4 in NPS x 720 psig Low-Temp Vertical Vessel

Design Conditions

Loadings Considered (UG-22)

The vessel is analyzed for the loadings listed in UG-22. Governing loads for this configuration are:

• Internal design pressure of 720 psig at 100 °F, with simultaneous excursion to the design MDMT of -50 °F at full pressure.
• Weight of the vessel and its contents at design and at hydrostatic test. The vessel is small (12 lb empty, 21 lb water-filled), so weight loads are negligible.
• Bolted skid mounting via two welded brackets on the lower cover. Wind, seismic, and external pressure are screened out.

The vessel is non-cyclic per UG-22(e) screening, so an explicit fatigue check is not required.

Toughness Basis

The Minimum Design Metal Temperature is -50 °F at the full design pressure of 720 psig. Each pressure-boundary component is qualified to the design MDMT by one of three pathways under UCS-66, UCS-66(b), UCS-66(g), and UCS-66.1:

• Shell, SA-333 Grade 6 seamless. The SA-333 Gr 6 spec includes mandatory Charpy V-notch impact tests at -50 °F. Per UCS-66(g), materials that meet impact tests at the design MDMT by their material spec are qualified without additional testing. Shell PASS at -50 °F by spec.
• Top and bottom covers, SA-350 Grade LF2 Class 1 forged. The SA-350 LF2 Cl 1 spec also includes mandatory Charpy V-notch impact tests at -50 °F. Covers PASS at -50 °F by spec.
• Nozzle couplings, SA-105. SA-105 is not impact-tested by spec. The couplings are UG-44 standard pressure parts (ASME B16.11 NPS 1 Class 6000), which puts them under the UCS-66(b)(3) exemption for standard parts. As a rigorous secondary check, the UCS-66.1 stress-ratio reduction is applied: the couplings see a hoop stress of 2,767 psi against a 20,000 psi allowable, giving a stress ratio of 0.138. From Fig UCS-66.1 the full 140 °F MDMT reduction credit applies, which lands the coupling-rated MDMT below -135 °F. The couplings PASS at -50 °F by either pathway; the worked walkthrough is in section 5 example 3.

The published rated MDMT is -50 °F at full MAWP. The vessel could be published colder still (the cover and shell are exposed at -50 °F by spec, and the couplings are exposed below -135 °F by UCS-66.1 credit), but -50 °F matches typical sour-adjacent and low-ambient field requirements and is the conservative published rating.

Hydrostatic Test Basis

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:

SA-333 Gr 6, SA-350 LF2 Cl 1, and SA-105 all carry their full allowable stress at the 70 °F test temperature relative to the 100 °F design temperature (no derating in this range), so the LSR is 1.00. The hydrotest is performed with the vessel in the vertical as-shipped orientation, water at 70 °F, vented at the top during fill.

Sour-Adjacent Service (NACE MR0175)

The vessel is qualified for sour-adjacent service per NACE MR0175 / ISO 15156. All wetted carbon-steel components are ordered to MR0175 supplementary requirements:

• SA-333 Gr 6 shell: maximum hardness 22 HRC (or 237 HBW), normalized condition, supplementary S-tempering not required at this thickness.
• SA-350 LF2 Cl 1 forged covers: maximum hardness 22 HRC, normalized and tempered per the LF2 spec.
• SA-105 couplings: standard B16.11 forgings, hardness verified at material receipt against the 22 HRC limit.
• Weld procedures qualified to give HAZ hardness ≤ 250 HV10. Post-weld heat treatment is not required at the cover-to-shell weld thickness (0.531 in) per UCS-56 Table UCS-56-1 exemption for SA-333 Gr 6 at less than 1.50 in thickness.

Pressure by Component

The forged flat covers govern the vessel MAWP at 1,280 psig at 100 °F. The shell carries the design pressure with a margin of seven to one between t_avail and t_req, and the couplings are exempt per UG-36(c)(3)(a) on the basis of opening size (NPS 1 bore at 1.049 in is well below the 3.5 in exemption limit for a 0.465 in corroded shell wall).

Governing Thickness, UG-16(b)

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). The shell at 0.465 in (mill-min) is 0.402 in above the UG-16(b) floor; the covers at 0.500 in are 0.438 in above.

Hydrostatic Test Summary

Calculated test stresses are below the 0.9 × S_y limit of UG-99(b) Note 35 for both controlling components. No bolt or gasket check is required (no bolted joints in this configuration). The couplings carry the same test pressure with very large margin to their B16.11 6,000 psi rating.

Material Schedule

Nozzle Schedule

All three openings are below the size limit of UG-36(c)(3)(a) (3.5 in finished diameter for a shell of 0.465 in corroded wall) and are exempt from detailed UG-37 reinforcement calculation. The verification calculation in section 5 example 5 confirms the area exemption with very large margin.

Radiography

Both Category A cover-to-shell circumferential welds are examined by full radiography per UW-11(a)(1), giving E = 1.00. The seamless shell carries E = 1.00 in Table UW-12 without further examination (no long seam). Nozzle attachment welds are examined by surface NDE (MT or PT) per UW-50. NACE MR0175 hardness checks are performed on the cover-to-shell weld HAZ at material receipt and after welding.

