2/2003AvestaPolarit Corrosion Management and Application Engineering

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2/2003 1(10)

Anders Olsson – Ph.D.

AvestaPolarit AB (publ)

This paper addresses the use of high strength stainless steels for storage tanks.

It has been shown that despite the fact that the corrosion resistance of type

304 austenitic stainless steel grades is sufficient for many applications, large

potential cost reductions if high strength stainless steels are utilized. The poten-

tial cost reduction is depending on the design standard used. Out the grades

considered herein, minimum shell thickness is, with exception of 304, higher

according to API 650 than the corresponding thickness according to BS 2654.

Possible design solutions comprising high strength stainless steel are

supported by means of a case: Three storage tanks for marble slurry designed

according to the British standard BS 2654. Three different grades were utilized

to arrive at a tank design optimised with respect to corrosion as well as

structural resistance. Grades used were: The austenitic 304 for the roof and

top courses, the duplex S32304 for the middle part and bottom whereas the very

high strength martensitic 1.4418 was used for the bottom part.

Introduction

Historically, storage tanks havebeen built in carbon steel with a corrosion allowance. However,due to corrosion and high main-tenance many storage tanks havebeen designed with an innerstainless steel lining, coating orcathodic protection. For decadesstorage tanks have also beendesigned and built in austeniticstainless steels. These grades dohave a corrosion resistance highenough for many applications inthe pulp and paper industry. It is

however possible to further reducethe cost of storage tanks by utiliz-ing high strength stainless steels.

This paper addresses the useof high strength stainless steel instorage tanks. Corrosion propertiesare discussed, but mechanicalproperties and design codes areemphasized. Corrosion propertiesare of course very important andthe main reason to consider stain-less steels. However, in additionto the corrosion properties, thefull potential of the mechanicalproperties have to be fully utilized

in order to arrive at a design opti-mised with respect to corrosion aswell as structural resistance.Several of the design codes oftenused for storage tank design docurrently restrict the use of highstrength stainless steels, e.g. byrestrictions with respect to themaximum allowable design stress.There is hence a need to addressthe structural resistance and designcodes. Recent examples haveshown that stainless steel gradeswith very high mechanical prop-erties can be effectively utilized inthe design of storage tanks.

Corrosion Resistance

Corrosion resistance or in the caseof carbon steel, lack of corrosionresistance, is the main reason forusing stainless steels for storagetanks. Even in not very corrosiveenvironments carbon steel showthinning and consequently has tobe protected or designed with acorrosion allowance. Due to thecorrosion problems with carbonsteel, stainless steels are frequentlyused in the pulp and paper in-dustry. Whether the environment

Utilizing High Strength Stainless Steel for Storage Tanks

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is mildly or highly corrosive, thereare suitable stainless steel grades.The chemical composition and thePRE of some stainless steel gradesis shown in table 1.

The PRE is a general approxi-mate rating of the pitting corrosionresistance, but is still used for ageneral ranking between gradeswith respect to corrosion resistance.It is here shown to give an idea ofthe relative corrosion resistancefor the stainless steel grades con-sidered in this paper. Jean-PierreAudouard et.al. have in a seriesof papers, [1], [2], [3], presentedextensive reviews and data oncorrosion problems in connectionwith storage tanks in differentservice environments. The generalconclusion drawn is that the corrosion resistance of type 304and 316 austenitic stainless steelsis sufficient for many applications.Considering also stress corrosioncracking it is well known that theresistance of duplex grades issuperior to the one of the auste-nitic grades. Also the resistance to wear is, due to their higherhardness, higher for the duplexgrades.

Shell Design

Cylindrical walls of storage tanksand silos are usually designed tocarry internal pressure from thestored media. This means that theshell thickness usually vary alongthe shell. In service, also loadingscomprising external pressure, e.g.wind load on the empty tank, mayoccur. Hence requiring checking of the buckling resistance of thestorage tank.Often used design codes for designof storage tanks are:

API 650 – American standardBS 2654 – British standardDIN 4119 – German standardCODRES – French standard

In this paper the first two areaddressed, i.e. API 650 [4] and BS2654 [5]. Furthermore, reference ismade to the Shell StabilityHandbook, edited by Eggwertzand Samuelsson [6], regardingshell stability.

