Vertical Vessels: Combining Structural Loads
Circumferential stresses from internal pressure usually control the design of cylindrical pressure vessels. However, in tall vertical vessels, five other factors may contribute to longitudinal stresses, and must be considered. These are the following:
Environmental loads (never considered to act simultaneously):
• Wind load
• Seismic loads
• Dead weight
• Operating pressure (or vacuum)
The dead weight of the vessel must include all appurtenances such as internal trays and catalyst, as well as external attachments such as platforms. Appendix F contains weight curves to help estimate vessel weights.
The operating pressure used in these calculations should be the Maximum Allowable Working Pressure (MAWP) as defined in Section 420. External pressure (vacuum) rarely controls the design, but it must be reviewed in each case. (Vacuum design is discussed in Section 430.)
The first step to calculate structural loads is to determine which environmental load (wind, seismic or vibration) contributes the greatest stress, and thereby controls the design. Section 445 below provides details on calculating environmental loads.
The greatest environmental stress is then combined with stresses from internal/external pressures. Figure 400-16 illustrates how to combine stresses for the inservice conditions. Note that signs (+ or -) of calculated stress values must be closely monitored. For an example demonstrating how to combine in-service stress conditions, see Figure 400-17.
Determining the shell thickness is an iterative process. The design procedure is as follows:
1. Calculate shell stresses due to internal pressure.
2. Determine minimum shell thickness required for internal pressure.
3. Calculate longitudinal shell stresses due to wind or earthquake, whichever controls, using the minimum shell thickness required for internal pressure.
4. Combine longitudinal shell stresses, including stress from dead weight.
If the combined stresses are greater than the allowable stresses, increase the shell thickness until the combined, calculated shell stresses are less than or equal to those allowable.
The shell stresses should be calculated at the following locations:
• At the point of attachment to the support structure (i.e., skirt, ring girder, etc.)
• At the joint of the skirt to the head
• At the bottom head-to-shell joint
• At all changes of diameter or thickness of the vessel
Pre-service conditions cover fabrication, transportation, erection, and testing. Vertical vessels are typically fabricated in the horizontal position and lifted into the vertical position. Maximum bending stresses must be calculated using erection weights, which may not include internal trays or catalyst. Furthermore, operating pressures are typically zero during this phase of the work. Test conditions usually consist of a hydrostatic test, and all stress conditions must be considered. Hydrostatic testing should also be made the responsibility of the designer by contract. For additional guidance, contact an engineering specialist.
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