I performed a finite element stress analysis to validate current pressure and vacuum allowable limits for API 12F shop welded tanks. I was invited to speak at the American Petroleum Institute 2014 Fall Refining and Equipment Standards Conference in Denver CO, on the topic of pressure studies of API 12F series tanks. The work was performed for the API Committee on Refinery Equipment, Subcommittee on Above-ground Storage Tanks (SCAST), Work Group on 12 Series documents, and Task Group on 12F Pressure Research, and was used as a basis for further consultation and pressure studies commissioned by the API.

1. 11
API 12F 750 Bbl TankAPI 12F 750 Bbl Tank
Finite Element Analysis- Shop Welded TankFinite Element Analysis- Shop Welded Tank
Performed by – Kieran ClaffeyPerformed by – Kieran Claffey
Baker Consulting GroupBaker Consulting Group

2. 22
BackgroundBackground
Kieran ClaffeyKieran Claffey
 Bsc Mechanical EngineeringBsc Mechanical Engineering
 Vacuum Chamber Design forVacuum Chamber Design for
Semiconductor Industry,Semiconductor Industry,
 Chemical Vapor Deposition ReactorChemical Vapor Deposition Reactor
Design,Design,
 Damage Mechanics, Fatigue, ImpactDamage Mechanics, Fatigue, Impact
Analysis, R&DAnalysis, R&D
 Stress Analysis and FE for Above GroundStress Analysis and FE for Above Ground
Storage TanksStorage Tanks

3. 33
Objective of AnalysisObjective of Analysis
1.1. Validate the current API 12F standardValidate the current API 12F standard
requirements for recommended sizesrequirements for recommended sizes
and pressure and vacuum limits.and pressure and vacuum limits.
2.2. Determine if the pressure rating of APIDetermine if the pressure rating of API
12F shop welded tanks can be12F shop welded tanks can be
increased.increased.

4. 44
SummarySummary
1.1. Modeling of 750 bbl tank sitting on medium density sand.Modeling of 750 bbl tank sitting on medium density sand.
2.2. Results for vapor space pressure of 10 psi, 4.5 psi, 2 psi, 1 psi andResults for vapor space pressure of 10 psi, 4.5 psi, 2 psi, 1 psi and
0.5 with a full tank of water.0.5 with a full tank of water.
3.3. Results of hard vacuum analysis.Results of hard vacuum analysis.
4.4. Results with 0.5 oz/in^2 vacuum in vapor space.Results with 0.5 oz/in^2 vacuum in vapor space.
5.5. Results for double fillet corner weld model.Results for double fillet corner weld model.
6.6. Results for hybrid joint comprising of fillet on inside corner weld andResults for hybrid joint comprising of fillet on inside corner weld and
bevel on outside.bevel on outside.
7.7. Points of interest - stress concentration at flush clean-out, bottom toPoints of interest - stress concentration at flush clean-out, bottom to
shell joint, shell to roof joint, dome/nozzle to roof joint, etc.shell joint, shell to roof joint, dome/nozzle to roof joint, etc.
8.8. Buckling instability under vacuum conditions.Buckling instability under vacuum conditions.
9.9. Tank DeflectionTank Deflection
10.10. Validation of model.Validation of model.
11.11. Recommendations from FE study.Recommendations from FE study.
12.12. Summarize resultsSummarize results

5. 55
Selection of Stress Intensity (Tresca Criterion)Selection of Stress Intensity (Tresca Criterion)
for API Stress Analysisfor API Stress Analysis
 Reference from API 579 B.2.1.2bReference from API 579 B.2.1.2b
 The American Petroleum Institute recommends the use of VonThe American Petroleum Institute recommends the use of Von
Mises and/or Tresca (Stress Intensity) stress to the tankMises and/or Tresca (Stress Intensity) stress to the tank
engineer/stress analyst when analyzing above ground storageengineer/stress analyst when analyzing above ground storage
tanks.tanks.
 Solid Works Simulation software allows the stress analyst to viewSolid Works Simulation software allows the stress analyst to view
the effective stress experienced at a point in a material, independentthe effective stress experienced at a point in a material, independent
of the type of loading or failure mechanism whether it be pureof the type of loading or failure mechanism whether it be pure
compression, pure tension, pure shear, torsion, bending or buckling.compression, pure tension, pure shear, torsion, bending or buckling.
This effective stress can be calculated within the finite elementThis effective stress can be calculated within the finite element
software in the form of Tresca stress (stress intensity P1-P3).software in the form of Tresca stress (stress intensity P1-P3).
Tresca criterion calculates principal stress in 3D space, (σ1, σ2,Tresca criterion calculates principal stress in 3D space, (σ1, σ2,
σ3); thus providing an assessment of stress through out theσ3); thus providing an assessment of stress through out the
thickness of the steel plate. Tresca analysis tends to highlight stressthickness of the steel plate. Tresca analysis tends to highlight stress
concentrations better than equivalent Von Mises Stress.concentrations better than equivalent Von Mises Stress.