Design of storage tanks comprises calculation of a mini-mum thickness of the shell. Thethickness of each shell course isaccording to both the consideredstandards based on the circum-

ferential stress in a section 0.3 mabove the bottom of each course.Minimum shell thickness is according to the considered designcodes obtained as:

API 650 – THE AMERICANSTANDARDThe expressions in API 650 for calculating the minimum shellthickness are:

where

td is the design shell thickness, [mm]

tt is the hydrostatic shell thickness, [mm]

D is the tank diameter, [m]

H is the distance from the course under consideration to the top of the tank shell or to the over flow designed to limit thefluid height

G is the density of the stored liquid, [g/ml]

Table 1: Chemical composition and PRE for some stainless steel grades

Standard – Grade

304316LS31254S32304S32205b)

S32750–

1.43011.44321.45471.43621.44621.44101.4418

0.040.020.010.020.020.020.03

0.050.050.200.100.170.270.04

18.116.920.023.022.025.016.0

8.310.718.04.85.77.05.0

–2.66.10.33.14.01.0

––

Cu––––

AusteniticAusteniticAustenitic

DuplexDuplexDuplex

Martensiticc)

19264326354320

ASTM EN C N Cr Ni Mo Other

Chemical composition Structure PREa)

a) PRE = %Cr + 3.3*%Mo+16*%N b) Exists also as S31803 c) Approximately 80% martensite, 15% austenite and 5% ferrite.

td = 4.9D(H–0.3)G +CASdE

tt = 4.9D(H–0.3)StE

(1)

(2)

Sd is the design stress, [MPa]

St is the hydrostatic test design stress, [MPa]

E Is the joint efficiency factor, 1.0, 0.85 or 0.7

CA is the corrosion allowance, [mm]

For shells where 500Dt > 2H,the shell thickness shall be basedon an elastic analysis showing the circumferential stress to be belowthe allowable design stress at thespecified temperature. No coursemay be thinner than the courseabove.

BS 2654 – THE BRITISH STANDARDThe minimum shell thickness isexpressed somewhat differently inBS 2654, but besides the internalpressure the equations are equal:

where

t is the minimum shell thickness

D is the tank diameter, [m]

S is the design stress, [MPa]

w is the density of the stored liquid, [g/ml], but w shall not be less than 1.0

H is the distance from the course under consideration to the top of the tank shell or to the overflow designed to limit the fluid height

p is the design pressure, [mbar]

c is the corrosion allowance,[mm]

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2/2003 3(10)

the minimum yield strength or260 MPa, whichever is the lower.Hence limiting the standard togrades with yield strength equalto or less than 390 MPa.

Allowable design stresses atroom temperature for some stain-less steel grades calculated accord-ing to the two standards API 650and BS 2654 respectively are pre-sented in table 2. API 650 designstresses for the duplex grades areobtained by extrapolation.

It can be discussed whether thedesign stress for the duplex stain-less steel grades should be obtain-ed according to the austeniticstainless steel or the carbon steelrules. Considering the designstresses according to API 650shown in table 2, it can be notedthat design stresses for the duplexand marten-sitic grades calculatedas for the austenitic grades arerelatively low compared with theminimum yield stress. Correspond-ing stresses calculated accordingto the carbon steel rules results ina design stress – minimum yieldstress ratio closer to the ones forthe austenitic grades. A resultexplained by the ratio Rp0.2/Rm,which is higher for the duplexand martensitic grades. Despitethe higher design stress obtainedby means of the carbon steelrules, the ratio Sd/Rp0.2 rangesfrom 0.40 to 0.49 for the duplexand martensitic grades whereas itranges from 0.75 to 0.85 for thetwo austenitic grades. The corre-sponding ratio range for designstresses extrapolated according tothe rules for austenitic stainlesssteels is 0.37 to 0.45. It is worth-

(3)t = D [98w(H –0.3)+p]+c20S

However, if materials with different mechanical propertiesare used and:

The minimum thickness of theupper course is calculated as

Indices U and L respectively in(4) refer to the upper and lowercourses with respect to the changeof mechanical properties. Futhermore, also according toBS 2654 no course may be thinnerthan the course above.

