Pressure Vessel and Storage Tank Design: ASME VIII vs API 650

Pressure vessels and storage tanks both contain fluids, but the codes behind them differ in philosophy, scope and cost drivers. Choosing the wrong one early leads to rework or a non-compliant design. This guide compares ASME Section VIII with API 650 and API 620 and walks through the decisions that shape pressure vessel and storage tank design.

Pressure vessels during fabrication. Photo: W.carter, CC0 via Wikimedia Commons

Which Code Applies

Pressure, geometry and service set the governing code. The owner's specification and local regulations confirm it.

Code	Typical scope	Key characteristic
ASME VIII Div 1	Pressure vessels above 15 psig (about 100 kPa gauge)	Design by rule, with a 3.5 design margin on tensile strength
ASME VIII Div 2	Heavy-wall, high-pressure or cyclic vessels	Design by rule or analysis, with a 2.4 margin, thinner walls and stricter requirements
API 650	Welded atmospheric tanks, up to 2.5 psig (about 17 kPa gauge)	Vertical cylindrical, aboveground storage
API 620	Large low-pressure tanks, up to 15 psig	Covers higher pressures than API 650, including refrigerated service

ASME VIII Div 3 covers very high pressures. The Div 1 versus Div 2 choice is mostly economic. Div 2 allows thinner walls on heavy, high-pressure vessels but adds analysis, material, inspection and documentation effort. Check jurisdictional rules and the owner's specification before fixing the code.

Core Design Inputs

Four inputs drive most of the calculation:

• Design pressure: set above the maximum operating pressure with a margin, including static head and any vacuum or external pressure.
• Design temperature: the metal temperature coincident with design pressure. It sets the allowable stress.
• Corrosion allowance: based on service and expected life, and added to every exposed surface. Carbon steel often falls between 1.5 and 3 mm, but follow the project specification.
• Joint efficiency: for Div 1 butt joints, E = 1.0 with full radiography, 0.85 with spot radiography and 0.70 with none. More NDE costs more but allows thinner plate.

Also record fluid density, hydrotest conditions, cyclic duty and any lethal or special service rules.

Shell and Head Thickness

For a cylindrical shell under internal pressure, the Div 1 circumferential stress formula usually governs: t = PR / (SE − 0.6P). P is design pressure, R is the inside radius in the corroded condition, S is allowable stress and E is joint efficiency. It applies to thin shells (t ≤ R/2 and P ≤ 0.385SE).

Worked example. Take a design pressure of 1.5 MPa, an inside diameter of 2,000 mm, SA-516 Gr 70 with S = 138 MPa, spot radiography (E = 0.85) and a 3 mm corrosion allowance. The corroded radius is R = 1,000 + 3 = 1,003 mm.

t = (1.5 × 1,003) / (138 × 0.85 − 0.6 × 1.5) = 1,504.5 / 116.4 = 12.9 mm

Adding the 3 mm corrosion allowance gives about 15.9 mm, so 16 mm plate is the next standard thickness, subject to mill tolerance and minimum thickness checks. A 2:1 ellipsoidal head under the same conditions uses t = PD / (2SE − 0.2P) and comes out near 12.8 mm before corrosion allowance. The allowable stress here is for illustration; take the value at your design temperature from Section II Part D.

Also check external pressure and vacuum. They often govern on large-diameter, thin-walled shells and may call for stiffening rings.

Nozzle Reinforcement and Local Loads

Every opening removes metal. The Div 1 area replacement rules in UG-37 check that the shell, nozzle neck and any reinforcing pad supply enough material around the opening, and UG-45 sets the minimum neck thickness.

Reinforcement addresses pressure only. Piping loads create local stresses in the shell at the nozzle, which need a separate check. WRC 537 (which updates WRC 107) covers local loads on attachments to cylindrical and spherical shells, and WRC 297 covers nozzles on cylindrical shells. Where loads or geometry fall outside their limits, use finite element analysis. Take nozzle loads from the piping stress analysis rather than assuming them. For API 650 tanks, Annex P gives a method for shell openings subject to external piping loads.

