TECTULby IMR

Fluid conveyance · Carbon steel flanges

TECHNICAL DATA SHEET

FT-TC-BRI-AC-001

Issue date: August 5, 2026 · Rev. 001

Check the current version

Forged ASTM A105 carbon steel flanges, Classes 150, 300 and 600 (ASME B16.5) — the class is not the pressure

TECTUL · Dispatch across Colombia · Exports to Central & South America

Forged ASTM A105/A105M-24 carbon steel flanges with ASME B16.5 dimensions, drilling and classes, covering the Classes 150, 300 and 600 in the TECTUL catalog and the five types we sell: slip-on, welding neck, blind, threaded and socket weld. The backbone of this sheet is the correction no Colombian supplier publishes: a flange class is not its working pressure. A Class 150 A105 flange is rated 19.6 bar (285 psi) between −29 °C and 38 °C, and above roughly 290 °C it is rated below the 150 psi its name suggests. The sheet publishes the full material group 1.1 pressure-temperature table in bar and psi, the three dimensional tables with drilling and stud length in millimetres and inches, and explains why the socket weld and welding neck references state the pipe schedule (SCH 40, SCH 80).

SKU VT-86931 (carbon steel flange, Class 150, slip-on) · 11 carbon steel references in Classes 150, 300 and 600 · slip-on, welding neck, blind, threaded and socket weld types · NPS 1/2″–12″ by reference · sold by the unit

1. Technical specifications

Class 150 dimensions and drilling — ASME B16.5

The millimetre values are those of Table 7 and Table 8 of ASME B16.5; the inch values are those of Tables II-7 and II-8 of Mandatory Appendix II of the same standard. They are not our conversions: the standard publishes both series. Bolt and bolt-hole diameters are always expressed in inches, even in the metric tables, by the standard’s own provision. Stud length corresponds to a 2 mm (0.06 in) raised face and does not include the height of the points.

Class 300 dimensions and drilling — ASME B16.5

Class 600 dimensions and drilling — ASME B16.5

In Class 600 the raised face is 7 mm (0.25 in) rather than 2 mm, and the tabulated stud length corresponds to that face. The bore B of Class 600 welding neck and socket weld flanges is specified by the purchaser: the standard does not tabulate it (Table 16, note 4). The value published here in the “bore B slip-on and socket weld” column for Class 600 is the slip-on bore, which is tabulated.

The t_f thickness includes the raised face

This is the most frequent dimensional confusion when measuring a flange at goods receipt. The minimum thickness t_f of Tables 8, 11 and 16 corresponds to the flange with its standard raised face —2 mm (0.06 in) in Classes 150 and 300, 7 mm (0.25 in) from Class 400 up—. The note to those tables states it explicitly: when the flange is supplied flat faced, its thickness is either the full t_f plus 2 mm, or the t_f without the raised face height. That is: a flat-faced Class 150 NPS 4 flange may measure 22.3 mm (the tabulated t_f, if the raised face was removed) or 24.3 mm (if the raised face was added to the thickness), and both comply with the standard.

Material: forged carbon steel ASTM A105/A105M-24

The upper hardness limit moved from 187 HB to 197 HBW in the 2021 edition of A105 and is retained in the 2024 edition: a certificate reading 195 HBW complies with the current standard and would not have complied with a pre-2021 edition. Whatever figure appears on the material certificate must be read against the edition that certificate declares.

General specifications

2. Engineering basis: the class is not the pressure

The rule in one line

A Class 150 carbon steel flange does not withstand 150 psi: it withstands 285 psi (19.6 bar) between −29 °C and 38 °C. A flange class is a series designation —the name of a column in the ASME B16.5 pressure-temperature table—, not the working pressure. The allowable pressure comes from the intersection of class, material group and service temperature. For forged A105 carbon steel the group is 1.1, and this is its table.

