that 68 vessels (approximately 50% of the vessels inspected in 1983) showed cracking in welds and adjacent heat-affected zones (HAZ) resulting from corrosion fatigue.9Of the three re
17、ported storage vessel ruptures, one resulted in fatalities and considerable plant downtime. Other literature on deterioration of deaerators noted that investigations of various systems indicated that cracks in the welds and HAZ of longitudinal and circumferential seams were the cause of some of the
18、problems.10Corrosion, another major cause, had occurred at a more rapid rate in the weld HAZ in some instances, and problems had reportedly occurred in both the welds and the base metal caused by shell thinning to levels that could not support the load. Periodic internal inspections combined with no
19、ndestructive examinations (NDE) were recommended to detect deaerator deterioration. Other reports of the seriousness of deaerator weld cracking also were published at this time.11-15 TG 244 has continued to actively collect data and information on reinspections. The information obtained from the rei
20、nspections has been used to update this standard. This standard practice was originally prepared in 1990 by T-7H-7 under the guidance of NACE Unit Committee T-7H on Corrosion and Its Control in Steam Generating Systems and issued by NACE under the auspices of NACE Group Committee T-7 on Corrosion by
21、 Waters. It was revised by T-7H-7 in 1996, and in 2006 by TG 244 on Deaerators: Prevention, Detection, and Correction of CrackingReview of NACE Standard RP0590. TG 244 is administered by Specific Technology Group (STG) 11 on Water Treatment and is sponsored by STG 34 on Petroleum Refining and Gas Pr
22、ocessing, STG 36 on Process Industry: Materials Performance in Chemicals, STG 38 on Process Industry: Pulp and Paper, STG 41 on Energy Generation, and STG 60 on Corrosion Mechanisms. This standard is issued by NACE under the auspices of STG 11. The root causes of deaerator cracking have not been det
23、ermined; therefore, this standard is based on good practices. In NACE standards, the terms shall, must, should, and may are used in accordance with the definitions of these terms in the NACE Publications Style Manual, 4th ed., Paragraph 7.4.1.9. Shall and must are used to state mandatory requirement
24、s. Should is used to state something considered good and is recommended but is not mandatory. May is used to state something considered optional. _ (1)Technical Association of the Pulp and Paper Industry (TAPPI), 15 Technology Parkway South, Norcross, GA 30092. _ SP0590-2007 ii NACE International _
25、NACE International Standard Practice Prevention, Detection, and Correction of Deaerator Cracking Contents 1. General. 1 2. Crack Characteristics . 3 3. Nondestructive Detection of Cracks. 4 4. Repair, Design, and Fabrication. 6 5. Operation and Water Chemistry. 8 6. Conclusions 12 References 12 Bibl
26、iography 13 Glossary of Acronyms. 14 Appendix A: Analysis of Data Collected Through Industry Surveys (Refer to Paragraph 1.5) (Nonmandatory) . 14 Figure 1a: Typical Mechanical Feedwater Deaerator 1 Figure 1b: Weld Areas Associated with Typical Unit . 1 Figure 1c: Saddle-Type Mechanical Deaerator . 1
27、 Figure 1d: Weld Areas Associated with Saddle-Type Unit 1 Figure A1: Deaerator Cracking Mini-Survey Sample Distribution by Years in Service 16 Figure A2: Cracking Distribution in Mini-Survey by Degree. 16 Figure A3: Type C Cracks Detected in Mini-Survey as a Percentage of Deaerators Inspected by Age
28、 Category. 17 Figure A4: Total Cracks Detected in Mini-Survey by Vessel Age 17 Figure A5: Re-inspection Intervals vs. Cracking Results Excluding Mud/Steam Drums . 18 Figure A6: Inspection Interval vs. Discontinuity Category 18 Figure A7: Deaerator Inspection and Repair Flow Chart. 19 _ SP0590-2007 N
29、ACE International 1 _ Section 1: General 1.1 The objective of this section is to identify important factors influencing boiler feedwater (BFW) deaerator cracking based on literature references and case history analyses. 1.2 Deaerator Design 1.2.1 The function of the deaerator in the steam plant cycl
30、e is to reduce oxygen and other dissolved gases in the feedwater to acceptable levels. Usually, oxygen content can be reduced to less than 10 g/L (ppb) as shown in Table A1 (see nonmandatory Appendix A). Two typical designs of mechanical feedwater deaerators are shown in Figures 1a and 1c. Figures 1
31、b and 1d show the weld areas associated with each type of unit. The steam used in these systems raises feedwater temperature, which lowers the solubility of oxygen. With proper venting, the steam also serves as the stripping gas, removing the oxygen from the system. In these systems, the deaerator p
32、rovides feedwater storage and proper mechanical conditions for the feedwater pump. FIGURE 1a Typical Mechanical Feedwater Deaerator FIGURE 1b Weld Areas Associated with Typical Unit FIGURE 1c Saddle-Type Mechanical Deaerator When a vertical heater is welded directly to the storage vessel, the saddle
