Determining the shell thickness of a pressure vessel involves several factors and considerations to ensure its safety and compliance with relevant standards and codes. Here’s a general outline of the steps involved:
Identify Design Parameters:
Gather the design parameters of the pressure vessel, such as the internal and external pressure (P_int and P_ext), vessel diameter (D), material properties (yield strength, tensile strength, etc.), and operating temperature.
Determine Design Code and Standards:
Identify the applicable design code and standards for pressure vessels in your region or industry. Common codes include ASME Boiler and Pressure Vessel Code (BPVC), EN 13445, PD 5500, etc. Adhering to the relevant code is crucial for ensuring the safety and legality of the pressure vessel.
Calculate Required Shell Thickness:
Based on the design parameters and the selected code, calculate the required minimum shell thickness using appropriate formulas. Different codes may have different equations for this purpose.
For example, in the ASME BPVC, the formula for calculating minimum required thickness is based on the maximum allowable working pressure (MAWP) and material properties.
Consider Corrosion Allowance:
To account for potential corrosion over the vessel’s service life, add a corrosion allowance to the calculated minimum thickness. The corrosion allowance is typically specified in the design code or industry standards.
Evaluate Longitudinal and Circumferential Stresses:
Depending on the vessel’s shape and operating conditions, evaluate the longitudinal and circumferential stresses using appropriate formulas.
Check that the calculated stresses are within allowable limits specified by the design code.
Buckling and Stability Analysis:
Perform buckling and stability analysis to ensure the pressure vessel’s shell can withstand the external pressure without buckling or failing.
Non-Destructive Examination (NDE):
If required by the design code, perform non-destructive examination methods such as ultrasonic testing or radiography to ensure the integrity of the pressure vessel’s shell.
Fabrication Tolerances and Quality Control:
Consider fabrication tolerances and ensure appropriate quality control measures are in place during the construction of the pressure vessel to meet the required specifications.
Review and Certification:
Have the design calculations, drawings, and fabrication details reviewed by a qualified engineer or an authorized third-party inspection agency.
Obtain the necessary certifications and approvals from relevant authorities before putting the pressure vessel into service.
It’s essential to note that designing pressure vessels can be a complex task, and safety is of utmost importance. Always consult with qualified engineers or pressure vessel specialists with experience in your industry to ensure compliance with relevant standards and safe operation of the pressure vessel.
Example of pressure vessel shell thickness calculation:
Let’s go through a simplified example of calculating the shell thickness of a pressure vessel. Please note that this is a basic illustration, and actual pressure vessel design requires more detailed and comprehensive analysis.
Example:
Consider a cylindrical pressure vessel with the following parameters:
Internal pressure (P_int): 10 MPa (megapascal) = 10,000,000 Pa
External pressure (P_ext): 0 MPa (assuming atmospheric pressure)
Diameter (D): 1 meter
Material: Carbon Steel (SA-516 Gr. 70)
Operating temperature: 150°C
Step 1: Identify Design Parameters.
We have already listed the design parameters above.
Step 2: Determine Design Code and Standards.
For this example, we will use the ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 1.
Step 3: Calculate Required Shell Thickness.
The formula for calculating the required minimum shell thickness according to ASME BPVC Section VIII,
Division 1 is as follows:
Minimum Required Thickness (t_min) = (P * D) / (2 * S * F * E – 0.6 * P)
Where:
P = Maximum Allowable Working Pressure (MAWP)
D = Inside diameter of the shell
S = Allowable Stress value of the material at operating temperature
F = Design Factor (typically 1.0 for most cases)
E = Joint Efficiency (typically 1.0 for seamless construction, lower for welded)
Step 3.1: Calculate MAWP.
To calculate MAWP, we need to consider the operating temperature and the design pressure (maximum expected pressure during operation). Let’s assume a design pressure (P_design) of 12 MPa.
MAWP = max(P_int, P_design) + P_ext
MAWP = max(10 MPa, 12 MPa) + 0 MPa
MAWP = 12 MPa
Step 3.2: Determine Allowable Stress (S) value.
For Carbon Steel SA-516 Gr. 70 at 150°C, the allowable stress value (S) can be obtained from the material properties table in ASME BPVC Section II, Part D. Let’s assume S = 140 MPa.
Step 3.3: Design Factor (F) and Joint Efficiency (E).
For this example, let’s assume F = 1.0 and E = 1.0 (seamless construction).
Step 3.4: Calculate the minimum required thickness.
t_min = (12 MPa * 1 m) / (2 * 140 MPa * 1.0 * 1.0 – 0.6 * 12 MPa)
t_min ≈ 8.57 mm
Step 4: Consider Corrosion Allowance.
Assume a corrosion allowance of 3 mm.
Total Required Thickness = t_min + Corrosion Allowance
Total Required Thickness ≈ 8.57 mm + 3 mm
Total Required Thickness ≈ 11.57 mm
Step 5: Evaluate Longitudinal and Circumferential Stresses.
Perform longitudinal and circumferential stress analysis using appropriate formulas and compare them with the allowable stresses based on material properties. Ensure that the stresses are within allowable limits.
Step 6: Buckling and Stability Analysis.
Perform buckling and stability analysis to ensure the pressure vessel’s shell can withstand the external pressure without buckling or failing.
These calculations provide a basic outline of the process, but in practice, pressure vessel design involves more detailed analysis, consideration of additional loads, material properties at different temperatures, and comprehensive safety checks. Always consult with qualified engineers and refer to the relevant design code for accurate and safe pressure vessel design.