This worked example documents the ASME Section VIII Division 1 design of a 12 in NPS vertical pressure vessel built from Schedule 80 pipe, at a design pressure of 600 psig and 200 °F. The vessel uses a B16.9 cap on top, a welded flat cover on the bottom, an NPS 4 Class 300 RFWN process inlet through the shell, and small threaded couplings for drain and vent. MAWP is 675 psig at the Class 300 flange per the B16.5 P-T table at 200 °F, and the shop hydrostatic test pressure is 878 psig.
The analysis follows the 2023 Edition of ASME Section VIII Division 1. Every code clause used in the rating is listed in section 4, and the four governing calculations (shell wall, B16.9 cap, flat cover, nozzle area replacement with UG-44 flange P-T governance) are written out with full substitutions in section 5. The teaching point of this page is that the registered MAWP of 675 psig is set by the standard-fitting flange rating, not by the pressure-boundary calc: the shell rates at 1,030 psig and the 1.5 in cover at 888 psig, both well above the 600 psig design. This page is a generic worked example, not a project record, and should not be used as the basis for a built unit.
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 | Vertical pressure vessel | |
| Design pressure | 600 psig | UG-21 |
| 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 (seamless pipe per Table UW-12) | UW-12 |
| Joint efficiency, cover weld | 1.00 (full RT, Cat. A circumferential) | UW-11, UW-12 |
| Radiography | Full RT on Cat. A welds, surface NDE on nozzle welds | UW-11, UW-51, UW-50 |
| Service | Non-lethal, non-cyclic | UG-22, UW-2 |
The vessel is analyzed for the loadings listed in UG-22. Governing loads for this configuration are:
External pressure, wind, seismic, and cyclic service are not credible for this configuration. The vessel is non-cyclic per UG-22(e) screening, so an explicit fatigue check is not required.
The Minimum Design Metal Temperature is -20 °F at the full design pressure of 600 psig. Each pressure-boundary component is qualified to the design MDMT by the methods of UCS-66 and UCS-66.1:
The bottom cover governs the published rated MDMT. The vessel is rated to -40 °F at full MAWP, which is 20 °F colder than the -20 °F design MDMT requirement.
The shop hydrostatic test follows UG-99(b). Test pressure is 1.3 times MAWP at design temperature multiplied by the lowest stress ratio across pressure-boundary components, with water at 70 °F in the vertical orientation:
The lowest stress ratio is 1.00 because both SA-53 Grade B and SA-516 Grade 70 carry their full allowable stress at 70 °F relative to 200 °F. The Class 300 RFWN flange hydrostatic test rating per ASME B16.5 is 1.5 × 740 = 1,110 psig at room temperature, well above the 878 psig vessel test, so the flange does not constrain the test.
| Component | Material | t_req (in) | t_avail (in) | MAWP (psig) | Clause |
|---|---|---|---|---|---|
| Cylindrical shell | SA-53 Gr B | 0.215 | 0.375 | 1,030 | UG-27(c)(1) |
| Top cap, ASME B16.9 | SA-234 WPB | 0.181 | 0.4375 min | matches pipe (1,030) | UG-44, B16.9 Table 4 |
| Bottom flat cover | SA-516 Gr 70 N | 1.182 | 1.4375 | 888 | UG-34(c)(2) |
| Inlet nozzle, NPS 4 Cl 300 RFWN | SA-105 forging | 0.058 neck | 0.337 neck | 675 (flange P-T) | UG-37, UG-44, B16.5 Table 2-1.1 |
The Class 300 RFWN process inlet sets the vessel MAWP at 675 psig per ASME B16.5 Table 2-1.1 at 200 °F. The pressure boundary itself (shell, cap, cover) rates between 888 and 1,030 psig, all well above the 600 psig design and all above the flange limit. This is a typical case of UG-44 standard-fitting governance: the registered MAWP is capped by the standard pipe-fitting rating, not by the pressure-boundary thickness calc.
