Welded-joint efficiencies

Allowable tensile stresses are one-fourth the ultimate tensile strength of the material of construction. Values at different temperatures are given in Table 18.4 for some steels of which shells and heads are made. Welded joint efficiencies vary from 100% for double-welded butt joints that are fully radiographed to 60% for... 

Table 6.1 Weld Joint Efficiencies for Ellipsoidal and Torispherical Heads (Source Adapted from Ref. 10). 

When vessels are designed to other international standards, the standard should always be consulted to determine the rules for joint efficiency, as different codes treat it in different ways. For example, welded-joint efficiency factors are not used, as such, in the design equations given in BS PD 5500 instead, limitations are placed on... 

If we allow for the welded-joint efficiency, E, this becomes... 

The structure of this formula can quickly be related to the thin-walled pressure vessel cylinder equation. Using the equation that calculates the stress at the center of the vessel wall, ux = P R + 0.5t)/t, and rearranging to solve for the thickness, results m. t = PR/ ux — 0.5P. The addition of the weld joint efficiency, E, and changing the coefficient before P to 0.6 results in the ASME code formula, t = PR/ SE — 0.6P), which they feel best represents the minimum wall thickness required to contain an internal pressure, P, in a cylindrical vessel having a radius, R, and made of a material with an allowable stress, S. 

The ASME code formula for the thickness of a cylindrical shell is listed in UG-27, as t = PR/(SE — 0.6P)S In this formula, t is the minimum thickness of the shell (in.), P is the maximum allowable working pressure (MAWP) (psi), R is the internal radius of the vessel (in.), S is the allowable stress in the material listed in ASME Section II, and E is the weld joint efficiency. 

Vessels can be designed with thinner walls when full radiography is performed to give a weld joint efficiency of 1.0. Conversely, the cost of radiographic examination can be traded for the increased cost and weight of a thicker vessel having a weld joint efficiency of 0.7. Such trade-offs need to be considered early in the design process. Welds that fail the examination must be reworked and retested. 

Y. Weld joint efficiency (E) accounts for the proven quality of a welded joint. The longitudinal weld seam often found in... 

The equations in the ASME Boiler and Pressure Vessel Code are based on equating the maximum membrane stress to the allowable stress corrected for weld joint efficiency. The allowable stress, S, is replaced by the term SE (to be explained later). In ASME Boiler and Pressure Vessel Code, Section VIII Division 1, the Eqs. (5.27) and (5.29) are modified as ... 

The required nozzle thickness (tm-) is given by (using Eq. (5.15) with weld joint efficiency E = 1 or 100 percent and S = Sm) ... 

Setting yt. = 0.3 and E = weld joint efficiency within the flat plate,... 

The value of t, obtained from any of the methods given above is used only to determine the required area of reinforcement. The value of tr used to set the minimum required thickness of the shell or head is based on the thickness formulas that consider aU the design loadings and weld joint efficiencies. 

Example 16.9. The vessel given in Example 16.3 is to be constructed from SA-516 Grade 60 material and designed for 475 psi at 650°F. The weld joint efficiency is E = 1.0. What are the total longitudinal stresses on both the windward and leeward sides at the support line What are the allowable tensile and compressive stresses ... 

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