ASME VIII Div 1 vs Div 2: Cost and Wall Thickness

Last updated: 2026-10-17 · Reviewed by: Senior Metallurgical Engineer, {BRAND}

For a pressure oxidation autoclave at 4.5 MPa and 4.0 m inside diameter in SA-516 Grade 70, Division 1 requires roughly 74 mm of shell where Division 2 requires roughly 50 mm. That 24 mm difference is about 23 tonnes of steel on a 10 m shell, and it usually pays for the extra design-by-analysis work three to five times over. Below about 2 MPa and 2.5 m diameter the arithmetic reverses and Division 1 is cheaper.

Key Takeaways

• The thickness difference comes from allowable stress basis and mandatory joint efficiency, not from different formulas.
• Div 2 typically cuts wall thickness by 25–35% at pressures above 3 MPa on large-diameter shells.
• Div 2 adds 300–600 engineering hours, a user design specification, and heavier non-destructive examination.
• Below roughly 2 MPa and 2.5 m diameter, minimum thickness governs and Div 2 costs more with no benefit.
• Use Div 2 when Div 1 wall exceeds 50 mm, when the duty is cyclic, or when plate availability is the constraint.

What Actually Differs Between the Two Divisions?

The two divisions use the same shell formula. What differs is the allowable stress, the joint efficiency you are permitted to claim, the amount of analysis required to justify the design, and the examination that proves it was built that way. That is why a fair comparison has to be run on a specific vessel rather than on a general preference for “lighter” or “simpler”.

What Is Hydrometallurgy? Definition, Process, Applications

ASME Section VIII Division 1 is a rules-based code that prescribes design formulas, sets allowable stress from a fixed margin on tensile strength, and lets the designer demonstrate compliance by calculation plus prescribed examination. For a cylindrical shell under internal pressure the circumferential thickness is t = P·R / (S·E − 0.6·P), where P is internal design pressure, R is inside radius, S is allowable stress from Section II Part D and E is joint efficiency.

ASME Section VIII Division 2 is a code that combines design-by-rule with design-by-analysis: it keeps rule-based formulas for common geometries, but allows the designer to demonstrate higher load capacity through elastic or elastic-plastic analysis, and it demands a user design specification, a fatigue screening, and full examination of all pressure-retaining welds.

Design margin is the ratio between the material’s specified minimum tensile strength and the allowable stress the code permits, and it is where most of the thickness difference originates. Division 2 permits a smaller margin on tensile strength for many material groups than Division 1, so the same SA-516 Grade 70 plate carries a higher allowable stress at the same temperature — typically 1.3 to 1.45 times the Division 1 value in the temperature range used by hydrometallurgical autoclaves.

Joint efficiency (E) is a multiplier that derates the allowable stress to account for the quality of the welded joint and the extent of radiographic or ultrasonic examination. Division 1 permits the designer to trade examination against efficiency — spot radiography gives E = 0.85, no radiography gives 0.70 on some joint types. Division 2 requires full examination, which is exactly why it can claim E = 1.0 across the shell.

Post-weld heat treatment (PWHT) is a controlled heating cycle that is applied after welding to relax residual stress and temper the heat-affected zone, and in Division 1 it becomes mandatory for P-No. 1 carbon steel above 38 mm thickness under UCS-56. The thickness at which PWHT is triggered often matters more to the cost comparison than the thickness itself, because a full furnace cycle on a 4 m diameter, 20 t vessel is days of shop time and a real schedule item.

That last point is the one most often missed. A Division 1 vessel specified with spot radiography at E = 0.85 can be 15–18% thicker than the same vessel with full radiography at E = 1.0, before the allowable stress difference is counted at all. Comparing “Div 1 with spot RT” against “Div 2” therefore overstates the Div 2 advantage; the honest comparison is Div 1 with full RT against Div 2, and the remaining saving is real but smaller.

How Much Wall Thickness Does Div 2 Actually Save?

Table 1 works the shell calculation for four vessels that appear in hydrometallurgical plants. All four use SA-516 Grade 70, inside diameter as stated, a corrosion allowance of 3 mm, and allowable stresses read from Section II Part D at design temperature — 138 MPa for Division 1 and 202 MPa for Division 2 at 50 °C, and 125 MPa and 183 MPa at 220 °C. The autoclave figures are illustrative of a pressure oxidation autoclave duty.

