PED, BS PD 5500 and EN 13445 —
Pressure Vessel Design
Explained

The regulatory and technical framework surrounding pressure vessel design in the UK involves at least three overlapping layers — the regulatory framework (what legal requirements apply), the design code (how the vessel is designed and calculated), and the conformity assessment route (how compliance is demonstrated and documented). Confusion between these three layers is common, and it has practical consequences: vessels built without meeting regulatory requirements cannot lawfully be placed in service, and vessels designed without understanding the code framework may not be safe.

This article explains how the Pressure Equipment Directive and its UK successor relate to the principal design codes — BS PD 5500, BS EN 13445, and ASME VIII — and what each layer requires of the engineer and manufacturer.

The Regulatory Framework — PED and UK PER

The Pressure Equipment Directive (PED 2014/68/EU)

The Pressure Equipment Directive is an EU directive that applies to the design, manufacture and conformity assessment of pressure equipment and assemblies with a maximum allowable pressure greater than 0.5 bar. It establishes essential safety requirements (ESRs) that pressure equipment must meet before it can be placed on the EU market, marked with the CE mark, and put into service.

The PED applies to: pressure vessels, piping, safety accessories, and pressure accessories. It does not itself specify how a vessel must be designed — it specifies the outcome that must be achieved (the essential safety requirements) and defines the conformity assessment routes through which compliance with those requirements is demonstrated.

UK Pressure Equipment (Safety) Regulations 2016 — UK PER

Following Brexit, the PED was retained into UK law as the Pressure Equipment (Safety) Regulations 2016 (UK PER), with UKCA marking replacing CE marking for equipment placed on the UK market. The technical requirements of UK PER are substantively identical to the EU PED — the same essential safety requirements, the same categorisation system, the same conformity assessment routes — but the administrative framework differs: UK Approved Bodies (UKABs) replace EU Notified Bodies (NBs) for UK market placement, and UKCA marking replaces CE marking.

Exclusions from PED/UK PER

Not all pressure equipment falls under the Regulations. Key exclusions include:

• Equipment at or below 0.5 bar gauge maximum allowable pressure
• Simple pressure vessels covered by the Simple Pressure Vessels (Safety) Regulations
• Pipelines for gas, oil and water supply covered by other legislation
• Steam boilers and associated pressure parts covered by separate regulations
• Equipment on ships and aircraft
• Equipment in nuclear applications

PED/UK PER Categorisation — Determining What Applies

The Regulations classify pressure equipment into four categories (I through IV, plus SEP — Sound Engineering Practice for the lowest risk equipment) based on the combination of maximum allowable pressure, volume or DN, and the hazard of the contained fluid. Higher categories attract more stringent conformity assessment requirements.

Fluid Groups

Fluids are classified into two groups that significantly affect the categorisation:

• Group 1 (Dangerous): Explosive, flammable, highly flammable, extremely flammable, toxic, highly toxic, or oxidising fluids. Hydrocarbons, hydrogen, chlorine, ammonia and most process chemicals fall into Group 1.
• Group 2 (Non-dangerous): Fluids not classified as dangerous — water, air, nitrogen, inert gases, steam, and many non-hazardous process fluids.

The combination of fluid group, design pressure, and vessel volume (or pipe DN) determines the PED category via the conformity assessment charts in Annex II of the Regulations. Group 1 vessels and piping reach higher categories at lower pressures and smaller sizes than Group 2, reflecting their greater hazard potential.

The Four Categories and What They Require

The Module letters refer to the conformity assessment procedures defined in Annex III of the Regulations. Module B is EC/UK Type Examination of the design. Module D is production quality assurance. Module G is unit verification — the most rigorous route, requiring NB/UKAB review of every individual vessel. For Category IV equipment, Module G or B+D is typically required.

