ISO 13704:2001
(Main)Petroleum and natural gas industries — Calculation of heater-tube thickness in petroleum refineries
Petroleum and natural gas industries — Calculation of heater-tube thickness in petroleum refineries
Industries du pétrole et du gaz naturel — Calcul de l'épaisseur des tubes de fours de raffineries de pétrole
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 13704
First edition
2001-12-15
Petroleum and natural gas industries —
Calculation of heater-tube thickness in
petroleum refineries
Industries du pétrole et du gaz naturel — Calcul de l'épaisseur des tubes
de tours de raffineries de pétrole
Reference number
ISO 13704:2001(E)
©
ISO 2001
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ISO 13704:2001(E)
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ii © ISO 2001 – All rights reserved
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ISO 13704:2001(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
2 Terms and definitions .1
3 General design information.3
3.1 Information required.3
3.2 Limitations for design procedures .3
4 Design.4
4.1 General.4
4.2 Equation for stress .6
4.3 Elastic design (lower temperatures) .6
4.4 Rupture design (higher temperatures).7
4.5 Intermediate temperature range.7
4.6 Minimum allowable thickness .7
4.7 Minimum and average thicknesses .7
4.8 Equivalent tube metal temperature.8
4.9 Return bends and elbows.11
5 Allowable stresses .13
5.1 General.13
5.2 Elastic allowable stress .14
5.3 Rupture allowable stress .14
5.4 Rupture exponent .14
5.5 Yield and tensile strengths.14
5.6 Larson-Miller parameter curves .14
5.7 Limiting design metal temperature.15
5.8 Allowable stress curves.15
6 Sample calculations .16
6.1 Elastic design.16
6.2 Thermal-stress check (for elastic range only).17
6.3 Rupture design with constant temperature .20
6.4 Rupture design with linearly changing temperature .22
Annex A (informative) Estimation of remaining tube life .26
Annex B (informative) Calculation of maximum radiant section tube skin temperature.30
Annex C (normative) Thermal-stress limitations (elastic range) .40
Annex D (informative) Calculation sheets .43
Annex E (normative) Stress curves (SI units).45
Annex F (normative) Stress curves (US customary units) .84
Annex G (informative) Derivation of corrosion fraction and temperature fraction .124
Annex H (informative) Data sources .132
Bibliography.137
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ISO 13704:2001(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of technical committees is to prepare International Standards. Draft International Standards adopted
by the technical committees are circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 13704 was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore structures for
petroleum and natural gas industries, Subcommittee SC 6, Processing equipment and systems.
Annexes C, E and F form an integral part of this International Standard. Annexes A, B, D, G and H are for
information only.
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ISO 13704:2001(E)
Introduction
[30]
This International Standard is based on API standard 530 , fourth edition, October 1996.
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INTERNATIONAL STANDARD ISO 13704:2001(E)
Petroleum and natural gas industries — Calculation of heater-tube
thickness in petroleum refineries
1 Scope
This International Standard specifies the requirements and gives recommendations for the procedures and design
criteria used for calculating the required wall thickness of new tubes for petroleum refinery heaters. These
procedures are appropriate for designing tubes for service in both corrosive and non-corrosive applications. These
procedures have been developed specifically for the design of refinery and related process fired heater tubes
(direct-fired, heat-absorbing tubes within enclosures). These procedures are not intended to be used for the design
of external piping.
This International Standard does not give recommendations for tube retirement thickness; annex A describes a
technique for estimating the life remaining for a heater tube.
2 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
2.1
actual inside diameter
D
i
inside diameter of a new tube
NOTE The actual inside diameter is used to calculate the tube skin temperature in annex B and the thermal stress in
annex C.
2.2
corrosion allowance
d
CA
additional material thickness added to allow for material loss during the design life of the component
2.3
design life
t
DL
operating time used as a basis for tube design
NOTE The design life is not necessarily the same as the retirement or replacement life.
