Pressure Vessel FAQ

How do you calculate the thickness of a pressure vessel?

To calculate the thickness of a pressure vessel, you will need to use a pressure vessel design code, such as the ASME Boiler and Pressure Vessel Code (BPVC) or the EN 13445 (for vessels in Europe).

These codes provide rules and guidelines for the design, fabrication, and inspection of pressure vessels.

They specify minimum required thicknesses for various materials and construction types, based on the design pressure and temperature.

How to calculate pressure vessel thickness

To calculate the thickness of a pressure vessel using a design code, you will need to follow these steps:

  • Determine the design pressure and temperature of the vessel.
  • Select the appropriate material of construction based on the design pressure and temperature, and any other requirements (e.g., corrosion resistance).
  • Select the appropriate type of construction based on the design pressure and temperature, and any other requirements (e.g., ease of fabrication).
  • Look up the minimum required thickness for the selected material and construction type in the design code.

ASME BPVC thickness calculation

Here is an example calculation using the ASME BPVC:

Suppose we are designing a pressure vessel made of carbon steel, with a design pressure of 100 psig and a design temperature of 350°F. The vessel will have a welded construction.

According to the ASME BPVC, the minimum required thickness for a carbon steel vessel with a welded construction and a design pressure of 100 psig is given by the following equation:

t = PD/(2SE + 0.6P)

where:

  • t is the minimum required thickness (in inches)
  • P is the design pressure (in psig)
  • D is the inside diameter of the vessel (in inches)
  • S is the allowable stress for carbon steel at the design temperature (in psi)
  • E is the joint efficiency for a welded joint (assumed to be 0.85 for this example)

Suppose the inside diameter of the vessel is 36 inches. The allowable stress for carbon steel at 350°F is 12,500 psi. Plugging these values into the equation, we get:

t = (100 psig)(36 inches)/[2(12,500 psi)(0.85) + 0.6(100 psig)] = 0.72 inches

This is the minimum required thickness for the vessel, based on the design pressure and temperature and the material and construction type.

It’s important to note that this is just a simple example, and there may be additional factors that need to be considered when calculating the thickness of a pressure vessel, such as corrosion allowance, nozzle loads, and flange ratings. You will need to consult the appropriate design code for guidance on these issues.

Here are some additional details to consider when calculating the thickness of a pressure vessel:

Corrosion allowance: In addition to the minimum required thickness, you may need to consider a corrosion allowance when designing a pressure vessel. A corrosion allowance is an additional thickness added to the vessel wall to allow for corrosion over the expected service life of the vessel.

The amount of corrosion allowance needed will depend on the material of construction, the design pressure and temperature, the expected service environment (e.g., chemical exposure), and the expected service life of the vessel. The design code will provide guidance on how to calculate the corrosion allowance.

Nozzle loads: If the pressure vessel has nozzle connections (i.e., openings for pipes or other attachments), you will need to consider the loads that will be applied to these nozzles during operation. These loads can include external loads (e.g., weight of attached piping or equipment), internal loads (e.g., pressure or thermal expansion), and miscellaneous loads (e.g., wind or earthquake loads).

The design code will provide guidance on how to calculate the required thickness of the vessel at the nozzle locations to withstand these loads.

Flange ratings: If the pressure vessel has flanged connections (i.e., openings with a flange around the edge), you will need to consider the flange rating. The flange rating is the maximum allowable pressure at the flanged connection, and is typically based on the flange material, size, and rating class (e.g., 150#, 300#, 600#).

The design code will provide guidance on how to calculate the required thickness of the vessel at the flanged connection to meet the flange rating.

Factor of safety: The factor of safety (FoS) is a measure of the margin of safety in the design of a pressure vessel. It is the ratio of the minimum burst pressure (the pressure at which the vessel fails) to the design pressure.

The higher the FoS, the more conservative the design. The design code will specify a minimum required FoS for various materials and construction types.

Related post: Understanding the Pressure Vessels (Safety) Rules 1995

It’s also worth noting that the thickness calculation is just one step in the design process for a pressure vessel. There are many other factors that must be considered, including the size and shape of the vessel, the type of closure (e.g., flanged or threaded), the type of nozzle connections, and the type of support (e.g., saddle or legs). You will need to consult the appropriate design code for guidance on these issues.

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