1. Scope and document structure
EN 13445 is the European harmonised standard for unfired pressure vessels — pressure-containing equipment that is not directly heated by combustion. It covers vessels used in process plants, offshore topsides, HVAC systems, cryogenic equipment, and hydraulic accumulators operating under gauge pressure above 0.5 bar.
The standard is published in six parts:
| Part | Title | Chunks in KB |
|---|---|---|
| EN 13445-1 | General — definitions, scope, referenced standards | 36 |
| EN 13445-2 | Materials — permitted materials, traceability, certificates | 81 |
| EN 13445-3 | Design — formulas, stress analysis, fatigue, creep | 459 |
| EN 13445-4 | Fabrication — welding, forming, heat treatment | — |
| EN 13445-5 | Inspection and testing — NDE, hydrostatic, pneumatic | 43 |
| EN 13445-6 | Requirements for design and fabrication of pressure vessels from spheroidal graphite cast iron | — |
Part 3 (Design) is by far the densest — 459 chunks against 36–81 for the surrounding parts. It contains all the thickness formulas, stress analysis methodology, fatigue assessment rules, and special design cases. This article focuses on Part 3, with cross-references to Parts 2 and 5 where relevant.
2. Equipment Directive categories I–IV
The PED classifies pressure equipment into four risk categories (I = lowest risk, IV = highest). The category determines which conformity assessment route is required and the level of third-party involvement in design and production verification.
Category assignment depends on three parameters: fluid group (Group 1 = hazardous fluids including flammable/explosive/toxic; Group 2 = other including steam and compressed air), design pressure PS, and volume V. These parameters are mapped through classification tables in the Directive. The boundaries are roughly:
| Category | Typical scope | Conformity route | Notified Body involvement |
|---|---|---|---|
| Cat I | Low pressure, small volume, Group 2 fluids | Module A — internal production control | None required |
| Cat II | Moderate pressure/volume, Group 2; low PS×V for Group 1 | Module A2, D1, or E1 | Quality system audit |
| Cat III | High pressure/volume, Group 2; moderate for Group 1 | Module B+D, B+F, B+E, G, or H | Type examination + QA or individual verification |
| Cat IV | Highest risk — large Group 1 vessels above defined thresholds | Module B+D, B+F, G, or H1 | Full design review + enhanced QA or individual verification |
For offshore applications, the relevant categories are typically III and IV for process vessels containing hydrocarbons or steam. Category determination must be done before choosing a conformity module — the design documentation, weld examination scope, and final test requirements all depend on it.
3. Design by Formula vs Design by Analysis
EN 13445-3 provides two primary design routes:
Design by Formula (DBF) — §5 and §7–§16
DBF uses closed-form analytical expressions for standard geometry (cylindrical shells, spherical shells, hemispherical/ellipsoidal/flat heads, conical sections, nozzles, flanges). The designer selects the applicable formula for the component geometry, inputs material properties and dimensions, and computes the required minimum wall thickness.
DBF is the default route for standard pressure vessel components. It is computationally straightforward, well-auditable, and accepted by all Notified Bodies. The limitations are that it applies only to geometries covered by the standard and does not capture stress concentrations in detail.
Design by Analysis (DBA) — §17–§18
DBA uses numerical stress analysis (typically FEA) or direct stress analysis methods to demonstrate that a vessel design meets the safety objectives where the geometry deviates from standard cases or where the DBF would give overly conservative results. DBA is also the route for vessels in the creep regime or under complex combined loading.
EN 13445-3 §17 (Direct Route DBA) classifies loads and failure modes:
- Gross plastic deformation (GPD) — static failure; controlled by design action effects vs plastic limit load
- Progressive plastic deformation (PD) — ratcheting under cyclic loading
- Instability (I) — buckling under compression or external pressure
- Fatigue (F) — cyclic stress failure
- Static equilibrium (SE) — overturning, sliding
For most offshore process vessels, DBF is used for shell and head design and DBA is reserved for nozzle junction zones, flat heads with irregular penetrations, and saddle support regions where stress concentrations exceed what DBF can address.
4. Nominal design stress f
All DBF thickness formulas in EN 13445-3 are expressed in terms of the nominal design stress f — the material strength property reduced by safety factors appropriate to the design temperature and material type.
Safety factor derivation
For ferritic steels (the most common vessel material) at temperatures below the creep regime, the nominal design stress is:
The safety factors C_r = 2.4 and C_s = 1.5 are defined in EN 13445-3 §6.1. They represent the margin between design stress and material failure — the proof strength factor 1.5 guards against gross yielding at operating conditions; the tensile strength factor 2.4 guards against rupture at ultimate load.
