ISO EN 10357-D is the part of the European EN 10357 standard that defines dimensions, tolerances, surface finishes, and material requirements for ISO-series hygienic stainless steel tubes — also referenced as ISO 2037. It specifies outer diameters, wall thicknesses, and dimensional tolerances for welded tubes used in food, dairy, beverage, pharmaceutical, and biotech processing, along with the surface roughness classes and material grades those tubes must meet. By establishing a common dimensional language, ISO EN 10357-D ensures that tubes, fittings, clamps, and weld ends from different manufacturers are fully interchangeable within hygienic pipework systems.
The "-D" suffix distinguishes this ISO dimension series from EN 10357-A (the DIN 11850 dimension series used predominantly in Germany) and from BS 4825 (the British standard). While surface and material quality requirements are comparable across these standards, the outer-diameter values differ — which means fittings must always match the correct dimension series.
In this guide, we cover the six core areas engineers and procurement professionals need to understand when specifying ISO EN 10357-D tubing: tube dimensions, surface roughness requirements, stainless steel material grades, pressure ratings, hygienic connection types, and testing requirements. Each section includes a reference table you can use during specification work or supplier discussions.
- ISO EN 10357-D (ISO 2037) defines hygienic tube outer diameters from 25 mm to 152.4 mm with specified wall thicknesses and tolerances.
- Five surface roughness classes range from Ra 0.8 µm (general hygienic) to Ra 0.25 µm (ultra-clean), achieved by mechanical polishing, electropolishing, or electropolishing plus passivation.
- The standard-grade material is 316L (1.4404); other compliant grades include 304L, 316Ti, Duplex 2205, and 904L.
- Maximum allowable pressure at 20 °C ranges from 40 bar (25 mm OD) down to 9 bar (152.4 mm OD) for 316L tubes.
- Hygienic connection types include Clamp per ISO 2852, Threaded per ISO 2853, Butt Weld, Socket Weld, and Flanged.
- Testing per ISO 2037 includes hydrostatic pressure tests, visual inspection, surface roughness measurement, dimensional checks, material verification per ISO 1127, and weld inspection per ISO 5817.
How ISO EN 10357-D works — and why it matters
EN 10357 is the European harmonised standard for welded stainless steel tubes intended for the food and chemical industries. Part D — the ISO dimension series — aligns directly with ISO 2037, which has long served as the international reference for hygienic tube dimensions. When an engineer specifies "ISO EN 10357-D" tubing, they are calling for tubes whose outer diameters, wall thicknesses, and tolerances follow the ISO 2037 dimension series, manufactured and tested to EN 10357 requirements.
The practical reason this standard exists is interchangeability. Any ISO EN 10357-D tube will mate correctly with compliant fittings, clamp connections, and orbital-weld ends — regardless of which manufacturer produced each component. This dramatically simplifies procurement, system design, and maintenance across multi-vendor installations.
Any ISO EN 10357-D tube will mate correctly with compliant fittings, clamp connections, and orbital-weld ends — regardless of which manufacturer produced each component.
ISO EN 10357-D does not exist in isolation. It works alongside a family of related standards that together define a complete hygienic pipework system: ISO 2852 governs clamp connections, ISO 2853 covers threaded and flanged connections, ISO 1127 addresses stainless steel tube dimensions and material verification, ISO 4288 specifies surface roughness measurement methods, and ISO 5817 sets weld quality acceptance criteria.
If you are familiar with the DIN tube world, the key distinction is straightforward: EN 10357-A follows DIN 11850 outer diameters (e.g. 38 mm, 51 mm), while EN 10357-D follows ISO 2037 outer diameters (e.g. 38.1 mm, 50.8 mm). The surface quality, material, and testing requirements are comparable — but the dimensional series are not interchangeable. An ISO-dimension fitting will not seal correctly on a DIN-dimension tube. For a deeper comparison between the two dimension series, see our related guide on DIN 11850 and EN 10357 hygienic tubes.