Per UG-16(b), the minimum required thickness of any pressure-retaining wall is the larger of the calculated required thickness and 0.0625 in (1/16 in). All components meet UG-16(b) with substantial margin. The shell at 0.375 in (corroded, mill-min) is 0.313 in above the UG-16(b) floor. The cover at 1.4375 in (corroded) is 1.375 in above.
| Component | Calc Stress at 878 psig (psi) | 0.9 × S_y at 70 °F (psi) | Status |
|---|---|---|---|
| Shell, UG-99 Note 35 | 14,580 | 31,500 | PASS |
| Bottom cover | 19,770 | 34,200 | PASS |
Calculated test stresses are well below the 0.9 × S_y limit of UG-99(b) Note 35 for both controlling components. The Class 300 RFWN flange test rating from ASME B16.5 paragraph 2.6 is 1,110 psig at room temperature, well above the 878 psig vessel test, so the flange does not constrain the hydrostatic test.
| Component | Specification | S at 200 °F (psi) | UCS-66 Curve | Form |
|---|---|---|---|---|
| Shell | SA-53 Grade B, seamless | 17,100 | B | NPS 12 Sch 80 pipe |
| Top cap | SA-234 Grade WPB | 20,000 | B | NPS 12 ASME B16.9 cap, wall ≥ pipe nom |
| Bottom cover | SA-516 Grade 70, normalized | 20,000 | D | 1.5 in plate, OD 12.75 in disc |
| Process inlet flange and neck | SA-105 forging | 20,000 | B | NPS 4 Class 300 RFWN per ASME B16.5 |
| Drain half-coupling | SA-105 forging | 20,000 | B | NPS 1 Cl. 6000 per ASME B16.11 |
| Vent half-coupling | SA-105 forging | 20,000 | B | NPS 0.5 Cl. 6000 per ASME B16.11 |
| Inlet flange bolting | SA-193 Gr B7 studs, SA-194 Gr 2H nuts | 25,000 | n/a | 8 studs x 3/4 in dia per B16.5 Table 6 |
| Tag | Function | Size and Class | Location | Type | Weld |
|---|---|---|---|---|---|
| N1 | Process inlet | NPS 4 Class 300 RFWN, SA-105 forging | Shell | Set-on, radial | Full pen butt + fillet per UW-16(c) |
| N2 | Drain | NPS 1 Cl. 6000 threaded half-coupling | Bottom cover | Set-on | Full pen + fillet per UW-16(f) |
| N3 | Vent / instrument | NPS 0.5 Cl. 6000 threaded half-coupling | Top cap | Set-on | Full pen + fillet per UW-16(f) |
The NPS 4 Class 300 RFWN inlet is above the UG-36(c)(3) size exemption and requires the full UG-37 area-replacement calc, worked in section 5. The drain and vent couplings are below the exemption limit of 2.375 in finished diameter for a shell over 0.375 in thick, so detailed reinforcement is not required for them. The cover and cap are thick enough that the small couplings on each attach with no area concern.
The Category A weld between the bottom cover and the shell is examined by full radiography per UW-11(a)(1), giving E = 1.00. The seamless shell pipe carries E = 1.00 in Table UW-12 without further examination. The Category C weld between the top cap and the shell is examined by full radiography (or 100% UT per UW-11(a)(7) at the manufacturer's option). The Category D nozzle attachment welds are examined by surface NDE (MT or PT) per UW-50, with 100% UT on the NPS 4 inlet attachment per the spec on this page.
| Item | Value |
|---|---|
| Shell OD | 12.75 in |
| Shell wall, nominal (Sch 80) | 0.500 in |
| Shell wall, mill-min (-12.5%) | 0.4375 in |
| Shell wall, corroded (mill-min less CA) | 0.375 in |
| Shell tangent-to-tangent length | 60 in |
| Bottom cover thickness | 1.5 in |
| Bottom cover OD | 12.75 in (matches shell OD) |
| Top cap (ASME B16.9 NPS 12 Sch 80) | per B16.9 dimensional table |
| Process inlet flange OD | 10.00 in (NPS 4 Class 300 RFWN per B16.5) |
| Internal volume, new | 6,645 in³ (28.8 US gal) |
| Empty weight, new | 445 lb |
| Empty weight, corroded | 430 lb |
| Test weight, water-filled | 685 lb |
Every clause applied in the analysis, grouped by topic. The substitutions and results for the most-asked clauses are written out in section 5.
