EVALUATION OF PRESSURE AND THERMAL STRESSES IN DISSIMILAR WELDS PIPE SPOOL RELATED TO OTC FAILURE ANALYSIS AND DESIGN ANALYSIS OF NEW SOLUTION

Transcription

1 EVALUATION OF PRESSURE AND THERMAL STRESSES IN DISSIMILAR WELDS PIPE SPOOL RELATED TO OTC FAILURE ANALYSIS AND DESIGN ANALYSIS OF NEW SOLUTION Miša Jočić* and Zdravko Ivančić** PIPETECH Jocic, Haegelerstrasse 75, 5400 Baden, Switzerland* NUMIKON Ltd., Dragutina Golika 63, Zagreb, Croatia** Abstract: Due to an incident at the Combined Cycle Power Plant where a weld seam at HP OTC (High Pressure Once Through Cooler) steam outlet failed producing considerable damage, an investigation into the incident was carried out. The aim was to find possible design reasons for failure and generate necessary corrective design actions, which shall rectify the situation and produce a new solution ensuring future integrity of this type of piping designs. The development of a new design solution and the evaluation of pressure and thermal stresses in dissimilar (bi-metallic) welds (DMW) have been performed in accordance with generalized extension of the method recommended in API 579 for high temperature evaluations. The modified method follows PRG (Paulin Research Group) approach, which addresses a more general situation than that from API standard. The method has been combined with stress results at dissimilar weld points from ASME B31.1 beam type CAESAR II analysis, as well as from shell type elements FEM / FEPipe analysis, of the models that contain complete OTC Steam piping system as well as OTC Air System. The Air System was included in the analysis to properly identify, evaluate and integrate its impact on OTC Steam System, thus bringing even more clarity to the investigation. The new design solution for dissimilar welds, described with the selected Material Combination Option, fulfils the requirements of API 579 and ASME B31.1 as well as ASME Sec VIII Div 2 for both, Stress Fraction Approach and Strain approach for the required design life of to hrs, correlating well with plants C inspection intervals. Key words: Power Plant, OTC, weld, piping, analysis 1. DESIGN AND EVALUATION METHOD The method involves two parts: a secondary/fatigue stress fraction and a primary stress fraction. The sum of the primary and secondary stress fractions must be less than one.

2 [ Secondary Fatigue Stress Fraction + Primary Stress Fraction ] < 1 for beam elements type analysis using B31.1 Code stresses: And for shell elements based FEM / FEPipe stress analysis: Bimetallic welds induce additional secondary stresses and strains in the local vicinity of the weld. The stresses due to these welds shall be added to the fatigue evaluation for the component and the strains limited to 1% while the bimetallic weld allowable thermal strain should be ε < The thermal and pressure strain contributions can be approximated by finding the following summation for B31.1 analysis: and for ASME VIII Div 2 FEM / FEPipe based analysis Where: S L = Maximum Sustained Stress from all sustained load cases as used in the analysis of the piping system and S h = the lesser of the B31.1 Code hot allowable as used in the analysis of the piping system, the ASME Section VIII Division 1 allowable, or the minimum rapture strength at time and temperature and S mh = the ASME Sec VIII Div 2 allowable, i.e. Design Stress Intensity at temperature from Sec II, Part D 2. FAILURE INVESTIGATION The following step-by-step method shown in following two chapters was used in support of the failure investigation.

3 2.1. Review of the actual design Existing Caesar II model of the complete Air piping, OTC s, steel structure, spring supports and snubbers was completed with the addition of Steam piping. The aim was to compare the results with the existing Steam piping report once that a more detailed computer model was available. In conclusion to this comparative analysis, it could be said that nothing major was discovered which would have lead to a decisive conclusion that a faulty piping design calculations were the cause of failure. However, during this phase of investigation it has been noted that a peculiar situation with the DMW at the connection of OTC Steam pipe to the OTC was not addressed in the calculation. Picture 1 shows model of the cooling air system. Picture 2 shows system model with added steam system lines (yellow colour). Picture 1 Cooling Air System Pictures 3 to 10 show Caesar II results from OTC Steam Piping analysis. Picture 11 shows comparison of Caesar II results with Project Pipe Report.