Key components of ISO EN 10357-D
Tube dimensions per ISO 2037
The ISO dimension series covers nominal diameters from 25 mm up to 152 mm. Note that the outer diameter values differ from those in the DIN series — for example, a nominal 38 mm tube has an OD of 38.1 mm under ISO 2037, compared with 38.0 mm under DIN 11850. This seemingly small difference is critical for gasket sealing and clamp fit.
Wall thickness is 1.5 mm for tube sizes up to 76.1 mm OD, increasing to 2.0 mm for the 101.6 mm and 152.4 mm sizes. OD tolerances tighten at smaller diameters (±0.15 mm for 25–38.1 mm OD) and widen at larger sizes (±0.4 mm for 152.4 mm OD), reflecting the manufacturing realities of cold-drawn tube production. Wall thickness integrity is maintained by using fabrication-grade minimum-wall tubing for all cold-formed products.
| Nominal Diameter (mm) | Outer Diameter (mm) | Wall Thickness (mm) | Tolerance OD (mm) | Tolerance Wall (mm) |
|---|---|---|---|---|
| 25 | 25 | 1.5 | 0.15 | 0.15 |
| 32 | 32 | 1.5 | 0.15 | 0.15 |
| 38 | 38.1 | 1.5 | 0.15 | 0.15 |
| 51 | 50.8 | 1.5 | 0.2 | 0.15 |
| 63 | 63.5 | 1.5 | 0.2 | 0.15 |
| 76 | 76.1 | 1.5 | 0.25 | 0.15 |
| 101 | 101.6 | 2 | 0.3 | 0.2 |
| 152 | 152.4 | 2 | 0.4 | 0.2 |
Table 1 — Tube Dimensions per ISO 2037
Surface roughness requirements
Surface roughness — measured as Ra (arithmetic average roughness) in micrometres — directly affects cleanability. A lower Ra value means fewer microscopic crevices where bacteria can harbour and product residue can accumulate. ISO EN 10357-D and its associated standards define distinct surface roughness classes for different application levels, from general hygienic processing through to ultra-clean pharmaceutical environments.
| Application Class | Internal Ra (µm) | External Ra (µm) | Finish Method | Applicable Standard |
|---|---|---|---|---|
| General Hygienic | 0.8 | 1.6 | Mechanical polish | ISO 2037 |
| Dairy / Food | 0.8 | 1.6 | Mechanical polish | ISO 2037 / EN 10357 |
| Pharmaceutical | 0.4 | 0.8 | Electropolish | ISO 14159 |
| Aseptic | 0.4 | 0.8 | Electropolish | EHEDG / ISO 14159 |
| Ultra-clean | 0.25 | 0.5 | Electropolish + passivation | Pharmaceutical grade |
Table 2 — Surface Roughness Requirements by Application Class
Three finish methods are used to achieve these roughness classes. Mechanical polishing uses a progressive series of abrasives from low to high grit, producing a consistent internal finish suited for optimal and economical cleaning. Electropolishing goes a step further — it promotes a chromium-enriched surface layer that maximises corrosion resistance and minimises bacterial build-up in surface cavities. For ultra-clean pharmaceutical applications, electropolishing plus passivation delivers the tightest possible surface specification.
Electropolishing promotes a chromium-enriched surface layer that maximises corrosion resistance and minimises bacterial build-up in surface cavities.
Surface finish is verified using a calibrated profilometer to ensure the Ra maximum is not exceeded. The Alfa Laval Hygienic range delivers an internal surface finish of Ra < 0.8 µm as standard, while the UltraPure range offers Ra < 0.8 µm either electropolished or mechanically polished. Tubular fittings in the Alfa Laval range exceed the requirements of DIN 11852 and meet both EHEDG and 3A surface roughness requirements.
Stainless steel material grades
The default material for ISO EN 10357-D hygienic tubing is 316L (European designation 1.4404, AISI 316L). Its low carbon content — a maximum of 0.03 % — minimises carbide precipitation during welding, while molybdenum content provides enhanced resistance to chloride-induced corrosion. For most dairy, food, and beverage applications, 316L offers the ideal balance of corrosion resistance and cost-effectiveness. Alfa Laval's years of experience in the installation material business have confirmed 1.4404 as the best match for the majority of customer processes.