DESIGN STRESSStainless steel grades consideredin the American standard API 650,2001 edition, are: 304, 304L, 316,316L, 317 and 317, i.e. all austeniticgrades. Austenitic-ferritic orduplex stainless steel grades arecurrently not covered by the standard. The maximum designstress for the austenitic stainlesssteel grades is obtained as the lesser of: 0.3 times the minimumtensile strength or 0.9 times theminimum yield strength.Corresponding rules for carbonsteel grades are: The lesser of 2/3 times the yield stress and 0.4times the tensile strength.

The Brittish standard BS 2654does not refer to a standard forstainless steels, but states allow-ance for use of suitable materialsagreed between the purchaser andthe manufacturer. The maximumdesign stress shall be two-thirds of:

(4)HU –0.3

≥HL–0.3

SU SL

(5)t = D [98wH+p]+c20S

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indicative relative cost is used tovisualise the principle of potentialcost reductions possible with highstrength stainless steel grades.From the minimum shell thick-ness shown in figure 1, figure 2and the indicative relative cost intable 2, a minimum relative shellthickness can be obtained. Theseare shown in figure 3 and figure 4.The method used to calculateallowable design stresses is reflect-ed also in the relative thickness.The range, i.e. cost reductionpotential, is clearly wider for the:API 650 – carbon steel rules andBS 2654 – no consideration ofupper stress limit.

while to note that despite the higher design stresses obtainedby means of the carbon steel rules,the extrapolated design stressesfor the duplex grades are lowerthan the design stresses accordingto BS 2654.

Considering eq. (1) and (3) it isobvious that the relation betweenthe various design stresses intable 2 and the minimum shellthicknesses for the different stainless steel grades is linear.The minimum shell thicknessesbased on austenitic stainless steelrules and carbon steel rules areshown in figure 1 and figure 2respectively. Hypothetical designconditions assumed are:

• Tank height: 30 m• Diameter: 12 m• Specific weight of stored media:

1 850 kg/m3

As can be seen in figure 1 the mini-mum shell thickness according toBS 2654 for the stainless steel grades S32304, S32205 and 1.4418is the same. A fact due to the upper

allowable design stress limit, 260MPa. In figure 2 correspondingminimum thicknesses obtainedwithout consideration of the upperlimit are shown. The difference inminimum shell thickness is evi-dent. Also the difference betweenallowable design stress for theduplex and martensitic gradescalculated with austenitic stainlesssteel and carbon steel rules accord-ing to API 650 is clearly shown.

COST COMPARISONThe potential of high strengthstainless steels is emphasized bymeans of a simple cost compari-son. A cost comparison based onminimum shell thickness accord-ing to the two standards API 650and BS 2654. It is worthwhile tonotice that in addition to reducedweight, a reduced plate thicknessalso results in reduced weldingtime, i.e. the cost may be furtherreduced. Indicative relative cost(European alloy prices, April–May2002) of some stainless steel grades is shown in table 3. The

Table 2: Allowable design stress at room temperature according to API 650 and BS 2654 respectively. Design stresses according to API 650 for the duplex grades and the martensitic grade are extrapolated.

Standard – Grade

185/155

153/145

360/180

405/186

612/252

515/520

485/520

600/630

620/640

680/840

205/210

170/220

400/400

450/460

–

1.4301

1.4432

1.4362

1.4462

1.4418

304

316L

S32304

S32205

–

–

–

266/240

300/248

453/336

186

155

360a)

405a)

612a)

140

146

260 (267)b)

260 (307)b)

260 (453)b)

ASTM EN ASTM/EN Austenitic

Rp0.2

[MPa]Rm [MPa] API 650 BS 2654

Sd [MPa]

Carbon

a) Extrapolated.b) Design stresses within brackets calculated with no consideration of the 260 MPa limit.

St [MPa] S [MPa]

Table 3: Indicative relative cost forsome stainless steel grades

304

316L

S32304

S31803

–

1.4301

1.4432

1.4362

1.4462

1.4418

100

150

130

150

150

Standard – Grade

ASTM EN

Relative cost

Fig 1. Minimum shell thickness according to API 650 and BS 2654. Minimum shell thickness according to API650for the duplex and martensitic grades are based on design stresses extrapolated using austenitic stainless steel rules.