Table 1 — Shell thickness, Division 1 versus Division 2, 3 mm corrosion allowance

The ordered thickness column is where the money is, because calculated thickness is rounded up to a commercially available plate and then checked against minimum thickness rules. In the smallest case the calculation gives 2.6 mm, but the practical minimum for a shop-fabricated carbon steel shell — 8 mm in most fabricators’ standards, above the code minimum — governs both designs, and Div 2 buys nothing at all.

In the largest case the difference is 24 mm on a shell measuring roughly 125 m² of plate for a 10 m length. At a steel density of 7,850 kg/m³, 24 mm over 125 m² is 23.6 t, which at an indicative USD 1,800/t for pressure-vessel-quality plate is about USD 42,000 of material — before counting the weight saved in supports, the reduced welding volume at a 52 mm rather than 76 mm edge preparation, and the easier plate procurement.

Table 2 puts the other side of the ledger on the same page. The numbers are indicative ranges from fabrication experience, not quotations, and they move with fabricator loading and material grade.

Table 2 — Indicative cost of the Div 2 route versus Div 1, by vessel size

The two tables produce a rule that holds across most equipment in a hydrometallurgical flowsheet: the Div 2 benefit scales with thickness saved times wetted area, while the Div 2 cost is roughly fixed per vessel. That is why the crossover sits at a particular size rather than being a matter of preference.

Where Does the Div 2 Saving Disappear?

Four conditions eliminate the saving, and each occurs often enough to be checked before the ASME VIII Div 1 vs Div 2 decision is written into a purchase specification. They turn up across the leaching train as often as in the autoclave house, because the same few millimetres of allowance that protect a leach tank also flatten the code comparison.

Corrosion allowance dominating the wall. When the alloy needs 6 mm of allowance because the duty is aggressive, the pressure-driven part of the thickness is a small fraction of the total, and a 30% cut in the pressure part becomes a 12% cut in the ordered plate. A mixer-settler feed tank in 316L with a 6 mm allowance in a high-chloride duty is the classic case: Div 2 saves nothing worth the analysis.

Minimum thickness and stiffness governing. Small vessels, tanks under 1 m diameter and low-pressure vessels are sized by handling, support and minimum-thickness rules rather than by pressure. Div 2 then adds engineering cost with no material benefit, as the first row of Table 1 shows.

Cyclic duty that forces a fatigue assessment either way. If the vessel sees more than about 1,000 significant pressure or temperature cycles, the fatigue question has to be answered regardless of division. Div 2 gives a formal route through Part 5.5, but a Div 1 vessel in cyclic service often ends up with the same screening analysis and the same conclusion, so the marginal cost of Div 2 is smaller — while the benefit is also smaller, because the wall is often thickened for fatigue rather than for pressure.

Client or jurisdiction specification. Many owner specifications in copper and gold name Division 1 as the contractual basis, and some export destinations require a CE marking route under the Pressure Equipment Directive with EN 13445 or a recognised equivalent. In those cases the code is not a technical choice at all, and the only decision is how to document the deviation if Div 2 would have been better.

There is a fifth, less technical condition: fabricator competence. A Division 2 vessel demands a fabricator with analysis capability, a written user design specification process and qualified procedures for the thicker sections it commonly handles. If the shortlisted shop has built fifty Div 1 tanks and two Div 2 vessels, the schedule risk of the Div 2 route is real and should be priced.

Which Code Should You Specify for a Hydrometallurgical Vessel?

Table 3 converts the analysis into a decision matrix by pressure and diameter, with conditional conclusions written out rather than left as “it depends”. The matrix assumes carbon or low-alloy steel shells at temperatures up to 250 °C, full radiography available from the fabricator, and a 20-year design life.

Table 3 — Code selection matrix for hydrometallurgical pressure vessels

For a 4.5 MPa, 4.0 m autoclave, Division 2 is preferred because the 24 mm reduction removes 23 t of steel, shortens the weld cycle and brings the plate back inside the rolling range of more than one mill. For a 1.6 MPa, 2.4 m leach tank, Division 1 is preferred because the 4 mm saving is worth about USD 2,600 in plate while the additional analysis costs USD 20,000 or more.

