DIN 11850 vs ASME BPE: metric and inch hygienic tube compared

ASME BPE and DIN 11850 both cover thin-wall stainless tube for hygienic process lines, and they are not interchangeable. ASME BPE sizes tube by its outside diameter in inches, with one wall thickness per size. DIN 11850 sizes tube by a metric nominal diameter (DN) that follows the bore, and offers each bore in three wall thicknesses. Apart from a couple of sizes that land close by coincidence, a BPE tube and a DIN tube of the "same" size have different outside diameters, different bores and different clamp ferrules.

That answers the question as it's usually asked. The detail matters if you're installing a European skid in a US plant, running US single-use assemblies on a European line, or chasing a clamp joint that keeps failing swab tests for no obvious reason.

Two ideas of what "size" means

Inch hygienic tube is sized by OD. A 1" ASME BPE tube is 1.000" across the outside with a 0.065" wall, so the bore is 0.870". Every BPE fitting, valve end and ferrule for 1" tube is made to that OD and that bore, and the orbital weld heads, clamps and gaskets on site are all built around the same numbers.

DIN 11850 works from the inside. The DN number is roughly the bore in millimeters, and the standard wraps three wall series around it. At DN25:

Same bore, three outsides. Series 2 is what you'll find on most drawings, and when someone says "DIN tube" and leaves it there, Series 2 is almost always what they mean.

Which means DN25 has no inch twin. DN25 Series 2 is 29 mm OD (1.142") with a 26 mm (1.024") bore. A 1" BPE tube is 25.4 mm OD with a 22.1 mm bore. You can't butt-weld one to the other, and whatever connects them is a reducer, with the drainage and hold-up questions a reducer always brings.

Tube dimensions side by side

Each ASME BPE size below is paired with the nearest DIN 11850 Series 2 size. Where two DN sizes bracket one inch size, both are listed.

Look at the first two rows.

3/4" BPE and DN15 Series 2 are 0.05 mm apart on OD. 1/2" and DN10 are 0.3 mm apart. These are the pairs that cause trouble, because nothing looks wrong on the floor: the tube goes into the weld head and the clamp closes. The walls are still different (1.65 mm against 1.5 mm), so the bores differ by 0.25 to 0.6 mm, and the finished joint doesn't match the dimension tables of either standard. It may well pass visual inspection. It won't match the turnover package.

Every other size is obviously different, which is safer. Nobody welds 53 mm tube to 2" tube by accident.

Clamp ferrules: where mismatches hide

Hygienic clamp ferrules group several tube sizes onto one clamp face diameter, and the groupings don't care which standard the tube came from.

On inch tube, 1" and 1-1/2" ferrules share a 1.984" (50.5 mm) clamp face. DIN 32676 Series A, the clamp standard written for DIN 11850 tube, uses a 50.5 mm face for DN25, DN32 and DN40. So a DN40 ferrule will clamp to a 1-1/2" BPE ferrule. The gasket seats and the joint holds a pressure test.

The bores are 38 mm and 34.8 mm. That's a 1.6 mm step all the way around.

On a horizontal run, that step is a ledge that holds liquid after the line drains. In a CIP circuit it's a spot where the flow separates and wall shear falls off. The gasket, cut for one bore, sticks into the flow on one side and leaves a recess on the other, and residue collects in both. The same pairing happens at 2" and DN50, which share a 64 mm face, and again at 4" and DN100 on a 119 mm face.

If one clamp joint keeps showing up in swab results and the gasket looks fine every time it comes out, check that both ferrules were made to the same standard.

DIN 32676 does have a Series C for inch OD tube, dimensionally aligned with ASME BPE, because so much inch equipment ends up in European plants. A European drawing that calls out DIN 32676 without a series letter is incomplete. Ask which one they meant.

Two other European connections turn up in mixed plants. DIN 11851 is the threaded "dairy" union with the slotted round nut, common in food and beverage and not compatible with anything inch. DIN 11864 covers aseptic flange, union and clamp connections that seal on an O-ring in a groove with a metal-to-metal stop; you'll see them on European bioprocess equipment where a flat clamp gasket isn't considered good enough.

DIN 11850, EN 10357, DIN 11866 and ISO 1127

DIN 11850 is the name everyone uses, but strictly it isn't the current European standard anymore. EN 10357 replaced it and carried the DIN 11850 Series 2 dimensions across as its own Series A, so the table above still holds. EN 10357 also contains an ISO OD series and an inch series, which makes "EN 10357 tube" no more specific than "DIN tube."

For pharma work the one to know is DIN 11866. It sorts hygienic tube into three rows:

• Row A: DIN 11850 metric dimensions
• Row B: ISO 1127 OD-based metric dimensions (13.5, 17.2, 21.3, 26.9, 33.7, 42.4, 48.3, 60.3, 76.1, 88.9 and 114.3 mm)
• Row C: ASME BPE inch dimensions

It then adds hygienic classes that tie a surface roughness requirement to the tube. Of the European documents, it's the closest to BPE in intent.

ISO 1127 sizes follow pipe ODs rather than tube bores, and they're common in Swiss, Scandinavian and some German pharma plants. They produce another near miss, too. 76.1 mm ISO tube and 3" BPE tube (76.2 mm) are 0.1 mm apart on OD, with walls of 2.0 mm and 1.65 mm.

So a European spec that says "DIN tube" could point at any of three dimensional families. The series or row is the part of the callout that actually carries information.

316L and 1.4435 are close, not identical

ASME BPE tube is 316L (UNS S31603), normally to ASTM A270 with BPE's supplementary requirements. One of those requirements is a sulfur range of 0.005 to 0.017% on 316L tube and fittings intended for autogenous orbital welding.