6. 66
Model ParametersModel Parameters
 The tank is modeled in Solid Works FEA Simulation software as aThe tank is modeled in Solid Works FEA Simulation software as a
series of events where there is pressure differential between theseries of events where there is pressure differential between the
inside and outside of tank.inside and outside of tank.
 No fixed boundary conditions were close to the shell to bottom weldNo fixed boundary conditions were close to the shell to bottom weld
or close to the shell to roof weld. This was done in order to allow fullor close to the shell to roof weld. This was done in order to allow full
motion of these joints as would occur in reality as most 12F tanks sitmotion of these joints as would occur in reality as most 12F tanks sit
on soils or sand foundations that are allowed to move and bend withon soils or sand foundations that are allowed to move and bend with
the foundation.the foundation.
 The natural frequency and mass was used to calculate the rigidity-The natural frequency and mass was used to calculate the rigidity-
stiffness of the bottom and roof plate.stiffness of the bottom and roof plate.
 The modulus of sub-grade reaction for medium density sand wasThe modulus of sub-grade reaction for medium density sand was
used to calculate the rigidity-stiffness of the sand underneath tank.used to calculate the rigidity-stiffness of the sand underneath tank.
 These stiffness values were added to the model in order to simulateThese stiffness values were added to the model in order to simulate
how the materials deflect in real world situations.how the materials deflect in real world situations.

7. 77
Model DescriptionModel Description
 Tank size - 15 ft 6” inches diameter and 24 ft high.Tank size - 15 ft 6” inches diameter and 24 ft high.
 Three shell courses (8ft high ea), flat bottom tank.Three shell courses (8ft high ea), flat bottom tank.
 Gravity effects are taken into consideration in all of theGravity effects are taken into consideration in all of the
FE models.FE models.
 All material in the tank was modeled as A 283 Grade CAll material in the tank was modeled as A 283 Grade C
steel which is the lowest strength material in API 12F.steel which is the lowest strength material in API 12F.
(Yield Strength = 30,000 psi)(Yield Strength = 30,000 psi)
 Minimum thickness shell plate (courses 1 through 3)Minimum thickness shell plate (courses 1 through 3)
per API 12F.per API 12F.
 Minimum thickness flat bottom plate per API 12F.Minimum thickness flat bottom plate per API 12F.
 Sloped deck (1:12 gradient per API 12F).Sloped deck (1:12 gradient per API 12F).

8. 88
ConstraintsConstraints
 Bottom is free to deflect into typical sandBottom is free to deflect into typical sand
foundation.foundation.
 Bottom of 1m thick sand is given a fixedBottom of 1m thick sand is given a fixed
constraint.constraint.
 Top of sand surface assigned a stiffnessTop of sand surface assigned a stiffness
value.value.
 Tank bottom assigned a stiffness value.Tank bottom assigned a stiffness value.
 Roof assigned a stiffness value.Roof assigned a stiffness value.

9. 99
Configurations StudiedConfigurations Studied
 Double fillet weld at shell to bottom joint.Double fillet weld at shell to bottom joint.
 Hybrid bevel/fillet weld at shell to bottomHybrid bevel/fillet weld at shell to bottom
joint.joint.
 Shell to roof joint – fillet welded.Shell to roof joint – fillet welded.
 Top nozzle/dome - ANSI 20 nozzle at topTop nozzle/dome - ANSI 20 nozzle at top
of deck/dome.of deck/dome.
 Flush Clean Out (36” high x 24” wide)Flush Clean Out (36” high x 24” wide)
with rectangular corners as per API 12F.with rectangular corners as per API 12F.
 Horizontal and vertical weld joints.Horizontal and vertical weld joints.