Clause-level detail matters when the two codes are compared side by side, because a single trigger can swamp the material saving. Two dominate: post-weld heat treatment under UCS-56 for P-No. 1 carbon steel above 38 mm, and impact testing under UCS-66 or UHA-51 for the minimum design metal temperature. Dropping from 40 mm to 30 mm crosses the PWHT trigger and can save a full furnace cycle, which on a 4 m diameter vessel is several days of shop time. Dropping from 76 mm to 52 mm crosses nothing, so the saving is material only.

How Do You Run the Comparison Yourself?

The comparison takes about a day with the right inputs, and it should be run before the enquiry package is issued rather than after the first round of quotations.

1. Fix the design conditions. Design pressure, design temperature, minimum design metal temperature, corrosion allowance and the design life that justifies the allowance. Vague inputs produce a comparison that cannot be defended at the design review.
2. Pull allowable stress for both divisions. Use Section II Part D at design temperature for the exact specification, grade and product form. Do not scale from another grade; the ratio between Division 1 and Division 2 allowable stress is not constant across temperatures or material groups.
3. Calculate shell and head thickness for both, with the same corrosion allowance. Use UG-27 for the shell and UG-32 for the head in Division 1, and the equivalent Part 4 rules in Division 2. The full arithmetic is worked in pressure vessel wall thickness calculation.
4. Round both to commercially available plate and apply minimum thickness. This step is where small vessels lose the Div 2 advantage entirely; skipping it produces an unrealistically large saving.
5. Cost the delta in steel, welding, PWHT, NDE and engineering hours. Use local plate price and shop rates; the ratio of engineering cost to steel cost varies by a factor of two between regions and moves the crossover point.
6. Add schedule and procurement risk. Check whether the Div 1 thickness is available from more than one mill within the required window. A 76 mm plate with a single qualified source is a schedule risk that does not appear in the cost table.

Run in that order, the comparison usually produces a clear answer in one direction, and a document that survives a client design review — worth as much as the saving.

What About Fatigue, Minimum Design Metal Temperature and Certification?

Fatigue, low-temperature toughness and certification route are the three criteria that most often override a cost-based code decision.

Fatigue is screened in Division 2 Part 5.5 using the elastic stress or elastic-plastic route, and the screening is mandatory rather than optional. An autoclave depressurised once a month for 20 years accumulates 240 cycles, below the usual screening threshold. A flash vessel that cycles daily accumulates 7,000 cycles and will need a proper assessment, at which point the code choice is largely made for you.

Minimum design metal temperature drives impact testing. A vessel in a Canadian or Kazakh winter, or one that can be depressurised to a low temperature through a Joule-Thomson effect, needs impact-tested steel with a specified Charpy energy. Both divisions require it, but the exemption curves differ, and a Div 1 exemption that would have allowed plain SA-516 Grade 70 may not be available under Div 2 at the same thickness.

Certification is the third override. A Division 2 vessel requires the owner to issue a user design specification, which is a document the purchaser must produce and own — a real obligation, not a formality, and one that many owner teams underestimate. For export to the European Union, the Pressure Equipment Directive route with EN 13445 may be required by the notified body regardless of which ASME division was used for the original design.

None of these three criteria is a cost argument, and any one of them can settle the ASME VIII Div 1 vs Div 2 question before the cost comparison is run. That is why the sequence matters: fix fatigue, minimum design metal temperature and certification route first, then run the arithmetic on whatever freedom is left. Where the vessel carries a corrosion-resistant overlay rather than solid alloy, the examination requirements add a fourth procedural layer, and the failure modes that examination is designed to catch are set out in weld overlay failure in hydrometallurgical vessels.

Case Data: Two Autoclaves, One Duty, Different Codes

Two pressure oxidation autoclaves with the same process duty illustrate the arithmetic and its practical consequences. Both are anonymised, both are in copper-gold concentrate service.

Vessel A — Division 1, 4.2 MPa, 3.6 m ID, 210 °C. Shell ordered at 72 mm, SA-516 Grade 70, full radiography, PWHT at 620 °C for four hours, 11.5 m tangent-to-tangent. Plate weight 68 t. Two mills could roll the plate, one could deliver within 22 weeks. Total fabrication and erection cost indexed at 1.00.

