Pressure Vessels Reinforced Openings
Because stresses around openings are higher than the normal design stresses for the plate thickness, additional material must be provided to carry the additional stress in the shell around the opening. The additional material provided is referred to as reinforcement.
The basic concept of reinforcement of openings is that the cross-sectional area of material removed by an opening must be replaced by adding additional material adjacent to the opening. It is assumed that the material added adjacent to the opening has the same load carrying capabilities as the material removed for the opening.
The two basic requirements for reinforcement are:
1. Enough metal reinforcement must be added to compensate for the weakening effect caused by the opening, while still preserving the general strain pattern in the vessel. Adding an excessive amount of material for reinforcement will create a “hard spot” on the vessel that will not allow its natural deformation under pressure, creating local overstressing.
2. The reinforcing material must be placed immediately adjacent to the opening, but suitably disposed in profile and contour so as not to introduce a stress concentration itself.
The reinforcement is usually provided by a separate welded reinforcing pad, or by extra thickness in the shell and nozzle wall.
It is most common for the reinforcement to be added to the outside of the vessel, as shown in Figure 100-15a. However, on some vessels, the reinforcement is added on the inside, as shown in Figure 100-15b. The best configuration is the “balanced reinforcement,” shown in Figure 100-15c, which consists of about 35% to 40% of the reinforcement on the inside and the remainder on the outside. A balanced reinforcement introduces very little local bending moments and stresses. The stress concentration factor in this case is 20% lower than for outside reinforcement only. It may, however, be difficult to place reinforcement on the inside of a vessel, either because the vessel interior is not accessible or because the reinforcement would interfere with the flow or drainage.
Stress concentration factors can be further reduced by using integrally reinforced nozzles (Figure 100-16) which provide a more gradual transition in thickness between the shell, reinforcement, and nozzle. Equally important is the detailed shape of the integral reinforcement. The use of generous transition radii between the shell and the nozzle minimizes stress concentrations due to discontinuities.
A common practice in vessel design, in excess of Code requirements, is to replace all the metal area removed by an opening. This is a Company practice for the design of new vessels, in order to take full advantage of all the strength of the weld.
When several openings are closely spaced, their arrangement requires special consideration, because their individual effects and reinforcements become overlapping. Keeping the spacing between two openings at no less than the sum of their diameters—measured from their centerlines—will maintain the basic average membrane stress in the vessel wall. If the distance is less than the sum of their diameters, the ASME Code sets special rules for reinforcement of multiple openings.
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