2.4
design metal temperature
T
d
tube metal, or skin, temperature used for design
NOTE This is determined by calculating the maximum tube metal temperature (T in annex B) or the equivalent tube
max
metal temperature (T in 2.7) and adding an appropriate temperature allowance (see 2.15). A procedure for calculating the
eq
maximum tube metal temperature from the heat flux density is included in annex B. When the equivalent tube metal
temperature is used, the maximum operating temperature can be higher than the design metal temperature.
2.5
elastic allowable stress
s
el
allowable stress for the elastic range (see 5.2)
NOTE See 3.2.3 for information about tubes that have longitudinal welds.
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ISO 13704:2001(E)
2.6
elastic design pressure
p
el
maximum pressure that the heater coil will sustain for short periods of time
NOTE This pressure is usually related to relief valve settings, pump shut-in pressures, etc.
2.7
equivalent tube metal temperature
T
eq
calculated constant metal temperature that in a specified period of time produces the same creep damage as does
a linearly changing metal temperature (see 4.8)
2.8
inside diameter
∗
D
i
inside diameter of a tube with the corrosion allowance removed; used in the design calculations
NOTE The inside diameter of an as-cast tube is the inside diameter of the tube with the porosity and corrosion allowances
removed.
2.9
minimum thickness
d
min
minimum required thickness of a new tube, taking into account all appropriate allowances [see equation (5)]
2.10
outside diameter
D
o
outside diameter of a new tube
2.11
rupture allowable stress
s
r
allowable stress for the creep-rupture range (see 4.4)
NOTE See 3.2.3 for information about tubes that have longitudinal welds.
2.12
rupture design pressure
p
r
maximum operating pressure that the coil section will sustain during normal operation
2.13
rupture exponent
n
parameter used for design in the creep-rupture range
See figures in annexes E and F.
2.14
stress thickness
d
σ
thickness, excluding all thickness allowances, calculated from an equation that uses an allowable stress
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ISO 13704:2001(E)
2.15
temperature allowance
T
A
part of the design metal temperature that is included for process- or flue-gas maldistribution, operating unknowns,
and design inaccuracies
NOTE The temperature allowance is added to the calculated maximum tube metal temperature or to the equivalent tube
metal temperature to obtain the design metal temperature (see 2.4).
3 General design information
3.1 Information required
The usual design parameters (design pressures, design fluid temperature, corrosion allowance, and tube material)
shall be defined. In addition, the following information shall be furnished:
a) the design life of the heater tube;
b) whether the equivalent-temperature concept is to be applied, and if so, furnish the operating conditions at the
start and at the end of the run;
c) the temperature allowance, if any;
d) the corrosion fraction (if different from that shown in Figure 1);
e) whether elastic-range thermal-stress limits are to be applied.
If any of items a) to e) are not furnished, use the following applicable parameters:
f) a design life equal to 100 000 h;
g) a design metal temperature based on the maximum metal temperature (the equivalent-temperature concept
shall not apply);
h) a temperature allowance equal to 15 °C (25 °F);
i) the corrosion fraction given in Figure 1;
j) the elastic-range thermal-stress limits.
3.2 Limitations for design procedures
3.2.1 The allowable stresses are based on a consideration of yield strength and rupture strength only; plastic or
creep strain has not been considered. Using these allowable stresses might result in small permanent strains in
some applications; however, these small strains will not affect the safety or operability of heater tubes.
3.2.2 No considerations are included for adverse environmental effects such as graphitization, carburization, or
hydrogen attack. Limitations imposed by hydrogen attack can be developed from the Nelson curves in
[15]
API RP 941 .
3.2.3 These design procedures have been developed for seamless tubes. When they are applied to tubes that
have a longitudinal weld, the allowable stress values should be multiplied by the appropriate joint efficiency factor.
Joint efficiency factors shall not be applied to circumferential welds.
