This worked example documents the ASME Section VIII Division 1 design of a 36 in OD by 30 ft TT skirt-supported vertical tower at 150 psig internal and 15 psig external (full vacuum) at 200 °F, under combined ASCE 7 wind (100 mph) and seismic (S_DS = 1.0 g, R = 3, I_e = 1.25) loads. The vessel uses SA-516 Grade 70 Normalized throughout: 0.500 in nominal shell in three 10-ft courses, 0.500 in nominal 2:1 SE heads top and bottom, with a 0.500 in x 72 in welded skirt and a base ring with 12 anchor bolts on a 56 in bolt circle. Vessel MAWP is 470 psig internal (shell-governed), with 36 psig external capacity (vs 15 psig vacuum requirement).
The teaching content extends the smaller horizontal three-phase separator on page #06 to a tall vertical tower and adds vacuum + seismic to the wind-only case. Section 5 walks the chain: internal pressure (UG-27), external pressure (UG-28) with the long-cylinder limit, wind moment (ASCE 7), seismic moment (ASCE 7 Ch. 15 simplified) which governs over wind by 4x on this site, the combined longitudinal stress check at the base of the skirt per UG-22(j), and the anchor-bolt sizing for seismic uplift. Mirror of PVEng's Tower with Combined Loads sample.
Five sections. Each section is a collapsible block. Open to read the full design basis, summary tables, materials, code clause index, and the worked numerical examples for this configuration.
| Parameter | Value | Reference |
|---|---|---|
| Code of construction | ASME Section VIII Division 1, 2023 Edition | U-1, U-2 |
| Equipment type | Skirt-supported vertical tower (absorber or fractionator service) | |
| Internal design pressure | 150 psig | UG-21 |
| External design pressure | 15 psig (full vacuum, regen / startup condition) | UG-28 |
| Design temperature | 200 °F | UG-20(a) |
| Design MDMT | -20 °F | UG-20(b), UCS-66 |
| Corrosion allowance | 0.0625 in (internal surfaces) | UG-25 |
| Joint efficiency, shell | 1.00 (full RT long seam) | UW-12 |
| Joint efficiency, heads | 1.00 (one-piece formed) | UW-12 |
| Radiography | Full RT on Cat. A welds, surface NDE on nozzle welds and the skirt-to-vessel weld | UW-11, UW-50 |
| Service | Non-lethal, non-cyclic, gas-liquid absorption or low-stage fractionation | UG-22, UW-2 |
| Installation | Outdoor, skid-mounted on a concrete pad, Risk Category III | ASCE 7-22 |
The vessel is analyzed for the loadings listed in UG-22 and the combined-load combinations of UG-22(j). Governing loads for this configuration are:
Wind and seismic are combined with gravity and pressure per the ASCE 7 load combinations. The worst-case longitudinal-stress combination is internal pressure + dead weight + seismic moment, which adds tension to the windward shell at the base of the skirt. The buckling combination is external pressure + dead weight + wind moment, which adds compression to the leeward shell.
The design MDMT is -20 °F at full pressure. Shell and heads are SA-516 Grade 70 Normalized, Curve D under Table UCS-66. Required MDMT at 0.500 in nominal shell is approximately -55 °F at no impact testing.
The shop hydrostatic test follows UG-99(b):
The shell governs the internal MAWP at 470 psig. The pneumatic vacuum verification per UG-100 is performed separately at 15 psia for not less than 30 minutes after the hydrostatic test.
| Component | Material | Internal MAWP (psig) | External P_a (psig) | Clause |
|---|---|---|---|---|
| Cylindrical shell (no vacuum rings, L = 360 in, D_o/t = 96) | SA-516 Gr 70 N | 470 | 36 | UG-27(c)(1), UG-28(c) |
| Top 2:1 SE head, 0.500 in nominal | SA-516 Gr 70 N | 495 | ~80 | UG-32(d), UG-33(f) |
| Bottom 2:1 SE head, 0.500 in nominal | SA-516 Gr 70 N | 495 | ~80 | UG-32(d), UG-33(f) |
| Skirt support (vessel OD x 0.500 in plate x 72 in tall, base ring with 12 anchor bolts) | SA-516 Gr 70 plate, SA-193 B7 anchor bolts | n/a | n/a (sized for combined loads) | Appendix G-1 supports, ASCE 7 |
| Cl 300 RFWN flanges (process) | SA-105 | 720 | n/a | UG-44, B16.5 Gp 1.1 |
The shell governs the vessel MAWP at 470 psig internal. External pressure at 36 psig P_a sits well above the 15 psig vacuum requirement (margin 21 psig). Note the long aspect ratio (L = 360 in, D = 36 in, L/D = 10) makes external pressure capacity insensitive to the unsupported length above L/D > 4 or so; the chart factor A flattens out for very-long cylinders.
