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8/13/2019 Guide to Thickness Calculation of Heater Tubes
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THICKNESS IN PETROLEUM
REFINERIES
API 530
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LIMITATIONS
A l to thin tubes with thickness to dia ratio less
than 0.15Apply to seamless tubes. When applied to welded
u es mu p y a owa e s ress w o n e c ency
No consideration for graphitization, carburization, or
h dro en attack
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DESIGN
design conditions: Elastic Design (lower temperature)
Rupture Design (higher temperature)
Creep rupture occurs in steel at highoperating temperature even at stress levels
well below the yield strength
Creep rupture is permanent deformation, afailure mode other than elastic/plastic
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DESIGN
When tube tem erature will be hi h enou h for cree
to be significant, tube will fail from creep rupture For steels operating at lower temperature, the effects
o creep w e neg g e e as c
Experience indicates that in elastic case tube will last
indefinitel unless a corrosion or oxidation
mechanism is active
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DESIGN
Elastic design Based on preventing bursting
Design in elastic range at lower temperature
Allowable stresses based on yield strength
ee
oes
PS
DPt
+
=
2
CAtt sm +=
w ere,
Pe = design pressure
Se = allowable stress in elastic range
o
ts = stress thickness
tm = minimum thickness
CA = corrosion allowance
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DESIGN
Rupture design Based on preventing creep rupture
Design in creep range at higher temperature
Allowable stresses based on rupture strength
rr
or
s PS
DP
t +=
2 fCAtt sm +=
where,
Pr = rupture design pressure
r
f = corrosion fraction function of B and n
B = CA/ts
n = ru ture ex onent at desi n metal tem erature
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DESIGN
Known values will be design pressure, design metal
temperature and outside diameter of tubes Determination of design criteria (elastic or rupture)
The same graph will provide the value of allowable
elastic or rupture strength ere, we w go t roug ot o es gns
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DESIGN
We take the exam le of a coil
Design pressure = 38 kg/cm2
Design temperature = 660 C
ater a o co -
Graph will define the type of design to be followed
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DESIGN
Ru ture desi n overns as allowable stress is
less than elastic marginal stress More data is required to get the value of
allowable rupture strength like design life of
tube (to be decided by the designer) and
design pressure
See the ra h on the next a e for clarit
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DESIGN
So allowable ru ture stren th comes out close to 41
MPa considering design life of 100,000 hrs Likewise, we can evaluate the thickness by using the
ormu as g ven a ove rup ure pressure s e ne
Elastic design is simple to practice design procedures
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