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AvestaPolarit Corrosion Management and Application Engineering
2/2003
Utilizing High Strength Stainless Steel
for Storage Tanks
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 potential 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 have
been built in carbon steel with
a corrosion allowance. However,
due to corrosion and high maintenance many storage tanks have
been designed with an inner
stainless steel lining, coating or
cathodic protection. For decades
storage tanks have also been
designed and built in austenitic
stainless steels. These grades do
have a corrosion resistance high
enough for many applications in
the pulp and paper industry. It is
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however possible to further reduce
the cost of storage tanks by utilizing high strength stainless steels.
This paper addresses the use
of high strength stainless steel in
storage tanks. Corrosion properties
are discussed, but mechanical
properties and design codes are
emphasized. Corrosion properties
are of course very important and
the main reason to consider stainless steels. However, in addition
to the corrosion properties, the
full potential of the mechanical
properties have to be fully utilized
in order to arrive at a design optimised with respect to corrosion as
well as structural resistance.
Several of the design codes often
used for storage tank design do
currently restrict the use of high
strength stainless steels, e.g. by
restrictions with respect to the
maximum allowable design stress.
There is hence a need to address
the structural resistance and design
codes. Recent examples have
shown that stainless steel grades
with very high mechanical properties can be effectively utilized in
the design of storage tanks.
Corrosion Resistance
Corrosion resistance or in the case
of carbon steel, lack of corrosion
resistance, is the main reason for
using stainless steels for storage
tanks. Even in not very corrosive
environments carbon steel show
thinning and consequently has to
be protected or designed with a
corrosion allowance. Due to the
corrosion problems with carbon
steel, stainless steels are frequently
used in the pulp and paper industry. Whether the environment
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Table 1: Chemical composition and PRE for some stainless steel grades
Chemical composition
Standard – Grade
ASTM
304
316L
S31254
S32304
S32205b)
S32750
–
Structure
EN
C
N
Cr
Ni
Mo
Other
1.4301
1.4432
1.4547
1.4362
1.4462
1.4410
1.4418
0.04
0.02
0.01
0.02
0.02
0.02
0.03
0.05
0.05
0.20
0.10
0.17
0.27
0.04
18.1
16.9
20.0
23.0
22.0
25.0
16.0
8.3
10.7
18.0
4.8
5.7
7.0
5.0
–
2.6
6.1
0.3
3.1
4.0
1.0
–
–
Cu
–
–
–
–
Austenitic
Austenitic
Austenitic
Duplex
Duplex
Duplex
Martensiticc)
PREa)
19
26
43
26
35
43
20
a) PRE = %Cr + 3.3*%Mo+16*%N
b) Exists also as S31803
c) Approximately 80% martensite, 15% austenite and 5% ferrite.
is mildly or highly corrosive, there
are suitable stainless steel grades.
The chemical composition and the
PRE of some stainless steel grades
is shown in table 1.
The PRE is a general approximate rating of the pitting corrosion
resistance, but is still used for a
general ranking between grades
with respect to corrosion resistance.
It is here shown to give an idea of
the relative corrosion resistance
for the stainless steel grades considered in this paper. Jean-Pierre
Audouard et.al. have in a series
of papers, [1], [2], [3], presented
extensive reviews and data on
corrosion problems in connection
with storage tanks in different
service environments. The general
conclusion drawn is that the
corrosion resistance of type 304
and 316 austenitic stainless steels
is sufficient for many applications.
Considering also stress corrosion
cracking it is well known that the
resistance of duplex grades is
superior to the one of the austenitic grades. Also the resistance
to wear is, due to their higher
hardness, higher for the duplex
grades.
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Shell Design
Cylindrical walls of storage tanks
and silos are usually designed to
carry internal pressure from the
stored media. This means that the
shell thickness usually vary along
the shell. In service, also loadings
comprising external pressure, e.g.
wind load on the empty tank, may
occur. Hence requiring checking
of the buckling resistance of the
storage tank.
Often used design codes for design
of storage tanks are:
API 650 – American standard
BS 2654 – British standard
DIN 4119 – German standard
CODRES – French standard
In this paper the first two are
addressed, i.e. API 650 [4] and BS
2654 [5]. Furthermore, reference is
made to the Shell Stability
Handbook, edited by Eggwertz
and Samuelsson [6], regarding
shell stability.
Design of storage tanks
comprises calculation of a minimum thickness of the shell. The
thickness of each shell course is
according to both the considered
standards based on the circum-
ferential stress in a section 0.3 m
above the bottom of each course.
