Two additional pressure vessel design criteria are required before vessel component thicknesses can be calculated (by the vendor) and in turn evaluated for acceptability (by Saudi Aramco engineers). These additional design criteria are:
• Weld joint efficiency • Corrosion allowance
Weld Joint Efficiency
Weld joint efficiency (E) is used to account for the quality of a welded joint and for the concentration of local stress. Determination of a stress concentration factor takes into consideration the fact that the stress in a localized region of a component or structure may be higher than would be calculated if normal static analysis were used. This higher local stress is due to local material or structural discontinuities. Stress concentration in welded joints arises from the following factors: • The geometry of the weld itself.
• The metallurgical structure of the weld with respect to the base metal.
• Weld defects, such as slag inclusions, shrinkage cracks, or porosity.
The last two factors are functions of the procedure that is used to make the weld. The first factor is the main source of stress concentration and can be controlled by the vessel design engineer. The net effect of the three stress concentration factors is to reduce the fatigue strength or efficiency of the weld. Weld joint efficiency must be considered in the vessel design.
Paragraph UW-12 of the ASME Code, Section VIII, Division 1, specifies the weld joint efficiencies to be used in the formulas for pressure vessel component thicknesses. These efficiencies depend on the type of weld joint design that is used and on the degree of weld radiographic examination that is made. Types of welded joints and weld inspection methods will be discussed in MEX 202.04. For our purposes here, it is only necessary to know that the efficiency of a weld joint is determined by the type of weld joint that is used and by the extent of its radiographic inspection. The ASME Code also defines weld joint categories based on the location of a joint in a vessel. The Code then specifies, by example, the weld joint designs that may be used in each category. MEX 202.04 discusses weld joint types and inspection further.
32-AMSS-004 requires 100% joint efficiency for all hydrogen, wet sour, lethal, cyclic and unfined steam drum services. 100° weld efficiency is also required for hydrocarbon, steam, amine and caustic services above 120°C (250°F).
Figure 26 in Work Aid 2A is excerpted from the ASME Code, Section VIII, Division 1, and identifies pressure vessel weld joint categories.
Figure 28 in Work Aid 2A is also excerpted from the ASME Code and defines weld joint efficiencies based on the type of weld (shown in Figure 27 of Work Aid 2A) and degree of radiographic examination. Figure 28 shows that the weld joint efficiency decreases as the degree of radiography decreases for a given type of weld joint. Note from Figure 28 that the direction of weaker weld joint designs is vertically downward and that lower weld joint efficiencies correspond with weaker weld joint designs.
The majority of pressure vessels use a Type 1 joint design. A Type 1 joint design has a weld joint efficiency of either 0.85 or 1.00; these values correspond with either spot or full radiographic examination. Later discussion of the ASME Code calculation formulas will show that the required shell and head thicknesses increase with decreasing weld joint efficiency. Work Aid 2A summarizes how to evaluate the acceptability of the specified weld joint efficiency. The degree of radiography and corresponding joint efficiency are specified on the Pressure Vessel Design Data Sheet.
Corrosion Allowance
Corrosion was discussed in COE 103 and COE 105. Corrosion, erosion, or abrasion causes the components of a pressure vessel to thin during the operating life of the vessel. In order to compensate for this thinning, components must have their thicknesses increased over those that are calculated based on the ASME Code design formulas. Internal corrosion/erosion- resistant linings are sometimes used as an alternative to the use of greater component thicknesses.
Process design and materials engineers typically specify the corrosion allowance for pressure vessel components. These allowances are based on determinations of the expected corrosion rate for the vessel material in the anticipated process environment. The expected corrosion rate is multiplied by the design life of the vessel (normally 20 years) to determine the corrosion allowance that is to be used in the vessel design. The expected corrosion rate is a major factor that influences material selection. A high corrosion rate for a material in a particular service requires a large corrosion allowance. This larger corrosion allowance requires thicker components and increases the cost of the pressure vessel. It is often possible to use a higher-alloy material that has a lower corrosion rate and corrosion allowance in the same service. In many cases, the greater cost per pound of the higher-alloy material is offset by the ability to use thinner components, and therefore, less material. Saudi Aramco minimum corrosion allowance requirements for carbon steel were discussed in MEX 202.02. The required corrosion allowance must be specified on the Pressure Vessel Design Data Sheet to permit the vendor to determine the required component thicknesses.
Work Aid 2B summarizes how to evaluate the acceptability of the specified corrosion allowance.
EVALUATING CONTRACTOR-SPECIFIED DESIGN CALCULATIONS FOR