Calculation of stresses should take into account circumferential, longitudinal, shear and equivalent stresses.
-Calculations should also take into consideration stress intensification factors that normally occur in components.
Calculation of stresses in loops, bends and offsets should be based on total range from min to max temperature normally expected.
STRESS FROM INTERNAL PRESSURE
It is calculated as:
where:
D = outside diameter of pipe, in. (mm)
Pi = internal design gage pressure, psi (bar)
SH = circumferential (hoop) stress due to internal pressure, psi (MPa)
t = wall thickness of pipe, in. (mm)
STRESS FROM EXTERNAL PRESSURE
It is calculated as:
where:
D = outside diameter of pipe, in. (mm)
Pe = external design gage pressure, psi (bar)
SH = circumferential (hoop) stress due to external pressure, psi (MPa)
t = wall thickness of pipe, in. (mm)
This code recognizes two restraint conditions:
- “Restrained” pipelines
-Sections of buried pipe.
-Above ground pipe sections attached to supports.
-Field bends in and adjacent pipe buried in stiff or consolidated soil.
- “Unrestrained” pipelines
-Above ground pipe configured to accommodate thermal expansion.
-Field bends and adjacent pipe buried in soft or unconsolidated soil.
:: ASCE Guidelines for the Design of Buried Pipelines: www.asce.org
STRESS FROM THERMAL EXPANSION
It is calculated as:
- Restrained pipe:
where:
E = Modulus of Elasticity
SE = thermal expansion stress, psi (MPE)
T1 = Temperature at installation or completion of final tie-in, 0F (0C)
T2 = operation temperature, 0F (0C)
a = coefficient of thermal expansion, in./in./0F (mm/mm/0C)
STRESS FROM THERMAL EXPANSION
It is calculated as:
- Unrestrained pipe:
where:
Sb = resultant bending stress, psi (MPa)
St = torsional stress, psi (MPa)
Sb is calculated as:
where:
ii = in-plane stress intensification factor. Note that ii, is 1 for pipe.
io = out-of-plane stress intensification factor. Note that io is 1 for pipe.
Mi = in-plane bending moment, in-Ib (Nm)
Mo = in-plane bending moment, in-Ib (Nm)
Z = Section modulus of the pipe or of the fitting outlet, in3 (cm3)
St is calculated as:
where:
Mt = torsional moment, in.-Ib (Nm)
where:
Mt = Torsional moment
Mi = In plane bending moment
Mo = Out of plane bending moment
Credit text: Tuan Muhammad Hafizul
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