Minimum shell thickness is
according to the considered design
codes obtained as:
API 650 – THE AMERICAN
STANDARD
The expressions in API 650 for
calculating the minimum shell
thickness are:
td = 4.9D(H–0.3)G +CA
Sd E
(1)
tt = 4.9D(H–0.3)
St E
(2)
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 the
fluid height
G
is the density of the stored
liquid, [g/ml]
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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
However, if materials with
different mechanical properties
are used and:
HU –0.3 HL–0.3
≥
SU
SL
The minimum thickness of the
upper course is calculated as
CA is the corrosion allowance,
[mm]
For shells where √500Dt > 2H,
the shell thickness shall be based
on an elastic analysis showing the
circumferential stress to be below
the allowable design stress at the
specified temperature. No course
may be thinner than the course
above.
BS 2654 – THE BRITISH
STANDARD
The minimum shell thickness is
expressed somewhat differently in
BS 2654, but besides the internal
pressure the equations are equal:
t = D [98w(H –0.3)+p]+c
20S
(3)
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 over
flow designed to limit the
fluid height
p is the design pressure, [mbar]
c is the corrosion allowance,[mm]
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(4)
t = D [98wH+p]+c
20S
(5)
Indices U and L respectively in
(4) refer to the upper and lower
courses with respect to the change
of mechanical properties.
Futhermore, also according to
BS 2654 no course may be thinner
than the course above.
DESIGN STRESS
Stainless steel grades considered
in the American standard API 650,
2001 edition, are: 304, 304L, 316,
316L, 317 and 317, i.e. all austenitic
grades. Austenitic-ferritic or
duplex stainless steel grades are
currently not covered by the
standard. The maximum design
stress for the austenitic stainless
steel grades is obtained as the
lesser of: 0.3 times the minimum
tensile strength or 0.9 times the
minimum yield strength.
Corresponding rules for carbon
steel grades are: The lesser of
2/3 times the yield stress and 0.4
times the tensile strength.
The Brittish standard BS 2654
does not refer to a standard for
stainless steels, but states allowance for use of suitable materials
agreed between the purchaser and
the manufacturer. The maximum
design stress shall be two-thirds of:
the minimum yield strength or
260 MPa, whichever is the lower.
Hence limiting the standard to
grades with yield strength equal
to or less than 390 MPa.
Allowable design stresses at
room temperature for some stainless steel grades calculated according to the two standards API 650
and BS 2654 respectively are presented in table 2. API 650 design
stresses for the duplex grades are
obtained by extrapolation.
It can be discussed whether the
design stress for the duplex stainless steel grades should be obtained according to the austenitic
stainless steel or the carbon steel
rules. Considering the design
stresses according to API 650
shown in table 2, it can be noted
that design stresses for the duplex
and marten-sitic grades calculated
as for the austenitic grades are
relatively low compared with the
minimum yield stress. Corresponding stresses calculated according
to the carbon steel rules results in
a design stress – minimum yield
stress ratio closer to the ones for
the austenitic grades. A result
explained by the ratio Rp0.2/Rm,
which is higher for the duplex
and martensitic grades. Despite
the higher design stress obtained
by means of the carbon steel
rules, the ratio Sd/Rp0.2 ranges
from 0.40 to 0.49 for the duplex
and martensitic grades whereas it
ranges from 0.75 to 0.85 for the
two austenitic grades. The corresponding ratio range for design
stresses extrapolated according to
the rules for austenitic stainless
steels is 0.37 to 0.45. It is worth3(10)
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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
ASTM
Rp0.2
[MPa]
EN
Rm [MPa]
API 650
St [MPa]
Sd [MPa]
ASTM/EN
Austenitic
BS 2654
S [MPa]
Carbon
304
1.4301
205/210
515/520
185/155
–
186
140
316L
1.4432
170/220
485/520
153/145
–
155
146
S32304
1.4362
400/400
600/630
360/180
266/240
360a)
260 (267)b)
260 (307)b)
260 (453)b)
S32205
1.4462
450/460
620/640
405/186
300/248
405a)
–
1.4418
–
680/840
612/252
453/336
612a)
a) Extrapolated.
b) Design stresses within brackets calculated with no consideration of the 260 MPa limit.
while to note that despite the
higher design stresses obtained
by means of the carbon steel rules,
the extrapolated design stresses
for the duplex grades are lower
than the design stresses according
to BS 2654.
Considering eq. (1) and (3) it is
obvious that the relation between
the various design stresses in
table 2 and the minimum shell
thicknesses for the different
stainless steel grades is linear.
