1995: fatigue cracking of adsorber on hydrogen psa unit

Fatigue Cracking of Adsorber onHydrogen PSA Unit

In May 1994 a leak was detected in a weld in the top dished head of the adsorber, which was repairedto be safely returned to service. The cause of cracking was examined, as well as inspection andNDTof this and other adsorbers to assess their suitability for further operation. Fracture mechanics wasassessed to establish defect acceptance criteria and an appropriate inspection interval to ensure the

ongoing integrity of the vessels.

R. Davies, S. Hewerdine, and J. ChapmanICI Chemicals and Polymers Ltd., Billingham, Cleveland TS23 11 E, England

Operational Details

The HYS1V unit at ICI Billingham is a ten-vesselpressure swing adsorber (PSA) unit designed topurify a hydrogen-rich stream from the nearby

methanol plant. The unit is arranged in two trains offive vessels with the capability to run on either fivevessel or ten vessel operation. The maximum produc-tion rate from the unit on ten vessel operation is 80ton/d.

Plant History

The HYS1V unit was commissioned in 1979 andafter some problems in 1980 with adsorbent carryover,one train of five vessels was discharged in 1981 for afull internal inspection. This inspection revealed nosignificant defects and the opportunity was taken todevelop an external ultrasonic examination technique.This external technique was then used for the inspec-tion of the other five vessels in 1981 and then for theroutine inspection of all ten vessels in 1985 and 1989.

In late 1993 the adsorbent was changed in all tenvessels and the opportunity was taken to carry out anextensive internal MPI inspection of those ten vessels(five at a time in order to maintain production). Alarge number of indications were reported at this timeand a policy was adopted of removal by grinding,measuring the depth of grind out, and hence estimat-ing the through-thickness dimension (depth) of thelonger defects. In the first train of five vessels nodefect was found to exceed 3 mm in depth, the policywas relaxed so that only a sample of longer defectswas ground out with again none being found greaterthan 3 mm in depth, and all the shorter defects report-ed were left when the vessels were recharged.

The unit then ran until May 1994 when a leak wasdetected audibly on the H adsorber, and on furtherexamination the Cl weld was found to be cracked.This is the crown to petal weld in the top head (seedetail B of Figure 1). On inspection, a through-thick-ness crack of outside length 100 mm was discovered.The train of five vessels involved was then shut downimmediately, and H adsorber was discharged to enable

AMMONIA TECHNICAL MANUAL 148 1996

detailed examination and repair. The plant is shown inPicture 1.

Vessel Repair

The crack in H adsorber was found to be in an areaof both double curvature and change of section.

To enable a full metallurgical investigation to be car-ried out, it was decided to cut out the cracked sectionintact and to repair the vessel with an elliptical shapedpatch.

The patch was fabricated from a piece of platepressed to the required radius along the longest axis ofthe patch (see Figure 2). The patch thickness was thatof the thinner plate at the repair site with the additionalmaterial required being added by weld overlay (seeFigure 3).

The elliptical hole in the vessel was cut using a high-pressure water cutting technique controlled by a com-puter controlled two axis manipulator. The same tech-nique was used to cut the repair patch as this gave apatch which was a good fit with the hole cut on thevessel. The weld prep was cut on to the patch as thisminimized the amount of on-site work required.

The hydrogen duty of the vessel led to a requirementto preheat the repair area to 150°C to prevent hydrogeninduced cracking. For this reason, it was also decidednot to carry out intermediate NDT on the repair weldas the cooling required could give rise to cold hydro-gen cracking.

During the welding, a small incident caused the pre-heat temperature to be lost rapidly and it was decidedto leave the repair to cool for 72 h to protect againstcold hydrogen cracking. Examination showed severalsmall surface breaking defects were found and thesewere removed prior to the preheat being reapplied.

On inspection of welding the repair area was cooledslowly (10°C/h), ground to the required profile, andsubjected to rigorous NDT examination.

During the repair every internal weld within the ves-sel was examined with MPI, and every indicationfound removed by grinding. No defect was found toexceed 2 mm in depth.