10. 1010
Conditions AnalyzedConditions Analyzed

Extreme positive pressure; Tank full of water, with 10 psiExtreme positive pressure; Tank full of water, with 10 psi
pressure in vapor space above water level. A sudden build uppressure in vapor space above water level. A sudden build up
of pressure in the vapor space is added to the liquid pressureof pressure in the vapor space is added to the liquid pressure
which varies with the height of the tank. When the tank is fullwhich varies with the height of the tank. When the tank is full
of liquid there is an uneven pressure distribution, with theof liquid there is an uneven pressure distribution, with the
maximum pressure at the bottom of tank and the minimummaximum pressure at the bottom of tank and the minimum
pressure in the vapor space. 10 psi is considered extremepressure in the vapor space. 10 psi is considered extreme
positive pressure condition.positive pressure condition.

Tank full of water, with 4.5, 2.0, 1.0 and 0.5 psi pressure inTank full of water, with 4.5, 2.0, 1.0 and 0.5 psi pressure in
vapor space above water level.vapor space above water level.

Tank full of water with 0.5 oz/in2 vacuum in vapor space.Tank full of water with 0.5 oz/in2 vacuum in vapor space.

Extreme negative pressure; Tank empty of product, underExtreme negative pressure; Tank empty of product, under
full/hard vacuum (400 inches of water), considered extremefull/hard vacuum (400 inches of water), considered extreme
negative pressure condition.negative pressure condition.

11. 1111
ConfigurationsConfigurations
Fig. 2Fig. 2 Cross-section of hybrid bottom to shell weld with fillet weld onCross-section of hybrid bottom to shell weld with fillet weld on
inside and bevel weld on outside of tank.inside and bevel weld on outside of tank.

12. 1212
Fig. 3Fig. 3 Exaggerated view of bottom deforming into sand foundationExaggerated view of bottom deforming into sand foundation
which was modeled as 1m thick.which was modeled as 1m thick.

13. 1313
ResultsResults
Tank DeflectionTank Deflection
Fig.4 Maximum deflection at tank dome when there is 1 psi inFig.4 Maximum deflection at tank dome when there is 1 psi in
vapor space and tank is full. Result = 0.096” at top nozzle.vapor space and tank is full. Result = 0.096” at top nozzle.

14. 1414
Fig. 5Fig. 5 Displacement of clean out with 1 psi in vapor space and tank full.Displacement of clean out with 1 psi in vapor space and tank full.
Result =0.032” (approx. 1/32”).Result =0.032” (approx. 1/32”).

15. 1515
Vacuum AnalysisVacuum Analysis
Fig. 6 Negative pressure can be created in tank due to wind fromFig. 6 Negative pressure can be created in tank due to wind from
outside AND/OR from process conditions within tank.outside AND/OR from process conditions within tank.

16. 1616
Fig. 7Fig. 7 400” water vacuum - displacement plot; Roof is more likely to400” water vacuum - displacement plot; Roof is more likely to
deflect before shell to roof joint fails.deflect before shell to roof joint fails.
 The tank was modeled inThe tank was modeled in
the extreme condition ofthe extreme condition of
hard vacuum – 400” ofhard vacuum – 400” of
water. This waswater. This was
performed in order to seeperformed in order to see
where the tank is weakestwhere the tank is weakest
under extreme andunder extreme and
improbable conditions inimprobable conditions in
order to find the weakestorder to find the weakest
point in the design forpoint in the design for
vacuum conditions.vacuum conditions.

17. 1717
Fig. 8Fig. 8 400 inches of water vacuum stress plot – Result – 155,000 psi at400 inches of water vacuum stress plot – Result – 155,000 psi at
deck to shell weld. Plastic non-linear analysis was not conducteddeck to shell weld. Plastic non-linear analysis was not conducted
however the stresses are so far beyond the yield point of steel that ithowever the stresses are so far beyond the yield point of steel that it
can be said that plastic instability is likely to occur at this joint undercan be said that plastic instability is likely to occur at this joint under
hard vacuum.hard vacuum.

18. 1818
Hard Vacuum ResultHard Vacuum Result
 From a vacuum perspective, the weakestFrom a vacuum perspective, the weakest
joint is the deck to shell weld whichjoint is the deck to shell weld which
experiences very high stresses (>155 ksi).experiences very high stresses (>155 ksi).
 The roof to shell joint will likely fail due theThe roof to shell joint will likely fail due the
extremely high stresses encountered atextremely high stresses encountered at
that point under hard vacuum conditions.that point under hard vacuum conditions.

