EN 13445 Unfired Pressure Vessels: Complete Guide to Parts 1, 2, 3 and 5

EN 13445 is the harmonised European standard for unfired pressure vessels — the primary design and fabrication reference for process vessels, heat exchangers, reactors, separators, and accumulators intended for the European market. It is the standard your PED conformity assessment is built on. With 619 chunks across Parts 1, 2, 3 and 5 in the Leide Navigator, this guide covers the complete design chain: from vessel categorisation and material selection through design calculations, weld joint quality, and final inspection requirements.

1. Scope and PED Alignment

EN 13445 applies to unfired pressure vessels — vessels not subject to direct flame exposure — made primarily from steel, with design pressure PS > 0.5 bar gauge. It provides the technical means to demonstrate conformity with the Pressure Equipment Directive (PED) 2014/68/EU, which governs vessels placed on the EU market.

Key scope boundaries:

• IN scope: Fixed pressure vessels ≥ PS 0.5 barg, operating temperatures from cryogenic to ≥ 500°C, carbon and alloy steels, austenitic stainless and duplex grades covered by EN 10028
• OUT of scope: Fired vessels, boilers, transportable pressure equipment (covered by TPED), pipelines (EN 13480), offshore pressure vessels under DNV class where the DNV rules govern (though EN 13445 is often used by reference)
• ASME relationship: EN 13445 uses design-by-formula (Part 3) with allowable stress at 2/3 R_e or 1/2.4 R_m; ASME VIII Div.1 uses 1/3.5 R_m — the different safety margins mean direct code substitution without recalculation is not acceptable

2. Part 1 — General: Vessel Categories and Essential Requirements

Part 1 establishes the overall framework and maps to the PED's risk-based vessel classification. The PED categorises pressure equipment into Categories I through IV based on fluid group (Group 1 = dangerous; Group 2 = other) and the product of maximum allowable pressure PS × volume V (for vessels) or PS × nominal size DN (for piping).

Part 1 also defines the minimum qualification requirements for personnel (welders per EN ISO 9606, NDE operators per EN ISO 9712), the documentation structure (Manufacturer's Design Report, MDR), and the required traceability of materials throughout fabrication.

3. Part 2 — Materials: Grouping, Impact Testing, and Elevated Temperature

Part 2 governs material selection and qualification. It does not specify which steel to use — that is in the EN 10028 product standards — but it defines how materials are grouped, what impact test evidence is required, and how properties at elevated temperature are used in design.

Material grouping

EN 13445-2 adopts the ISO/TR 15608 grouping scheme (Groups 1–11), which controls welding procedure qualification. A welding procedure qualification (WPQ) for a higher group covers lower groups within the same group range. Key groups for pressure vessel work:

Impact testing requirements

Part 2 §4.4 defines the minimum design temperature (MDT) and requires Charpy impact testing at or below that temperature. The acceptance criterion is 27 J minimum average (minimum individual 20 J) for most ferritic grades. For vessels operating below −10°C, the impact sub-grade must match: JR (0°C), J0 (−20°C), J2 (−40°C), K2 (−40°C, higher energy).

Properties at elevated temperature

When the design temperature exceeds ~300°C for carbon steel or ~400°C for austenitic stainless, the allowable stress must use elevated-temperature values. Part 2 Annex B provides yield strength R_p0.2/T and creep rupture strength R_u/T for standard grades. The design allowable is the lower of:

For austenitic stainless at temperatures above 500°C, the creep term controls — and the long-time creep rupture values (Annex B) diverge significantly from short-time tensile properties. Using room-temperature yield strength for a high-temperature austenitic vessel is a critical error.

4. Part 3 — Design: DBF vs DBA, Stress Categories, and Fatigue

Part 3 is the engineering core of EN 13445 — it contains the design-by-formula (DBF) rules for shells, heads, nozzles, and flanges, the design-by-analysis (DBA) methods for complex geometry, and the fatigue assessment approach. It is the largest part of the standard by far. Two separate deep-dive articles cover specific aspects:

• EN 13445-3: Design by Formula — shells, heads, nozzles, flanges
• EN 13445-3: DBF vs DBA — when to use each method

This section summarises the key concepts across Part 3.