4 Picture 2 Model with Steam System Picture 3 Maximum Sustained Stress in the HP OTC Steam System

5 Picture 4 Maximum Expansion Stress in the HP OTC Steam System Picture 5 Maximum Occasional Stress in the HP OTC Steam System

6 Picture 6 Maximum Sustained Stress in the vicinity of PT150 Nozzle Picture 7 Maximum Expansion Stress in the vicinity of PT150 Nozzle

7 Picture 8 Maximum Occasional Stress in the vicinity of PT150 Nozzle Picture 9 PT150 Operating Nozzle Loads

8 Picture 10 Operating Displacements Picture 11 Operating Displacements

9 2.1. Calculation of design life using CAESAR II beam model analysis Here is shown calculating design life of the dissimilar weld using CAESAR II beam elements model analysis for OTC HP Steam Outlet piping. In a combined ASME Sec VIII Div2, API 530/579 Stress Fraction and Strain Fraction approaches this analysis has shown that the actual design life of the welded joint would have corresponded to approximately hrs, coinciding well with the actual failure which occurred after hrs. Picture 12 shows proposed material and thickness for dissimilar welding. Picture 12 Proposed Material/Thickness for Dissimilar Welding Picture 13 shows pipe line pressure wall thickness calculation for different materials.

10 Picture 13 Pipe Line Pressure Wall Thickness Calculation Picture 14 Stresses in different points Following pictures show high temperature evaluation of bimetallic welds for stress fraction approach (picture 15) and for strain approach (picture 16).

11 Picture 15 High Temperature Evaluation of Bimetallic Welds Stress Fraction Approach Picture 16 High Temperature Evaluation of Bimetallic Welds Strain Approach 3. DESIGN AND ANALYSIS OF NEW SOLUTION The next two steps for finding a solution that would have assured a design life of hrs Calculation of Max Shear Stress distribution in the HP Steam Outlet Piping and validation of DMW relocation During this analysis, firstly the mapping of Maximum Shear Stress distribution was carried out and proved that by simple relocation of the DMW to the location of the smallest stress would double the design life of the welded joint. This finding was interesting because it has also shown that the existing

12 location of DMW was in the least favourable spot, at the location where the highest stress induced by external piping loads occurred. Pictures 17 and 18 show maximum shear stress distribution. Picture 19 shows details about relocation of dissimilar weld. Picture 20 shows system overview with changed location of dissimilar weld. Picture 17 Maximum Shear Stress Distribution Picture 18 Maximum Shear Stress Distribution

13 Picture 19 Relocation of DMW Picture 20 System Overview - Relocation of DMW Based on everything shown before, a practical most advantageous spot for possible relocation of the dissimilar welded joint was selected.

14 3.2. FE/Pipe shell elements based analysis of solution for relocation of the DMW The investigation up to this point has shown clearly that in systems with rapid temperature changes operating at elevated service temperatures, such as in OTC Steam Systems, high thermal stresses are produced where materials with significantly different coefficients of thermal expansion are joined. Experience in exploitation of DMW outlined in various studies and presented at conferences such as in the paper POTENCIAL EXPLOITATION PROBLEMS OF DISSIMILAR WELDED JOINTS P91 STEEL - LOW ALLOY HEAT RESISTANTE STEELS, presented at the International Symposium Powerplants 2008, recommend that the difference in thermal expansion coefficients of DMW should not be greater than 2 x10-6 mm/mm/ o C. Therefore to alleviate these discontinuity stresses, the piping transition should be constructed with a short spool piece of an intermediate coefficient of expansion between the adjoining pipes. Even more so, where severe temperature changes take place, and the materials being joined have radically different coefficients of thermal expansion, several buffer spools of different materials with progressively decreasing coefficients of expansion are required. Connecting stainless pipe to P-91 pipe falls into the most severe of these situations. Stainless pipe has a coefficient of expansion of 18.7x10-6 mm/mm/ o C and P-91 has a coefficient of expansion of 12.6x10-6 mm/mm/ o C. Hence, various combinations with numbers and materials for transition peaces has been analyzed in order to finally establish that the best practical option is with two transition spools made of : Alloy 800H (N08810) with coefficient of thermal expansion 17.1x10-6 mm/mm/ o C and Alloy 625 (N06625) with coefficient of thermal expansion 14.3x10-6 mm/mm/ o C. So, the progressively decreasing distribution of thermal expansion coefficients from stainless steel TP321H to P91 is achieved via two intermediate spools in the sequence: 18.7x x10-6 (Al 800H) x10-6 (Al 625) x10-6. This combination is analyzed as shown below and proven to satisfy the projected design life of hrs at design temperature of 585 o C and 19.3 MPa and at lower maximum operating conditions of 580 o C and below, even in the excess of hrs. This was sufficient to satisfy the C inspection intervals as typically set by the project requirements.