The top dome of the vessel was then post-weld heattreated using an internal bulkhead method with elec-tric elements. The repair area was then re-examined

prior to a hydraulic pressure test being applied.The hydraulic test pressure used was higher than

that used at the time of original manufacture in orderto achieve the maximum "bunting" effect on anydefects present within the vessel.

The repair weld was then examined one last timebefore the vessel was charged with adsorbent andreturned to service.

Metallurgical Investigation

Metallurgical investigation of the cause of crackingwas carried out by TWI (see Literature Cited) andincluded:

• Visual and scanning electron microscope (SEM)fractography (see Picture 2)

• Metallography of sections through the crack (seePicture 3)

• Hardness survey• Chemical analysis

The main conclusions from the investigation were:(1) Failure occurred by a fatigue mechanism with

crack initiation from a pre-existing defect adjacent tothe inner wall of the vessel.

(2) The pre-existing defect is believed to be an orig-inal heat affected zone fabrication hydrogen crack ofdepth 2 mrn and internal surface length 20 mm.

(3) The finished surface lengths of the through-thickness crack were 140 mm and 100 mm inside andoutside, respectively.

Design, Stress Analysis and FractureMechanics Assessment

Design concept

The adsorbers are registered within the ICI in-ser-vice inspection system as 'Limited Life—Fatigue'because of the requirement for frequent pressurecycling.

They were manufactured in 1978 by a sophisticated,high quality local fabrication company with design,manufacture, inspection and testing being in accor-dance with BS 5500 Category 1 (see Literature Cited).The specified fatigue duty was 375,000 pressurecycles from 3 to 35 barg and back every 16 min and


Picture 1. The plant.

DETAIL A

CIRC. SEAM CIDETAIL B

DETAIL A-NOZZLE

DIAGRAMMATIC VIEW OF ADSORBER

DETAIL B - CIRCUMFERENTIAL WELD C1

OUTSIDE

Figure 1. View of adsorber.

FIG 2

SEE FIG 3 FOR DETAIL

NOTE: If pressing of plats to spherical radius is not possible thenthe patch is to be rolled to 1710 radius along the major axis

Figure 2. Elliptical shaped patch.

DIMENSIONS ON PATCH ARE TYPICAL

GRIND WELD FLUSH PATCH

40 MIN lPETAL PLATE

V

FULL PEN WELDS CUT BACKTO SOUND METAL BEFORE WELDING(TYP)

FIG 3

CROWN PLATE

TO MAINTAIN 1:4 TAPERWELD OVERLAY, SEE WELDPROCEDURE

Figure 3. Dimensions on patch are typical.


Picture 2. Fracture surface of crack. Picture 3. Macrosection through crack.

10'

E10'£

M

S

E10'

10s

=4-

103 10' 10s

NUMBER OF CYCLES, W10'

Figure 4. S-N curve.

-2C-

Figure 5. Crack dimensions.


Figure 6. Effect of weid microstructure on da/dNin hydrogen atmosphere.

because, like ASME Division II (see Literature Cited),BS 5500 provides only fatigue data for exposure in airand ICI specified modifying requirements. The rea-sons for these requirements were as follows:

(a) The possession of experimental evidence show-ing the deleterious effect of high purity hydrogen oncrack propagation rates (CEGB, Literature Cited).

(b) The assumption that fabrication cracks of depth2 mm or less could escape detection and act as initia-tion points for fatigue crack propagation.

In order to simplify specification, the ICI designrequirements were expressed in the form of a maxi-mum permissible operating peak stress range whichwas significantly lower than that which would havebeen derived from the S-N curve (Figure 4) providedin Appendix C of BS 5500. The underlying conceptwas that an undiscovered, 2-mm-deep, surface planardefect could begin to propagate as a fatigue crackfrom the first cycle and that failure could be said tooccur when the crack had penetrated 12 mm into the35-mm-thick material.