Design by formula (DBF) — shell wall thickness

The fundamental shell wall thickness formula (§7.4.2 for cylindrical shells under internal pressure):

Design by analysis (DBA) — stress categories

When geometry is too complex for DBF formulas (branch intersections, non-standard head shapes, supports with combined loading), DBA per EN 13445-3 Annex B uses linear elastic stress analysis categorised into:

Fatigue assessment (Part 3 §18)

EN 13445-3 §18 provides a fatigue assessment method for vessels subjected to cyclic pressure, thermal cycling, or mechanical loading. The method uses fatigue curves (S-N curves) categorised by weld detail class, with cycles to failure N_A as a function of equivalent stress range Δσ_eq:

The standard provides separate fatigue curves for unwelded material, butt welds, and fillet/attachment welds. The weld curves have a characteristic knee point at ~2×10⁶ cycles and a slope change. For vessels with low cycle counts but high stress ranges (pressure cycling > 5% of design pressure over hundreds of cycles), Part 3 §18 fatigue screening must confirm that simplified assessment criteria are met before omitting the full analysis.

5. Weld Joint Coefficients and NDT in Part 3

Part 3 §8 defines weld joint coefficients z that directly affect the calculated wall thickness. The coefficient reflects the quality of weld examination — a lower z means less NDT, requiring thicker walls to compensate:

The choice of z=0.85 vs z=1.0 must be consistent with the inspection group selected in Part 5. In practice: inspection group a → z=1.0; groups b/c → z=0.85 is permissible (but check Part 5 §6.6 for details of extent required at each group).

6. Part 5 — Inspection: Groups, NDT Extent, and Pressure Test

Part 5 governs manufacturing inspection and testing — the activities that verify the vessel meets the design. It defines inspection groups, NDT extent by joint type and group, and the pressure test requirements.

Inspection groups

Part 5 §6 assigns each vessel to an inspection group (a, b, or c) based on the material group (from Part 2), design temperature, and fluid hazard. The inspection group controls the minimum NDT extent for each weld category:

NDT methods and acceptance criteria

Part 5 §7 specifies the NDT methods acceptable for each weld type and requires acceptance criteria per the relevant NDT product standard:

• Radiographic testing (RT): EN ISO 17636-1/2 — acceptance per EN ISO 10675-1 or as specified in Part 5
• Ultrasonic testing (UT): EN ISO 17640 + TOFD per EN ISO 10863 — acceptance per EN ISO 11666
• Magnetic particle testing (MT): EN ISO 17638 — surface and near-surface indications
• Penetrant testing (PT): EN ISO 3452 — surface-breaking indications for non-magnetic materials (austenitic, duplex)

Pressure test requirements

Part 5 §10 requires a hydrostatic pressure test before the vessel enters service. The test pressure is:

Where a hydrostatic test would damage the vessel (e.g. cryogenic liners, ceramic-lined vessels), a pneumatic test at 1.1 × PS is permitted per Part 5 §10.2.4 — subject to additional precautions (pressure relief valve, personnel exclusion zone, risk assessment). Pneumatic tests carry higher stored energy than hydrostatic and require NB witness for Cat III/IV.

7. Cross-Standard Reference Map

8. Common Engineering Errors Across All Parts

Design and material selection errors

• Selecting allowable stress at design temperature from room-temperature data — for carbon steel above ~300°C and austenitic stainless above ~400°C, the elevated-temperature properties (R_p0.2/T, R_u/T) from Part 2 Annex B are lower and control the design; ignoring this produces non-conservative wall thicknesses
• Using z = 1.0 in the DBF calculation without committing to group a NDT in the Inspection and Test Plan — the weld joint coefficient must match the actual NDT that will be performed; specifying z = 1.0 and then carrying out only spot RT (group b) is non-conforming
• Impact testing not repeated for parent material if a repair weld results in local heat treatment beyond the original PWHT temperature — Part 2 §4.4.6 requires retesting when PWHT parameters change material toughness

Inspection and documentation errors

• MDR (Manufacturer's Design Report) issued without referencing the Notified Body's type examination certificate for Cat III/IV vessels — the MDR must reference the NB reference number; a CE-marked vessel without this traceability is non-compliant regardless of the technical content of the calculations
• Pressure test performed with the vessel partially assembled (insulation, internals) so inspectors cannot access the welds during the hold — Part 5 §10.2 requires visual inspection of all accessible welds during the pressure hold; hidden welds must be confirmed accessible before test or inspected before closure
• Pneumatic test used for cost savings without the required risk assessment and exclusion zone — Part 5 §10.2.4 permits pneumatic testing only where hydrostatic is damaging or impractical; using it to avoid draining a water-sensitive vessel is not a valid justification without documented risk assessment and NB concurrence