The Design Codes — What They Are and How They Relate to the Regulations

The PED/UK PER do not themselves specify how a vessel must be designed. They define the outcome (the essential safety requirements) and leave the means to the designer and manufacturer. The design codes — BS PD 5500, BS EN 13445, ASME VIII — are the technical documents that specify calculation methods, material requirements, fabrication and inspection requirements, and testing requirements. Designing to a recognised code is the primary route to demonstrating compliance with the ESRs.

BS EN 13445 is a harmonised standard — its use creates a presumption of conformity with the EU PED, meaning that a vessel designed and manufactured to BS EN 13445 is presumed to meet the essential safety requirements without further justification. BS PD 5500 was a UK national standard and is not a harmonised EU standard, meaning it does not carry the same automatic presumption of conformity under the EU PED — though it is widely accepted in practice and its use can be justified under the alternative routes in the Directive.

BS PD 5500 — The UK Unfired Pressure Vessel Standard

BS PD 5500 (formerly BS 5500) is the primary UK design code for unfired fusion-welded pressure vessels. It covers vessels in carbon, carbon-manganese, alloy and stainless steel, aluminium and copper alloys. The "PD" designation (Published Document) reflects its current status — it was reclassified from a British Standard to a Published Document when BS EN 13445 was adopted as the harmonised European standard, but it remains widely used in the UK and internationally, particularly in the oil and gas sector.

Key features of BS PD 5500:

• Design by formula (Section 3) for standard components — cylindrical shells, heads, nozzles, flat ends, supports
• Design by analysis (Appendix A) using FEA for non-standard geometries
• Material tables with design stress values indexed by temperature
• Weld joint efficiency factors (class 1, 2, 3) based on non-destructive examination extent
• Hydrostatic test requirements (test pressure = 1.25 × MAWP × stress ratio)
• Comprehensive inspection and fabrication requirements

BS EN 13445 — The Harmonised European Standard

BS EN 13445 (Unfired Pressure Vessels) is the harmonised European standard for pressure vessel design and manufacture. It is published in multiple parts:

• Part 1: General
• Part 2: Materials
• Part 3: Design (the primary calculation standard)
• Part 4: Fabrication
• Part 5: Inspection and testing
• Part 6: Requirements for the design and fabrication of pressure vessels and pressure parts constructed from spheroidal graphite cast iron
• Part 8: Additional requirements for pressure vessels of aluminium and aluminium alloys

EN 13445 and BS PD 5500 produce similar results for most standard vessel configurations — both use the same underlying pressure vessel theory and similar safety factors. The main differences are in the specific calculation methodologies for certain components (particularly nozzle reinforcement calculations), material allowable stress tables, and the weld joint efficiency approach.

ASME VIII Division 1 and Division 2

The ASME Boiler and Pressure Vessel Code Section VIII covers pressure vessels. Division 1 uses design by rule with higher safety factors (3:1 on tensile strength, varying by material). Division 2 uses more rigorous design requirements including fatigue assessment where required, with lower safety factors (2.4:1 on tensile strength) that allow thinner walls. Division 2 is appropriate for higher-pressure vessels where the material savings justify the more complex design process.

ASME VIII is the dominant standard in the US and Middle East oil and gas sectors, and is widely accepted internationally. It is not a harmonised standard under the EU PED and its use requires justification or supplementary technical documentation for EU/UK PED compliance. ASME-stamped (U-stamp) vessels are not automatically CE/UKCA marked — these are separate certification routes.

Key Design Concepts

Design Pressure and Maximum Allowable Working Pressure

The design pressure is the pressure used as the basis for calculating vessel wall thicknesses and component sizes. It is typically set at the highest pressure the vessel will encounter in normal operation plus a margin — typically 10% above operating pressure, or the set pressure of the safety relief device, whichever is greater.

The maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at the top of the completed vessel in its operating position for a specific temperature. The MAWP is determined by calculation after the vessel is designed — it will be equal to or greater than the design pressure, reflecting the fact that the vessel as built may have more capacity than the minimum required. The MAWP is stamped on the nameplate.