3.2.4 These design procedures have been developed for thin tubes (tubes with a thickness-to-outside-diameter
ratio, d /D , of less than 0,15). Additional considerations may apply to the design of thicker tubes.
min o
3.2.5 No considerations are included for the effects of cyclic pressure or cyclic thermal loading.
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ISO 13704:2001(E)
3.2.6 The design loading includes only internal pressure. Limits for thermal stresses are provided in annex C.
Limits for stresses developed by mass, supports, end connections, and so forth are not discussed in this
International Standard.
3.2.7 Most of the Larson-Miller parameter curves in 5.6 are not Larson-Miller curves in the traditional sense but
are derived from the 100 000-h rupture strength as explained in H.3. Consequently, the curves might not provide a
reliable estimate of the rupture strength for a design life that is less than 20 000 h or more than 200 000 h.
4 Design
4.1 General
There is a fundamental difference between the behaviour of carbon steel in a hot-oil heater tube operating at
300 °C (575 °F) and that of chromium-molybdenum steel in a catalytic-reformer heater tube operating at 600 °C
(1 110 °F). The steel operating at the higher temperature will creep, or deform permanently, even at stress levels
well below the yield strength. If the tube metal temperature is high enough for the effects of creep to be significant,
the tube will eventually fail due to creep rupture, although no corrosion or oxidation mechanism is active. For the
steel operating at the lower temperature, the effects of creep will be non-existent or negligible. Experience indicates
that in this case the tube will last indefinitely unless a corrosion or an oxidation mechanism is active.
Since there is a fundamental difference between the behaviour of the materials at these two temperatures, there
are two different design considerations for heater tubes: elastic design and creep-rupture design. Elastic design is
design in the elastic range, at lower temperatures, in which allowable stresses are based on the yield strength (see
4.3). Creep-rupture design (which is referred to below as rupture design) is the design for the creep-rupture range,
at higher temperatures, in which allowable stresses are based on the rupture strength (see 4.4).
The temperature that separates the elastic and creep-rupture ranges of a heater tube is not a single value; it is a
range of temperatures that depends on the alloy. For carbon steel, the lower end of this temperature range is about
425 °C (800 °F); for Type 347 stainless steel, the lower end of this temperature range is about 590 °C (1 100 °F).
The considerations that govern the design range also include the elastic design pressure, the rupture design
pressure, the design life and the corrosion allowance.
The rupture design pressure is usually less than the elastic design pressure. The characteristic that differentiates
these two pressures is the relative length of time over which they are sustained. The rupture design pressure is a
long-term loading condition that remains relatively uniform over a period of years. The elastic design pressure is
usually a short-term loading condition that typically lasts only hours or days. The rupture design pressure is used in
the rupture design equation, since creep damage accumulates as a result of the action of the operating, or long-
term stress. The elastic design pressure is used in the elastic design equation to prevent excessive stresses in the
tube during periods of operation at the maximum pressure.
The tube shall be designed to withstand the rupture design pressure for long periods of operation. If the normal
operating pressure increases during an operating run, the highest pressure shall be taken as the rupture design
pressure.
In the temperature range near or above the point where the elastic and rupture allowable stress curves cross, both
elastic and rupture design equations are to be used. The larger value of d should govern the design (see 4.5). A
min
sample calculation that uses these methods is included in clause 6. Calculation sheets (see annex D) are available
for summarizing the calculations of minimum thickness and equivalent tube metal temperature.
The allowable minimum thickness of a new tube is given in Table 1.
All of the design equations described in this clause are summarized in Table 2.
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ISO 13704:2001(E)
B = d /d
CA σ
pD
ro
d =
s
2s + p
d is the corrosion allowance
rr
CA
D is the outside diameter p is the rupture design pressure
o r
n is the rupture exponent
s is the rupture allowable stress
r
a
Note change of scale.