As well as the additional design requirements, ICIalso applied stringent requirements aimed at the detec-tion and elimination of planar defects, because these

seriously reduce fatigue performance. These require-ments included grit blasting and visual examination ofplates on receipt as well as multistage nondestructiveexamination (NDE), post-stress relief and post-hydraulic test magnetic particle and ultrasonic exami-nation.

The vessels successfully completed their design lifeof 375,000 cycles in 1993 and authorization was givento extend the life. This decision was based on the orig-inal design concept that fatigue crack propagationwould commence immediately from an undiscoveredsurface planar defect 2-mm-deep. Since it was report-ed that no defects of this size had been found by rou-tine ultrasonic examination, it seemed logical to allowa further life of 375,000 cycles from the time that thefirst such defect was reported.

Design Details

The adsorbers are made from a fine-grain, carbonmanganese steel to specification BS 1501-224-32B(superseded) comparable in composition and mechani-cal properties to A516-70. A vessel is shown in Figure1. The adsorbers are of the simplest possible design,comprising a cylindrical shell, hemispherical headsand only two nozzles; an inlet and an outlet. There areno internal or external attachments welded to the shellexcept for the support skirt. Because of the size andshape, it was necessary to manufacture the heads bythe crown and petal plate procedure.

The following geometrical details were specificallychosen because of the fatigue requirements andbecause external ultrasonic examination was to be themajor in-service inspection tool:

(a) Protruding nozzles, which reduce the peak stressat the welds.

(b) Hemispherical heads, which were easier to ana-lyze from a stress point of view in 1978.

(c) An inside taper between the 35-mm petal platesand the 58-mm crown plate in the hemispherical headsin order to achieve a smooth outer surface for ultra-sonic probes.

In the conclusions and recommendations section, thewisdom of (b) and (c) will be discussed.

The vessels were fully post-weld heat treated inaccordance with code requirements.


AXIAL MISALIGNMENT 'e'

^—W/-SrFigure 7. Midsurface misalignment at butt weld. Figure 8. Angular misalignment at butt weld.

Figure 9. Out-of-roundness or ovality.

220

6 8TIME IN YEARS

12 14

Figure 10. Adsorber H: Crack depth-V-time.Slight mismatch at welded joint.

"so

TIME IM YEARS

Figure 11. Adsorber H: Length -V- time.Slight mismatch at welded joint.

15 20DEPTH, a IN mm

Figure 12. Adsorber H: 2C-V-a.Slight mismatch at welded joint.


Crack P r o p a g a t i o nCalculations

General

The rate of propagation of a crackcan be determined in accordancewith the methods of British StandardPD 6493 (see Literature Cited). Thebasic relationship use is known gen-erally as the Paris Equation and is asfollows:

do/dN = A (delta/0A m (1)

Table 1. Crack Propagation Parameters Use in Paris Equation.A and M for air are from AEA technology (Literature Cited).

Medium

HydrogenHydrogenHydrogenHydrogen

Air

Phase

1234

AU

From delta /ifMN/mA 3/2101117.622.5

Notapplicable

To deltasMN/«A 3/21117.622.560

Notapplicable

A

3.8xlOA(-13)1.5xlOA(-10)2.33xlOA(-29)2.27xlOA(-9)

9.49 x 10A (-19)

m

4.31.817

2.25

3

where do/dN is the increase in crack depth per cycle(m/cycle); A and m are experimentally derived valuesfor a given material in a given environment; delta K isthe range of crack tip stress intensity (MN/mA3/2).

Crack tip stress intensity is a linear elastic fracturemechanics concept which is a function of the stressfield at the crack position, the thickness of the crackedcomponent and the crack geometry.

K = function [stress (pi x )A0.5] (2)

and

delta K = function [delta stress (pi a)A 0.5] (3)

where x is the instantaneous crack dimension.Delta stress is the stress range at the crack tip includ-

ing all local geometrical effects (stress raisers) butexcluding residual welding stresses. The stress rangeis split into membrane and bending components; themembrane component being the normal sphericalhoop stress and the bending component being due tothe offset of the adjoining midsurfaces at the taperplus any unintentional geometrical imperfections (mis-match).