The minimum shell thicknesses
based on austenitic stainless steel
rules and carbon steel rules are
shown in figure 1 and figure 2
respectively. Hypothetical design
conditions 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 minimum shell thickness according to
BS 2654 for the stainless steel
grades S32304, S32205 and 1.4418
is the same. A fact due to the upper
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allowable design stress limit, 260
MPa. In figure 2 corresponding
minimum thicknesses obtained
without consideration of the upper
limit are shown. The difference in
minimum shell thickness is evident. Also the difference between
allowable design stress for the
duplex and martensitic grades
calculated with austenitic stainless
steel and carbon steel rules according to API 650 is clearly shown.
COST COMPARISON
The potential of high strength
stainless steels is emphasized by
means of a simple cost comparison. A cost comparison based on
minimum shell thickness according to the two standards API 650
and BS 2654. It is worthwhile to
notice that in addition to reduced
weight, a reduced plate thickness
also results in reduced welding
time, i.e. the cost may be further
reduced. Indicative relative cost
(European alloy prices, April–May
2002) of some stainless steel
grades is shown in table 3. The
Table 3: Indicative relative cost for
some stainless steel grades
Standard – Grade
ASTM
EN
Relative
cost
304
1.4301
100
316L
1.4432
150
S32304
1.4362
130
S31803
1.4462
150
–
1.4418
150
indicative relative cost is used to
visualise the principle of potential
cost reductions possible with high
strength stainless steel grades.
From the minimum shell thickness shown in figure 1, figure 2
and the indicative relative cost in
table 2, a minimum relative shell
thickness can be obtained. These
are shown in figure 3 and figure 4.
The method used to calculate
allowable design stresses is reflected also in the relative thickness.
The range, i.e. cost reduction
potential, is clearly wider for the:
API 650 – carbon steel rules and
BS 2654 – no consideration of
upper stress limit.
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30
25
304 – API
S32304 – API
20
H [m]
S32205 – API
304 – BS
S32304 – BS
15
S32205 – BS
1.4418 – BS
10
1.4418 – API
5
0
0
5
10
15
20
25
Minimum shell thickness [mm]
Fig 1. Minimum shell thickness according to API 650 and BS 2654. Minimum shell thickness according to API650
for the duplex and martensitic grades are based on design stresses extrapolated using austenitic stainless steel rules.
30
H [m]
25
20
304 – API
S32304 – API
15
S32205 – API
304 – BS
S32304 – BS
S32205 – BS
1.4418 – BS
1.4418 – API
10
5
0
0
5
10
15
20
25
Minimum shell thickness [mm]
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.
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30
25
304 – API
S32304 – API
20
H [m]
S32205 – API
304 – BS
S32304 – BS
15
S32205 – BS
1.4418 – BS
10
1.4418 – API
5
0
0
5
10
15
20
25
30
Minimum relative shell thickness [mm]
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.
30
25
304 – API
S32304 – API
20
H [m]
S32205 – API
304 – BS
S32304 – BS
15
S32205 – BS
1.4418 – BS
10
1.4418 – API
5
0
0
5
10
15
20
25
Minimum relative shell thickness [mm]
Fig 4. 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 carbon steel rules.
BS 2654 thicknesses are obtained with no consideration of the 260 MPa limit.
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STABILITY
Now, the indicative cost comparison depicted in figure 3 and
figure 4 shows that compared
with a storage tank designed in
304, cost reductions are possible
if high strength stainless steels
are utilized. It has though to be
emphasized that the comparison
is indicative, relative material
costs vary and welding of the
martensitic grade 1.4418 is more
complicated than welding in
austenitic or duplex grades.
Welding of the martensitic grade
1.4418 is briefly discussed in
connection with a case described
below. Furthermore, stability is
not considered in the comparison.
Nevertheless, the comparison
highlights high strength stainless
steels as cost effective.
Consider the minimum shell
thickness in figure 2, the relative
minimum shell thickness in
figure 4 and API 650. From these
the tentative tank design shown
in table 4 can be obtained. The
high strength grades are used in
the lower and middle parts
whereas the low strength 304
grade is used for the upper part.
However, in addition to the minimum thickness, the stability of
the tank has to be checked for the
load case: Empty tank subjected
to wind load.
According to API 650 the
maximum height, H1, of an
unstiffened shell is obtained as:
H1=9.47t√( t )3
D
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Table 4: Hypothetical tank design.
Based on minimum shell thickness and indicative relative shell thickness.