Other industry-standard grades used in ISO-compliant hygienic tubing address more specialised requirements:
| Common Designation | EN Number | AISI Equivalent | Max Carbon (%) | Key Property | Typical Application |
|---|---|---|---|---|---|
| 316L | 1.4404 | AISI 316L | 0.03 | Corrosion resistance + low carbon | Standard hygienic grade |
| 304L | 1.4307 | AISI 304L | 0.03 | General corrosion resistance | Non-aggressive media |
| 316Ti | 1.4571 | AISI 316Ti | 0.08 | Titanium stabilised | High temperature applications |
| Duplex 2205 | 1.4462 | UNS S31803 | 0.03 | High strength + corrosion | Aggressive media |
| 904L | 1.4539 | AISI 904L | 0.02 | High alloy resistance | Highly corrosive media |
Table 3 — Stainless Steel Material Grades for ISO-Compliant Hygienic Tubing
304L is suitable where the additional cost of molybdenum-bearing grades is not justified — typically non-aggressive aqueous media. 316Ti, Duplex 2205, and 904L address niche conditions involving high temperatures, high chloride concentrations, or highly corrosive chemicals respectively. All product-wetted stainless steel items in the Alfa Laval Hygienic range are delivered with a 3.1 certificate in accordance with EN 10204, providing full material traceability.
Types and variations: hygienic connection types
ISO EN 10357-D tubes are joined using several standardised connection methods, each governed by its own ISO standard and suited to different operational requirements.
| Connection Type | Governing Standard | Size Range (mm) | Seal Type | Typical Application |
|---|---|---|---|---|
| Clamp (Tri-Clamp) | ISO 2852 | 12 to 152 | EPDM / Silicone / PTFE | General hygienic process |
| Threaded coupling | ISO 2853 | 25 to 100 | Flat face gasket | Dairy and food |
| Weld end (butt weld) | ISO 2037 | 12 to 152 | No seal — welded | Permanent hygienic pipework |
| Weld end (socket weld) | ISO 2037 | 12 to 76 | No seal — welded | Compact installations |
| Flanged | ISO 2853 | 25 to 150 | Flat face gasket | High pressure hygienic |
Table 5 — ISO Hygienic Connection Types
Clamp connections (Tri-Clamp) per ISO 2852 are the industry standard for hygienic pipework. They use neuter-style ferrules that simplify design and installation, and all clamp fitting styles comply with 3A standards for CIP (clean in place). A connection consists of a plain ferrule, a clamp, and a gasket. Available seal ring materials for ISO-dimension clamp fittings include NBR, EPDM, FPM, PTFE, and Silicone (Q). One important caveat: PTFE gaskets tend to "cold flow" and are limited to a maximum temperature of approximately 93 °C (200 °F) due to possible leaking problems.
Clamp connections are the industry standard, with neuter-style ferrules that simplify design and installation.
Butt-weld ends per ISO 2037 are the choice for permanent hygienic pipework where joints do not need to be opened. Fittings designed for orbital welding use a machined square-cut end-facing method, which allows for accurate and consistent orbital weld results. Socket-weld ends serve compact installations up to 76 mm. Threaded couplings (ISO 2853) are common in dairy and food applications from 25 to 100 mm, while flanged connections (ISO 2853) handle high-pressure hygienic duties up to 150 mm.
When comparing dimension standards, remember: EN 10357-D (ISO series), EN 10357-A (DIN 11850 series), and BS 4825 each use different outer-diameter values for the same nominal tube size. The connection principles, surface requirements, and material specifications are comparable — but an ISO-dimension clamp ferrule will not seal on a DIN-dimension tube. Always verify the dimension series before ordering fittings. For more on the DIN dimension series, see our related guide on DIN 11850 and EN 10357 hygienic tubes. If you are integrating hygienic process valves into your system, the same dimension-matching principle applies.