19. 1919
Fig. 9Fig. 9 Buckling safety factor under hard vacuum is 0.0005 whichBuckling safety factor under hard vacuum is 0.0005 which
means plastic instability will occur at roof joint under worst case fullmeans plastic instability will occur at roof joint under worst case full
vacuum (unlikely event). The shell tends to hold its shape as it has avacuum (unlikely event). The shell tends to hold its shape as it has a
more rigid shape, however an initiator (initial weakness) in shell platemore rigid shape, however an initiator (initial weakness) in shell plate
could cause collapse in shell under full vacuum conditions.could cause collapse in shell under full vacuum conditions.

20. 2020
Buckling of tank under 0.5 oz/in2Buckling of tank under 0.5 oz/in2
 What is the buckling safety factor forWhat is the buckling safety factor for
current standard?current standard?
 API 12F allows forAPI 12F allows for 0.5 oz/in0.5 oz/in22
vacuum forvacuum for
this tank.this tank.
 Result – Buckling safety factor = 7.7Result – Buckling safety factor = 7.7
 No elastic instability atNo elastic instability at 0.5 oz/in0.5 oz/in22
 Good NewsGood News

21. 2121
Fig. 10Fig. 10 Half oz/inHalf oz/in22
of vacuum in vapor space with tank almost full;of vacuum in vapor space with tank almost full;
Buckling safety factor = 7.7 Note: elastic deformation of shell is highlyBuckling safety factor = 7.7 Note: elastic deformation of shell is highly
exaggerated.exaggerated.

22. 2222
Results cnt’dResults cnt’d
 API 579 B.4.2.1 type 1 buckling analysis for aAPI 579 B.4.2.1 type 1 buckling analysis for a
tank fitness for service evaluation recommendstank fitness for service evaluation recommends
an in-service buckling safety factor of 3;an in-service buckling safety factor of 3;
 Which means 0.5 oz/inWhich means 0.5 oz/in22
is within safeis within safe
parameters.parameters.
 Elastic deformation is likely to occur in the shellElastic deformation is likely to occur in the shell
at this low vacuum level but elastic instability isat this low vacuum level but elastic instability is
unlikely; i.e. the shape of the structure is unlikelyunlikely; i.e. the shape of the structure is unlikely
to be altered as a result of insufficient stiffness.to be altered as a result of insufficient stiffness.
 No stiffeners are required at the top of tank ifNo stiffeners are required at the top of tank if
the pressure differential between inside andthe pressure differential between inside and
outside of tank is kept below 0.5 oz/inoutside of tank is kept below 0.5 oz/in22
..

23. 2323
Compressive Stress in EmptyCompressive Stress in Empty
tank at atmospheric pressuretank at atmospheric pressure
Fig. 11 The compressive stress at the bottom of tank shell Result = 1800 psi
when tank is empty. Compressive stress is caused by the weight of the tank on
itself.

24. 2424
Fig. 12Fig. 12 Magnified view of outside bottom to shell joint. There is an inherentMagnified view of outside bottom to shell joint. There is an inherent
stress in the weld due to the weight of the tank (compressive in the first 3”stress in the weld due to the weight of the tank (compressive in the first 3”
inches on shell). The transition in shape from circular shell to flat bottom alsoinches on shell). The transition in shape from circular shell to flat bottom also
induces a tensile bending moment at the tank bottom when the tank is filled.induces a tensile bending moment at the tank bottom when the tank is filled.
Another factor which is considered is the tank’s ability to squash into the sandAnother factor which is considered is the tank’s ability to squash into the sand
foundation causing bending stress in the corner weld joint.foundation causing bending stress in the corner weld joint.

25. 2525
Weld Configuration Comparison – 10 psi inWeld Configuration Comparison – 10 psi in
Vapor SpaceVapor Space
(Extreme positive pressure)(Extreme positive pressure)
Fig. 13 Maximum Stress = 9,137 psi in fillet weld cross section with 10 psi
in vapor space and bottom allowed to deform into sand foundation.

26. 2626
Hybrid Fillet/Bevel WeldHybrid Fillet/Bevel Weld
Fig. 14 Maximum Stress = 4,753 psi in hybrid weld cross section with 10 psi in
vapor space and bottom allowed to deform into sand foundation. There is a
reduction of 96% in stress intensity by changing from fillet weld to hybrid fillet/bevel
weld. The hybrid fillet/bevel weld is a much stronger joint than traditional fillet weld.