33、 weld is of particular interest because of the fabrication stresses possible. FIGURE 1d Weld Areas Associated with Saddle-Type Unit MAKE UP WATER CONDENSATE SP0590-2007 2 NACE International 1.3 Oxygen Contamination 1.3.1 Severe oxygen contamination occurs in BFW when mechanical problems occur with d
34、eaerators, feedwater pumps, turbine gland seals, and systems operating under vacuum. Incomplete deaeration can be caused by many factors, including improper venting, operational changes that cause an influx of cold makeup water, improperly aligned trays, plugged or broken water spray nozzles, and op
35、eration with a temperature differential between the dome and storage sections that deviates from specifications. 1.4 System Design and Operation 1.4.1 Internal structural and component failures related to equipment design and manufacture, system design, and operation have occurred in deaerators. The
36、 deaerator is a relatively simple device when compared with typical boilers and turbines in a system. As such, it has often been neglected when unusual operating conditions are considered. 1.4.2 Deaerator design for utility systems has evolved as a result of several reported problems with utility de
37、aerators because of rapid load reduction, thermal steady state or shock stresses, vibration, or component shortcomings. For example, data from field reports obtained between 1969 and 1979 showed that damage to trays in utility deaerators was common prior to 1976.16 1.4.2.1 Of the 80 installations su
38、rveyed, 18 (22.5%) reported damage to trays or related hardware. Trays were dislodged and rattled inside the unit and, in some cases, were bent and broken beyond repair. 1.4.2.2 Tray pans, end clips, fasteners, and distribution troughs were also damaged. 1.4.2.3 Shrouding around the tray enclosure w
39、as damaged in some cases as a result of plant upsets and, in particular, following full-load rejection, which occurs when the turbine trips and turbine extraction steam is lost. This may cause flooding of the downcomers, resulting in water being blown upward against the tray bank. 1.4.2.4 Flashing o
40、f steam from the storage section to the deaerator section causes damage and is related to excessive pressure drop in the equalizers or excessive pressure drop across the tray bank; this problem was alleviated by adequately sizing the equalizers and providing sufficient height above the bottom of the
41、 deaerator. 1.4.2.5 Since 1976, the reported incidence of tray damage has decreased significantly. However, experience has shown that damage to trays and spray nozzles during operation still occurs. 1.4.3 Other problems also were reported when units were operated without regard to design limitations
42、.16 Severe thermal stresses occur, for example, when a deaerator is alternately subjected to hot steam and cold water; this can cause rapid failure of the unit. In one peaking unit, extensive failure occurred after a few months of operation because superheated steam had been fed at reduced pressures
43、 during offload periods and cold condensate was fed in slugs to maintain the level. In another situation, condensate was near steam temperatures between production batches, resulting in severe damage. 1.4.4 Water hammer or steam hammer can also occur in the steam plant. At the deaerator, steam hamme
44、r is usually caused by water entering a steam line or steam-filled space. In one installation, the water level was maintained at 762 mm (30 in.) above the recommended level when a load rejection occurred. This resulted in severe water/steam hammer, which damaged equipment supports. Other operational
45、 concerns are prolonged low-load operation, flows beyond design, operation at less than design temperature, and steam temperature above design level, all of which can increase localized stress intensities. 1.5 Inspection Statistics 1.5.1 Statistics were compiled by Work Group T-7H-7a using informati
46、on supplied by independent testing laboratories, insurance companies, and in-house maintenance inspection teams from the pulp and paper, chemical, and power industries. Refer to Appendix A for these statistics. 1.5.2 Table A1 shows typical performance of oxygen removal equipment in an attempt to def
47、ine operating parameters of most units. The oxygen levels are based on operation under design conditions. The statistics in Table A2 were developed by the task group and relate to all types of deaerators. 1.5.3 The study showed that of 650 vessels inspected, 269 (41%) were cracked at or near welds a
48、nd required repair. Wet fluorescent magnetic particle testing (WFMT) results were used to compile these statistics. 1.5.4 Analysis of inspection data showed no apparent statistical correlation between or among cracking frequency and manufacturer, operating pressure, size, age, materials, water treatment, or other variables. A survey conducted by T-7H-7e, however, found that water/steam hammer was a likely contributor to the problem. SP0590-2007 NACE International 3 1.5.5 A mini-survey of 174 ves
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