15 Picture 21 Material Combination Option 1 Picture 22 Material Combination Option 2

16 Picture 23 Material Combination Option 3 option 3. Following pictures show FE/Pipe Shell Elements Model for option 3 and results of analysis for Picture 24 FE/Pipe Shell Model Option 3

17 Picture 25 Calculated Primary Stress Option 3 Picture 26 Calculated Secondary Stress Option 3

18 Picture 27 High Temperature Evaluation of Bimetallic Weld Following pictures (27 and 28) show Summary for Various material combinations Picture 28 Summary for all 3 options

19 Picture 29 Summary of Options 4. CONCLUSION Piping systems exposed to internal pressure at elevated temperatures are designed and wall thickness selected to satisfy the highest component stress in relation to other stresses and which is usually the hoop stress due to pressure. However, in piping systems subject to 3-D state of stress other stresses are also present, such as stresses acting in shear direction for example, which are theoretically equal to one half of the normal stress. In any case, knowing all of these stresses is especially important in welded joints, because experience has shown that the largest number of failures of welded joints occurred around the circumference in the weld zone of the thick walled components, which is significantly different to what might be theoretically estimated. The reasons can be found in the complexity of the stress state produced by factors not taken into account during calculations, such as: residual stresses which have not been removed completely during heat treatment, residual stresses, which are the consequence of existence of different micro structural components and phases, which in the zone of the welded joint have different properties, system stresses produced by effects of various external loadings, stresses produced during transient processes of operation, stresses produced by geometrical factors of influence of the weld design.

20 All above-mentioned reasons contribute to the creation of 3-D state of stress in welded connections of thick wall piping, in such way that theoretical stress distribution usually doesn t correlate well with the real situation and consequently estimations of calculated design life becoming difficult to achieve accurately. DMW s like any other weld shall be done with PWHT to minimize or alleviate residual stress. B31codes have this specified as the only reason for PWHT. There are specialized software for calculation of welds, created with the sole purpose of designing the welding process and PWHT to achieve exactly this, while at the same time it has been acknowledged that heat affected zone is the area of concern, where failures have been usually detected. In his study, A. R. C. Markl and his team (1950 s) determined that girth butt-welds typically resulted in stresses approximately 1.7 to 2.0 times the stress in non-welded piping. As a result, all of the piping codes have been base lined to include the factor of 2.0 for girth welds via the application of the original SIFs which are still used in ASME piping Codes today in fatigue design due to cyclic loading. Another important thing is the relation between FSRF (Fatigue Stress Reduction Factor) and SIF factors: In terms of ASME Section 8, Division 2, Appendix 5 and finite element analysis (FEA) work, we could use the following equation SIF =Range of Peak Stress due to M / (2 * (Moment M) / (Section Modulus Z)), where the peak alternating stress (PL+PB+Q+F) is determined from finite element analysis. Normally, the peak stress is the product of the secondary stress and a FSRF. For instance: PL+PB+Q+F = (PL+Pb+Q)*FSRF / 2.0 and FSRF s are determined from testing or taken from references such as WRC 432. One could conclude that by using a factor of 0.50 on peak stress, Markl has essentially reduced the stress range to an alternating stress component. Using this conclusion, we can use Division 2 Appendix 4 & 5 peak stresses and which is exactly what we did in our analysis. As can be seen there is nothing about residual stress which should be used in the form of some stress concentration factor, however FSRF of 1.35 which was applied in our analysis based on 2007 Div 2 recommendations for modeling, could serve this purpose because from SIF * M / Z = PL+PB+Q+F = * (N) the number of cycles to failure N has been further reduced for the same alternating peak stress. In addition it should be also noted that we have limited our analysis to elastic only and as elastic analysis typically overestimate the creep damage it will result in a conservative estimate of design life. Finally, where dissimilar welding cannot be avoided in joining piping systems, it should be noted that this type of weld could definitely create a somewhat peculiar situation because of other reasons as well. In fact, as composition of the weld deposit will be controlled not only by the electrode or filler metal but also by the amount of dilution from the two base metals, the created joint material properties

21 could vary with the welding process, the operator technique, the joint design and may result in producing a joint weaker than expected. Hence, apart from being difficult to achieve, dissimilar metal welds could form weak adhesive rings and may act as stress concentration points, creating potentially situations where failure due to fatigue may occur much sooner than expected especially considering the 3D state of stress in such components. Therefore, in order to ensure reliability of our designs and without having control over the above-described processes, a fair dose of conservativeness was needed and found in the described design calculation procedure, which is also code compliant and contributes to achieving the highest practically possible integrity of this type of piping designs. It would be also strongly recommended, since OTC Steam Piping systems operate in the creep regime, are subjected to fatigue, transient temperatures and large external forces, that appropriate inspection strategies are set in place where DMW s are to be used. REFERENCES [1] ASME B : Power Piping [2] ASME Sec VIII, Div 1 and 2, 2007 : Pressure Vessel Code [3] API 530 Calculation of Heater Tube Thickness [4] API 579 Fitness For Service [5] Caesar II, version 5.20; COADE Inc., Houston, USA [6] FE/Pipe, Paulin Research Group, Houston, USA