Dimensional Summary

Every clause applied in the analysis, grouped by topic. The substitutions and results for the most-asked clauses are written out in section 5.

Wall Thickness

Nozzle Reinforcement and Standard Fittings

Welds

Low-Temperature Toughness

Sour-Service Materials (NACE)

Hydrostatic Test

Marking and Reports

Five worked examples. Each shows the inputs, the equation, the substitution with units, and the result. The UCS-66.1 stress credit walkthrough in example 3 is the central teaching point of this page.

Example 1. Shell Wall Thickness, UG-27(c)(1)

Required thickness of the SA-333 Grade 6 seamless cylindrical shell under 720 psig at 100 °F.

Required thickness with no corrosion allowance is 0.066 in. Mill-minimum supplied is 0.465 in, which is 0.399 in above the requirement. PASS with a margin of seven to one. The shell is not the governing component.

Shell MAWP is 4,506 psig, six times the design pressure.

Example 2. Forged Flat Cover Thickness, UG-34(c)(2) Sketch (h)

Both ends use a forged flat cover from 0.500 in SA-350 LF2 Class 1 bar stock. The 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. The cover diameter `d` used in the UG-34 formula is the nominal shell inside diameter (this is the convention held across the corpus, see the narrative note at the end of this example).

Required thickness with no corrosion allowance is 0.375 in. Actual cover thickness is 0.500 in. PASS with 0.125 in margin.

The forged flat covers set the vessel MAWP at 1,280 psig (rounded), 1.78 times the design pressure. A thicker cover or a hub-flange bolted closure would raise this number; for a small forged low-temperature vessel the 0.500 in welded cover is the standard build and the margin above design is healthy.

Note on `d` convention: ASME Section VIII Division 1 does not explicitly state whether the cover diameter `d` in UG-34(c)(2) Sketch (h) is the nominal or the corroded inside diameter, and practitioners interpret it both ways. This corpus uses `d` = nominal shell ID. Using the corroded ID (in this case 3.438 in, the same value because the corrosion allowance is zero) would give the same result here. On vessels with non-zero corrosion allowance, the corroded-ID interpretation would lower the cover MAWP by 5 to 10 percent.

Example 3. UCS-66.1 Stress-Ratio MDMT Reduction for the SA-105 Nozzle Couplings

The shell and the flat covers are both qualified to -50 °F by their material specifications (SA-333 Gr 6 includes a mandatory Charpy V-notch test at -50 °F, and SA-350 LF2 Cl 1 likewise). The SA-105 couplings are NOT impact-tested by their spec. Two exemption pathways are available for the couplings, demonstrated below.

Pathway A. UCS-66(b)(3) Standard-Part Exemption

Per UCS-66(b)(3), UG-44 standard pressure parts are exempt from impact testing without further analysis when:

• The part is built to a recognized standard (ASME B16.11 for these threaded couplings).
• The part's standard rating is at or above the vessel design pressure (NPS 1 Cl 6000 = 6,000 psi standard rating, well above the 720 psig design).
• The service is not lethal (confirmed in the design basis).

All three conditions are met, so the couplings are exempt from impact testing under the standard-part rule. PASS by UCS-66(b)(3). This is the shortcut and is sufficient for the certifying engineer.

Pathway B. UCS-66.1 Stress-Ratio Walkthrough (Rigorous)

Independent of the standard-part exemption, the UCS-66.1 stress-ratio reduction is computed as a secondary check. The required MDMT at the coupling body thickness without credit is found first, then the stress ratio is computed, then the credit reduction is applied.

From Figure UCS-66.1, a stress ratio of 0.138 gives the maximum allowable MDMT reduction. The figure caps at a 140 °F reduction for any stress ratio below approximately 0.35. So the rated MDMT for the coupling material is:

The coupling-rated MDMT with UCS-66.1 credit is -132 °F, which is 82 °F colder than the -50 °F design requirement. PASS with very large margin under either Pathway A or Pathway B. The vessel publishes -50 °F as its rated MDMT.

Example 4. Hydrostatic Test, UG-99(b) Note 35

The shop hydrostatic test pressure is computed per UG-99(b), and the resulting stress in each pressure-boundary component is checked against the 0.9 × S_y limit of Note 35.

Both the shell and the covers carry the test pressure with margin to the 0.9 × S_y limit. PASS.

Example 5. Nozzle Reinforcement Exemption, UG-36(c)(3)(a)

All three openings (NPS 1 process inlet on top cover, NPS 1 drain on bottom cover, NPS 1 vent / instrument on shell side) use NPS 1 Class 6000 threaded half-couplings, SA-105 forgings. The finished diameter of each opening is 1.049 in (NPS 1 ID).

All openings qualify under UG-36(c)(3)(a). A formal UG-37 area replacement calculation is not required by code. As a quick verification: A_required for the largest opening (NPS 1 on shell, d = 1.049 in) is 1.049 × 0.066 × 1.00 = 0.069 in², and the area available in the shell A1 = 1.049 × (0.465 - 0.066) = 0.418 in², six times the requirement. PASS.