Wind, Seismic and Supports

ASCE 7 or the local code defines wind and seismic loads. How they are carried depends on the equipment:

• Vertical vessels on skirts: check the skirt, base ring and anchor bolts for overturning moment, and check deflection.
• Horizontal vessels on saddles: use Zick's method to check longitudinal bending, shear and local stresses at the saddle. Saddle location and contact angle both affect the result, and one saddle should slide to allow thermal growth.
• Tanks: check wind overturning and shell buckling, which may require top stiffeners or intermediate wind girders. Seismic design follows API 650 Annex E, including liquid sloshing. Anchor the tank if its weight cannot resist overturning.

Atmospheric storage tanks at a refinery in Wyoming. Photo: Tony Webster, CC BY 4.0 via Wikimedia Commons

Storage Tank Shell Design and Foundations

API 650 shell thickness commonly uses the one-foot method for tanks up to 61 m in diameter, evaluating each course at a point 0.3 m above its bottom. Larger tanks, or designs that need more economy, use the variable-design-point method.

The foundation matters as much as the steel. Tanks sit on a ringwall or a compacted pad, and settlement must be controlled. Uniform settlement affects connected piping. Differential settlement distorts the shell and floor and can bind floating roofs. API 650 Annex B gives foundation guidance, and API 653 Annex B provides settlement evaluation. Feed predicted settlement into the nozzle and piping flexibility checks.

Materials and MDMT

Select material by temperature, corrosion and fabrication needs. Carbon steels such as SA-516 Gr 70 are common, low-temperature service calls for fine-grain or impact-tested steels, and stainless or clad materials handle corrosive duty.

The minimum design metal temperature (MDMT) is the lowest metal temperature at the coincident pressure. Consider startup, upset, auto-refrigeration and hydrotest conditions, not only normal operation. Div 1 uses the UCS-66 curves to find impact test exemptions, and allows an MDMT reduction when stress ratios are low. Thickness and post-weld heat treatment also affect toughness. API 650 has its own design metal temperature rules and material groups.

Design Tools

PV Elite and Compress are widely used for pressure vessels, and TANK covers API 650 and API 620 tanks. These tools automate the code equations but do not replace judgement. Check inputs, units, load combinations and warnings. Use finite element analysis for geometry or loads beyond code rules.

Deliverables to Expect

• Calculation report: design basis, loads, thickness, nozzle, support and MDMT checks.
• General arrangement drawings: dimensions, nozzle orientations, supports and weld details.
• Datasheets: design conditions, materials, NDE and testing requirements.
• Nozzle schedule and load summary: sizes, ratings and allowable loads.
• Foundation and anchor bolt loads: for civil and structural design.

How Fluxiss Delivers Pressure Vessel and Tank Design

Fluxiss is a multidisciplinary engineering consultancy, so vessel and tank design sits alongside piping and stress analysis and structural engineering. Nozzle loads, support reactions and foundation loads can move between disciplines without re-keying data. The pressure vessel and storage tank design service covers code selection, calculations, drawings and datasheets for your project.

Frequently Asked Questions

Should I design to ASME VIII Div 1 or Div 2?

Div 1 is simpler and suits most vessels. Div 2 can save material on heavy, high-pressure vessels, but it requires more analysis, stricter materials and more documentation. Compare total cost, including inspection and engineering time.

When do I use API 650 instead of ASME VIII?

API 650 covers welded aboveground storage tanks at near-atmospheric pressure, up to 2.5 psig. API 620 extends this to 15 psig. Above 15 psig, or for non-tank geometries, ASME VIII is the usual route.

Why does joint efficiency matter?

It sets the required shell thickness through the formula. Higher efficiency needs more radiography but allows thinner plate. The right choice balances NDE cost against material, weight and fabrication savings.

What is MDMT and why is it a design input?

MDMT is the lowest metal temperature at which the vessel can carry its design pressure without brittle fracture risk. It decides whether impact testing or a tougher material is needed, so it must account for upset, startup and auto-refrigeration cases.

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Related reading: Piping Design and Stress Analysis: A Practical ASME B31.3 Workflow | Structural Engineering for Industrial Plants: Pipe Racks, Skids and Foundations | Heat Exchangers