The calculation of minimum wall thickness is crucial in ensuring the safety and reliability of any pipeline project. This process is typically performed during the early stages of detailed design and involves selecting the appropriate wall thickness based on the specific design parameters. In this article, we will explain the methodology for calculating the minimum wall thickness for a liquid pipeline with a diameter of 10 inches, using a sample example. The pipeline is made of API 5L-Gr X52 material and has an outer diameter of 10.75 inches. The design pressure is 78 Bar-g, and the design temperature is 60 degrees Celsius.
Criteria for Minimum Pipeline Wall Thickness Calculation
To ensure the pipeline can withstand the internal pressure, the minimum wall thickness for a CS line pipe is calculated based on the permissible hoop stress. According to ASME B31.4, clause 403.2.1, the nominal wall thickness of straight sections of steel pipe should be equal to or greater than tn, which is determined using the equation tn ≥ t + A.
Here, A is the sum of allowances for threading, grooving, corrosion, erosion, and an increase in wall thickness if used as a protective measure. The nominal wall thickness tn must satisfy both the pressure requirements and the allowances. The pressure design wall thickness t is calculated in inches or millimeters. The value of A is determined based on the type of coating, anticipated erosion, and any other factors that may affect the pipe’s integrity.
The minimum wall thickness of a pipeline is a critical aspect in pipeline design, as it ensures the pipeline’s structural integrity under the design pressure. The wall thickness calculation is based on the permissible hoop stress due to internal pressure. The equation used to calculate the wall thickness is t = P * D / (2 * F * S * E), where P is the design pressure, D is the outside diameter of the pipe, F is the design factor, S is the specified minimum yield strength, and E is the longitudinal joint factor.
After calculating the wall thickness, allowances for threading, grooving, corrosion, erosion, and protective measures are added to arrive at the nominal wall thickness. However, various organizations have their guidelines for selecting the minimum wall thickness, considering aspects such as pipe rigidity, supporting, handling, and field bending. Therefore, some additional checks need to be performed before finalizing the wall thickness.
For instance, some organizations limit the use of metallic line piping with thickness less than 4.8 mm. Additionally, the diameter to wall thickness ratio should not exceed 96 for metallic pipelines for some organizations. In the case of 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), the calculated wall thickness was found to be 4.1 mm. After adding the allowances, the nominal wall thickness was calculated as 4.4 mm. Considering the additional checks, the selected thickness for the pipeline would be 4.8 mm.
Full Vacuum Collapse check
To ensure that a pipeline is capable of withstanding full vacuum conditions, it is recommended by some organizations to account for vacuum collapse in the design, even if vacuum conditions are not expected during operation.
The vacuum collapse check is conducted in accordance with the ASME Section VIII, Division 1 pressure vessel code, specifically UG-28. The calculations are performed using the nominal wall thickness of the pipeline without including the corrosion allowance.
As per UG 28 (f) of the ASME code, the selected pipeline wall thickness will be considered safe for full vacuum if it is able to withstand a net external pressure of 1.01325 bar (15 psi). To determine if the selected thickness is satisfactory, allowable external working pressure is calculated using UG 28 equations and graphs. If the allowable external working pressure is greater than the design external pressure (1.01325 bar), then the selected thickness is deemed sufficient for full vacuum conditions.
Equivalent Stress check
To ensure the pipeline’s mechanical integrity under operational loads, equivalent stress 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.
The equivalent stress is determined by the Von Mises equation, which takes into account the hoop stress (due to pressure), longitudinal stress (due to pressure, thermal expansion, and bending), and combined shear stress (due to torque and shear force). The equivalent stress calculations should be carried out using the nominal wall thickness, excluding the corrosion allowance.
According to ASME B31.4 Article 402 and ASME B31.8 Article 833, the equivalent stress shall not exceed certain values. These values are as follows:
- For carbon steel pipelines, the equivalent stress shall not exceed 0.9 times the specified minimum yield strength (SMYS) of the material.
- For pipelines made of materials other than carbon steel, the equivalent stress shall not exceed 0.9 times the lesser of the specified minimum yield strength (SMYS) and the specified minimum tensile strength (SMTS) of the material.
It is important to note that these requirements may differ from other standards or guidelines, and it is necessary to follow the specific code or standard applicable to the project.
Pipeline Wall Thinning Criteria Check
Pipeline wall thinning can occur due to changes in direction made by cold bending of the pipe or by installing factory-made bends or elbows. Cold bending can result in significant wall thinning, which can impact the structural integrity of the pipeline. Therefore, it is important to ensure that the wall thickness of finished bends, taking into account wall thinning at the outer radius, is not less than the calculated wall thickness required for Hoop Stress. This calculation should be carried out in accordance with BS 8010, which provides guidance on wall thinning criteria for pipelines.
BS 8010 provides a formula for calculating the percentage of wall thinning, which can be used to determine the extent of thinning that has occurred due to the bending process. However, it should be noted that this formula does not take into account other factors that may affect wall thinning, such as the specific bending process used or the quality of the bend manufacturer. For critical applications, it is important to consult with the bend manufacturer to ensure that the wall thinning is within acceptable limits.
The wall thinning formula provided in BS 8010 is:
t(thin) = 50/(n+1)
where t(thin) is the percentage of wall thinning, n is the inner bend radius (Ri) divided by the pipe outside diameter (D), and Ri is calculated as the inner bend radius = (Bend Radius x OD) – (OD/2). The value of wall thinning should be less than 2.5% to ensure the structural integrity of the pipeline.
Pipeline Strain Check
When a pipeline is bent along a radius R, it experiences a strain that can cause permanent deformation. The amount of strain induced in the pipeline can be calculated using the formula:
strain = (Pipe OD) / 2R
where R is the bend radius of the pipeline and Pipe OD is the outside diameter of the pipe. The strain is a measure of the amount of deformation that occurs as a result of the bending process.
To ensure the structural integrity of the pipeline, the permanent bending strain induced by the bending process should be within 2%. This means that the pipeline should not experience more than 2% permanent deformation as a result of the bending process. Excessive strain can cause the pipeline to become weakened or even fail, which can have serious safety and environmental consequences.
It is important to note that the strain-induced in the pipeline is influenced by several factors, including the material properties of the pipe, the wall thickness, the bend radius, and the bending process used. Therefore, it is crucial to consider these factors when assessing the strain-induced by bending a pipeline and to ensure that the permanent bending strain is within acceptable limits.