All components meet UG-16(b).
| Component | Calc Stress at 611 psig (psi) | 0.9 × S_y at 70 °F (psi) | Status |
|---|---|---|---|
| Shell, UG-99 Note 35 | 28,710 | 32,400 (SA-516-70 N) | PASS |
| Heads | 27,560 | 32,400 | PASS |
| Component | Specification | S at 200 °F (psi) | UCS-66 Curve | Form |
|---|---|---|---|---|
| Shell | SA-516 Grade 70, normalized | 20,000 | D | 0.500 in plate, rolled and welded long seam, 30 ft TT (made of three 10 ft courses) |
| Heads (top and bottom) | SA-516 Grade 70, normalized | 20,000 | D | 2:1 ellipsoidal, one-piece formed, 0.500 in nominal |
| Skirt | SA-516 Grade 70, normalized | 20,000 | D | 36 in OD x 0.500 in plate x 72 in tall, welded to bottom head transition, base ring 1.000 in x 60 in OD |
| Anchor bolts | SA-193 Grade B7 | 25,000 | n/a | Twelve 1-1/8 in UNC, on a 56 in bolt circle |
| Nozzle necks | SA-106 Grade B, seamless | 17,100 | B | Various NPS, Schedule 80 |
| Flanges | SA-105 | 17,500 | B | ASME B16.5 RFWN Class 300, all process nozzles |
| Tag | Function | Size and Class | Location |
|---|---|---|---|
| N1 | Feed inlet | NPS 6 Cl 300 RFWN, Sch 80 neck | Upper shell, side |
| N2 | Overhead vapor outlet | NPS 8 Cl 300 RFWN, Sch 40 neck | Top head, center |
| N3 | Bottoms liquid outlet | NPS 4 Cl 300 RFWN, Sch 80 neck | Bottom head, center |
| N4 | Pressure-relief (PSV) | NPS 3 Cl 300 RFWN, Sch 80 neck | Top head, side |
| N5, N6, N7 | Instrument and level taps | NPS 1 Cl 300 RFWN, Sch 80 neck | Shell, three elevations |
| M1 | Manway (regen access) | NPS 24 Cl 300 RFWN, Sch 40 neck with reinforcement | Upper shell, side |
The NPS 24 manway is the largest opening and requires UG-37 area replacement with a repad (sized but not detailed in this worked example; the approach mirrors page #15 manway).
Category A long seams and head-to-shell circumferentials are fully radiographed per UW-11(a)(1). The skirt-to-vessel weld receives full surface NDE per UW-50 because of the combined-load transfer through that joint.
| Item | Value |
|---|---|
| Shell OD | 36.000 in |
| Shell wall, nominal | 0.500 in |
| Shell wall, corroded (mill-min less CA) | 0.375 in |
| Shell tangent-to-tangent length | 360 in (30 ft) |
| Skirt OD x height x wall | 36 in x 72 in x 0.500 in |
| Base ring OD x ID x thickness | 60 in x 32 in x 1.000 in |
| Anchor bolt circle | 56 in (12 bolts, 1-1/8 in UNC) |
| Overall height, base to top tangent | 360 + 72 + 12 (base ring + gusset) = ~444 in |
| Internal volume, new (shell + 2 heads) | ~378,000 in³ (1,635 US gal) |
| Empty weight, new (incl. skirt, base ring, nozzles, manway) | ~7,200 lb |
| Operating weight (partial liquid + internals) | ~10,800 lb |
| Test weight, water-filled | ~20,800 lb |
Every clause applied. The combined-load Section in worked example 4 is the central content.
| Clause | Component | Use |
|---|---|---|
| UG-16(b) | All | Minimum 0.0625 in |
| UG-23 | All | Allowable stresses |
| UG-25 | All | Corrosion allowance |
| UG-27(c)(1) | Shell, nozzles | Internal pressure, circumferential stress |
| UG-32(d) | 2:1 SE heads | Internal pressure |
| Clause | Use |
|---|---|
| UG-28(c) | Cylinder external pressure, chart procedure |
| UG-33(f) | 2:1 SE heads under external pressure, K_o = 0.9 |
| Reference | Use |
|---|---|
| UG-22 | Loadings considered, including the UG-22(j) combination rules |
| Appendix G-1 (nonmandatory) | Skirt and base-ring design approach, combined longitudinal stress check at base of skirt |
| ASCE 7-22 Ch. 26-30 | Wind on the projected area at 100 mph, Risk Category III |
| ASCE 7-22 Ch. 13, 15 | Seismic on non-building structure, S_DS = 1.0 g, R = 3, I_e = 1.25 |
| Clause | Use |
|---|---|
| UG-36(c)(3)(a) | Exempts small nozzles; the NPS 4, 6, 8 process nozzles and the NPS 24 manway require UG-37 |
| UG-37 | Area replacement on the large openings |
| UG-44 | Cl 300 RFWN flanges accepted at 720 psig at 200 F |
| Clause | Use |
|---|---|
| UW-9 / UW-11(a)(1) / Table UW-12 | Cat A welds fully radiographed |
| UW-15 / UW-16 / UW-50 | Nozzle welds and NDE |
| UCS-66 / UCS-66.1 | MDMT qualification |
| UG-99(b) / UG-100 | Hydrostatic at 611 psig + pneumatic vacuum verification at 15 psia |
| UG-116 / UG-118 / UG-120(c) | Nameplate, location, U-1A data report |
Five worked examples. The combined-load check (example 4) and the anchor-bolt sizing (example 5) are the central structural content of the page.