Grade – ASTM/EN
H [m]
15
2
304/1.4301
5
14
4
304/1.4301
5
13
6
304/1.4301
5
12
8
S31803/1.4462
5
11
10
S31803/1.4462
5
10
12
S31803/1.4462
7
9
14
S31803/1.4462
7
8
16
S31803/1.4462
8
7
18
S31803/1.4462
9
6
20
S31803/1.4462
9
5
22
–/1.4418
9
4
24
–/1.4418
9
3
26
–/1.4418
9
2
28
–/1.4418
10
1
30
–/1.4418
10
where
t
For the tentative tank in table 4 it
is obtained:
H1 =12.73 and ∑ Wtri =15.73,
is the thickness of the top
shell course.
i.e. intermediate wind stiffeners
or increased shell thickness is
required. A possible solution
would be to increase the thickness of the five upper courses
from 5 to 6 mm, resulting in:
H1 =20.08 and ∑ Wtri =18.44.
D is the nominal diameter of
the tank.
The maximum height, H1,
according to (6) shall be larger
than a transposed shell height
obtained as:
∑Wtri =∑Wi
√
tuniform
tactual
5
(7)
where
Wtr
is the transformed width of
the ith shell course.
Wi
is the width of the ith shell
course.
i
Thickness [mm]
Course No.
tuniform is the thickness of the top
shell course.
(6)
tuniform is the thickness of the ith
shell course.
A stability check according to the
Shell Stability Handbook, edited
by Eggwertz and Samuelsson [6],
using the same conditions as
above, results in a maximum
external uniform pressure, e.g.
caused by wind load, of 1.4 kPa.
The following assumptions was
made: Reduction factor for
tolerances and manufacturing
method – 0.9, partial coefficient
for determination of allowable
stress – 1.2.
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10
17.25
304
S32304
27.250
t =7
6
304
13
S32304
t =7–1.5
8.25
1.4418
t =11.5 –14
t = 6–8
t = 8–14
27.25
12.8
12.8
Fig 5. Tank designs considered for storage of marble slurry.
Case:
Storage Tanks for Marble Slurry
A manufacturer of storage tanks
in Norway has designed and
built three 3500 m3 storage tanks
for 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
Fig 6. Parts of storage tanks before assembly.
The service environment, mildly
corrosive, allowed also low alloy
grades such as 304 and 1.4418 to
be considered as potential materials. Hence alternative solutions
were possible. The principles of
the two alternative designs considered in the final stage are shown
in figure 5. One where the austenitic grade 304 and the duplex
grade S32304 were used, and one
with the three grades, 304, S32304
and 1.4418. Alternative two, with
three grades, resulted in a weight
reduction of 15%. The weight of
the two alternatives was 129 and
110 metric tonnes respectively. The
second alternative furthermore
proved to be the most cost efficient of the two and was selected
for the three storage tanks.
Welding of the martensitic
grade 1.4418 did require special
considerations regarding welding
method and consumables to be
used. After tests with respect to
welding and obtained properties,
consumables used were the same
as used for the duplex grade
S32205. Welding methods used
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were submerged arc welding,
SAW, and flux core arc welding,
FCAW. Nor did welding of the
martensitic grade to the duplex
grade did not cause any problems.
The tank shells were welded
in sections with a maximum
weight of 75 metric tonnes at the
manufacturer and subsequently
transported to the customer for
assembly, see figure 6. The weight
limit was due to the maximum
lifting capacity of the crane available. The tanks were insulated
before taken into service.
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Discussion and Conclusions
The results presented in this
paper imply that large potential
cost reductions for storage tanks
are possible if high strength
stainless steels are utilized.
Depending on the design code
used, the potential cost reduction
varies. Difference in design
resistance is not unique for the
area of storage tank design, but
still has to be addressed. There
are, 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 continued harmonization of standards.
From the results presented in this
paper it is concluded:
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
• There are differences between
design codes with respect to
allowable design stress and
hence minimum shell thickness
of storage tanks.
• High strength stainless steel can
be, 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.
2/2003
"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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acom is distributed to persons actively involved in process industry
development and other areas where stainless steels are important.
All rights reserved.
If you have any queries regarding acom,
please contact me at jan.olsson@avestapolarit.com or by
telephone on +46 (0) 226 812 48, fax +46 (0)226 813 05.
Jan Olsson
Technical Editor, acom
AvestaPolarit AB
AvestaPolarit AB
Research and Development
SE-774 80 Avesta, Sweden
Tel: +46 (0)226 810 00
Fax: +46 (0)226 813 05
ISSN 1101–0681
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