0

5

10

15

20

25

30

0 5 10 15 20 25

Minimum shell thickness [mm]

H [

m]

304 – API

S32304 – API

S32205 – API

304 – BS

S32304 – BS

S32205 – BS

1.4418 – BS

1.4418 – API

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Fig 2: Minimum shell thickness according to API 650 and BS 2654. Shell thickness according to API 650 for the duplex and martensitic grades are based on design stresses extrapolated using carbon steel rules. BS 2654 thicknesses are obtained with no consideration of the 260 MPa limit.

0

5

10

15

20

25

30

0 5 10 15 20 25

Minimum shell thickness [mm]

H [

m]

304 – API

S32304 – API

S32205 – API

304 – BS

S32304 – BS

S32205 – BS

1.4418 – BS

1.4418 – API

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Fig 3. Indicative minimum relative shell thickness based on relative prices in table. Relative thicknesses according to API 650 for the duplex and martensitic grades are based on design stresses extrapolated using austenitic stainless steel rules.

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Minimum relative shell thickness [mm]

H [

m]

304 – API

S32304 – API

S32205 – API

304 – BS

S32304 – BS

S32205 – BS

1.4418 – BS

1.4418 – API

Fig 4. Indicative minimum relative shell thickness based on relative prices in table. Relative thicknesses according toAPI 650 for the duplex and martensitic grades are based on design stresses extrapolated using carbon steel rules.BS 2654 thicknesses are obtained with no consideration of the 260 MPa limit.

0

5

10

15

20

25

30

0 5 10 15 20 25

Minimum relative shell thickness [mm]

H [

m]

304 – API

S32304 – API

S32205 – API

304 – BS

S32304 – BS

S32205 – BS

1.4418 – BS

1.4418 – API

where

t is the thickness of the top shell course.

D is the nominal diameter of the tank.

The maximum height, H1, according to (6) shall be largerthan a transposed shell heightobtained as:

where

Wtriis the transformed width ofthe ith shell course.

Wi is the width of the ith shell course.

tuniform is the thickness of the top shell course.

tuniform is the thickness of the ith shell course.

tuniformtactual

tH1=9.47t ( )3

D

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STABILITYNow, the indicative cost com-parison depicted in figure 3 andfigure 4 shows that comparedwith a storage tank designed in304, cost reductions are possibleif high strength stainless steelsare utilized. It has though to beemphasized that the comparisonis indicative, relative materialcosts vary and welding of themartensitic grade 1.4418 is morecomplicated than welding inaustenitic or duplex grades.Welding of the martensitic grade1.4418 is briefly discussed in connection with a case describedbelow. Furthermore, stability isnot considered in the comparison.Nevertheless, the comparisonhighlights high strength stainlesssteels as cost effective.

Consider the minimum shellthickness in figure 2, the relativeminimum shell thickness in figure 4 and API 650. From thesethe tentative tank design shownin table 4 can be obtained. Thehigh strength grades are used inthe lower and middle parts whereas the low strength 304grade is used for the upper part.However, in addition to the mini-mum thickness, the stability ofthe tank has to be checked for theload case: Empty tank subjectedto wind load.

According to API 650 the maximum height, H1, of anunstiffened shell is obtained as:

For the tentative tank in table 4 itis obtained: H1 =12.73 and ∑ Wtri =15.73, i.e. intermediate wind stiffeners or increased shell thickness is required. A possible solutionwould be to increase the thick-ness of the five upper coursesfrom 5 to 6 mm, resulting in: H1 =20.08 and ∑ Wtri =18.44.

A stability check according to theShell Stability Handbook, editedby Eggwertz and Samuelsson [6],using the same conditions asabove, results in a maximumexternal uniform pressure, e.g.caused by wind load, of 1.4 kPa.The following assumptions wasmade: Reduction factor for tolerances and manufacturingmethod – 0.9, partial coefficientfor determination of allowablestress – 1.2.

Table 4: Hypothetical tank design. Based on minimum shell thickness and indicative relative shell thickness.

Course No.