27. 2727
Double Fillet Configuration – 10 psi in VaporDouble Fillet Configuration – 10 psi in Vapor
Space (Extreme Condition)Space (Extreme Condition)
Fig. 15 Maximum stress with 10 psi in vapor space and with corner fillet weld.

28. 2828
Results cnt’dResults cnt’d
 Stress at the flush cleanout corner joint =Stress at the flush cleanout corner joint =
107,261 psi107,261 psi
 The nozzle joint stress at the dome =The nozzle joint stress at the dome =
44,652 psi.44,652 psi.
 Clearly 10 psi is too great a pressure inClearly 10 psi is too great a pressure in
the vapor space in addition to thethe vapor space in addition to the
hydrostatic pressure from full tank of waterhydrostatic pressure from full tank of water

29. 2929
Reduce PressureReduce Pressure
Double Fillet Configuration – 4.5 psi in VaporDouble Fillet Configuration – 4.5 psi in Vapor
SpaceSpace
Fig. 16 Maximum stress with 4.5 psi in vapor space with fillet weld.

30. 3030
Results cnt’dResults cnt’d
 The 90° corner at flush cleanout has the largestThe 90° corner at flush cleanout has the largest
stress = 77,382 psistress = 77,382 psi
 The next largest stress occurs at the nozzle =The next largest stress occurs at the nozzle =
19,887 psi.19,887 psi.
 4.5 psi is too high a vapor pressure due to the4.5 psi is too high a vapor pressure due to the
stress at the flush cleanout.stress at the flush cleanout.
 It is worth noting that the stresses in the shell toIt is worth noting that the stresses in the shell to
bottom joint and shell to roof joint are less thanbottom joint and shell to roof joint are less than
those seen in the top nozzle at 4.5 psi in vaporthose seen in the top nozzle at 4.5 psi in vapor
space.space.

31. 3131
Reduce Pressure againReduce Pressure again
Double Fillet Weld Configuration – 2 psi inDouble Fillet Weld Configuration – 2 psi in
Vapor SpaceVapor Space
Fig. 17 Maximum stress with 2 psi in vapor space with fillet weld.

32. 3232
Results cnt’dResults cnt’d
 The 90° corner at flush cleanout has theThe 90° corner at flush cleanout has the
largest stress = 63,835 psi.largest stress = 63,835 psi.
 2.0 psi is too high a vapor pressure due to2.0 psi is too high a vapor pressure due to
the stress at the flush cleanout.the stress at the flush cleanout.
 Note: If the flush clean-out were given aNote: If the flush clean-out were given a
stress reducing radius it may be possiblestress reducing radius it may be possible
to increase the pressure to 2 psi in vaporto increase the pressure to 2 psi in vapor
space for this size tank (with carefullyspace for this size tank (with carefully
made design changes in welds etc.).made design changes in welds etc.).

33. 3333
Reduce Pressure againReduce Pressure again
Double Fillet Weld Configuration – 1 psi inDouble Fillet Weld Configuration – 1 psi in
Vapor SpaceVapor Space
Fig. 18 Tank stress with 1 psi in vapor space and double fillet weld at tank bottom.

34. 3434
Results cnt’dResults cnt’d
 Flush cleanout general stress concentration ofFlush cleanout general stress concentration of
11,432 psi exists in vicinity of cleanout with11,432 psi exists in vicinity of cleanout with
concentrated maximum stress at 90° corner ofconcentrated maximum stress at 90° corner of
33,356 psi.33,356 psi.
 Yield strength of A283-Gr C is 30,000 psi whichYield strength of A283-Gr C is 30,000 psi which
means an increase in pressure rating of API 12Fmeans an increase in pressure rating of API 12F
tanks should not occur without applying stresstanks should not occur without applying stress
reducing radius to top corners of flush cleanout.reducing radius to top corners of flush cleanout.
 Increasing the height of API 12F will alsoIncreasing the height of API 12F will also
increase the pressure further and should not beincrease the pressure further and should not be
considered without applying stress reducingconsidered without applying stress reducing
radius to top corners of flush cleanout.radius to top corners of flush cleanout.

35. 3535
Strain for tank with 1 psi inStrain for tank with 1 psi in
Vapor SpaceVapor Space
Fig. 19 Maximum strain for 1 psi in vapor space.