Vessel B — Division 2, 4.2 MPa, 3.6 m ID, 210 °C. Shell ordered at 50 mm, same grade, full radiography plus TOFD on all longitudinal seams, PWHT unchanged, same dimensions. Plate weight 47 t. Four mills could supply. Design engineering ran 520 hours against 165 hours for Vessel A. Total fabrication and erection cost indexed at 0.87 — 13% lower despite the additional engineering, with a 6-week shorter plate lead time that pulled the schedule forward.

The three numbers worth carrying into a specification are 72 mm versus 50 mm at 4.2 MPa and 3.6 m, 13% lower erected cost for the Division 2 route, and 21 t of plate avoided. All three are specific to this duty and this fabricator; the ratio moves with plate price and shop rates, and the numbers should be confirmed by local quotation before use in a business case.

The judgement that comes from having run both: the Div 2 decision is rarely lost on the material saving and rarely won on the engineering cost. It is usually decided by whether the thinner plate can be procured inside the schedule window, and by whether the owner can produce a defensible user design specification without adding three weeks to the design freeze.

FAQ

Is Division 2 always lighter than Division 1 for the same duty?
No. Division 2 can produce a thinner shell where pressure governs, but where minimum thickness, corrosion allowance or stiffness governs, both divisions produce the same ordered plate and Div 2 simply costs more engineering.

Can I mix the two divisions in one plant?
Yes. Each vessel is certified individually, so a plant can have Division 1 tanks and Division 2 autoclaves. What must not be mixed is a design basis within one vessel; a Division 2 shell with Division 1 heads is a documentation and certification problem, not a saving.

Does Division 2 require finite element analysis on every vessel?
No. Part 4 provides rule-based formulas for common geometries, and FEA is only needed where geometry or loading falls outside those rules, or where the designer chooses design-by-analysis to justify a higher load. The mandatory additions are the user design specification, the fatigue screening and the examination requirements.

What is a user design specification and who writes it?
It is the document in which the owner states design pressure, temperature, corrosion allowance, design life, loads and any additional requirements, and it is a Division 2 obligation on the purchaser. It is normally drafted by the owner’s engineer or the EPC with metallurgical input, and it must exist before the fabricator can complete the design.

How much does the code choice change delivery time?
Expect two to four weeks of additional engineering for Division 2 and, in the 50–76 mm range, two to six weeks less plate lead time. The net effect is usually neutral to favourable, but only if the user design specification is ready at enquiry stage.

Can a Division 1 vessel be re-rated to Division 2 later?
Rarely in a way that saves money. Re-rating requires a full design review, a user design specification, updated fatigue screening and often additional examination of existing welds, and the original fabrication may not meet Division 2 examination requirements.

Which code applies to a titanium-clad autoclave?
Division 1 with the UCL rules for clad plate is the common route, because the cladding and bond examination requirements are well established and the base metal thickness is rarely the governing cost. Division 2 is used where the base metal is thick enough that the thickness saving matters.

Conclusion

For hydrometallurgical pressure vessels, the ASME VIII Div 1 vs Div 2 decision is a size-dependent economic question with three hard triggers: 50 mm of Div 1 wall, a shell diameter above 3.5 m at pressures above 3 MPa, and a duty cyclic enough to force a fatigue assessment. Run the thickness and cost comparison on the actual vessel rather than on a rule of thumb, and settle the code before the enquiry goes out.

If you have a vessel in the 40–80 mm wall range and want the comparison run against your duty, send the design conditions through our turnkey plant engineering route and we will return both thickness cases with a cost delta, or start from the pressure vessel material selection guide to fix the alloy first.

References

1. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — UG-27, UG-32, UCS-56, UCS-66, UCL; current edition.
2. ASME Boiler and Pressure Vessel Code, Section VIII, Division 2 — Part 2, Part 4, Part 5.
3. ASME Boiler and Pressure Vessel Code, Section II, Part D — Properties (Metric), Tables 1A and 1B.
4. SME Mineral Processing Handbook — autoclave and pressure vessel practice. smenet.org
5. USGS Mineral Commodity Summaries — copper and gold production context. usgs.gov
6. MetSoc Conference of Metallurgists (COM) proceedings — pressure oxidation design and operation papers.
7. EN 13445 Unfired Pressure Vessels — European alternative basis for CE marking.

Changelog

• 2026-10-17 — First publication. Thickness comparison, cost ledger, decision matrix, two anonymised autoclave cases.

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