| Clause | Component | Use |
|---|---|---|
| UG-16(b) | All | Minimum required thickness floor of 0.0625 in |
| UG-23 | All | Allowable stress tables, S-values at design temperature |
| UG-25 | All | Corrosion allowance, 0.0625 in on internal surfaces |
| UG-27(c)(1) | Cylindrical shell | Required thickness for internal pressure, circumferential stress |
| UG-32(d) | Top cap | 2:1 ellipsoidal-head check on the B16.9 cap, used as verification against B16.9 Table 4 rating |
| UG-34(c)(2) | Bottom cover | Welded flat cover, Figure UG-34 Sketch (h), C = 0.33 |
| Clause | Use |
|---|---|
| UG-36(c)(3)(b) | Size-exemption check: NPS 4 inlet exceeds the 2.375 in finished diameter limit for a shell over 3/8 in thick, so detailed UG-37 calc is required. Drain and vent couplings remain exempt. |
| UG-37 | Area replacement procedure for the NPS 4 inlet, fully worked in section 5 |
| UG-41 | Strength of reinforcement, satisfied by shell-and-nozzle wall area alone (no reinforcing pad required) |
| UG-43(a) | Welded nozzle attachment, full penetration butt weld for the NPS 4 RFWN |
| UG-44 | Flanges and pipe fittings. The Class 300 RFWN sets the vessel MAWP per the B16.5 P-T table at design temperature. ASME B16.9 cap and B16.11 couplings also enter at this clause. |
| UG-45 | Nozzle minimum-thickness check on the inlet neck and on each coupling |
| Clause | Use |
|---|---|
| UW-9 | Design of welded joints, Category A and Category C |
| UW-11(a)(1) | Full radiography on the Category A cover weld |
| Table UW-12 | Joint efficiency E = 1.00 for fully radiographed butt welds and seamless shell |
| UW-13.2 | Welded flat-head attachment, weld sizing for Sketch (h) |
| UW-15 | Strength of attachment welds, nozzle to shell |
| UW-16(c) | Set-on nozzle attachment, full penetration butt weld for NPS 4 inlet |
| UW-16(f) | Attachment-weld details for the threaded couplings (drain, vent) |
| UW-50 | Final NDE on nozzle attachment welds (MT or PT, full surface; 100% UT on the NPS 4 inlet per page spec) |
| Clause | Use |
|---|---|
| UCS-66 | Required MDMT at the actual component thickness, Fig UCS-66 curves |
| UCS-66.1 | Stress-ratio MDMT reduction, Fig UCS-66.1 |
| UCS-66(b) | Exemption rules for fittings and standard products |
| Clause | Use |
|---|---|
| UG-99(b) | Standard hydrostatic test, 1.3 × MAWP × LSR |
| UG-99(b) Note 35 | Stress at test pressure check against 0.9 × S_y |
| B16.5 para 2.6 | Hydrostatic shell test for Class 300 flange, 1,110 psig at room temp, not constraining here |
| Clause | Use |
|---|---|
| UG-116 | Nameplate marking, U stamp |
| UG-118 | Nameplate location and attachment |
| UG-120(c) | Manufacturer's Data Report, Form U-1A |
Four worked examples. Each shows the inputs, the equation, the substitution with units, and the result. Example numbering is independent of the clause numbers used elsewhere on the page.
Required thickness of the SA-53 Grade B cylindrical shell under 600 psig at 200 °F.
| Symbol | Value | Source |
|---|---|---|
| P | 600 psig | Design pressure |
| R | 6.000 in | Inside radius, corroded: ½ × (12.75 - 2 × (0.4375 - 0.0625)) |
| S | 17,100 psi | SA-53 Gr B at 200 °F, ASME II-D Table 1A |
| E | 1.00 | Seamless pipe, Table UW-12 |
Adding corrosion allowance gives t_total = 0.2151 + 0.0625 = 0.278 in required. Mill-minimum supplied is 0.4375 in (Sch 80), which leaves 0.375 in available after corrosion. PASS, with 0.097 in margin between t_avail and t_total.
Shell MAWP is 1,030 psig at the mill-minimum corroded condition. The shell is not the governing component for this vessel.
The top closure is an ASME B16.9 NPS 12 Schedule 80 cap, SA-234 Grade WPB. Per UG-44, ASME B16.9 fittings are accepted at the pressure rating of the matching pipe, which is 1,030 psig at this configuration.