Pressure-temperature table for material group 1.1 (A105) — in bar

The same table in imperial units — in psi

Both tables are those of ASME B16.5 (Table 2-1.1 in bar and Table II-2-1.1 of Mandatory Appendix II in psi) for material group 1.1, which groups the A105 and A350 grade LF2 and LF3 forgings, the A216 grade WCB casting and the A515 and A516 grade 70 plates. The temperature steps of the metric and imperial tables do not correspond to each other (38 °C is not exactly 100 °F, 538 °C is not exactly 1,000 °F): each table is read in its own units and values are not interpolated between them. Above 425 °C the carbide phase of the steel may convert to graphite: the standard permits it but does not recommend it for prolonged service.

How to read the table: a four-step method

1. Take the design temperature of the service, not the ambient temperature nor the normal running fluid temperature: the design one, which is what the line code governs.
2. Move up to the next tabulated step. If your design temperature is 215 °C, read the 250 °C row, never the 200 °C one. Interpolating downwards is what produces joints that weep on a hot start-up.
3. Compare the allowable pressure of that row with the design pressure. If the design pressure exceeds it, the class does not serve: you must move up a class, not tighten harder.
4. Check the gasket and the bolting, which are almost always the weakest link: the gasket material may have a thermal ceiling far below what the table allows the steel.

What the class does determine, besides the pressure

Moving from Class 150 to Class 300 at the same NPS does not only change the allowable pressure: it changes the bolt circle, the quantity and diameter of the studs, the outside diameter and the thickness. At NPS 2″, Class 150 carries 4 studs of 5/8″ on a 120.7 mm circle and Class 300 carries 8 studs of 5/8″ on a 127.0 mm circle. They are geometrically incompatible flanges: a Class 150 flange cannot be bolted against a Class 300 even at the same nominal diameter, because the holes do not line up. They do not line up at NPS 1/2″ either, the only diameter in the catalog where all three classes share bolt quantity and diameter: the bolt circle is 60.3 mm in Class 150 and 66.7 mm in Classes 300 and 600.

3. Why the catalog states the schedule: bore, types and welding

A flange bore is not set by the flange: it is set by the pipe

Four catalog references carry the pipe schedule in their name —“Class 150 SCH 40 socket weld”, “Class 300 SCH 40 socket weld”, “Class 300 SCH 80 welding neck”— and that is not a commercial label: it is the datum without which the part cannot be made. On a slip-on, blind or threaded flange, the inner hole is tabulated in ASME B16.5 and depends only on NPS. On a welding neck or socket weld flange, the inner hole must reproduce the inside diameter of the pipe it will be welded to, and that inside diameter changes with the schedule.

The standard resolves it this way: the bore that Table 8 tabulates for Class 150 and 300 welding neck and socket weld flanges corresponds to the inside diameter of standard wall pipe per ASME B36.10M —which at NPS 10″ and smaller is the same as Schedule 40—, and that is the bore supplied unless the purchaser specifies otherwise. In Class 600 the standard does not tabulate the bore of welding neck and socket weld flanges: Table 16 refers it explicitly to the purchaser.

Practical consequence: the flange and the pipe are ordered together

Ordering “NPS 3 Class 300 welding neck flange” without stating the schedule leaves the specification incomplete: the flange will arrive with the standard wall bore and, if the line is SCH 80, an internal step will remain at the weld root. That step concentrates stress, retains solids and is a fatigue crack initiation point. That is why our welding neck and socket weld references name the schedule: it is not redundancy, it is the only way for the part to match the pipe. The pipe dimensional data by schedule live in the carbon steel pipe family data sheets, not here.

The double weld of the slip-on flange

The slip-on flange slides over the pipe and is welded with two fillet welds: an outer one at the back face of the hub and an inner one at the pipe end, against the flange face. The inner weld is not optional on a joint that will contain pressure: without it, fluid enters the clearance between pipe and flange and the joint fails by crevice corrosion and by fatigue of the single outer weld. This is the assembly detail most often skipped in the field and it has no relation to the class: a badly welded Class 600 slip-on fails just like a Class 150 one.

The welding neck flange, by contrast, is joined with a single butt weld with bevel preparation, and that weld is radiographable end to end. That is the real difference between the two types: not the pressure they are rated for —at the same class and material the pressure-temperature table is identical— but the quality and inspectability of the welded joint, and the behaviour under fatigue and thermal cycling, where the tapered neck spreads the stress that the slip-on concentrates in the weld.