Design Temperature

The design temperature is the temperature at which the component operates, used to determine the allowable stress from the material tables. Both maximum and minimum design temperatures must be considered — high temperature reduces material strength (lower allowable stress, thicker required wall), while low temperature reduces material toughness and drives Charpy impact testing requirements.

Allowable Stress and Safety Factors

The allowable design stress is the maximum stress the material is permitted to carry in pressure vessel design at the design temperature. It is derived from the material's yield strength and tensile strength at temperature, divided by the applicable safety factor:

*ASME VIII Div.1 safety factor was reduced from 4:1 to 3.5:1 in the 2007 edition.

The difference in safety factor between ASME VIII Div.1 and the European codes is why ASME Div.1 vessels have thicker walls than equivalent BS PD 5500 or EN 13445 vessels at the same design pressure — the lower allowable stress requires more material.

Cylindrical Shell Thickness

The minimum required shell thickness for a cylindrical vessel under internal pressure — the most fundamental pressure vessel calculation — is given by:

e = PD_i / (2f·z − P)

Where e is the minimum required wall thickness (mm), P is the design pressure (MPa), D_i is the internal diameter (mm), f is the allowable design stress (MPa), and z is the weld joint efficiency factor (1.0 for fully examined welds, 0.85 for partially examined, lower for spot examination). This is the formula from BS PD 5500 — EN 13445 uses a near-identical form.

The calculated e is a minimum — the actual specified thickness must be e plus any corrosion allowance, plus sufficient margin to ensure the nominal ordered thickness, after mill tolerance, meets the minimum.

Weld Joint Efficiency

The weld joint efficiency factor z (or joint coefficient in ASME) accounts for the reduced confidence in weld integrity compared to parent material when non-destructive examination is not comprehensive. In BS PD 5500, three examination classes apply:

• Class 1 (z = 1.0): 100% radiographic or ultrasonic examination of all butt welds. Full confidence — no penalty on design stress.
• Class 2 (z = 0.85): Spot radiographic examination. Partial confidence — effective design stress reduced by 15%.
• Class 3 (z = 0.7): Visual examination only. Lowest confidence — significant design stress reduction, resulting in thicker walls.

Selecting a lower examination class to reduce inspection cost increases material cost through thicker walls. At higher pressures and larger diameters, the crossover point where Class 1 examination pays for itself in saved material is frequently reached. This trade-off should be evaluated at the design stage, not assumed.

Pressure Vessel Heads

The ends (heads) of a pressure vessel take several standard forms, each with different structural efficiency and cost:

• Hemispherical: The most structurally efficient — requires approximately half the wall thickness of the cylindrical shell for the same pressure. Expensive to form. Used on high-pressure vessels where the material saving justifies the forming cost.
• 2:1 Semi-ellipsoidal: The most common head form. Requires approximately the same thickness as the shell. Cost-effective to form by hot or cold pressing. Standard for most process vessels.
• Toriconical (Torispherical): Shallower than ellipsoidal, cheaper to form, but requires a slightly thicker wall. Common on lower-pressure storage vessels and tanks.
• Flat end: The least efficient — requires a very thick plate, typically with stay bolts or other reinforcement. Used on small-diameter high-pressure vessels, heat exchangers, and where head access (manholes, inspection covers) is required.
• Conical: Used where a transition from one diameter to another is required — column transitions, hoppers, cyclone separators. Requires careful treatment of the cone-to-cylinder junction where bending stresses are generated.

Nozzles, Openings, and Reinforcement

Every opening in a pressure vessel shell (for nozzles, manholes, vents, drains, instrumentation connections) reduces the structural integrity of the shell at that location. The removed material must be compensated by reinforcement — additional material in the shell, the nozzle wall, or a dedicated reinforcing pad — to restore the integrity lost by the opening.