There is much inconsistency in fracture mechanicsover the terminology for the crack leading dimensions;the terms used for 'length' and 'depth' are often inter-changed. For clarity, the term 'depth' will be used forthe penetration into the component thickness and 'sur-face length' for its surface dimension (Figure 5). The

dimensions are generally given the symbols a fordepth and 2C for surface length.

To calculate the total time for a crack to advance aspecified distance, the distance is split into many smallincrements, dx, and Eq. 1 is solved for the number ofcycles dN. The numbers of cycles for each incrementare then added to find a total and this is multiplied bythe cycle time to arrive at a total propagation time.The crack dimension used for each increment is con-servatively taken as that at the end of the cycle andtherefore it is more accurate to choose many smallincrements. It is important to note that the functionused in Eqs. 2 and 3 depends on both depth and sur-face length, and both are increasing at different rates.It is therefore necessary to solve for dx = da and dx =d(2Q.

Complexity Introduced by Hydrogen

As a result of testing carried out by CEGB Research,it is known that the crack propagation velocity inhydrogen follows distinct phases (Figure 6).Thesephases can be characterized by their values of A and mas shown in Table 1.

This phenomenon makes the numerical integrationof total cycles somewhat irksome because:

(a) Three distinct regions have to be calculatedinstead of just one as for air

(b) In normal circumstances, the difference in prop-agation rates for a and 2C are geometry-dependentonly and simplifying relationships can be introduced.


In hydrogen, the depth and surface length propagationrates are also dependent on which A and m band placetheir respective instantaneous values of delta K, e.g., amay be growing in band 2 while 2C is still growing inband 1. This results in additional reiterations of thecalculations.

Complexity Due to Tolerances

Although the weld is ground flush in this case, thereis still the possibility of the stress being increased byother joint imperfections which fall within the designcode tolerances. These are

(a) Axial misalignment between the parts to bejoined

(b) Angular misalignment between the parts to bejoined

(c) Out-of-roundness of the shell, sometimesreferred to as ovality.

Each of these is shown in Figures 7, 8 and 9 andtheir net effect is to produce an additional bendingstress across the plate thickness. At the joint in ques-tion, ovality is not relevant and it is only included forcompleteness. However, a comparison of stressesbetween a geometrically perfect joint and one whichutilizes full tolerances permitted by the code for axialand angular misalignment is shown in Table 2.

This means that in the absence of actual measure-ments, or any other evidence to the contrary, it is pru-dent to assume that the full tolerances have been usedand that the additional bending stresses are present.This will have a profound effect on any prediction ofthe fatigue life.

Procedure

Table 2. Possible Range of Applied Stress at Peak ofEach Cycle Depending on Joint Fit-up Quality

Joint Class

PerfectPermitted

MembraneStress MPa

81.681.6

BendingStress MPa

56.898.5

Total SurfaceStress MPa

138.4180.1

Therefore, the 35 mm was divided into three unequalphases based on the delta K, A, and m values detailedin Table 1.

The starting points for Phase 1 were the measuredsize of the initial fabrication defect and the stressranges based on a perfect joint. Reiteration was carriedout to get a match between the crack dimensions andthe delta K limit at the end of the phase. Phases 2 and3 were then calculated with the end point dimensionsof one phase being the start point dimensions for thenext.

The whole process was then reiterated using anincreasing bending stress to allow for increasing jointimperfection until a total fatigue life was obtainedwhich was consistent with experience.

A sample spread sheet calculation for the three phas-es is provided as Appendix A and the results are chart-ed in Figures 10, 11 and 12.

Results for H

Some observations on these results are:(a) The calculated time for propagation of the origi-

nal fabrication defect to the final through-thicknesscrack directly agreed with experience at a stress rangeless than 1 % higher than that which would apply to a

Since Adsorber H had surviveda through-thickness crack ofabout 100-mm length without suf-fering brittle fracture, it is evidentthat the material was toughenough to allow the full 35-mmthickness to be available for prop-agation, i.e., there was no criticaldepth at which another failuremechanism would intervene.