Applications: where ISO EN 10357-D tubing is used
Dairy and food processing is the core application for ISO EN 10357-D tubing. Standard hygienic-grade tubes — 316L with Ra 0.8 µm mechanical polish — handle milk, cream, sour milk products, and other foodstuffs. Tri-Clamp connections enable CIP-compliant quick assembly and disassembly, and EPDM seals are rated highly suitable for dairy products and hot-water CIP cycles.
Beverage production — including brewing, soft drinks, and wine — uses similar specifications. For non-aggressive media such as water-based beverages, 304L may be sufficient where the additional cost of 316L is not justified. EPDM seals are rated highly suitable for brewery products, while FPM may be preferred when handling concentrated fruit juices or essential oils.
Pharmaceutical and biotech applications demand tighter surface specifications. Electropolished surfaces at Ra ≤ 0.4 µm (or Ra ≤ 0.25 µm for ultra-clean environments) minimise bacterial harbourage. The Alfa Laval UltraPure range is specifically designed for these industries, with all components individually capped and bagged, and delivered with a 3.1 certificate per EN 10204 or a Mill Test Report. For readers building out complete pharmaceutical process systems, our guide on pharmaceutical pumps covers the pump side of the equation.
Personal care and cosmetics manufacturing also relies on hygienic-grade tubing with CIP capability to prevent cross-contamination between product batches.
How to choose the right ISO EN 10357-D tube specification
Selecting the correct tube specification is a step-by-step process that starts with your application class and narrows down through material, size, pressure, and connection requirements.
Step 1 — Identify your application class. This determines the surface roughness requirement. Refer to Table 2: general food and dairy applications need Ra 0.8 µm (mechanical polish), pharmaceutical processes need Ra 0.4 µm (electropolish), and ultra-clean or aseptic processes need Ra 0.25 µm (electropolish plus passivation).
Step 2 — Select the material grade. For most hygienic processes, 316L (1.4404) is the standard choice. Use 304L only for non-aggressive media. Consider Duplex 2205 or 904L for highly corrosive or aggressive chemicals — refer to Table 3.
Step 3 — Size the tube. Determine the nominal diameter based on your flow requirements. Use the standard dimension series from Table 1 to confirm the outer diameter and wall thickness.
Step 4 — Check the pressure rating at your operating temperature. Maximum allowable pressures decrease with both increasing tube diameter and increasing temperature:
| Outer Diameter (mm) | Wall Thickness (mm) | Max Pressure at 20 °C (bar) | Max Pressure at 100 °C (bar) | Material |
|---|---|---|---|---|
| 25 | 1.5 | 40 | 32 | 316L |
| 38.1 | 1.5 | 26 | 21 | 316L |
| 50.8 | 1.5 | 20 | 16 | 316L |
| 63.5 | 1.5 | 16 | 13 | 316L |
| 76.1 | 1.5 | 13 | 10 | 316L |
| 101.6 | 2 | 13 | 10 | 316L |
| 152.4 | 2 | 9 | 7 | 316L |
Table 4 — Maximum Allowable Pressure Ratings by Tube Dimension for 316L
Note that these are tube body ratings. Connection ratings — for clamps, flanges, or threaded couplings — may be the limiting factor in your system and should be verified separately.
Maximum allowable pressures decrease with both increasing tube diameter and increasing temperature — and connection ratings may be the limiting factor in your system.
Step 5 — Choose the connection type. Use clamp connections (ISO 2852) for lines that need frequent opening for inspection or product changeover. Use butt-weld ends for permanent installations. Threaded couplings suit smaller dairy and food lines. Flanged connections handle high-pressure hygienic duties.
Step 6 — Verify seal material compatibility. Match the gasket material to your process medium and temperature. EPDM is broadly suitable for dairy, water, and CIP chemicals. FPM handles fats and oils well. PTFE offers wide chemical resistance but is limited to approximately 93 °C due to cold-flow tendencies.
