Calculation of minimum wall thickness of a given pipeline diameter and selection of actual thickness is one of the most important basic design considerations for any pipeline projects. This is one of the basic activities that is performed at the initial stages of any detailed design project. In this article, the pipeline wall thickness calculation methodology will be explained for a liquid pipeline of 10-inch diameter (API 5L-Gr X52, 10.75-inch OD, Design Pressure=78 Bar-g, Design Temp=60 Deg. C) with a sample example.
Criteria for Minimum Pipeline Wall Thickness Calculation
The wall thickness for the CS Line pipe shall be calculated based on permissible hoop stress due to internal pressure. In accordance with ASME B31.4, clause 403.2.1, The nominal wall thickness of straight sections of steel pipe shall be equal to or greater than tn determined in accordance with the following
Equation : tn ≥ t + A
- A: sum of allowances for threading, grooving, corrosion, and erosion and an increase in wall thickness if used as a protective measure
- tn: nominal wall thickness satisfying requirements for pressure and allowances
- t: pressure design wall thickness as calculated in inches (millimeters)
The line pipe wall thickness (t) to withstand the internal design pressure is calculated as below:
t = P * D / (2 * F *S * E)
- t : Calculated Wall thickness (mm)
- P : Design pressure for the pipeline (kPa)=78 bar-g=7800 KPa
- D : Outside diameter of pipe (mm)= 273.05 mm
- F : Design factor = 0.72
- S : Specified Minimum Yield Strength (MPa)=359870 KPa for the specified material.
- E : Longitudinal joint factor = 1.0
Hence Calculated wall thickness (t, mm) = (7800*273.05)/ (2*0.72*359870*1) = 4.10
If the sum of allowances for threading, grooving, corrosion, and erosion and an increase in wall thickness if used as protective measure=0.3 mm
Then nominal wall thickness satisfying requirements for pressure and allowances= 4.1+0.3= 4.4 mm.
So, any available thickness greater than 4.4 mm can be used as a selected thickness.
Now various organizations have their own guidelines for minimum thickness selection considering pipe rigidity, supporting, handling, field bending and other aspects relating to construction and in-situ integrity of the pipeline and those need to be checked. Based on these, few checks need to be performed before deciding the final wall thickness. These are listed below:
Few organizations limit the use of metallic line piping with thickness less than 4.8 mm. Hence 4.8 mm will be the selected thickness.
The diameter to wall thickness ratio should not exceed 96 for metallic pipelines for some organizations. Here D/T=273.05/4.8=56.88.
Full Vacuum Collapse check
As per a few organizations, collapse due to vacuum conditions shall be accounted for in the design of all pipelines, even when vacuum conditions are not expected to occur in service.
The calculations are carried out in accordance to pressure vessel code ASME Section VIII, DIV 1, UG-28. All vacuum collapse calculations are carried with nominal wall thickness excluding corrosion allowance.
As per UG 28 (f) of ASME section VIII, the selected pipeline wall thickness will be safe for full vacuum, if it is capable of withstanding net external pressure of 1.01325 bar (15 psi).
Now following UG 28 equations and graphs calculate allowable external working pressure. If allowable external working pressure is more than design external pressure (i. e, 1.01325 bar) then the selected thickness is satisfactory.
Equivalent Stress check
The equivalent stresses calculations must be carried out as per ASME B31.4.
The wall thickness initially derived from hoop stress considerations based on design factors, should be such that the longitudinal, shear and equivalent stresses in the pipe wall under functional and environmental loads do not exceed certain values. This is covered in ASME B31.4 Article 402 and of ASME B31.8 Article 833. Because the requirements in these various articles differ from each other, it is recommended to use a single approach for all pipelines as detailed below.
The equivalent stress can be defined as follows:
Seq = (Sh2 + SL 2– ShSL+ 3Ss2)1/2 (Von Mises equation)
- Seq = equivalent stress
- Sh = hoop stress (due to pressure)
- SL = longitudinal stress (due to pressure, thermal expansion and bending)
- Ss = combined shear stress (due to torque and shear force)
The stress calculations for the operational phase shall be carried out with the nominal wall thickness excluding the corrosion allowance. The equivalent stress shall not exceed the values given below:
Pipeline Wall Thinning Criteria Check
Changes in direction may be made by cold bending of pipe or installing factory-made bends or elbows. The bending of the pipe will result in a significant wall thinning. Hence, the wall thickness of finished bends, considering wall thinning at the outer radius, should be not less than the calculated wall thickness for Hoop Stress. The wall thinning calculations should be carried out in accordance with BS 8010.
As per BS 8010; An indication of wall thinning as a percentage can be calculated using the following equation:
This formula does not take into account other factors that depend on the bending process, and the bend manufacturer should be consulted where wall thinning is critical.
Here, n=inner bend radius (Ri) divided by pipe outside diameter(D) for wall thinning formulae
Ri=Inner bend radius=(Bend Radius*OD)-(OD/2)
The value of bend thinning shall be less than 2.5%.
Pipeline Strain Check
The strain-induced in a pipeline by bending it along a radius R is=(Pipe OD)/2R (Bend Radius) the permanent bending strain should be within 2%.
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