This worked example documents the ASME Section VIII Division 1 design of a 66 in OD horizontal firetube treater vessel at 110 psig and 450 °F, with a 14 in x 28 in obround firetube manway in the shell. The vessel is SA-516 Grade 70 Normalized at 0.500 in nominal wall, two 2:1 SE heads of the same material, supported on two saddles at 120 ° wrap. MAWP is 185 psig at 450 °F (flange-limited by the Class 150 RFWN process flanges).
The teaching content is the UG-37 area-replacement calculation for the obround firetube manway. The 14 x 28 in opening sits at the 2:1 long-to-short ratio limit of UG-36(c)(2), the upper bound for treating non-round openings under standard UG-37 rules without an Appendix-style special analysis. The area calc is run in both the hoop direction (d = 14 in, F = 1.0) and the longitudinal direction (d = 28 in, F = 0.5). The hoop direction governs and requires a repad; a 0.500 in x 4.000 in plate repad welded full-perimeter around the obround opening provides 4.00 in² of additional reinforcement in the hoop direction, closing the 0.146 in² deficit with 2.4 times the required area.
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 | Horizontal firetube treater (indirect-fired vessel with removable U-shaped firetube) | |
| Design pressure | 110 psig | UG-21 |
| Design temperature | 450 °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 including the obround manway perimeter | UW-11, UW-50 |
| Service | Non-lethal, non-cyclic. Operating temperature varies with firetube cycling, but the design point envelopes peak operating. | UG-22, UW-2 |
The vessel is analyzed for the loadings listed in UG-22. Governing loads for this configuration are:
The vessel is non-cyclic by UG-22(e) screening despite firetube cycling, because the operating pressure cycles below 50% of design and the thermal gradient zone is bounded by the manway repad. An explicit fatigue check is not required.
The Minimum Design Metal Temperature is -20 °F at the full design pressure of 110 psig. The shell and heads are SA-516 Grade 70 Normalized, Curve D under Table UCS-66. Required MDMT at the shell thickness (0.500 in nominal) is approximately -45 °F at no impact testing.
The shop hydrostatic test follows UG-99(b):
SA-516 Grade 70 at 70 °F test temperature has the same allowable stress as at the 450 °F design temperature for ratio purposes; the LSR is 1.00 for the material because the derating from S = 18,800 at 450 °F vs 20,000 at ≤ 300 °F only matters above 300 °F. The flange P-T rating ratio (Cl 150 at 70 °F = 285 psig vs at 450 °F = 185 psig) is favorable for test.
| Component | Material | t_req (in) | t_avail (in) | MAWP (psig) | Clause |
|---|---|---|---|---|---|
| Cylindrical shell | SA-516 Gr 70 N | 0.192 | 0.375 | 215 | UG-27(c)(1) |
| 2:1 ellipsoidal heads (each) | SA-516 Gr 70 N | 0.191 | 0.388 | 223 | UG-32(d) |
| 14 x 28 in obround firetube manway | SA-105 hub forging, 0.750 in wall on neck, 0.500 x 4 in repad | UG-37 PASS with repad | 0.750 | matches shell | UG-36(c)(2), UG-37 obround |
| Cl 150 RFWN flanges (process) | SA-105 | standard | standard | 185 | UG-44, B16.5 Gp 1.1 at 450 F |
The Class 150 process flanges govern the vessel MAWP at 185 psig at 450 °F (between the table values of 200 psig at 400 F and 170 psig at 500 F). Shell and heads are well above the flange rating. The 14 x 28 in obround manway is the design challenge of the page.