36. 3636
Strain Results cnt’dStrain Results cnt’d
 Maximum Strain Result occurs at flushMaximum Strain Result occurs at flush
cleanout = 1.029 x 10cleanout = 1.029 x 10-3-3
..
 The allowable Hookean strain for A283-GrThe allowable Hookean strain for A283-Gr
C is 1.024 x 10C is 1.024 x 10-3-3
, which means that area of, which means that area of
the tank is over-strained with 1 psi inthe tank is over-strained with 1 psi in
vapor space.vapor space.
 BCG does not recommend an increase inBCG does not recommend an increase in
pressure without applying stress reducingpressure without applying stress reducing
radius to top corners of flush cleanout.radius to top corners of flush cleanout.

37. 3737
Reduce Pressure againReduce Pressure again
Double Fillet Weld Configuration – 0.5 psi inDouble Fillet Weld Configuration – 0.5 psi in
Vapor Space (Current API 12F spec.)Vapor Space (Current API 12F spec.)
Fig. 20 Stress concentration at corner joint in flush cleanout with 0.5 psi in
vapor space. Result = 28,619 psi.

38. 3838
Results cnt’dResults cnt’d
 The current API 12F standard allows for 8 oz/in2 (0.5 psi).The current API 12F standard allows for 8 oz/in2 (0.5 psi).
 The yield strength of A283-Gr C is 30,000 psi which means the yieldThe yield strength of A283-Gr C is 30,000 psi which means the yield
safety factor = 1.048 (which is too close to unity).safety factor = 1.048 (which is too close to unity).
 The API 650 allowable hydrostatic stress for A283-Gr C is 22,500The API 650 allowable hydrostatic stress for A283-Gr C is 22,500
psi which means the stress in this corner joint (28,619 psi) exceedspsi which means the stress in this corner joint (28,619 psi) exceeds
the API allowable stress for that material by 27%.the API allowable stress for that material by 27%.
 Serious consideration should be made to providing a stressSerious consideration should be made to providing a stress
reducing radius at these 2 corners.reducing radius at these 2 corners.
 Brittle fracture is unlikely because the plate thicknesses are lessBrittle fracture is unlikely because the plate thicknesses are less
than 0.5” but leaks are likely to occur at this joint, especially ifthan 0.5” but leaks are likely to occur at this joint, especially if
corroded.corroded.
 This is the weakest point in the design of API 12F shop weldedThis is the weakest point in the design of API 12F shop welded
tanks.tanks.

39. 3939
FatigueFatigue
 It is assumed that the pressure/vacuum valve preventsIt is assumed that the pressure/vacuum valve prevents
continuous cycling due to minimal pressure differentialscontinuous cycling due to minimal pressure differentials
and that fatigue is generally not an issue for these tanks.and that fatigue is generally not an issue for these tanks.
 However, it is worth noting that fatigue starts to becomeHowever, it is worth noting that fatigue starts to become
an issue when the stress is greater than the fatiguean issue when the stress is greater than the fatigue
strength of lowest grade steel Se = 27,550 psi.strength of lowest grade steel Se = 27,550 psi.
 The stress of 28,600 psi at corner joint in flush cleanoutThe stress of 28,600 psi at corner joint in flush cleanout
is susceptible to fatigue failure in older tanks that haveis susceptible to fatigue failure in older tanks that have
experienced more than 10,000 cycles (filling, emptyingexperienced more than 10,000 cycles (filling, emptying
causing pressure change at cleanout).causing pressure change at cleanout).
 Leaks initiated by fatigue are likely to occur in olderLeaks initiated by fatigue are likely to occur in older
tanks with this type of 90° joint.tanks with this type of 90° joint.

40. 4040
Shell to Bottom WeldsShell to Bottom Welds
 The stress in the weld at the tank bottom to shellThe stress in the weld at the tank bottom to shell
joint can be complicated.joint can be complicated.
 An analysis of the fillet weld configuration isAn analysis of the fillet weld configuration is
given here for the three Cartesian directions; X,given here for the three Cartesian directions; X,
Y, Z normal stress in order to explain the stressY, Z normal stress in order to explain the stress
regime at this critical joint in ASTs.regime at this critical joint in ASTs.
 The final figure shows the stress intensityThe final figure shows the stress intensity
(Tresca results) for the combined loading(Tresca results) for the combined loading
situation which takes X,Y, and Z normal stressessituation which takes X,Y, and Z normal stresses
and combines them into a useful engineeringand combines them into a useful engineering
stress.stress.

