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PRESSURE VESSELS

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A pressure vessel, as a type of unit, is one of the most important components in industrial and petrochemical process plants. In the broad sense, the term pressure vessel encompasses a wide range of unit heat exchangers, reactors, storage vessels, columns, separation vessels, etc. (See also Mechanical Design of Heat Exchangers.) Because of the risks that would be associated with any accidental release of contents, in many countries the production and operation of pressure vessels are controlled by legislation. This legislation may define the national standard to which the pressure vessel is to be designed, the involvement of independant inspection during construction, and subsequently the regular inspection and testing during operation. Some national pressure vessel standards such as ASME VIII (1993) or BS5500 (1994) have effectively the status of defacto international standards.

The national legislation and/or standard generally define when a vessel is to be treated as a pressure vessel. A definition of minimum pressure (typically 5.104 N/m2) will exclude low pressure tanks and a minimum of a few liters will exclude piping and piping components. Note that vessels operating at vacuum are often defined as pressure vessels to ensure that the design, construction etc. are of acceptable quality.

For design and construction purposes, the pressure vessel is generally defined as the pressure vessel proper including welded attachments up to, and including, the nozzle flanges, screwed or welded connectors, or the edge to be welded at the first circumferential weld to connecting piping. Figure 1 shows a typical pressure vessel envelope.

Several organizations are involved in the production and operation of a pressure vessel. These can be considered as follows:

  1. The Regulating Authority is the authority in the country of installation that is legally charged with the enforcement of the requirements of law and regulations relating to pressure vessels.

  2. The User operates the plant and thus the pressure vessel. He is responsible to the regulating authority for the continued safe operation of the vessel.

  3. The Purchaser is the organization that buys the finished pressure vessel for its own use or on behalf of the purchaser.

  4. The Manufacturer is the organization that designs, constructs and tests the pressure vessel in accordance with the purchaser's order. Note that the design function may be carried out by the purchaser or by an independant organization.

  5. The Inspecting Authority is the organization that verifies that the pressure vessel has been designed, constructed and tested in accordance with the order and with the standard.

Pressure vessels, as components of a complete plant, are designed to meet requirements specified by a team, typically comprising process engineers, thermodynamicists and mechanical engineers. The full design procedure is described in detail in Bickell and Ruiz (1967) and the interaction between the elements of the procedure is shown in Figure 2.

Operational Requirements

The first step in this design procedure is to set down the operational requirements. These are imposed on the vessel as part of the overall plant and include the following:

  1. Operating pressure. As well as the normal steady operating pressure, the maximum maintained pressure needs to be defined. Regulations and/or standards will define how this maximum pressure is translated into vessel design pressure.

  2. Fluid conditions. Maximum and minimum fluid temperatures will need to be specified and translated into metal design temperatures. Fluid physical and chemical properties will influence material choice and specific gravity will effect support design.

  3. External loads. Loads to be considered include wind, snow, and local loads such as piping reactions and dead weight of equipment supported from the vessel.

  4. Transient conditions. Some vessels may require an assessment of cyclic loads resulting from operational pressure, temperature, structural and accoustic vibration loading.

Functional Requirements

Next the functional requirements, which cover geometrical parameters, are defined. Some of these parameters are again defined by the plant design team whilst some are left to the discretion of the pressure vessel designer. The functional requirements include the following:

  1. Size and shape of the vessel.

  2. Method of vessel support.

  3. Location and size of attachments and nozzles.

Pressure vessel envelope.

Figure 1. Pressure vessel envelope.

Pressure vessel design procedure.

Figure 2. Pressure vessel design procedure.

Materials

Next the main materials are selected. Some national standards list acceptable materials with acceptable temperature ranges and design stresses. Design stresses are set using safety factors applied to material properties, which include:

  1. Yield strength at design temperature.

  2. Ultimate tensile strength at room temperature.

  3. Creep strength at design temperature.

(See Stress in Solid Materials; Fracture of Solid Materials.)

The standard will have selected the materials based upon the above material properties together with knowledge of the following properties that influence fabrication and operation:

  1. Elongation and reduction of area at fracture.

  2. Notch toughness.

  3. Ageing and embrittlement under operating conditions.

  4. Fatigue strength.

  5. Availability.

The range of materials used for pressure vessels is wide and includes, but is not limited to, the following:

  1. Carbon steel (with less than 0.25% carbon).

  2. Carbon manganese steel (giving higher strength than carbon steel).

  3. Low alloy steels.

  4. High alloy steels.

  5. Austenitic stainless steels.

  6. Non-ferrous materials (aluminum, copper, nickel and alloys).

  7. High duty bolting materials.

Clad materials are accepted by national standards but often only the base material thickness can be used in design calculations.