Table 3. Fatigue Performance in Different Phases in Hydrogen and Air

Phase

123

Distancemm

2 to 7.27.2 to 21.S21.5 to 35

HydrogenCycles

304,97392,8154,069

Time

10.1663.0940.136

AirCycles

196,805164,55282,172

Time

6.565.4852.739

RatioAir/H

0.6451.772

20.194


Table 4. Effects of Hydrogen on Toughness (CTOD)at 10°C * NACE TM-01-77

LocationParent plate

Weld metal

HAZ

ConditionUnchargedCharged*

UnchargedCharged

UnchargedCharged

Mean CTOD (mm)1.160.24

1.060.27

0.630.12

perfect joint fit-up. The calculated total time was 13.4years; each year being 8,000 operating hours.

(b) The calculated value for the crack surface lengthat breakthrough is 124 mm, which is a close matchwith the physical measurement.

(c) The times taken and the distances propagatedduring each phase are shown in Table 3. This includesa direct comparison with what might be expected inair.

It can be readily seen that the propagation rate inhydrogen increases above that in air at a delta K of17.5 MN/mA(3/2) or greater and that the accelerationis catastrophic above 22.5 MN/mA(3/2). However, ifthe delta K value can be kept below 17.5 MN/mA(3/2)the fatigue performance is better than in air.

(d) Using Figures 10 and 11 to work back fromcrack breakthrough, we arrive at crack dimensions forFebruary 1993 of about 13-mm-deep x 37-mm-longsurface. The sample examined by TWI showed abeach mark of approximately these dimensions associ-ated with a vessel entry at that time (see Picture 2).

(e) Reference to Figure 10 validates the originaldesign concept in that it took 12.2 years for the 2-mmdeep initial surface defect to propagate to the designlimit of 12-mm deep.

(f) Fracture mechanics calculations, based on thefact that the vessel survived a through-thickness defectof surface length 125 mm means that the fracturetoughness expressed as a crack tip opening displace-ment (CTOD) value exceeds 0.112 mm; this is in goodagreement with TWI tests on hydrogen charged mater-ial of the same specification (Table 4).

Inspection and NOT

Extensive internal examination and magnetic parti-

cle flaw detection (MPFD) was carried out on all tenadsorbers during an adsorbent change at the end of1992/beginning of 1993. Numbers of defect indica-tions were detected. A sample of these were surveyedby external manual ultrasonic examination; none wasmeasured at greater than 3 mm through thicknessdimension (depth).

Defects selected on the basis of the length of MPFDindications were removed by careful grinding; thisconfirmed the ultrasonic flaw detection findings inthat none was greater than 3-mm-deep.

Although MPFD indications were detected in theregion of the subsequent through-thickness crack, theywere not ground out or surveyed ultrasonically.

Following detection of the leak, a full internal exam-ination was carried out on adsorber H, including mag-netic particle flaw detection (MPFD) of all weldseams. Additionally, a full external manual ultrasonicflaw detection survey was carried out on all buttwelds, following a modified procedure developedfrom tests on the through-thickness crack, otherknown cracks detected by internal MPFD, and artifi-cial defects made in the plate from which the repairpatch was cut.

Similar ultrasonic surveys were carried out on theother nine adsorbers. Time of flight diffraction(TOFD) examination was also carried out on buttwelds on two other adsorbers for comparison purposes(AEA Technology, Literature Cited). The TOFD workwas complemented with measurements on a sectioncontaining part of the through-thickness crack and theplate containing the artificial defects.

Results

Manual pulse-echo ultrasonic examination

External ultrasonic flaw detection (USFD) of weldseams on Adsorber H gave a close match with thefindings of internal magnetic particle flaw detection.

Further examination of welds on other vessels usingthe modified external manual USFD procedure detect-ed a number of defects, but no significant depth wasmeasurable except for one defect which was measuredat 2-3-mm deep in weld B on Adsorber F.