The five catalog types, compared

4. Frequently asked questions

How much pressure does a Class 150 carbon steel flange withstand?

19.6 bar (285 psi) between −29 °C and 38 °C, if it is forged A105 carbon steel or any other ASME B16.5 group 1.1 material. It is not 150 psi: the class is a series designation, not the working pressure. The allowable pressure falls with temperature in a tabulated way: 13.8 bar at 200 °C, 10.2 bar at 300 °C and 1.4 bar at 538 °C. The “150” designation only coincides with the allowable pressure at around 290 °C; below that the flange is rated higher and above it, lower.

Can I bolt a Class 150 flange against a Class 300 of the same diameter?

No. The drilling is different: at NPS 2″ Class 150 has 4 holes of 3/4″ on a 120.7 mm circle and Class 300 has 8 holes of 3/4″ on a 127.0 mm circle. Neither the quantity nor the position match, so there is no way to pass the studs. They do not match at NPS 1/2″ either, where both classes carry 4 studs of 1/2″ but on circles of 60.3 mm and 66.7 mm respectively. When two systems of different class must be joined, a spool with one flange of each class is used, or a transition flange made for the case.

What is the real difference between a slip-on and a welding neck flange?

At the same class and the same material, the allowable pressure is identical: ASME B16.5 tabulates them in the same column of the same pressure-temperature table. The difference lies in the welded joint. The slip-on slides over the pipe and needs two fillet welds, outer and inner; the welding neck is joined with a single butt weld, bevelled and radiographable end to end, and its tapered neck spreads the stress instead of concentrating it in the weld. That is why the welding neck is the one specified for cyclic service, fatigue, high temperature and wherever the weld must be certified by non-destructive examination; the slip-on is cheaper, faster to align and sufficient in static service.

Why does the socket weld reference say SCH 40 and the welding neck one say SCH 80?

Because in those two constructions the inner hole of the flange must reproduce the pipe inside diameter, and that diameter changes with the schedule. ASME B16.5 tabulates the bore of Class 150 and 300 welding neck and socket weld flanges for standard wall pipe per ASME B36.10M —which at NPS 10″ and smaller is Schedule 40— and supplies it that way unless the purchaser asks for another. In Class 600 the standard does not tabulate it: the purchaser specifies it. Ordering a welding neck flange without declaring the schedule leaves an internal step at the weld root if the pipe is not standard wall.

How many studs, and what size, does an NPS 4 flange take?

It depends on the class. Class 150: 8 studs of 5/8″, length 90 mm (3.50 in), on a 190.5 mm (7.50 in) circle. Class 300: 8 studs of 3/4″, length 115 mm (4.50 in), on a 200.0 mm (7.88 in) circle. Class 600: 8 studs of 7/8″, length 145 mm (5.75 in), on a 215.9 mm (8.50 in) circle. The lengths are those of the standard raised face and do not include the height of the stud points. Material, tightening torque and sequence live in the flange stud and bolting data sheet.

Does the thickness I measure with a caliper include the raised face?

Yes. The t_f thickness that ASME B16.5 tabulates corresponds to the flange with its standard raised face: 2 mm (0.06 in) in Classes 150 and 300, and 7 mm (0.25 in) from Class 400 up. That is why a flat-faced flange may measure the tabulated t_f minus the raised face, or t_f plus 2 mm, and both versions comply with the standard. If you are comparing a measured thickness against a table, first check which of the two cases you are holding.

Does the same A105 flange serve for potable water and for steam?

The flange does, within its pressure-temperature table; the joint as a whole does not always. In potable water the problem is not pressure but corrosion: carbon steel rusts in permanent contact with water and that rust ends up on the sealing face. In steam the problem is the gasket and the thermal cycling, not the steel. In both cases the material decision is taken against the fluid and not against the class: when the fluid demands corrosion resistance, selection moves to forged A182 stainless steel flanges and to their own pressure-temperature table, which is not the same.

5. Application notes

6. Technical notice and limitation of liability