AMERICAN PETROLEUM INSTITUTE API RP 581 RISK BASED INSPECTION BASE RESOURCE DOCUMENT BALLOT COVER PAGE

Transcription

1 Ballt ID: Title: USING LIFE EXTENSION FACTOR (LEF) TO INCREASE BUNDLE INSPECTION INTERVAL Purpse: 1. Prvides a methd t increase a bundle s inspectin interval t accunt fr LEF. 2. Clarifies Table Als clarifies ERL in a sectin that was prpsed t be related. Impact: Rewrites Sectin and clarifies Table 8.6 Ratinale: Actin items frm ballting the changes incrprated int API RP 581, 3 rd editin expressed cncern abut (1) the way in which LEF are used t mdify the start date, (2) nt prperly defining Estimated Remaining Life, (3) and varius issues with Table 8.6. Technical Reference(s): Primary Spnsr: Name: Cmpany: Rbert Sladek Asset Optimizatin Cnsultants Phne: rsladek@accrp.cm Cspnsrs: Name/Cmpany: Mark Harmdy / Equity Engineering Name/Cmpany: Andrew Mrey / Dw Chemical Cmpany Ken Gttselig / BP Prducts Nrth America Tracking Status Submitted t Task Grup Submitted t SCI Submitted t Master Editr Date Reslutin Date Reslutin Date Added Dcument Rev 0 6/11/2012 Page 1 f 7

2 Prpsed Changes and/r Wrding {attach additinal dcumentatin after this pint} The prpsed Changes are redlined in the attached dcument. These changes are prpsed t reslve the fllwing screcarded API 581 Task Grup Actin Items: : ERL (estimated remaining life) is nt explained adequately. Where is it calculated and hw? - The authr shuld prvide the prper references and/r definitins. -- add (ERL) t sentence after estimated remaining life. SCORECARD the idea f prviding guideline fr mechanisms like cracking - Antni : Bundle life extensin shuld be added t the remaining bundle life, nt t the past bundle life. This culd be tied int the ERL apprach f valid cmment. Need t d it the way as prpsed : Install Spare Bundle - Add a cmment that indicates the spare bundle cnditin is knwn thrugh inspectin. Shuld the LEF be defined by the cnditin f the bundle? : This table has several issues: 1) Wuldn't the LEF fr a partial retube really depend n hw much f it was retubed? 2) Wuldn't the LEF fr the installatin f a spare bundle really depend n the cnditin f that bundle and its histry? 3) Where d the LEF numbers derive frm? 4) D the LEF numbers reflect a gain f a percentage mre life? <negative vte issue> - 1) This is truly dependent upn the percentage f tubes that were replaced and their lcatin. It is t variable t prvide in the table as a fixed value. Recmmend stating that it is user definable, with guidance n hw t d that. 2) This is truly dependent upn the cnditin f the spare bundle and its previus histry. It is t variable t prvide in the table as a fixed value. Recmmend stating that it is user definable, with guidance n hw t d that. 3) The authr shuld prvide the prper references. 4) This can be further explained in Sectin 8.6.5, alng with a reference t this table Inspectin Planning with Inspectin Histry a) Effect f Inspectin n POF The infrmatin gained frm an inspectin f the tube bundle can be used t assess the actual cnditin f the bundle and t make ustments t the POF rate curves as necessary. An inspectin prvides tw things: Dcument Rev 0 6/11/2012 Page 2 f 7