The reinforcement area method (used in BS PD 5500 and ASME VIII Div.1) calculates the cross-sectional area removed by the opening and requires an equivalent area to be provided in the reinforcement zone around the nozzle. EN 13445 uses a similar approach. The nozzle calculation is one of the more complex standard calculations in pressure vessel design and is a frequent source of error — particularly when the nozzle is inclined, the opening is large relative to the shell diameter, or the nozzle carries significant external loading from connected pipework.

The Technical Documentation Package

For CE/UKCA-marked pressure equipment, the manufacturer must produce and retain a technical documentation package. This is not optional — it is a legal requirement under the Regulations and must be available to enforcement authorities on request for a minimum of ten years after the last unit is placed on the market. The package includes:

• General description of the equipment
• Design and manufacturing drawings, including material specifications
• Design calculations demonstrating compliance with the ESRs
• Reference to harmonised standards or other specifications used
• Copy of the Declaration of Conformity
• Fabrication records including material certificates, weld records, NDE reports
• Test reports (hydrostatic test certificate, NDT records)
• Notified Body / Approved Body certificate where applicable

Post-Brexit: UKCA vs CE — The Current Position

Since 1 January 2021, equipment placed on the UK market requires UKCA marking and must use a UK Approved Body (UKAB) for conformity assessment at Categories II, III and IV. Equipment placed on the EU/EEA market requires CE marking and a EU Notified Body. The same equipment can carry both marks if it meets both sets of requirements and uses both a UKAB and an EU NB — which is the common approach for manufacturers supplying both markets.

The UK government has extended the transitional period during which CE-marked equipment is accepted for the UK market multiple times. As of the time of writing, some CE-marked equipment continues to be accepted, but the transitional arrangements have been changing — verify the current position with the Office for Product Safety and Standards (OPSS) before relying on CE marking for UK market placement.

Common Misconceptions

1. "We have a CE mark so we comply with PED." CE marking is evidence of conformity with one or more directives/regulations — but it does not specify which one. A CE mark from the Machinery Directive is not evidence of PED compliance. Check that the Declaration of Conformity explicitly references the Pressure Equipment Directive (2014/68/EU) or UK PER.
2. "It's only low pressure so PED doesn't apply." The PED/UK PER applies to equipment above 0.5 bar gauge. Much equipment that appears low pressure — compressed air vessels, hot water calorifiers, expansion vessels — exceeds this threshold. Check the actual maximum allowable pressure, not the operating pressure.
3. "We designed it to ASME so it's CE marked." ASME certification (U-stamp) and CE/UKCA marking are entirely separate certification schemes. A U-stamped vessel is not CE or UKCA marked unless separate conformity assessment has been carried out under the PED/UK PER.
4. "The Notified Body stamps the design." In most conformity assessment modules, the NB/UKAB assesses the quality management system or the design dossier — they do not design the vessel. The design responsibility remains entirely with the manufacturer and the responsible engineer. NB involvement is not a substitute for competent pressure vessel design.
5. "We can use any design code." The PED/UK PER permits the use of harmonised standards (which carry a presumption of conformity) or alternative technical specifications. If using a non-harmonised standard (ASME VIII, BS PD 5500), the manufacturer must demonstrate equivalence to the ESRs — this requires more documentation and is more difficult to defend under audit.

Summary

Pressure vessel design in the UK sits at the intersection of a regulatory compliance requirement (UK PER/PED), a technical design discipline (BS PD 5500, EN 13445 or ASME VIII), and a quality assurance system (conformity assessment modules, NDE, material certification). Understanding which layer each question belongs to is the first step to answering it correctly.

The Regulations determine whether marking and third-party assessment are required and at what level. The design code determines how the vessel is sized, what materials can be used, and what fabrication and inspection is required. The conformity assessment module determines how compliance with the Regulations is documented and by whom. These three are distinct and must all be addressed — a well-calculated vessel with no conformity assessment documentation does not meet the Regulations; a conformity-assessed vessel with an inadequate design calculation is unsafe.