- Match your application class to the required surface roughness — Ra 0.8 µm for general hygienic, Ra 0.4 µm for pharmaceutical, Ra 0.25 µm for ultra-clean.
- Select material grade based on process medium aggressiveness — 316L for standard, 304L for non-aggressive, Duplex 2205 or 904L for highly corrosive media.
- Size the tube using nominal diameter from Table 1 and check maximum allowable pressure at your operating temperature against Table 4.
- Choose the connection type based on whether the joint needs to be frequently opened (clamp) or permanently sealed (butt weld).
- Verify that seal material is compatible with your process medium and temperature — PTFE is limited to approximately 93 °C due to cold flow.
Maintenance: CIP compatibility, gaskets, and inspection
ISO EN 10357-D tubing is designed for CIP (Clean-in-Place) compatibility. Smooth internal surfaces at Ra ≤ 0.8 µm, combined with crevice-free joints, ensure that cleaning solutions can reach all wetted surfaces effectively. Clamp fittings comply with 3A standards for CIP, and electropolished surfaces further reduce cleaning time and chemical usage. For a deeper look at how CIP systems work with hygienic process equipment, see our article on CIP pumps for cleaning-in-place.
Gasket replacement deserves careful attention. It is not possible to state a universal lifetime for elastomer seals — chemical attack, temperature, and mechanical wear all influence degradation rates. Extreme temperatures, even within generally accepted limits, can worsen other forms of attack and reduce seal life. Establish replacement intervals based on your actual process conditions. As a general guide: EPDM seals suit dairy products and hot-water or weak-lye CIP cycles; FPM is preferred for fats, oils, and mineral oils; Silicone handles high dry-heat applications; and PTFE offers broad chemical resistance but must not exceed approximately 93 °C.
Quality assurance for ISO EN 10357-D tubing involves a structured testing programme:
| Test Type | Method | Acceptance Criteria | Standard | Frequency |
|---|---|---|---|---|
| Hydrostatic pressure test | Fill with water — pressurise | No leakage or deformation | ISO 2037 | Per batch |
| Visual inspection | Manual or optical | No cracks, pits, or surface defects | ISO 2037 | 100 percent |
| Surface roughness measurement | Profilometer | Ra per application class | ISO 2037 / ISO 4288 | Per batch |
| Dimensional check | Calliper / gauge | Within stated tolerances | ISO 2037 | Per batch |
| Material verification | Positive material identification | Grade confirmation | ISO 1127 | Per heat |
| Weld inspection | Visual and dye penetrant | No porosity or undercut | ISO 5817 | Per weld |
Table 6 — Testing Requirements per ISO 2037 and Related Standards
All fittings undergo 100 % visual inspection, and ovality and squareness tolerances are checked with calibrated equipment. Surface finish is verified with a calibrated profilometer.
In the Alfa Laval production process, every fitting undergoes 100 % visual inspection, while ovality and squareness are checked with calibrated equipment and surface finish is verified using a calibrated profilometer to ensure the Ra maximum is not exceeded. In service, we recommend periodic re-verification of surface roughness, particularly after aggressive CIP cycles or any mechanical damage to the tube interior.
Summary and next steps
ISO EN 10357-D (ISO 2037) provides a complete specification framework for hygienic stainless steel tubes — from dimensions and tolerances through surface finish classes, material grades, pressure ratings, connection types, and testing requirements. By following a structured selection process — application class, surface roughness, material grade, tube size, pressure verification, and connection type — engineers and procurement professionals can confidently specify tubing that meets both regulatory requirements and process performance goals.
If you are specifying ISO EN 10357-D tubes and fittings for a dairy, food, beverage, or pharmaceutical project in southern Germany, our team can help. As an Alfa Laval Master Distributor, we support engineers, plant builders, and system suppliers in selecting the right Hygienic or UltraPure components — including tube dimensions, surface finishes, connection types, and seal materials — matched to your process requirements. Contact Euroflow for technical advice, quotations, and 3.1-certified product sourcing.