Per UG-16(b), the minimum required thickness is 0.0625 in. All components meet UG-16(b) by a wide margin.
| Component | Calc Stress at 240 psig (psi) | 0.9 × S_y at 70 °F (psi) | Status |
|---|---|---|---|
| Shell, UG-99 Note 35 | 20,880 | 32,400 (SA-516-70 N) | PASS |
| Heads | 20,160 | 32,400 | PASS |
| Obround manway hub flange (FEA-verified in production) | varies, bounded by 18,000 psi peak | 32,400 | PASS |
| Component | Specification | S at 450 °F (psi) | UCS-66 Curve | Form |
|---|---|---|---|---|
| Shell | SA-516 Grade 70, normalized | 18,800 | D | 0.500 in plate, rolled and welded long seam |
| Heads (2 ea) | SA-516 Grade 70, normalized | 18,800 | D | 2:1 ellipsoidal, one-piece formed, 0.500 in nominal |
| Obround manway neck | SA-105 hub forging | 17,500 | B | 14 in x 28 in oblong forging, 0.750 in wall on the neck, 2.500 in flange face |
| Obround manway repad | SA-516 Grade 70, normalized | 18,800 | D | 0.500 in x 4.000 in wide, rolled to match the obround perimeter, full-circumference fillet |
| Process nozzle necks | SA-106 Grade B, seamless | 15,900 | B | NPS 4 / 3 / 2 Sch 80 pipe |
| Process flanges | SA-105 | 17,500 | B | ASME B16.5 RFWN Class 150, all process nozzles |
| Saddles | SA-36 plate | n/a | n/a | 120 ° wrap, 10 in saddle width, two skid-mounted |
| Tag | Function | Size and Class | Location | Type | Weld |
|---|---|---|---|---|---|
| M1 | Firetube manway (removable U-tube firetube) | 14 in x 28 in obround, custom SA-105 hub flange with 20 studs on 19 in bolt circle (short dim) | Shell, side wall, long axis longitudinal | Set-in obround, with 0.500 in x 4.000 in repad full perimeter | Full pen + fillet, surface NDE per UW-50 at every quadrant |
| N1 | Process inlet | NPS 4 Cl 150 RFWN | Top head | Set-in radial, no repad (UG-37 PASS without repad) | Full pen + fillet per UW-16(f) |
| N2 | Treated-oil outlet | NPS 3 Cl 150 RFWN | Bottom head (downstream of weir) | Set-in radial, exempt UG-36(c)(3)(a) | Full pen + fillet per UW-16(f) |
| N3 | Water draw-off | NPS 3 Cl 150 RFWN | Shell, bottom, midspan | Set-in radial, exempt UG-36(c)(3)(a) | Full pen + fillet per UW-16(f) |
| N4 | Gas outlet to stack | NPS 4 Cl 150 RFWN | Shell, top (downstream of weir) | Set-in radial, no repad | Full pen + fillet per UW-16(f) |
| N5 | Pressure relief (PSV) | NPS 2 Cl 150 RFWN | Shell, top | Set-in radial, exempt UG-36(c)(3)(a) | Full pen + fillet per UW-16(f) |
The 14 in x 28 in obround firetube manway is the largest opening on the vessel and is the design challenge of the page. The UG-37 area calc for the obround is in section 5 example 3.
Category A welds (long seam and head-to-shell circumferentials) are fully radiographed per UW-11(a)(1). The obround manway perimeter weld is examined by surface NDE at every quadrant per UW-50. Process nozzle welds are surface NDE per UW-50.
| Item | Value |
|---|---|
| Shell OD | 66.000 in |
| Shell wall, nominal | 0.500 in |
| Shell wall, mill-min (-12.5%) | 0.438 in |
| Shell wall, corroded (mill-min less CA) | 0.375 in |
| Shell tangent-to-tangent length | 240 in (20 ft) |
| Head nominal thickness, 2:1 SE | 0.500 in |
| Obround manway, short axis (height) | 14 in |
| Obround manway, long axis (length, parallel to vessel axis) | 28 in |
| Obround ratio (long / short) | 2.00 (at the UG-36(c)(2) limit) |
| Obround manway flange face OD (short direction) | 19.250 in |
| Saddle wrap angle | 120 ° |
| Saddle width | 10 in |
| Internal volume, new (shell + 2 heads) | ~948,000 in³ (4,105 US gal) |
| Empty weight, new | ~7,800 lb (incl. obround forging and repad) |
| Test weight, water-filled | ~42,600 lb |
Every clause applied, grouped by topic. The obround nozzle calc is the central content of this page.