Figure 1 — Corrosion fraction
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ISO 13704:2001(E)
4.2 Equation for stress
In both the elastic range and the creep-rupture range, the design equation is based on the mean-diameter equation
for stress in a tube. In the elastic range, the elastic design pressure (p ) and the elastic allowable stress (s ) are
el el
used. In the creep-rupture range, the rupture design pressure (p ) and the rupture allowable stress (s ) are used.
r r
The mean-diameter equation gives a good estimate of the pressure that will produce yielding through the entire
tube wall in thin tubes (see 3.2.4 for a definition of thin tubes). The mean-diameter equation also provides a good
correlation between the creep rupture of a pressurized tube and a uniaxial test specimen. It is therefore a good
[16], [17], [18] and [19]
equation to use in both the elastic range and the creep-rupture range . The mean diameter
equation for stress is as follows:
ppʈD D
ʈ
oi
s = -=11+ (1)
Á˜
Á˜
˯
22˯dd
where
1)
s is the stress, expressed in megapascals [pounds per square inch ];
p is the pressure, expressed in megapascals (pounds per square inch);
D is the outside diameter, expressed in millimetres (inches);
o
D is the inside diameter, expressed in millimetres (inches), including the corrosion allowance;
i
d is the thickness, expressed in millimetres (inches).
The equations for the stress thickness (d ) in 4.3 and 4.4 are derived from equation (1).
σ
4.3 Elastic design (lower temperatures)
The elastic design is based on preventing failure by bursting when the pressure is at its maximum (that is, when a
pressure excursion has reached p ) near the end of the design life after the corrosion allowance has been used up.
el
With the elastic design, d and d (see 4.6) are calculated as follows:
σ min
*
pD pD
el o el i
dd==or (2)
ss
22ss+-p p
el el el el
d = d + d (3)
min σ CA
where
*
D is the inside diameter, expressed in millimetres (inches), with corrosion allowance removed;
i
s is the elastic allowable stress, expressed in megapascals (pounds per square inch), at the design metal
el
temperature.
1) 2
The unit “pounds per square inch (psi)” is referred to as “pound-force per square inch (lbf/in )” in ISO 31.
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ISO 13704:2001(E)
4.4 Rupture design (higher temperatures)
The rupture design is based on preventing failure by creep rupture during the design life. With the rupture design,
d and d (see 4.6) are calculated as follows:
σ min
*
pD pD
ro ri
dd==or (4)
ss
22ss+-p p
rr rr
d = d + f d (5)
min σ corr CA
where
s is the rupture allowable stress, expressed in megapascals (pounds per square inch), at the design metal
r
temperature and the design life;
f is the corrosion fraction given as a function of B and n in Figure 1;
corr
where
B = d /d
CA σ
n is the rupture exponent at the design metal temperature (shown in the figures given in annexes E
and F).
The derivation of the corrosion fraction is described in annex G. It is recognized in this derivation that stress is
reduced by the corrosion allowance; correspondingly, the rupture life is increased.
This design equation is suitable for heater tubes; however, if special circumstances require that the user choose a
more conservative design, a corrosion fraction of unity (f = 1) may be specified.
corr
4.5 Intermediate temperature range
At temperatures near or above the point where the curves of s and s intersect in the figures given in annexes E
el r
and F, either elastic or rupture considerations will govern the design. In this temperature range, both the elastic and
rupture designs are to be applied. The larger value of d shall govern the design.
min
4.6 Minimum allowable thickness
The minimum thickness (d ) of a new tube (including the corrosion allowance) shall not be less than that shown
min
in Table 1. For ferritic steels, the values shown are the minimum allowable thicknesses of Schedule 40 average
wall pipe. For austenitic steels, the values are the minimum allowable thicknesses of Schedule 10S average wall
pipe. (Table 5 shows which alloys are ferritic and which are austenitic). The minimum allowable thicknesses are
0,875 times the average thicknesses. These minima are based on industry practice. The minimum allowable
thickness is not the retirement or replacement thickness of a used tube.
4.7 Minimum and average thicknesses
The minimum thickness (d ) is calculated as described in 4.3 and 4.4. Tubes that are purchased to this minimum
min
thickness will have a greater average thickness. A thickness tolerance is specified in each ASTM specification. For
most of the ASTM specifications shown in the figures given in annexes E and F, the tolerance on the minimum
0 0
thickness is % for hot-finished tubes and % for cold-drawn tubes. This is equivalent to tolerances on
(+28 ) (+22)
the average thickness of ±12,3 % and ±9,9 %, respectively. The remaining ASTM specifications require that the
minimum thickness be greater than 0,875 times the average thickness, which is equivalent to a tolerance on the
average thickness of +12,5 %.