Proprietary materials are used for special applications by agreement between the designer and purchaser although standard bodies will require evidence of previous successful applications before accepting as a material to be listed in the standard. (See Metals; Steels.)

Design Rules

Figure 2 illustrates the overall pressure vessel design procedure. The design rules in standards will give minimum thicknesses or dimensions of a range of pressure vessel components. These thicknesses will ensure integrity of vessel design against the risk of gross plastic deformation, incremental collapse and collapse through buckling. The components covered by the design rules in standards are described in more detail in the Mechanical Design of Heat Exchangers.

The thicknesses determined by the relevant equations are minimal to which should be added various allowances, including allowances for corrosion, erosion, material supply tolerances and any fabrication thinning.

The preliminary thicknesses of components are generally obtained by using the relevant internal or external pressure equations of the standard. These thicknesses are then checked for the other loads that have been identified in the operational requirements.

Where components or loads are not covered by explicit equations in the standard additional analysis may be required and this is by agreement between the designer, purchaser and inspecting authority. An example of the assessment of additional analysis is Appendix A of BS5500 (1994), which identifies the general design criteria to be used in these circumstances. For further reading see Bickell and Ruiz (1967).

Before construction starts, the manufacturer is often required to submit fully dimensioned drawings of the main pressure vessel shell and components for approval by the purchaser and inspecting authority. In addition to showing dimensions and thicknesses, these drawings include the following information:

  1. Design conditions.

  2. Welding procedures to be applied .

  3. Key weld details.

  4. Heat treatment procedures to be applied.

  5. Non-destructive test requirements.

  6. Test pressures.

The manufacturer is generally required to maintain a positive system of identification for the materials used in construction so that all material in the completed pressure vessel can be traced to its origin. The forming of plates into cylinders or dished ends will be either a hot or cold process depending on the material, its thickness and finished dimensions. The standard will define the allowable assembly tolerances and forming tolerances of cylinders and ends. These tolerances limit the stresses resulting from out-of-roundness and joint misalignment. Additional tolerances may be specified by the purchaser to allow, for example, for the insertion of internals. The standards will usually show typical acceptable weld details for seams and attachment of components.

Depending on material and thickness at the weld joint, preheating and post-weld heat treatment may be required. Preheat is applied locally to the weld area but post-weld heat treatment is preferably applied to the complete vessel in an enclosed furnace.

Inspection and Testing

Each pressure vessel is inspected by the inspecting authority during construction. The standard specifies the stages from material reception through to completed vessel at which inspection by this authority is mandatory. The purchaser may require additional inspection, for example, to check internals.

The manufacturer identifies the welding procedures required in the pressure vessel construction, together with test pieces that are representative of the materials and thicknesses used in the actual vessel. The production and testing of these test pieces are generally witnessed by the inspecting authority unless previously authenticated test pieces are available.

Welders have to pass approval tests which are designed to demonstrate their competence to make sound welds similar to those used in the actual vessel. These welder approvals are again authenticated by a recognized inspecting authority.

The national standard defines the level of nondestructive testing that is applied during the construction. This nondestructive testing is usually one or more of the following:

  1. Magnetic particle or dye penetrant (for weld surface flaws).

  2. Radiography (for weld internal flaws).

  3. Ultrasonic (for weld internal flaws).

The degree of nondestructive testing depends upon material and thickness (i.e. upon the difficulty of welding). Some standards use a "joint factor" approach, which allows a reduced amount of nondestructive testing if the designed thickness is increased. This joint factor is chosen and applied at the initial design stage.

Before delivery, most standards require a pressure test which is witnessed by the inspecting authority. Water is the preferred test fluid because of its incompressibility. If air is the only possible test fluid, special precautions have to be taken and consultations are needed with the inspecting authority and other relevant safety authorities. The test pressure is usually between 1.2 and 1.5 times the design pressure and this test pressure is gradually applied in stages and held for an agreed time to demonstrate the adequacy of the vessel.

Once delivered and placed into operation, the user picks up the responsibility for safe service. Legislation will often require inspection at regular intervals during the vessel life and for some critical contents may require the involvement of the regulating authority.

REFERENCES

ASME VIII Division 1, ASME Boiler and Pressure Vessel Code (1993) Rules for the Construction of Pressure Vessels, ASME New York.

BS5500 British Standard for the Specification for Unfired Pressure Vessels (1994) BSI London.

Bickell, M. B. and Ruiz, C. (1967) Pressure Vessel Design and Analysis, Macmillan, London.

References

  1. ASME VIII Division 1, ASME Boiler and Pressure Vessel Code (1993) Rules for the Construction of Pressure Vessels, ASME New York.
  2. BS5500 British Standard for the Specification for Unfired Pressure Vessels (1994) BSI London.
  3. Bickell, M. B. and Ruiz, C. (1967) Pressure Vessel Design and Analysis, Macmillan, London.

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