| Clause | Component | Use |
|---|---|---|
| UG-16(b) | All | Minimum 0.0625 in |
| UG-23 | All | Allowable stresses at design temperature (SA-516 Gr 70 derates to 18,800 at 450 F) |
| UG-25 | All | Corrosion allowance 0.0625 in |
| UG-27(c)(1) | Cylindrical shell | Internal pressure, circumferential stress |
| UG-32(d) | 2:1 SE heads | Internal pressure |
| Clause | Use |
|---|---|
| UG-36(c)(2) | Permits obround openings with the long-to-short ratio not exceeding 2:1 to be designed under standard UG-37 rules (the obround here is at the 2:1 limit) |
| UG-36(c)(3)(a) | Exempts the NPS 3, NPS 2 process openings from UG-37 |
| UG-37 | Area replacement procedure, run on the 14 x 28 in obround in worked example 3 and verified on the NPS 4 inlet and gas outlet |
| UG-41 | Strength of reinforcement, repad-to-shell weld sized per UG-41 |
| UG-44 | Cl 150 RFWN flanges at the B16.5 Gp 1.1 P-T rating (185 psig at 450 F) |
| UG-45 | Nozzle minimum-thickness check on each neck |
| Clause | Use |
|---|---|
| UW-9 / UW-11(a)(1) / Table UW-12 | Cat. A and Cat. C weld design, full RT on Cat. A, E = 1.00 |
| UW-15 / UW-16 / UW-50 | Attachment weld design and NDE; the obround manway perimeter weld receives surface NDE at every quadrant |
| Clause | Use |
|---|---|
| UG-22 / Appendix G-1 | Loadings and Zick saddle analysis |
| UCS-66 / UCS-66.1 | MDMT qualification |
| UG-99(b) / Note 35 | Hydrotest at 1.3 × 185 = 240 psig, stress under 0.9 S_y |
| UG-116 / UG-118 / UG-120(c) | Nameplate, location, U-1A data report |
Five worked examples. The obround manway UG-37 area calc in example 3 is the central content. Examples 1 and 2 are routine pressure-containment. Example 4 is the NPS 4 inlet check (also UG-37 because of size). Example 5 is the hydrotest summary.
| Symbol | Value | Source |
|---|---|---|
| P | 110 psig | Design pressure |
| R | 32.625 in | Inside radius, corroded: (66 - 2 × 0.375) / 2 |
| S | 18,800 psi | SA-516 Gr 70 at 450 °F, ASME II-D Table 1A |
| E | 1.00 | Full RT long seam, Table UW-12 |
Required thickness with CA is 0.254 in. Mill-minimum supplied is 0.438 in (0.500 nominal less 12.5%), corroded to 0.375 in. PASS with 0.121 in margin. Shell MAWP at corroded thickness 0.375 in is 215 psig.
| Symbol | Value | Source |
|---|---|---|
| P | 110 psig | Design pressure |
| D | 65.250 in | Inside diameter at the head skirt (shell ID, corroded) |
| S | 18,800 psi | SA-516 Gr 70 at 450 °F |
| E | 1.00 | One-piece formed head |
| K | 1.00 | 2:1 ellipsoidal aspect ratio |
Required thickness with CA is 0.254 in. Head supplied at 0.500 nominal, formed-corroded min 0.388 in. PASS with 0.134 in margin. Head MAWP at 0.388 in formed-corroded is 223 psig.