41. 4141
Double Fillet Weld Stress with 1Double Fillet Weld Stress with 1
psi in Vapor Spacepsi in Vapor Space
Fig. 21 Double fillet weld X normal stress with 1 psi in vapor space - bending in
toe of internal fillet due to movement downwards on tank bottom pressing into
sand is the dominant stress in the X-X direction.

42. 4242
Fig. 22Fig. 22 Double fillet weld Y normal stress –with 1 psi in vapor space.Double fillet weld Y normal stress –with 1 psi in vapor space.
Bending stress in the head of internal fillet weld and the fluidBending stress in the head of internal fillet weld and the fluid
longitudinal stress are dominant in this stress direction (Y-Y direction).longitudinal stress are dominant in this stress direction (Y-Y direction).

43. 4343
Fig. 23Fig. 23 Double fillet weld Z normal stress when 1 psi is in vapor spaceDouble fillet weld Z normal stress when 1 psi is in vapor space
– the membrane stress from the inside of shell is dominant causing a– the membrane stress from the inside of shell is dominant causing a
tensile stress in fillet weld; along with secondary bending stress fromtensile stress in fillet weld; along with secondary bending stress from
the bottom pushing down into the sand are stress factors in the Z-Zthe bottom pushing down into the sand are stress factors in the Z-Z
direction.direction.

44. 4444
Result for Flexible FoundationResult for Flexible Foundation
Fig. 24Fig. 24 Weld stress intensity at cross section with 1 psi in vapor spaceWeld stress intensity at cross section with 1 psi in vapor space
double fillet weld; Result = 4,443 psi max stress occurs at center ofdouble fillet weld; Result = 4,443 psi max stress occurs at center of
internal fillet (corner weld).internal fillet (corner weld).

45. 4545
Result for Rigid FoundationResult for Rigid Foundation
Fig. 25Fig. 25 Higher weld stresses due to rigid bottom compared with flexibleHigher weld stresses due to rigid bottom compared with flexible
sand foundation. There is an approximate increase of 15% in thesand foundation. There is an approximate increase of 15% in the
maximum stress due to the increased rigidity (4,443 psi max stress onmaximum stress due to the increased rigidity (4,443 psi max stress on
flexible sand vs. 5,133psi on completely rigid foundation.flexible sand vs. 5,133psi on completely rigid foundation.

46. 4646
Roof Stress IntensityRoof Stress Intensity
Fig. 26 Bending stress in roof plate at dome-nozzle junction with 1 psi in the
vapor space.

47. 4747
Results cnt’dResults cnt’d
 Stress at Dome-Nozzle at very top of Tank =Stress at Dome-Nozzle at very top of Tank =
4,550 psi4,550 psi
 There is a high bending stress approximately 2”There is a high bending stress approximately 2”
away from the welded joint caused by the steelaway from the welded joint caused by the steel
at that point attempting to move upwards due toat that point attempting to move upwards due to
internal tank pressure, yet being constrained byinternal tank pressure, yet being constrained by
the rigidity and weight of the nozzle.the rigidity and weight of the nozzle.
 Yield safety factor of 6.52 (FYield safety factor of 6.52 (Fyy = 30,000 psi)= 30,000 psi)

48. 4848
FE Model Reality CheckFE Model Reality Check
Fig. 28 Stress along height of tank when 1 psi exists in the vapor space.

49. 4949
Model ValidationModel Validation
 Model reality check. Calculated value (handModel reality check. Calculated value (hand
calculation) of hoop stress = 5,658 psicalculation) of hoop stress = 5,658 psi
 Correlates well with FE result of 5,620 psi atCorrelates well with FE result of 5,620 psi at
tank bottom.tank bottom.
 Stress concentrations exist at shell plate joints,Stress concentrations exist at shell plate joints,
nozzle and flush cleanout joints.nozzle and flush cleanout joints.
 Stress concentrations correlate with R.E.Stress concentrations correlate with R.E.
Peterson’s book “Stress Concentration Factors”.Peterson’s book “Stress Concentration Factors”.
 Hand calculations predict a stress concentrationHand calculations predict a stress concentration
of 28,158 psi at cleanout; FE predicts 28,619of 28,158 psi at cleanout; FE predicts 28,619
psi.psi.

