The Fabrication process for Pressure Vessels

A variety of different types of steel are used to make pressure vessels in both the private and the industrial sectors. Due to the diverse uses of pressure vessels, different types of steel must be used, such as stainless steel, carbon steel, carbon manganese steel, etc. An accurate cutting, molding, welding, and assembly of metals for the fabrication of pressure vessels is a very detailed process that involves the cutting, molding, welding, and assembly of metals that require precision. The following major design steps need to be taken before the fabrication process can begin, in order to ensure that it goes smoothly:

  • Design Concept – Define the scope of the project along with all the technical specifications that are required. The shape, size, and dimensions of the vessel must be specified. The fabricators of pressure vessels will also choose the material for the components that are required for manufacture based on the intended use of the vessel.
  • Stress & Strain Analysis – It is important to perform calculations based on mechanical principles in order to determine the strength of the material that will be used in the fabrication of pressure vessels. 
  • Fabrication Drawings – Prepare fabrication drawings or assembly drawings for the fabrication process that include material parts, manufacturing standards, viewing sections, welding / bolting information, specific instructions, and serve as a guide for the execution of the fabrication process in the best possible way.

A building material which is commonly used for fabrication is generally a plate, pipe, forging, structural shape, welding rod or wire, etc., which is made of different types of materials. As per the vessel requirements, these components are molded and forged from the raw material to meet the specifications of the vessel. Some of the parts may also require machining if additional precision is required. Following the welding process, the welded components of the pressure vessel are assembled and are then cooled and sandblasted before primer and paint can be applied to them.

Fabrication Techniques for High Pressure Vessels:

A high pressure process, as the pressure required increases, the thickness necessary to maintain stresses within a specified range increases as the amount of pressure required increases. As a result, the circumferential stress distribution becomes non-uniform. It has its maximum value at the innermost radius, while it has its minimum value at the outermost radius.

These vessels can be manufactured in a variety of ways:

  1. Mono-block or thick walled vessel.
  2. Layered vessels.

It is common for multi-layered vessels to consist of numerous layers wrapped tightly around an inner shell to create a pressure retaining envelope that is surrounded by multiple layers. The construction of layered vessels can be done in a variety of ways. The difference between these methods is in the thickness of individual layers, wrapping procedure and welding technique. There are three types of layered vessel construction that can be categorized.

1. Concentric or spiral wrapped method:

In this method, layers are formed by connecting segments together in a spiral or concentric manner to form the thickness that is required.

2. Shrink fit method:

It is a method of forming layers by shrinking individual cylinders on top of one another so that they can be combined into a single layer.

3. Coil wrapped method:

A cylinder is formed by winding continuous sheets or strips in a spiral or helical pattern in order to form a continuous strip or sheet.

A thick walled vessel: In designing a vessel for high pressure by mono-block construction, two fundamental modes of failure should be considered when designing the vessel.

  1. It is based on elasticity theory that elastic failure occurs.
  2. It is based on the theory of plasticity that plastic failure occurs.

An elastic failure occurs when the elastic limit of a material has been reached. It is measured by the tensile strength, yield strength, and rupture strength of the material. The three most commonly used theories in order to predict the elastic failure are:

(a) Maximum principal stress theory:

As per this theory, failure is described as the event when one or more of the principal stresses (tangential, radial, and axial) has reached the elastic limit of the material and is determined from tension and compression tests performed on the material.

(b) Maximum Shear Stress theory:

According to this theory, failure occurs when the maximum shear stress equals the shear stress at the elastic limit of the material. It is determined that the maximum shear stress limit can be reached by testing pure shear. In the case of three principal stresses, the maximum shear stress is defined as one half of the algebraic difference between the largest and the smallest of the three principal stresses.

(C) Maximum Strain Energy theory:

This theory is also known as the distortion energy theory. According to this theory, failure occurs when the distortion energy accumulated in the part under stress reaches the elastic limit, which is determined by the distortion energy that is accumulated in the part through tension or compression tests.