5

5

5

5

5

7

7

8

9

9

9

9

9

10

10

304/1.4301

304/1.4301

304/1.4301

S31803/1.4462

S31803/1.4462

S31803/1.4462

S31803/1.4462

S31803/1.4462

S31803/1.4462

S31803/1.4462

–/1.4418

–/1.4418

–/1.4418

–/1.4418

–/1.4418

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

H [m] Grade – ASTM/EN Thickness [mm]

(7)√∑Wtri=∑Wi

√ (6)

5

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Case:

Storage Tanks for Marble Slurry

A manufacturer of storage tanksin Norway has designed andbuilt three 3500 m3 storage tanksfor a suspension of marble dust.The design conditions were:

• Volume: 3500 m3

• Calcium carbonate, CaCO3, specific weight 1850 kg/m3, no pressure

• Design temperature: 90°C

• Design standard: BS 2654

The service environment, mildlycorrosive, allowed also low alloygrades such as 304 and 1.4418 tobe considered as potential ma-terials. Hence alternative solutionswere possible. The principles ofthe two alternative designs consid-ered in the final stage are shownin figure 5. One where the auste-nitic grade 304 and the duplexgrade S32304 were used, and onewith the three grades, 304, S32304and 1.4418. Alternative two, withthree grades, resulted in a weightreduction of 15%. The weight of

the two alternatives was 129 and110 metric tonnes respectively. Thesecond alternative furthermoreproved to be the most cost effici-ent of the two and was selectedfor the three storage tanks.

Welding of the martensiticgrade 1.4418 did require specialconsiderations regarding weldingmethod and consumables to beused. After tests with respect towelding and obtained properties,consumables used were the sameas used for the duplex gradeS32205. Welding methods used

were submerged arc welding,SAW, and flux core arc welding,FCAW. Nor did welding of themartensitic grade to the duplexgrade did not cause any problems.

The tank shells were welded in sections with a maximumweight of 75 metric tonnes at themanufacturer and subsequentlytransported to the customer forassembly, see figure 6. The weightlimit was due to the maximum lifting capacity of the crane avail-able. The tanks were insulatedbefore taken into service.

Fig 5. Tank designs considered for storage of marble slurry.

Fig 6. Parts of storage tanks before assembly.

10 t=6–8t=7

t=7–1.5

t=11.5–14

t=8–1417.25

27.250 27.25

6

13

8.25

12.812.8

S32304

304304

S32304

1.4418

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Discussion and Conclusions

The results presented in thispaper imply that large potentialcost reductions for storage tanksare possible if high strength stainless steels are utilized.Depending on the design codeused, the potential cost reductionvaries. Difference in design resistance is not unique for thearea of storage tank design, butstill has to be addressed. Thereare, as mentioned several times in this paper, relatively large differences between design codes.Differences, due to tradition and design philosophies. The mechanics are however the same,thus implying a need for con-tinued harmonization of standards.From the results presented in thispaper it is concluded:

• There are differences between design codes with respect to allowable design stress and hence minimum shell thickness of storage tanks.

• High strength stainless steel canbe, and have been, successfully utilized in order to obtain cost effective storage tanks.

• Combining grades in order to optimise storage tanks with respect to corrosion as well as structural resistance has been shown to be cost effective.

References

[1] Audouard, J-P. et.al. Duplex stainless steels for tanks in the pulp & paper industry. Proceedings: INDUSTEEL – 10th ISCPPI,

Helsink, Finland, August 2001

[2] Audouard, J-P. et.al. Duplex stainless steels for tanks in the pulp & paper industry. Proceedings: TAPPI 2001

[3] Audouard, J-P. and Grocki, J. Duplex stainless steels for storage tanks. Proceedings: NACE 2002, Denver Colorado, USA, April 2002

[4] API Standard 650, Tenth edition incl, addedum 1(2000) and addendum 2(2001) (1998). Welded Steel Tanks for Oil Storage,

American Petroleum Institute, Washington, USA

[5] BS 2654:1989 incl. amendment No. 1. (1989). Specification for:

Manufacture of vertical steel welded non-refrigerated storage tanks

with butt-welded shells for the petroleum industry, BSI, British standard Institute

[6] Shell Stability Handbook (1992). Ed. by Samuelsson, L-Å and Eggwertz, S, Elsevier Science Publishers Ltd, ISBN 1-85166-954-X

"This paper was originally presented

at TAPPI Engineering Conference

in Anaheim – USA in 1999.

Republished with the kind permission

of the authors and TAPPI".

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1/2003 10(10)

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