3 1) Reductin in uncertainty due t the effectiveness f the inspectin resulting in the use f a mre accurate failure rate curve, e.g., mving frm a 50% AU curve (n inspectin histry) t a curve 20% AU curve (usually effective inspectin). See Sectin b) fr a discussin f inspectin effectiveness. 2) Knwledge f the true cnditin f the bundle. This can result in a shift f the failure rate curve t the right r t the left. The current cnditin f the bundle culd either be quantified by remaining wall thickness data r by an estimate f the remaining life that cmes directly frm an actual inspectin, see Sectin c). b) Reductin in Uncertainty Due t Inspectin Effectiveness If the tube bundle has been inspected, the uncertainty is reduced (the POF curve mves t the right) and the POF at any time decreases. In this way, inspectin knwledge reduces the POF and the calculated risk. At this pint, the cncept f inspectin effectiveness is intrduced, similar t the methdlgy used in ther mdules. As imprved inspectin techniques are used, the amunt f uncertainty decreases and the Weibull plt shifts t the right. Using this cncept will result in mre rigrus inspectin techniques being implemented as the bundle reaches end f life. In the example bundle prblem, the impact f mre rigrus inspectin techniques can be seen by evaluating the predicted atin as a functin f inspectin effectiveness in Errr! Reference surce nt fund..table 2.C.4.1 The discussin f inspectin effectiveness is cntinued in Annex 2.C. c) Shift f POF Curve Due t Knwledge f True Bundle Cnditin 1) General Nt nly des an inspectin reduce the uncertainty in the data, it als prvides knwledge f the current cnditin f the bundle. It may be determined that the bundle is in excellent cnditin and that the curve being used fr the POF is t cnservative. Cnversely, it may als be determined that the bundle is in wrse cnditin than what has been predicted using data frm bundles in similar service. Where general crrsin is the cntrlling damage mechanism in the bundle, the average measured tube thickness data may be used t predict the future bundle failure date. Fr ther damage mechanisms (vibratin, tube end thinning, etc.) r where measured thickness data des nt exist, a qualitative estimate f the remaining life can be used t predict the future bundle failure date. Whichever methd is used t calculate a future failure date based n an inspectin recrd, the predicted future failure data can be used t mdify the base POF curve. 2) Predicted Future Failure Date Based n Measured Thickness Data The thinning rate f the tube bundle may be determined using the average furnished wall thickness and the average wall thickness as measured at the last inspectin as fllws: t t t rig insp rate (1.71) t With the failure pint defined as a fractin f remaining wall thickness, RWT, the predicted bundle life usted fr inspectin, PBL, can be calculated using Equatin (1.72). f Dcument Rev 0 6/11/2012 Page 3 f 7

4 PBL RWT t f rig (1.72) trate As an example, let s assume that the example bundle frm Sectin had the fllwing inspectin details: i) Installatin Date January 1992 with riginal wall thickness, t rig, f 0.12 inches. i) June 2000 Bundle had minr general crrsin thrughut n utside diameter (OD), 10% f the tubes were sampled using Ellit gages/calipers and fund t have an average wall thickness f 0.11 inches (8.5% wall lss). Bundle was hydrtested withut leaks. Inspectin effectiveness graded as C. ii) September 2003 Bundle shwed minr wall lss t inches average thickness (13% wall lss). Bundle was hydrtested withut leaks. Inspectin effectiveness graded as C. iii) A failure definitin f 50% remaining wall thickness is used, i.e., RWTf 0.5. Fr the example prblem, the bundle was inspected in September 2003 after 11 3/4 years in service. Over this time perid, the average measured wall thickness went frm 0.12 inches dwn t inches. The thinning rate is calculated using Equatin (1.73) as fllws: trate inches/year (1.73) The predicted bundle life usted fr inspectin, (1.74). PBL PBL, is then calculated using Equatin years (1.74) At this rate, the average wall thickness wuld reach 50% f the riginal wall thickness r the remaining wall thickness ( RWT ) in 44.1 years, r in September f 2047January f This f bundle is in better cnditin than predicted by bundles in similar service. d) Predicted Future Failure Date Based n Estimated Remaining Life As an alternative, when tube wall thickness data is unavailable fr calculatin f a bundle tube thinning rate, r when the damage mechanism is smething ther than general crrsin, the estimated remaining life (ERL) f the bundle frm the last inspectin can be used t calculate predicted bundle life. The ERL may be btained using inspectin data cmbined with accepted fitness fr service calculatins based n the damage mechanism(s) knwn r anticipated. PBL t ERL (1.75) e) Adjustment t Failure Rate Curve Based n Actual Cnditin f Bundle Once the predicted bundle life based n the last inspectin has been determined using either Equatin (1.72) r (1.74), a mdified characteristic life (Weibull parameter) fr the bundle may be determined using Equatin (1.76). It is mdified by adding the usted predicted life f the bundle as an additinal failure pint as fllws: Dcument Rev 0 6/11/2012 Page 4 f 7

