Joint efficiency, welded joints

In Eqs. (5.30) and (5.32), R stands for the inside radius, J i, whereas in Eqs. (5.31) and (5.33) it stands for the outside radius, Ro- In both of the above equations, S is the allowable stress and E is the joint efficiency. This joint efficiency is employed because cylindrical shells are often fabricated by welding. The values of E depend on the t5qje of radiographic examination performed at various welded seams of the shell. 

This subsec tion includes rules governing weided-joint designs and the degree of radiography, with efficiencies for welded joints specified as functions of the quahty of joint. These efficiencies are used in the formulas in Subsec. A for determining vessel thicknesses. 

Moximum Allowable Joint Efficiencies for Arc and Gas Welded Joints... 

E = joint efficiency for welded vessel joint, plate to plate to heads. See ASME Par. UW-12, nominal = 85% = 0.85... 

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... 

The necessary wall thickness for metal vessels is a function of (1) the ultimate tensile strength or the yield point of the metal at the operating temperature, (2) the operating pressure, (3) the diameter of the tank, and (4) the joint or welding efficiencies. Table 4 presents a summary of design equations and data for use in the design of tanks and pressure vessels based on the ASME Boiler and Pressure Vessel Code as specified in Section VIII of Division 1. 

Vessels larger in diameter than about 30 in (0.672 m) and above are fabricated from plates, which are formed into cylinders, called shells, and welded longitudinally. Shells smaller than 30 in (0.672) may be extraded and thus will not contain a longitudinal weld. Shells may then be joined by welding circumferentially to form longer shells. After fabricating the shell, end caps, called heads, are welded to the shell to form the vessel. Because the weld may have imperfections, the radial stress will be less than its maximum value. Thus, S is multiplied by a joint or weld efficiency, s, which depends on the type of x-ray inspection of the weld. Thus,... 

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 ratio of the knuckle to crown radii should not be less than 0.06 to avoid buckling, and the crown radius should not be greater than the diameter of the cylindrical section. Any consistent set of units can be used with equations 13.43 to 13.46. For formed heads (no welds or joints in the head), the joint efficiency E is taken as 1.0. 

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