The 14 in x 28 in obround opening in the shell is the design challenge. The obround ratio is 28 / 14 = 2.00, at the upper limit of UG-36(c)(2) for treating obround openings under standard UG-37 rules. Two area calcs are performed: one in the hoop (circumferential) direction using the short axis as `d`, and one in the longitudinal direction using the long axis as `d` with F = 0.5. Both must pass.
| Symbol | Value | Source |
|---|---|---|
| d_short (hoop direction) | 14 in | Short axis, perpendicular to vessel axis |
| d_long (longitudinal direction) | 28 in | Long axis, parallel to vessel axis |
| t_r | 0.192 in | Shell required thickness from Example 1, F = 1.0 |
| t | 0.375 in | Shell wall, corroded mill-min |
| t_n | 0.750 in | Obround manway neck wall (custom SA-105 forging) |
| F (hoop direction) | 1.00 | Radial section, full circumferential stress |
| F (longitudinal direction) | 0.50 | Longitudinal section, Fig UG-37 chart at θ = 90 ° |
| f_r1 | 0.931 | S_n / S_v = 17,500 / 18,800 (SA-105 in SA-516-70 N shell at 450 F) |
Hoop-direction A1 = 2.562 in² against A_required = 2.708 in². Shell wall alone is insufficient by 0.146 in². A repad is required.
Longitudinal-direction A1 = 7.812 in² against A_required = 2.698 in². PASS in the longitudinal direction without repad. The hoop direction governs.
| Symbol | Value | Source |
|---|---|---|
| Repad thickness, t_p | 0.500 in | 1/2 in plate, SA-516 Gr 70 N |
| Repad width per side, w_p | 4.000 in | 4 in beyond the opening edge |
| Repad effective length in hoop direction | 2 × w_p = 8 in (each side beyond the d_short) |
Total area available with repad in the hoop direction:
Margin = 3.854 in², or 142% of A_required. PASS. The 0.500 in x 4 in repad provides 2.4 times the required reinforcement in the hoop direction, with the longitudinal direction already passing without repad. The repad also serves as a thermal-mass buffer at the firetube manway, reducing the through-thickness gradient during firetube cycling.
The NPS 4 process inlet on the top head exceeds the UG-36(c)(3)(a) 3.5 in finished-diameter exemption limit. A UG-37 area calc is run.
| Symbol | Value |
|---|---|
| d (NPS 4 ID corroded) | 4.151 in |
| t_r (head, from example 2) | 0.191 in |
| t (head, corroded formed) | 0.388 in |
| t_n (NPS 4 Sch 80 wall corroded) | 0.232 in |
| F | 1.00 (radial in 2:1 SE head) |
PASS with 73% margin. No repad required on the NPS 4 inlet.
| Component | Stress at test (psi) | 0.9 × S_y at 70 °F (psi) | Status |
|---|---|---|---|
| Shell (hoop) | 240 × 32.625 / 0.375 = 20,880 | 32,400 (SA-516-70 N) | PASS |
| Head (membrane) | 240 × 65.250 / (2 × 0.388) = 20,180 | 32,400 | PASS |
| Obround manway hub flange | ~17,500 (FEA-bounded in production) | 32,400 | PASS |
All components carry the test pressure with margin to the 0.9 × S_y limit. The obround manway flange is the highest-stressed component and is verified by FEA in production to bound the local peak below 0.9 S_y; for the code calc the UG-37 area replacement is the controlling rule.
66 in horizontal firetube treaters are common in upstream oil and gas as heated free-water knockouts and emulsion treaters, where a removable U-shaped firetube provides the heat to break the oil-water emulsion. The teaching value of the example is the UG-37 area-replacement calc applied to the firetube obround manway. Obround openings up to 2:1 long-to-short ratio are permitted under standard UG-37 rules per UG-36(c)(2), with separate hoop-direction and longitudinal-direction checks. The hoop direction governs and a repad is required to close the area deficit; a 0.5 in x 4 in plate repad welded full perimeter is the standard solution.