Time of flight diffraction (TOFD) ultrasonic exami-nation

Trials on a section of the through-thickness cracklead to the conclusion that TOFD is not so successfulin inspecting the fully penetrating cracking in theelliptical section sample. Despite this limitation, rea-sonable sizing accuracy can be achieved using TOFD,typically the length of a defect (around the circumfer-ence of a weld). This can be sized within the range -1mm to +3 mm and the depth (from the outer surface ofthe material) +/-1 mm' (AEA Technology, LiteratureCited).

TOFD detected various small original weldingdefects in welds Cl and C4 on Adsorbers B and F.Additionally, crack-like indications were recorded inboth Cl welds; the worst of these were estimated to beup to 7.2- and 8.5-mm-deep from the internal vesselsurface and 35 and 23 mm in surface length, respec-tively. On further analysis and processing using a syn-thetic aperture focusing technique (SAFT), the lengthsreduced to 18 and 5 mm, respectively, although thesurface lengths remained unchanged.

Further discussion and analysis of results has indi-cated that these are more likely to be sub-surfacewelding defects rather than internal surface-breakingfatigue cracks.

Conclusions and Recommendations

(1) A through-thickness crack propagated from anoriginal heat-affected zone hydrogen crack in a regionwhere an internal surface-breaking flaw had beendetected at a previous in-service inspection, butbecause of the sampling approach used, it had notbeen removed by grinding, nor had a specific attemptbeen made to measure its through-thickness dimensionusing external ultrasonic flaw detection procedures.

(2) Using Paris' Law, in conjunction with dataspecifically derived for crack propagation in carbon-manganese steel in a hydrogen atmosphere, has beendemonstrated to give reliable results. This validatesthe calculational technique and data being used in theassessment of the remaining life óf the vessel givenaccurate defect sizes determined from ultrasonicexaminations.

(3) For the majority of the time taken to reachthrough-thickness status, the subject crack had beenpropagating in a stable manner, i.e., up to midwaythrough stage 2. However, crack propagation thenaccelerates dramatically and over 20 mm of thicknessis traversed in only a few months. Because of this, itcan be said that the original choice of 12 mm as thefailure depth was validated.

(4) A more general way of setting a satisfactory fail-ure limit would be to limit delta K to about 18.5MN/mA(3/2) which would cover combinations ofdepth and surface length. Future ultrasonic results forcracks should be assessed on this basis using a plustolerance of 1 mm on reported crack dimensions.

(5) The speed of propagation of cracks is criticallydependent on applied stress and this in turn is grosslyaffected by the geometrical factors introduced by fab-rication imperfections which are permitted by nationalpressure vessel codes. It seems sensible further torestrict the relevant imperfections of axial and angularmisalignment at welds and ovality of the shell bytightening the tolerances for vessels with a significantcyclic duty. The measurements should be recordedduring fabrication for possible future use in connec-tion with rémanent life prediction. In the absence ofmeasurements, calculations should be performed forthe worst permissible imperfections; this could reducepredicted lives by about 70% compared to the assump-tion of perfect construction.

(6) Curves showing crack propagation rates can beused as a valuable tool for setting inspection intervals.

(7) Using the procedure developed for externalmanual ultrasonic flaw detection based on work onactual cracks and artificial defects, correlated withMPFD examination, fatigue cracking can be detectedwith a high degree of confidence, even when crackingis relatively shallow; 2-3 mm-deep from the internalsurface.

(8) If the opportunity for vessel entry occurs, aninternal surface crack examination should always becarried out and if possible any indications ground out.Local removal of metal could be considered on a case-by case basis and should involve the prediction of anew rémanent life.

(9) If periodic monitoring of known defects is nec-essary and permissible from a fracture mechanics


viewpoint, then TOFD offers advantages over manualmethods in terms of sensitivity and repeatability.

(10) Retrospectively, the wisdom of design of amultipiece hemispherical head with a thicker crownplate is questionable because of the local stress-raisingfeature; in ellipsoidal heads there is usually sufficientmaterial in the crown to adequately compensate fornozzles without further thickening. The location of thetaper on the inside also ensures that the bending stressis tensile on the inside and that, consequently, the localstress is highest on the inside. This means that the out-side surface was made smooth in order to aid ultrason-ic detection of cracks which the design ensured wouldstart on the inside.