Temperature dependence
At elevated design temperatures, R_p0.2,T decreases. The governing safety factor (C_s / R_p0.2,T) then typically controls, and f decreases accordingly. EN 13445-2 (Materials) provides the minimum elevated-temperature proof strength data for permitted materials — referenced during the f calculation. For austenitic stainless steels (EN 10028-7), an additional factor of 1.2 applies to the proof strength path, reflecting the higher ductility and strain-hardening capacity of austenitic grades.
f vs allowable stress — terminology note
The EN 13445 nominal design stress f is conceptually equivalent to "allowable stress" in ASME VIII Div.1 but is derived differently. ASME uses min(S_y/3, S_u/3.5) for carbon steels at room temperature; EN 13445 uses min(R_p0.2/1.5, R_m/2.4). These produce different absolute values for the same steel. Designs produced under one system cannot simply be re-stamped for the other without recalculation.
5. Weld joint coefficients z
The weld joint coefficient z (also called weld efficiency) appears in every shell thickness formula in EN 13445-3. It reduces the effective strength of the shell at welded joints to account for the fact that welds may contain defects not detected by examination — with more extensive NDE, a higher z is justified because the probability of undetected defects is lower.
EN 13445-3 Table 5.6-1 defines three z values:
| z value | Joint testing group | NDE requirement | Volume examined |
|---|---|---|---|
| z = 1.0 | Testing Group 1 | 100% volumetric (RT or UT) on all butt welds | All seams |
| z = 0.85 | Testing Group 2 | Spot volumetric examination (10–25% of weld length) | Selected joints |
| z = 0.7 | Testing Group 3 | Visual examination only — no volumetric NDE required | Surface only |
The testing group selection is linked to the PED category. Category III and IV vessels are generally required to use Testing Group 1 (z = 1.0) for longitudinal and circumferential seams. For Category I–II vessels, lower testing groups (and hence lower z) are permitted.
How z affects required thickness
Consider a cylindrical shell under internal pressure with P = 25 bar, D_i = 800 mm, f = 133 MPa, and three z values. The minimum wall thickness (excluding corrosion allowance) varies as follows:
| z | e_min (mm) | Relative thickness increase vs z=1.0 | NDE cost implication |
|---|---|---|---|
| 1.0 | 7.6 | — | Full 100% RT/UT — higher inspection cost |
| 0.85 | 8.9 | +17% | Spot examination — moderate |
| 0.7 | 10.8 | +43% | Visual only — lowest inspection cost |
The penalty for not performing volumetric NDE is significant — 43% more material for the same design pressure. For large offshore process vessels where plate weight is a direct cost driver, using z = 1.0 and performing full radiography is typically more economical than the z = 0.7 wall thickness increase, in addition to the PED category obligation.
6. Shell and head thickness formulas
Cylindrical shell under internal pressure (§7.1)
This is the workhorse formula of EN 13445-3. It applies when the wall is thin relative to the radius (e/D_i ≤ 0.16). For thicker walls — pressure vessels with e/D_i > 0.16 — the thick-wall formula in §7.1.4 applies.
Spherical shell under internal pressure (§7.2)
Dished ends — hemispherical (§7.2), ellipsoidal (§7.3.1), torispherical (§7.3.2)
The three common head types have different efficiency factors:
- Hemispherical heads: Same formula as spherical shell — most efficient, thinnest head, highest fabrication cost
- Semi-ellipsoidal heads (2:1 ratio): Required thickness approximately equal to cylindrical shell — standard choice for most vessels
- Torispherical (Klopper-Geerlings) heads: Thickness governed by crown radius and knuckle radius; may require greater thickness than semi-ellipsoidal in some configurations due to knuckle yielding; cheaper to form but heavier
For all dished ends, EN 13445-3 §7.3 also requires checking the knuckle zone for plastic instability (external pressure on the knuckle in suction service, or internal pressure with a thin knuckle relative to the crown radius).
Flat ends (§7.4)
Flat ends are the least efficient head geometry — they resist pressure in bending rather than membrane tension. Required thickness is proportional to diameter squared:
For large flat ends (flanged covers, blind flanges), the required thickness quickly becomes impractical. Most large-diameter pressure-containing closures use dished heads or bolted flanged ends rather than flat plates.
7. Nozzle and opening reinforcement (§9)
Every nozzle (pipe connection, manway, instrument tap) penetrates the shell and locally reduces its load-carrying cross-section. EN 13445-3 §9 requires that the removed material be compensated by reinforcement — either integral (thickened shell or nozzle neck) or pad reinforcement welded around the opening.
Area replacement method
The fundamental check is that the cross-sectional area of reinforcement available within an influence zone around the opening is at least equal to the area of shell removed by the opening:
The influence zone width is typically 2.5 × √(D_i × e_s) on each side of the nozzle centreline. Reinforcement outside this zone does not contribute.
Set-on vs set-through nozzles
Set-in (set-through) nozzles, where the pipe penetrates the shell wall and is welded from both sides, contribute more to reinforcement than set-on nozzles attached to the shell surface only. The weld geometry also affects the required NDE — a full-penetration butt weld is required for nozzles on Category III/IV vessels, with volumetric examination.