5 md 1 N ti r i1 1 (1.76) Fr the example bundle with matching data f Table 8.2, a mdified parameter, by adding the 44.1 year predicted failure life t the riginal data set as fllws: md, is calculated md (44.1) 27.2 years (1.77) Pltting this as the failure pint n the Weibull diagram results in a shift t the right as shwn in Figure 8.4. Nte that the parameter (Weibull slpe parameter) was kept the same as the riginal curves frm data btained frm similar bundles. This is the basis f Weibaye s analysis that assumes that similar failure mechanisms will prduce similar slpe values. The new POF curve in Figure 8.3 (secnd frm left) shws the impact f the September 2003 inspectin. In the example prblem, tw ustments t the right were made. The uncertainty was reduced frm 50% (n inspectin) t 30% ( C Inspectin) as a result f the Ellit gauging/calipers measurements taken t estimate the remaining wall thickness. Additinally, the base curve cntaining the raw data was shifted t the right f the riginal raw data curve because the bundle was nt in as pr cnditin as was expected using the initial curve. As a result, the recmmended inspectin interval at the maximum acceptable POF f 50% fr the bundle was increased frm 7.1 years t 17.1 years r t February This calculatin can be made using Equatin (1.78) as fllws: tube tint md ln 1 P f,max 27.2 ln years tube tinsp md ln 1 P f,max 27.2 ln years (1.78) Effects f Bundle Life Extensin Effrts In general, ing an inspectin, nly minr repairs and cleaning peratins are perfrmed n exchanger bundles. Bundles are nt returned t an as new cnditin and therefre, the bundle POF is typically calculated at a service atin (i.e., time in service) based n the bundle s riginal installatin date. Hwever, there are several life extensin methds that may be made t a bundle ing shutdwns that serve t return the bundle back int service in an imprved (nt as-new) cnditin. In these situatins, it wuld be t cnservative t calculate the future bundle POF based n the riginal service dateatin, s an ustment is made t the start date fr calculatin purpses. Table 8.6 prvides a list f life extensin methds and the credit given as represented by the life extensin factr, LEF. An usted service atin, t, is then calculated using the LEF in accrdance with Equatin (1.79). Dcument Rev 0 6/11/2012 Page 5 f 7

6 t t AMERICAN PETROLEUM INSTITUTE 1 LEF t 1 LEF t (1.79) insp t In Equatin (1.80), the actual service atin,, is calculated as the time perid (years) calculated frm the bundle riginal installatin date t the inspectin date when the life extensin methd was perfrmed. t Inspect Date Install Date (1.80) The usted installatin date frm which aall subsequent POF calculatins are made is determined by subtracting using the usted service atin, methd was perfrmed. as shwn in Equatin (1.81). t (years), frm the date when the life extensin New install date = Inspect Date t (1.81) Fr the example bundle abve, if tubes were plugged ing the September 2003 inspectin, the recmmended inspectin interval wuld be increased by 10% (see Table 8.6) t 18.8 years, r Nvember The fllwing calculatin shws the effect f plugging tubes n service atin: t (1 LEF) t (1 0.1) years insp Table Effects f Bundle Life Extensin Methds Life Extensin Methd Life Extensin Factr (LEF) 3 Plug Tubes Bundle Rtatin 0.50 Partial Re-tube 0.50 Ttal Re-tube 0.90 Install Spare Bundle Install Tube Ferrules Nte: 1. This LEF is nly valid when the tube ferrules are installed fr prtectin against lcalized, tube-end damage due t ersin, crrsin r impingement. 2. TheA spare bundle s cnditin is knwn t be gd thrugh prir inspectins. Depending n the knwn cnditin, LEF may be usted. An LEF f 0.5 assumes an unknwn cnditin. If the cnditin f the spare bundle is knwn t be gdexcellent, a higher LEF can be used. 3. LEFs prvided in this table are suggestins. It is the respnsibility f the wner-user t define life extensins fr use fr the bundle life extensin methds. Dcument Rev 0 6/11/2012 Page 6 f 7

7 Table Numerical Values Assciated with POF and Financial-Based COF Categries fr Exchanger Bundles Prbability Categry (1) Cnsequence Categry (2) Categry Range Categry Range ($) 1 POF 0.1 A COF $10, POF 0.2 B $10,000 COF $50, POF 0.3 C $50,000 COF $150, POF 0.5 D $150,000 COF $1,000, POF 1.0 E COF $1,000,000 Ntes: 1. In terms f the ttal DF, see Part 2, Sectin In terms f cnsequence area, see Part 3, Sectin Dcument Rev 0 6/11/2012 Page 7 f 7