Acknowledgments

Grateful acknowledgments are made to A.M.Barnes, C. I. K. Sinclair, and S. Manteghi of TWI whocarried out the examination of the cracked section. C.E. Bull. P. C. Jones and M. G. Silk of AEATechnology NDT Centre carried out TOFD ultrasonic

examination of selected welds and part of the crackedsection. P. H. Pumphrey, P. Mclntyre and D. J.Goddard of CEGB Research carried out the work onhydrogen fatigue crack growth rates in pressure vesselsteel.

Literature Cited

AEA Technology NDT Centre Report, "Inspection ofHydrogen Pressure Vessel Samples."

ASME Boiler and Pressure Vessel Code Div IIMaterials Specification.

B S PD 6493, "Guidance on Methods for Assessing theAcceptability of Flaws in Fusion WeldedStructures".

BS 5500, "Unfired Fusion Welded Pressure Vessels."CEGB Research Report," The Influence of High

Pressure Hydrogen Gas on the Rate of FatigueCrack Growth in Welds of Pressure Vessel Steel toBS1501-224 Grade 32B."

TWI Report, "Investigation of Cracked HydrogenVessel."

1. State membrane and bending components of total linearised

Membrane component, d sigma(m) = 81.6 MPa

2. Stito Pöris Luw oud HDolicBbte Dflrsinotefs

da / <JN = A (delta K) «m and here A(a) = 3.8E-13 m(a) =

stress range excluding weld toe effects

Bending component, d sigma(b) =

4.3 A(b) 1.5E-10

CASE:STAGE:REFERE!

Adsorber B - slight mismatch1 delta K to 17.6 MN/m«(3/2)1CE:PD6493: 1991

57 MPa

m(b) » 1.8

3. State dimensions and number of increments and calculate log Increment

Start depth. ao= 0.007m Starting length, 2C = 0.018m

Increments, N = 23 From App. H, x =

Final depth, A(f)

(tog(af)-tog(ao))/N

0.0105 m Thk. B = 0.035

0.007656m

4. Calculate cycles from oo to of together with final surface length 2Cf, (Mk - 0 for flush ground weld)

Step Q) a] aJB dKC/dKa dC/da Cj

0 0.007 02 0.003t 0.007124 0.203557 0.982456 0.968642 0.0092412 0.007251 0.207177 0.978575 0.961767 0.0094853 0.00738 0.210862 0.974927 0.955322 0.0097314 0.007511 0.214612 0.971496 0.949279 0.0099815 0.007645 0.218429 0.968268 0.943609 0.0102336 0.007781 0.222314 0.965232 0.93829 0.0104887 0.007919 0.226268 0.962375 0.933297 0.0107468 0.00806 0.230292 0.959688 0.928612 0.0110089 0.008204 0.234388 0.957162 0.924217 0.011273

10 0.008349 0.238557 0.95478B 0.920094 0.01154111 0.008498 0.242799 0.952558 0.91623 0.01181312 0.008649 0.247118 0.950465 0.912809 0.01208913 0.008803 0.251513 0.948503 0.909221 0.01236914 0.00896 0.255986 0.946665 0.906052 0.01265315 0.009119 0.260539 0.944947 0.903094 0.0129416 0.009281 0.265172 0.943343 0.900337 0.01323217 0.009446 0.269889 0.941848 0.897771 0.01352918 0.009614 0.274689 0.940459 0.895389 0.0138319 0.009785 0.279574 0.939171 0.893183 0.01413520 0.009959 0.284546 0.937981 0.891148 0.01444521 0.010136 0.289607 0.936886 0.889276 0.0147622 0.010317 0.294758 0.935883 0.887562 0.0150823 0.0105 0.3 0.934969 0.886002 0.015405

aj/Cj

0.7777780.7709510.7644990.7583930.7526090.7471210.7419090.7369530.7322350.72774

0.7234520.7193580.7154450.7117020.7081170.7046820.7013860.6982220.69518

0.6922550.6894380.6867240.6841050.681578

Mm1.0744881.07587

1XJ772431.0786091.0799731.0813381.0827061.08408

1.0854631.0868581.0882671.0896931.0911371.0926021.0940911.0956051.0971461.0987171.10032