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ISO 13704:2001(E)
0
With a % tolerance, a tube that is purchased to a 12,7 mm (0,500 in) minimum-thickness specification will
(+28 )
have the following average thickness:
(12,7)(1 + 0,28/2) = 14,5 mm (0,570 in)
To obtain a minimum thickness of 12,7 mm (0,500 in) in a tube purchased to a ± 12,5 % tolerance on the average
thickness, the average thickness shall be specified as follows:
(12,7) / (0,875) = 14,5 mm (0,571 in)
All thickness specifications shall indicate whether the specified value is a minimum or an average thickness. The
tolerance used to relate the minimum and average wall thicknesses shall be the tolerance given in the ASTM
specification to which the tubes will be purchased.
Table 1 — Minimum allowable thickness of new tubes
Minimum thickness
Tube outside diameter
Ferritic steel tubes Austenitic steel tubes
mm (in) mm (in) mm (in)
60,3 (2,375) 3,4 (0,135) 2,4 (0,095)
73,0 (2,875) 4,5 (0,178) 2,7 (0,105)
88,9 (3,50) 4,8 (0,189) 2,7 (0,105)
101,6 (4,00) 5,0 (0,198) 2,7 (0,105)
114,3 (4,50) 5,3 (0,207) 2,7 (0,105)
141,3 (5,563) 5,7 (0,226) 3,0 (0,117)
168,3 (6,625) 6,2 (0,245) 3,0 (0,117)
219,1 (8,625) 7,2 (0,282) 3,3 (0,130)
273,1 (10,75) 8,1 (0,319) 3,7 (0,144)
4.8 Equivalent tube metal temperature
In the creep-rupture range, the accumulation of damage is a function of the actual operating temperature. For
applications in which there is a significant difference between start-of-run and end-of-run metal temperatures, a
design based on the maximum temperature might be excessive, since the actual operating temperature will usually
be less than the maximum.
For a linear change in metal temperature from start of run (T ) to end of run (T ), an equivalent tube metal
sor eor
temperature (T ) can be calculated as shown below. A tube operating at the equivalent tube metal temperature
eq
will sustain the same creep damage as one that operates from the start-of-run to end-of-run temperatures.
T = T + ƒ (T − T ) (6)
T
eq sor eor sor
where
T is the equivalent tube metal temperature, expressed in degrees Celsius (Fahrenheit);
eq
T is the tube metal temperature, expressed in degrees Celsius (Fahrenheit), at start of run;
sor
T is the tube metal temperature, expressed in degrees Celsius (Fahrenheit), at end of run;
eor
ƒ is the temperature fraction given in Figure 2.
T
The derivation of the temperature fraction is described in annex G. The temperature fraction is a function of two
parameters, V and N:
*
ʈ
ʈ
DTA
Vn= ln
0Á˜
Á˜
*
˯s
T
˯ 0
sor
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ISO 13704:2001(E)
ʈ
Dd
Nn=
0
Á˜
˯d
0
where
n is the rupture exponent at T ;
0 sor
∗
∆T (= T − T ) is the temperature change, expressed in kelvins (degrees Rankine), during operating
eor sor
period, K (°R);
∗
T = T + 273 K (T + 460 °R);
sor
sor sor
ln is the natural logarithm;
∆d = f t is the change in thickness, expressed in millimetres (inches), during the operating period;
corr op
f is the corrosion rate, expressed in millimetres per year (in inches per year);
corr
t is the duration of operating period, expressed in years;
op
d is the initial thickness, expressed in millimetres (inches), at the start of the run;
0
s is the initial stress, expressed in megapascals (pounds per square inch), at start of run using
0
equation (1);
...
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