50. 5050

51. 5151
ConclusionsConclusions
1.1. There is an acceptable amount of tank deflection with current pressure limits.There is an acceptable amount of tank deflection with current pressure limits.
2.2. From a vacuum perspective, the weakest area is the deck to shell weld.From a vacuum perspective, the weakest area is the deck to shell weld.
3.3. From a positive pressure perspective, the weakest area is 2” away from the corner inFrom a positive pressure perspective, the weakest area is 2” away from the corner in
the flush cleanout.the flush cleanout.
4.4. Elastic instability is unlikely to occur with current allowable vacuum of 0.5 oz/inElastic instability is unlikely to occur with current allowable vacuum of 0.5 oz/in22
i.e.i.e.
the shape of the structure is unlikely to buckle as a result of insufficient stiffness.the shape of the structure is unlikely to buckle as a result of insufficient stiffness.
5.5. The hybrid fillet/bevel weld is a stronger joint than traditional fillet weld.The hybrid fillet/bevel weld is a stronger joint than traditional fillet weld.
6.6. The API 650 allowable hydrostatic stress for A283-Gr C is 22,500 psi which meansThe API 650 allowable hydrostatic stress for A283-Gr C is 22,500 psi which means
the stress in the clean-out corner joint (28,619 psi) exceeds the API allowable stressthe stress in the clean-out corner joint (28,619 psi) exceeds the API allowable stress
for that material by 27%.for that material by 27%.
7.7. The allowable Hookean strain for A283-Gr C is 1.024 x 10The allowable Hookean strain for A283-Gr C is 1.024 x 10-3-3
, which means the clean, which means the clean
out area of the tank is over-strained with 1 psi in vapor space.out area of the tank is over-strained with 1 psi in vapor space.
8.8. Serious consideration should be made to providing a stress reducing radius at theseSerious consideration should be made to providing a stress reducing radius at these
2 corners.2 corners.
9.9. Leaks initiated by fatigue, could possibly occur in older tanks with 90° corner joint atLeaks initiated by fatigue, could possibly occur in older tanks with 90° corner joint at
clean out (>10,000 cycles).clean out (>10,000 cycles).
10.10. Brittle fracture is unlikely to occur at the flush cleanout as material is thinner than 0.5”.Brittle fracture is unlikely to occur at the flush cleanout as material is thinner than 0.5”.
Failure mechanism is more likely ductile cleavage mechanism and not brittleFailure mechanism is more likely ductile cleavage mechanism and not brittle
fracture.fracture.

52. 5252
Conclusions cnt’dConclusions cnt’d
11.11. An increase in pressure rating of API 12F tanks should not occur without applying stressAn increase in pressure rating of API 12F tanks should not occur without applying stress
reducing radius to top corners of flush cleanout.reducing radius to top corners of flush cleanout.
12.12. Increasing the height of API 12F tanks will increase the pressure further and should not beIncreasing the height of API 12F tanks will increase the pressure further and should not be
considered without applying stress reducing radius to top corners of flush cleanout.considered without applying stress reducing radius to top corners of flush cleanout.
13.13. The allowable pressure for this tank cannot be increased from 0.5 psi to 2 psi because thereThe allowable pressure for this tank cannot be increased from 0.5 psi to 2 psi because there
is a large stress concentration at clean out which is greater than the tensile strength of theis a large stress concentration at clean out which is greater than the tensile strength of the
shell plate.shell plate.
14.14. If the allowable pressure were to be increased from 0.5 psi (8 oz/inIf the allowable pressure were to be increased from 0.5 psi (8 oz/in22
), the 90° corner would), the 90° corner would
require rounding to reduce the stress concentration.require rounding to reduce the stress concentration.
15.15. Double fillet welds are sufficiently strong when there is a full tank of water and 1 psi in vaporDouble fillet welds are sufficiently strong when there is a full tank of water and 1 psi in vapor
space.space.
16.16. Bending stress at top nozzle was higher than expected.Bending stress at top nozzle was higher than expected.
17.17. If the flush clean-out were given a stress reducing radius it may be possible to increase theIf the flush clean-out were given a stress reducing radius it may be possible to increase the
pressure to 2 psi in vapor space for this size tank (with carefully made design changes inpressure to 2 psi in vapor space for this size tank (with carefully made design changes in
welds etc.).welds etc.).
18.18. An increase in allowable vacuum above 0.5 oz/inAn increase in allowable vacuum above 0.5 oz/in22
is possible. That limit value requires moreis possible. That limit value requires more
analysis to keep buckling safety factor above 3.analysis to keep buckling safety factor above 3.
19.19. Current pressure and vacuum allowable limits should not be changed without accompanyingCurrent pressure and vacuum allowable limits should not be changed without accompanying
design changes.design changes.