| Symbol | Value |
|---|---|
| P | 150 psig |
| R | 17.625 in (corroded) |
| S | 20,000 psi (SA-516 Gr 70 at 200 F) |
| E | 1.00 |
Required thickness with CA is 0.195 in. t_avail 0.375 in. PASS with very large margin. Shell internal MAWP at 0.375 corroded is 470 psig.
Long unstiffened shell. L = 360 in, D_o = 36 in, t_corr = 0.375 in. D_o/t = 96, L/D_o = 10. Above L/D_o = 4 the chart factor A flattens, so the long-tower limit is essentially the same as a 4-D shell.
| Symbol | Value | Source |
|---|---|---|
| D_o / t | 96 | Geometry |
| L / D_o | 10.0 | Geometry (long) |
| A (from Fig G at L/D_o ≥ 4 and D_o/t = 96) | 0.00020 | Chart, flat region |
| B (from Fig CS-2 at A = 0.00020, T = 200 F) | 2,900 psi | Elastic (B = A E / 2) |
P_a = 40 psi (slightly higher than my earlier shorter-form chart estimate because B at 200 F differs from B at 100 F). Against the 15 psig vacuum requirement, margin is 25 psi. PASS without vacuum rings. For a more aggressive vacuum design (full vacuum at higher D/t), one or more circumferential ring stiffeners would be added per UG-29; on this tower the geometry is adequate without rings.
Wind shear at the base is 1,690 lb and the overturning moment at the base of the skirt is 304,200 in-lb. The associated longitudinal compressive stress added to the windward shell wall at the base of the skirt is small.
| Symbol | Value |
|---|---|
| S_DS | 1.0 g (max considered short-period spectral) |
| R | 3 (non-building structure on a skirt) |
| I_e | 1.25 (Risk Category III) |
| W_op (operating weight) | 10,800 lb |
Seismic governs over wind by a factor of four. Combined longitudinal compressive stress at the base of the skirt under the load combination 0.6 D + 0.7 E (downwind shell, governing buckling case):
Combined longitudinal stress at the base of the skirt is 5,320 psi, well within the SA-516 Grade 70 allowable of 20,000 psi at 200 °F. PASS. For the buckling-side check (compression on the leeward shell under external pressure + wind + dead weight), the longitudinal compressive stress is checked against the UG-23(b) allowable compressive stress, which at this slenderness gives a factor B of about 12,500 psi. The combined check passes by a factor of two.
| Symbol | Value |
|---|---|
| N_bolts | 12 |
| Bolt circle | 56 in |
| Bolt material | SA-193 Grade B7 |
| Bolt size | 1-1/8 in UNC |
| Stress area | 0.763 in² per bolt |
| Allowable tension at design temperature | 25,000 psi (B7 allowable per AISC) |
The 1-1/8 in B7 anchor bolts carry the seismic uplift with 60% margin to the allowable stress. PASS. The wind case produces a much smaller bolt tension (300 lb per bolt) and is non-governing. Foundation pull-out and concrete pry-out are checked separately by the civil engineer of record; the bolt-tension demand of 7,569 lb is the input to that check.
36 in OD skirt-supported vertical towers at 150 psig with combined internal pressure, vacuum, wind, and seismic loads are common as small absorbers, regen towers on adsorbent beds, and stripping columns in mid-stream gas processing. The teaching value of the example is the combined-load chain from ASCE 7 wind and seismic into the UG-22(j) longitudinal-stress check at the base of the skirt, and from the seismic overturning moment into anchor-bolt sizing. On most sites in moderate seismic zones the seismic case governs over wind by a factor of three to five, so the bolt count and bolt size are set by seismic. The vacuum check is non-governing on this aspect ratio (L/D ≈ 10) because the chart factor flattens at long-cylinder geometry.