12 in NPS x 150 psig Horizontal Separator

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 150 psig at 200 °F.
• Weight of the vessel and its contents at design and at hydrostatic test, distributed across two saddle supports.
• Saddle reactions transmitted into the shell, evaluated by the Zick saddle method per the nonmandatory Appendix G-1.
• Skid-mounted, indoor or sheltered installation. Wind and seismic loadings are screened out for this configuration; they are addressed on larger horizontal separators in companion pages.

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 -20 °F at the full design pressure of 150 psig. Each pressure-boundary component is qualified to the design MDMT by the methods of UCS-66 and UCS-66.1:

• Shell, SA-516 Grade 70 Normalized. Curve D per Table UCS-66. Required MDMT at 0.406 in nominal wall is approximately -55 °F at no impact testing.
• Heads, SA-516 Grade 70 Normalized, 2:1 ellipsoidal. Curve D. Required MDMT at 0.375 in nominal is approximately -55 °F.
• Nozzle necks and flanges, SA-105 forgings and SA-106 Grade B pipe. Curve B. Required MDMT bounded by the head treatment after the UCS-66.1 stress-ratio reduction.

The shell and head govern the published rated MDMT. The vessel is rated to -55 °F at full MAWP. With a stress-ratio reduction from the flange-limited MAWP of 240 psig, the rated MDMT could be moved colder still; the no-credit value of -55 °F 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 horizontal as-shipped orientation:

SA-516 Grade 70 and SA-105 do not derate below 300 °F, so the material stress ratio is 1.00. The B16.5 Group 1.1 Class 150 flange rating is higher at the 70 °F test temperature (285 psig) than at the 200 °F design temperature (240 psig), so the flange ratio is favorable for test and is not the lowest ratio. Using LSR = 1.00 keeps the test at the standard UG-99(b) value.

Pressure by Component

The Class 150 RFWN flanges govern the vessel MAWP at 240 psig at 200 °F. The shell and heads are capable of 923 to 936 psig, so the pressure-retaining capacity sits well above the flange P-T rating. Moving to a Class 300 flange (740 psig at 200 °F) would lift the vessel MAWP toward the shell-and-head limit; for a 150 psig design the Class 150 flange is the obvious selection.

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) by a wide margin. The shell at 0.293 in (corroded, mill-min) sits 0.230 in above the UG-16(b) floor; the heads at 0.282 in (after the typical 10% forming thinning and corrosion allowance) sit 0.220 in above.

Hydrostatic Test Summary

Calculated test stresses on the shell and heads are an order of magnitude below the 0.9 × S_y limit of UG-99(b) Note 35. The flange test pressure of 312 psig is just above the Class 150 cold rating of 285 psig at the 70 °F test temperature; UG-99(b) permits test pressures to exceed the standard P-T rating during the controlled hydrotest as long as the flange-bolt stress stays below 0.9 × S_y, which the standard Class 150 stud-bolt arrangement satisfies for this small flange size.

Material Schedule

Nozzle Schedule

Four of the five openings are below the size limit of UG-36(c)(3)(a) (3.5 in finished diameter on a shell of 0.293 in corroded wall) and are exempt from detailed UG-37 reinforcement calculation. The NPS 4 inlet exceeds that limit (4.151 in finished diameter, corroded) and requires a UG-37 area check, written out as worked example 4 in section 5.

Radiography

The Category A long seam in the shell is examined by full radiography per UW-11(a)(1), giving E = 1.00. The Category A circumferential head-to-shell welds are also fully radiographed. The seamless one-piece formed heads carry E = 1.00 in Table UW-12 without further examination. Nozzle attachment welds are examined by surface NDE (MT or PT) per UW-50.

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

Supports and Loadings

Toughness

Hydrostatic Test

Marking and Reports

Five 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.

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

Required thickness of the SA-516 Grade 70 Normalized cylindrical shell under 150 psig at 200 °F.

Adding corrosion allowance gives t_total = 0.0458 + 0.0625 = 0.108 in required. Mill-minimum supplied is 0.355 in, which leaves 0.293 in available after corrosion. PASS, with 0.247 in margin between t_avail and t_total. The shell is not the governing component.

Shell MAWP is 936 psig, six times the design pressure.

Example 2. Head Thickness, 2:1 Ellipsoidal per UG-32(d)

Required thickness of the SA-516 Grade 70 Normalized 2:1 ellipsoidal head under 150 psig at 200 °F.

Adding corrosion allowance gives t_req = 0.0457 + 0.0625 = 0.108 in. The nominal head thickness ordered is 0.375 in. After the typical 10% forming thinning at the knuckle and the corrosion allowance, the minimum thickness in service is 0.282 in. PASS with 0.174 in margin.

Head MAWP is 923 psig, six times the design pressure.

Example 3. Vessel MAWP Set by Cl 150 Flange Rating, UG-44

The shell and heads are both rated near 925 psig, but the pressure-retaining capacity of the vessel cannot exceed the lowest-rated pressure-boundary component. Per UG-44, ASME B16.5 flanges are accepted at their standard pressure-temperature rating. All five process nozzles use ASME B16.5 Class 150 raised-face weld-neck flanges in Material Group 1.1 (carbon steel SA-105).

The flanges set the vessel MAWP at 240 psig, well below the shell and head capacity. PASS against the 150 psig design pressure. A move to Class 300 flanges (740 psig at 200 °F) would lift the vessel MAWP toward the shell-and-head limit; for a 150 psig design the Class 150 flange is the correct selection and the resulting MAWP of 240 psig is the published vessel rating.

Example 4. NPS 4 Inlet Nozzle Reinforcement, UG-37

The process inlet is a NPS 4 Class 150 RFWN flange on a Sch 80 neck. The finished opening (4.151 in corroded bore) exceeds the UG-36(c)(3)(a) exemption limit of 3.5 in for a shell of 0.293 in corroded wall, so a UG-37 area-replacement calculation is required.

A1 alone (1.026 in²) exceeds A_required (0.193 in²) by 4.3 times. PASS. No reinforcing pad is required. Nozzle wall excess area (A2) and weld leg area (A3) are not needed and are not computed.

For the other four openings (NPS 3, NPS 2, NPS 1.5, NPS 1), the finished diameter is below the 3.5 in exemption limit of UG-36(c)(3)(a), so UG-37 is not required. A check on the largest of those (NPS 3 at 3.07 in finished diameter) would give A_req < 0.15 in² against the same A1 > 0.75 in², so the exemption is conservative.

Example 5. Zick Saddle Analysis, Longitudinal Bending Stress S1

The horizontal vessel is supported on two saddles equally spaced from each end. Saddle reactions are evaluated per the nonmandatory Appendix G-1 using the Zick procedure. Five stresses are checked: S1 longitudinal bending at the saddle and at midspan, S2 tangential shear, S3 circumferential bending at the saddle horn, S4 additional stress in the head where it acts as a stiffener, S5 ring compression in the shell at the saddle horn. S1 at midspan is worked out below; the remaining four are summarized in the table that follows.

The Zick longitudinal bending stress at midspan is 219 psi, two orders of magnitude below the UG-23 allowable stress of 20,000 psi at design temperature. PASS with very large margin.

All five Zick stresses pass with very large margins on this 12 in NPS vessel. For larger horizontal separators (the 36 in OD three-phase separator on a companion page is a good comparison), the Zick stresses climb into the same order of magnitude as the allowables and become the governing design check. On this small unit, the Zick analysis is a formality and the saddle plates are sized for fabrication and shipping convenience rather than for stress.