1.1019571.10363

1.1053411.1070921.108885

Mb

0.7923750.7886190.7847520.7807770.7766940.7725050.7682090.7638070.7592990.7546850.7499640.7451360.7402010.7351580.7300050.7247420.7193670.71388

0.7082790.7025620.6967270.690774

0.68470.678502

phi

1.4025191.3975311.3928271.3883851.3841851.3802081.3764381.3728591.3694591.3662241.3631431.3602061.3574031.3547261.3521661.3497151.3473681.3451161.3429561.3408791.3388831.3369611.3351091.333324

deltaK14.0461114.2101914.3726714.5336814.6933714.8518615.0092615.1656715.3211915.4759

15.6298915.78323

15.93616.08825165400416.3914416.5424916.6932416.8437416.9940317.1441517.2941417.4440217.59384

N

6988.5366968.7526951.8786937.7046926.0416916.7196909.5876904.5076901.3576900.0246900.4076902.4146905.9626910.9736917.3776925.1126934.1196944.3446955.7386968.2566981.8576996.5047012.161

Total N

6988.53613957.2920909.1727846.8734772.9141689.6348599.2255503.7362405.0869305.1176205.5183107.9390013.8996924.86103842.2110767.4117701.5124645.8131601.6138569.8145551.7152548.2159560.3

2CJ

0.0180.0184820X11897

0.0194630.0199610.0204650.0209760.0214920.0220150.0225450.0230820.0236270.0241780.0247380.0253050.0258810.0264650.0270580.0276590.02827

0.0288910.0295210.0301610.030811

Vears

0.2329510.4652430.6969720.9282291.1590971.3896541.6199741.8501242.0801692.31017

2.5401842.7702643.0004633.2308293.4614083.6922453.9233824.15486

4.3867184.6189944.8517225.0849395.318678

>•o

z2?S

>


DISCUSSIONMax Walter, BASF: Do you fix your inspectionendeavors according to the crack propagation lineyou've calculated? And do you use crack monitoringusing external NDE?Hewerdine: Yes is the answer to both these questions.We do use the crack growth rates to define our inter-val, and we are carrying out NDE. We have modified,in fact, the ultrasonic procedure that we used based onour recent inspection experience but we are relying onexternal ultrasonic examination for periodic monitor-ing.Nick de Clercq, South Africa: Have your vessels gotany internal support welding? In other words, is thereany support welded on the inside?Hewerdine: No, there is no support welding inside atall. One of the key design features of these vessels isthat they're made as simple as possible to reduce anystress concentration feature. This is why, for instance,there are only two nozzles and they're each in thehead. There is no additional welding apart from theskirt.de Clercq: Have you had any experience of failures inthe bottom part of your vessels? Are your failures only

occurring in the top section of the vessel or is it ran-dom in welding?Hewerdine: No, it's not random in welding. The mostsignificant defects have certainly occurred in the topof the vessel in this particular scene. Some defectshave been found elsewhere, but are not of the signifi-cance of this one. One of the significances of toler-ances is that this vessel was actually made close to theperfect condition which came out with the length oftime that it actually lasted. However if the cylindricalshell had been made to more adverse tolerances, thenwe may well have expected the first crack, certainly ifthere was a pre-existing defect, to have gone throughin one of the main longitudinal welds in the vessel. Infact there is some experience in the U.S. with similarvessels of through thickness cracks developing in thelongitudinal seams, and they have also been identifiedwith excessive peaking of the weld. So, the tolerancesthrow some elements of chance into it.de Clercq: We're operating, it appears, a systemwhich we haven't opened up yet. We are now in oper-ation for three years.


1995: fatigue cracking of adsorber on hydrogen psa unit

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