Nozzle limitation
EN 13445-3 §9 is limited to isolated openings. When two nozzle influence zones overlap — closely spaced nozzles or clusters — the analysis becomes more complex and often requires DBA rather than the simple area-replacement check.
8. Common misuses and calculation errors
1. Using z = 1.0 without 100% volumetric examination
The most frequent non-conformity encountered in pressure vessel documentation audits. The designer selects z = 1.0 in the thickness calculation, resulting in a thinner wall; the fabrication specification then requires only spot or visual NDE. This is structurally unsafe — the design assumes full volumetric assurance of weld quality. Confirmation of the testing group used must appear in the fabrication specification and be traceable in the inspection records.
2. Omitting or understating the corrosion allowance c
EN 13445-3 requires the designer to add a corrosion allowance c to the calculated e_s to obtain the nominal ordered thickness. The standard does not prescribe c — it is determined by the designer based on process fluid, expected service life, and material selection. On offshore process equipment in sour service (H₂S-containing), corrosion allowances of 3–6 mm are typical for carbon steel vessels; austenitic stainless (from EN 10028-7) may require zero allowance but must then meet HISC and pitting resistance requirements.
3. Confusing design pressure with maximum allowable working pressure (MAWP)
The design pressure PS used in thickness formulas is the maximum pressure the vessel is designed to contain, including any pressure surge allowances. MAWP is the maximum pressure the as-built vessel can withstand, determined from the actual ordered thickness minus corrosion allowance minus manufacturing tolerance. MAWP is always ≥ PS; if MAWP = PS, the vessel has no thickness margin. For offshore vessels subject to wear or inspection interval constraints, maintaining some MAWP margin is often a project-specific requirement.
4. Applying EN 13445-3 to fired vessels or boilers
EN 13445 explicitly excludes fired pressure vessels (boilers, superheaters, reheaters) — these are covered by EN 12952 and EN 12953. Offshore heat-recovery steam generators (HRSGs), fired heaters, and waste-heat boilers fall outside EN 13445 scope. Applying EN 13445-3 formulas to a fired vessel is a misuse of the standard.
5. Treating EN 13445 and ASME VIII as interchangeable
Safety factors, material allowable stress basis, weld efficiency factors, hydrostatic test pressure multipliers, and NDE scope requirements all differ between EN 13445 and ASME VIII Div.1/Div.2. A vessel designed to ASME VIII cannot be CE-marked as complying with EN 13445 without recalculation and documentation rework — and vice versa.
9. Cross-reference: EN 13445-5 testing requirements
EN 13445-5 (Inspection and Testing) is the operational counterpart to Part 3 — it defines how the vessel design is validated before commissioning. The link between Parts 3 and 5 is bidirectional: Part 5 test requirements depend on the testing group selected in Part 3; and Part 3 allows higher z values precisely because Part 5 specifies more stringent examination for higher testing groups.
Hydrostatic pressure test
The standard hydrostatic test pressure (PT) is:
The hydrostatic test is performed at ambient temperature with the vessel filled with water (or other suitable liquid). The vessel is held at PT for a minimum period (typically 30 minutes) while inspected for leaks and deformation.
Pneumatic test
A pneumatic test with compressed gas at 1.1 × PS is permitted only when a hydrostatic test is impractical — for example, when the vessel cannot be supported under the water load or when residual water would damage the service process. Pneumatic testing carries significantly higher risk due to stored energy, and EN 13445-5 requires additional safety precautions and a lower maximum test pressure (1.1 × PS rather than 1.25 × PS).
NDE scope — Testing Group 1 vs Group 3
For Testing Group 1 vessels (z = 1.0), EN 13445-5 requires:
- 100% volumetric examination (RT or UT) of all longitudinal and circumferential butt welds
- 100% surface examination (MT or PT) of all butt welds and fillet welds of nozzles
- Visual examination of all welds
For Testing Group 3 (z = 0.7), visual examination is the primary requirement for weld quality, with no mandatory volumetric NDE — the thicker wall provides the structural compensation.
10. Using Leide Navigator for EN 13445 queries
All four EN 13445 parts that are currently ingested — Part 1 (36 chunks), Part 2 (81 chunks), Part 3 (459 chunks), and Part 5 (43 chunks) — are available in the Leide Navigator knowledge base. EN 13445-3 is one of the most extensively chunked standards in the corpus, covering shell and head design, DBA methodology, fatigue assessment, creep analysis, and flanged connections in detail.
Related ingested standards for offshore pressure vessel work include EN 10028-7 (pressure equipment austenitic and duplex steels, 45 chunks) and EN 10204 (material inspection certificate types 3.1 and 3.2, 3 chunks).
Ask Leide Navigator EN 13445 questions
Query clause-level questions about shell thickness formulas, weld joint coefficient selection, DBA failure mode assessment, or nozzle reinforcement rules — all covered in the 459-chunk EN 13445-3 corpus. EN 10028-7 (pressure equipment materials) and EN 13445-1, -2, and -5 are also available.