European hygienic tube is usually 1.4404, the general 316L equivalent, or 1.4435. Swiss and German pharma often specify 1.4435 to Basler Norm 2 (BN2), which puts molybdenum at 2.5% minimum, raises nickel and caps delta ferrite at 0.5%. For corrosion resistance that's a step up from ordinary 316L.

Welding it is another matter. Sulfur changes the way the weld pool circulates, and when the two sides of an orbital weld have very different sulfur contents the pool shifts toward the lower-sulfur side. The higher-sulfur side can end up without full penetration at the ID, which is the one place you can't see from outside. BPE's sulfur band exists to keep heat-to-heat welds predictable, and a low-sulfur 1.4435 heat made outside that system has no obligation to fall inside it.

That doesn't rule the combination out. It means the transition weld gets its own coupons and its own weld schedule, and someone reads the material certs for both sides before the weld head goes on. If you're buying European material for a BPE system, put the sulfur range on the purchase order.

Surface finish in two units

BPE writes roughness in microinches Ra. The usual numbers are 20 µin for mechanically polished product-contact surfaces and 15 µin for electropolished (the full set of finish designations is in What is ASME BPE?). European specs write micrometers, typically 0.8 µm or 0.4 µm, with 0.25 µm for the tighter electropolished classes.

0.4 µm and 15 µin get treated as the same number all the time. They aren't. 0.4 µm is the looser of the two, and a surface measuring 0.39 µm (15.4 µin) passes a metric acceptance criterion and fails an imperial one. Pick one unit for acceptance on a project and do the conversion once, in the spec, rather than on every inspection report.

Who uses which

In North America, biopharma runs on ASME BPE almost without exception. Food and dairy mostly work to 3-A with inch OD tube that shares BPE's outside diameters.

In Europe, food, dairy, beverage and brewing are DIN 11850 Series 2 territory, with DIN 11851 or DIN 32676 connections. European biopharma is mixed. Utilities and older plants lean DIN or ISO, while bioprocess equipment is increasingly inch, partly because single-use assemblies ship with inch clamp ends almost everywhere. A metric plant ends up with inch connections at every bag, filter and sensor manifold whether anyone planned it or not.

Elsewhere, the plant tends to follow whoever engineered it. A facility designed by a US engineering firm will usually be BPE. One built around a German equipment package usually won't be.

Running a plant that has both

Plenty of plants run both standards and get along fine. When it goes wrong, it's usually at a transition nobody planned. What tends to work:

Put transitions at defined boundaries, like a skid edge or an equipment nozzle, and show them on the P&ID and the isometrics. A transition that exists because the fitter ran out of one kind of tube is the one nobody can find two years later.

Use a machined transition piece with a blended bore. A tube-to-tube weld across a step and two ferrules from different standards that happen to clamp together both leave a ledge.

Write the standard and the series on every line in the line list: "DIN 11850 Series 2, DN40" or "ASME BPE, 1-1/2 in." Never "sanitary 40."

Qualify transition welds separately, with coupons cut from the actual heats, and check sulfur on both certs.

Keep the gaskets apart in stores. A DN40 clamp gasket and a 1-1/2" clamp gasket have the same outside diameter and different bores, and in a parts bin they look identical. Gaskets get replaced far more often than tube, so this is how a well-designed plant slowly turns into a mixed one without anyone deciding it should.

What this means for valves

A diaphragm valve's drainability and hold-up volume are worked out for a specific bore. Fit metric ends to an inch line, or the other way round, and the valve gets a step at each end, right where the flow path is supposed to stay continuous. The end connection has to be specified to the line standard, not approximated to it. Weir geometry and drain angle are covered in How diaphragm valves work.

Aquasyn machines valve bodies from bar stock rather than castings, so the end connection is set at machining instead of being fixed by a casting pattern. ASME BPE inch ends are standard, and metric tube ends are available on request. If your line spec crosses standards, send it with the configuration and engineering will match the ends to it.

Frequently asked questions

Is DIN 11850 the same as ASME BPE?

No. ASME BPE sizes hygienic tube by outside diameter in inches, with a fixed wall per size. DIN 11850 sizes it by a metric nominal diameter (DN) close to the bore, in three wall series. The sizes don't match, and apart from near misses at 1/2" and 3/4", the outside diameters are visibly different.

What is DIN 11850 Series 2?

The middle wall-thickness series of DIN 11850 and the one in widest use. The wall is 1.5 mm up to DN50 and 2.0 mm from DN65 to DN100. DN25 Series 2 is 29 × 1.5 mm with a 26 mm bore.

Can DIN 11850 tube be welded to ASME BPE tube?

Not directly at matching nominal sizes, because the outside diameters and bores differ. Use a machined transition fitting with a blended bore, and qualify the weld on the actual material heats, since European 1.4435 and BPE 316L can differ enough in sulfur content to shift the weld pool.

Is DIN 11850 still a current standard?

It was superseded by EN 10357, which carried the DIN 11850 Series 2 dimensions over as Series A. Most drawings and suppliers still call it DIN 11850.

Is 1.4435 the same as 316L?

Close, but no. 1.4435 has higher molybdenum and nickel than standard 316L, and when made to Basler Norm 2 it also carries a delta ferrite limit. It's at least as corrosion resistant. Its sulfur content isn't held to the ASME BPE range, though, which matters for orbital welding.

Do DIN and inch clamp ferrules fit together?

Some do, and that's the problem. DIN 32676 Series A ferrules for DN25 to DN40 share a 50.5 mm clamp face with 1" and 1-1/2" inch ferrules, so they clamp together even though the bores differ by up to 3.9 mm. Check the bore, not the clamp.