Avoid Costly Design Mistakes

Common errors keep plants from getting the most reliable and suitable vessels

By Chip Eskridge, Jacobs, Mike James, DuPont, and Steve Zoller, Enerfab

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4. Head choices. Functionality, not cost, should determine head choice; so understanding the functional differences is crucial. Dished heads for ASME vessels typically are available in three styles; elliptical (2:1), flanged and dished (F&D), and hemispherical (hemi-heads). Under 600 psig, elliptical heads are the most common and least expensive in terms of wall thickness and forming costs. Above 600 psig, hemi-heads are economically attractive due to their inherent low-stress shape; below 600 psig, they are the most expensive choice because they are constructed of welded, segmental parts not a single piece. F&D (torispherical) heads have the lowest profile (height/diameter ratio) and compete well with elliptical heads under 100 psig, although they have half the volume. The low profile of the F&D head only is advantageous when top head accessibility is required for maintaining instruments, agitator, etc., or when space is limited below or, for horizontal vessels, to the sides. For vessels 24 in. or less, off-the-shelf pipe caps (elliptical) provide the most economical design.

Flat heads have very limited use for pressure vessels more than 24 in. in diameter. Because of their flat geometry, they offer far less resistance to pressure than elliptical and F&D heads of the same thickness. Engineers occasionally will specify a flat head, but this practice is uneconomical for pressures above 15 psig–25 psig. If a large diameter flat head is necessary for code equipment, then stiffening the head with structural I-beams is possible but requires sophisticated finite elemental analysis, a skill that not all fabricators possess.

 Click on illustration for a larger image.

5. Jacket choices. As with heads, consider functionality, not cost. Choosing the correct jacket is paramount to achieve process needs. The three common types — conventional, half-pipe and dimple — each offer advantages and disadvantages with respect to process parameters, reliability and cost [1]. Table 1 compares them.

6. Cones. Conical sections (cones) are needed where there’s a change in diameter or as a bottom head, e.g., for a bin or hopper. The rule here is keep the transition angle (referred to as the half apex angle) to 30° or less unless process conditions govern, as exceeding 30° adds costs. The ASME code demands the piece have a rolled knuckle at both ends when the transition is greater than 30°; bending stresses complicate the calculation, putting it beyond the skill of many fabricators.

7. Nozzles loads and projections. The ASME Code [2] requires consideration of all loads. Designers routinely perform wind and seismic calculations but too often overlook nozzle loads due to thermal pipe stress — these can cause visible damage. If attached piping operates at more than 200°F we suggest providing the fabricator with the nozzle loads in Table 2 for a reasonable nozzle stiffening. By providing the fabricator with a reasonable nozzle load, the vessel fabrication and piping design can proceed in parallel and avoid pipe stress/nozzle loading issues months into fabrication.

Also, nozzle projections below the support ring or lugs shouldn’t stick out further than the support bolt circle or structural steel will have to be removed when setting the vessel. This is ill-advised for heavy equipment.

8. Rectangular tanks. It’s not cost effective to specify a rectangular vessel for pressure other than static head; therefore, only consider this configuration for atmospheric tanks. Flat surfaces are highly stressed under pressure (and vacuum) and the required thickness without adding stiffeners can be mind-boggling. An engineer needing a rectangular tank often incorrectly specifies API 650 or ASME. Neither API 650 nor any other API standard exists for rectangular tanks. Appendix 13 of the ASME Pressure Vessel Code provides a methodology but will lead to an expensive over-design. Most fabricators will apply the stress/strain formulas in Roark [3] to design a safe and economical tank that can operate under 15 psig.

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