As an independent check, the cap is verified using the UG-32(d) 2:1 ellipsoidal-head formula. The B16.9 cap geometry is closer to a 2:1 ellipsoid than a torispherical, so K = 1.00:
| Symbol | Value | Source |
|---|---|---|
| P | 600 psig | Design pressure |
| D | 12.000 in | Inside diameter, corroded (12.75 - 2 × 0.375) |
| S | 20,000 psi | SA-234 WPB at 200 °F |
| E | 1.00 | Seamless forging |
| K | 1.00 | 2:1 aspect ratio, UG-32(d) |
Adding corrosion allowance gives t_req = 0.1805 + 0.0625 = 0.243 in. The B16.9 cap minimum wall is 0.4375 in (mill-min Sch 80). PASS with 0.195 in margin. The cap is non-governing.
The bottom closure is a welded flat cover from 1.5 in SA-516 Grade 70 Normalized plate. Attachment is a full-penetration weld with a retainer fillet per UW-13.2, matching Figure UG-34 Sketch (h). The attachment factor C = 0.33 from Table UG-34 applies, with no credit taken for shell stiffness:
| Symbol | Value | Source |
|---|---|---|
| P | 600 psig | Design pressure |
| d | 11.875 in | Diameter at the weld (shell ID, mill-min) |
| S | 20,000 psi | SA-516 Gr 70 at 200 °F |
| E | 1.00 | Full RT cover weld |
| C | 0.33 | Figure UG-34 Sketch (h) |
Adding corrosion allowance gives t_req = 1.182 + 0.0625 = 1.244 in. Actual thickness is 1.5 in. PASS with 0.256 in margin.
The bottom flat cover MAWP is 888 psig. With the shell at 1,030 psig and the cover at 888 psig, the pressure boundary itself rates above 600 psig with substantial margin. The governing component is not yet the cover, see example 4.
The process inlet is a NPS 4 Class 300 RFWN forged flange, SA-105, set onto the shell with a full-penetration butt weld. Two checks govern the rating:
Check (a): Per UG-36(c)(3)(b), for a shell over 3/8 in thick the area-exemption limit is 2.375 in finished diameter. The NPS 4 inlet bore of 3.826 in (Sch 80) exceeds this limit, so the full UG-37 area-replacement calc is required.
| Symbol | Value | Source |
|---|---|---|
| d | 3.826 in | Finished opening, NPS 4 Sch 80 ID |
| t_r | 0.2151 in | Shell required thickness from Example 1, F = 1.0 |
| t | 0.375 in | Shell wall, corroded mill-min |
| t_n | 0.337 in | Neck wall, NPS 4 Sch 80 |
| t_rn | 0.0584 in | Nozzle neck required thickness, P R_n / (S_n E_n - 0.6 P) |
| F | 1.00 | Radial opening in cylindrical shell, Fig UG-37 |
| f_r1 | 1.00 | S_n / S_v capped at 1.0 (S_n = 20,000, S_v = 17,100) |
Total area available (A1 + A2) = 0.612 + 0.469 = 1.081 in². Required is 0.823 in². PASS with 31 percent margin. No reinforcing pad required.
Check (b): Per UG-44 and ASME B16.5 Table 2-1.1, the Class 300 RFWN flange in Group 1.1 material has a pressure-temperature rating of 740 psig at 100 °F and 675 psig at 200 °F. The flange P-T rating at design temperature caps the registered MAWP at:
This is the lowest MAWP in the vessel: shell 1,030 psig, cover 888 psig, flange 675 psig. The flange governs. The vessel is registered at MAWP = 675 psig and stamped accordingly. A higher MAWP would require either a Class 600 flange or a lower design temperature.
12 in NPS vertical vessels at 600 psig design pressure are common in upstream gas processing as compact knock-out drums, scrubber pots, sample vessels, and small filter housings on skid-mounted production trains. Building from Sch 80 pipe avoids rolling plate while keeping pressure-boundary cost low. The teaching value of this example is the demonstration of flange-limited MAWP: the shell and cover both rate well above the 600 psig design, but the Class 300 RFWN inlet caps the registered MAWP at 675 psig per the B16.5 P-T table at 200 °F. Moving to Class 600 flanges raises the cap to 1,350 psig at 200 °F and the cover becomes the next governing component.