Concept of Reinforcing Openings

When an opening is made in a pressure vessel shell or head, the ability of the nearby wall to retain pressure is significantly reduced. Reinforcing pressure vessel openings maintains the pressure retaining capabilities of the shell by the addition of wall thickness near the opening.

The basic rule of the Code is that the wall section around the opening of the vessel must be reinforced with an area of metal equal to the area of metal removed to create the vessel opening. The replaced area of material is called the opening or nozzle reinforcement. The reinforcement may be incorporated into the vessel wall, the nozzle wall, or an attached pad surrounding the nozzle.

The simple rule, however, needs further amplification, as follows:

1. The Code says it is not necessary to replace the removed amount of metal, but the amount of wall thickness required to resist the internal pressure. This required thickness at the openings is usually less than at other points of the shell or head, because of corrosion allowance and nominal size plates yielding extra thickness. In computing the allowable pressure for an existing vessel, most engineers follow the practice of using the lesser wall thickness at the openings, but when designing new vessels, openings can often be reinforced to full as-built shell thickness so vessels may be used up to the limits of their strength whenever required. In the design of new vessels, there will be no appreciable extra cost if the openings are reinforced for the full thickness of the new plate.

2. The plate actually used and nozzle neck are usually thicker than would be required according to calculation. According to the Code, the excess plate in the vessel wall (A1) and nozzle wall (A2) serves as reinforcements. (See Figure 400-7.) Likewise, the inside extension of the opening (A3) and the area of the weld metal (A4) can also be taken as credit for reinforcement. The recommended practice is to assume that A is equal to zero; i.e., no excess in vessel wall is credited to reinforcement. Instead, the excess should be assigned to corrosion allowance.

3. The reinforcement must be within certain dimensional limits.

4. The area of reinforcement must be proportionally increased if its stress value is lower than that of the vessel wall.

5. The area requirement for reinforcement must be satisfied for all planes through the center of opening and normal to the vessel surface.

Figure 400-7 presents the equations and terminology for determining nozzle reinforcement.

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