How to Specify the Right Flange for a Bolted Pipe Joint

The right steel flange is not selected by nominal pipe size or pressure class alone. A usable specification identifies the flange type, NPS, class, facing, material specification and grade, dimensional standard and project-required edition, and bore or matching pipe schedule where applicable. It must also account for service conditions, mating components, gasket and bolting requirements, documentation, and the approved assembly procedure.

This guide applies to steel pipe flanges, but its material comparison is intentionally limited: it discusses common 304L and 316L stainless grades without attempting to select among carbon steel, low-alloy steel, duplex stainless, or nickel alloys. Material selection for pressure service may require qualified piping, mechanical, corrosion, or materials engineering review. If the design basis or an existing component cannot be verified, stop before ordering, replacing, or installing the flange.

Steel flange basics: what it is and what must match

A steel pipe flange creates a detachable connection between pipe and components such as valves, fittings, equipment, or pressure vessels. A typical joint uses two compatible mating flanges, bolting and nuts, and a gasket.

This article concerns steel pipe flanges, not the flange portions of structural beams, wheels, ductwork, or unrelated components.

The flange is only one part of the pressure-boundary joint. The complete connection can include:

• The flange and its attachment to the pipe
• The flange face and mating component
• The specified gasket
• Studs or bolts and matching nuts
• Washers where required
• Lubricant where specified
• An approved alignment and tightening procedure
• Inspection and quality-control records

These parts have to work as a system. A gasket or bolt-tightening value cannot be selected safely from the words “steel flange,” or even from size and class, without the rest of the joint design.

Matching NPS is likewise insufficient. Flanges of the same nominal size can differ in class, dimensional standard, facing, material, bore, thickness, bolt-hole arrangement and, where a standard defines multiple series, series. Replacement work therefore requires a documented compatibility check rather than a visual size match.

Steel flange types and their practical roles

The flange type describes its form and how it attaches to the pipe or closes the system. Common options include weld-neck, slip-on, blind, threaded, socket-weld, and lap-joint flanges.

Flange type	How it connects	Typical role	Checks before selection
Weld-neck	Butt-welded to the pipe through a neck	Commonly considered for more demanding pressure or temperature service	Bore or schedule, weld requirements, material, loads, code and inspection
Slip-on	Slides over the pipe and is welded	Often marketed as an economical arrangement for less demanding service	Bore fit, required welds, material, loads, service and code permission
Blind	Bolts to a mating flange without a flow opening	Closes a pipe end, nozzle, or other opening	Rating, facing, material, gasket, bolting and imposed loads
Threaded	Screws onto compatible pipe threads	Commonly associated with smaller pipe sizes	Thread compatibility, sealing arrangement, material, service and code restrictions
Socket-weld	Pipe enters a socket and is welded	Commonly associated with smaller pipe sizes	Dimensions, approved welding details, material, service and code restrictions
Lap-joint	Rotates behind a separate stub end	Provides the bolted backing element in a stub-end arrangement	Stub-end compatibility, facing arrangement, dimensions, material and load path

That does not make every weld-neck flange suitable for severe service. Design pressure and temperature, external loads, cyclic duty, material, welding requirements, the governing code, and the project specification still control.

That commercial description is not universal permission to use them in every nominally low- or medium-pressure system.

A blind flange closes an opening rather than joining two through-flowing pipe sections. Although it has no center bore, its specification still needs to cover rating, material, facing, mating flange, gasket, bolting, and applicable loads.

Threaded and socket-weld flanges are commonly associated with smaller pipe sizes. That general association does not establish suitability for hazardous, cyclic, corrosive, or high-temperature service; those applications require code- and service-specific review.

It should not be treated as interchangeable with an integral flange merely because its nominal size and class appear to match.

Commercial catalogs also use labels such as backup, plate, paddle-blind, tank, and filler flange. Those labels can help locate products, but the governing standard, approved drawing, and manufacturer’s technical documentation must define what is actually being supplied.

B16.5 or B16.47? Map the standard by size, class, and type

Start by separating four specification layers. They work together; one does not replace another.

Specification layer	What it addresses	Typical reference
Governing design code	Overall design, service and pressure-boundary requirements	Project-selected piping or vessel code
Dimensions and ratings	Size range, class, dimensions, facings, tolerances and rating tables	ASME B16.5 or B16.47
Material and manufacture	Chemistry, properties, heat treatment and manufacturing requirements	Applicable ASTM material specification
Joint assembly	Cleaning, alignment, component installation, tightening and quality control	ASME PCC-1 and the approved project procedure

A dated ASME summary of the B16 standards reports the following scope:

• ASME B16.5: NPS 1/2 through NPS 24 for Classes 150, 300, 400, 600, 900, and 1500.
• ASME B16.5 Class 2500: NPS 1/2 through NPS 12, not through NPS 24.
• ASME B16.47: NPS 26 through NPS 60 for large-diameter weld-neck and blind flanges in Classes 75, 150, 300, 400, 600, and 900.

Those figures provide an initial scope map, not proof of current project applicability. For example, B16.5 may be the starting point for a standard NPS 20 flange, while B16.47 may be the starting point for an NPS 36 weld-neck or blind flange. Flange type, material, governing code, owner requirements, and the contractually adopted edition still need to be checked. For B16.47 work, confirm the required series rather than assuming Series A and Series B are interchangeable.

ASTM A182 is associated with material and manufacturing requirements for relevant forged or rolled alloy- and stainless-steel flanges and related components. It does not provide the complete dimensional and pressure-rating framework supplied by a dimensional standard such as B16.5 or B16.47. A supplier’s overview of stainless flange specifications illustrates that distinction, but the applicable ASTM text and project requirements must govern an actual purchase.

Similarly, PCC-1 addresses pressure-boundary bolted-flange joint assembly.

Edition control is essential. The ASME scope summary above was published in 2016, while a 2020 ASME Press reference chapter cites B16.5—2017, B16.47—2017, and PCC-1—2018. Those references do not establish the edition currently governing a particular job. Verify the edition incorporated by the contract, project specification, jurisdiction, and governing code rather than writing “latest edition” by habit.

Why Class 150 does not mean 150 psi

Class 150 is not a universal maximum pressure of 150 psi. A class designation belongs to a pressure-temperature rating system. Allowable pressure depends on the flange material and temperature and must be taken from the applicable official rating table, then checked against the governing design code and project requirements. This distinction is also summarized in the general technical overview of pipe flanges.

For a proposed NPS 2, Class 150 flange, use this bounded workflow:

1. Identify the exact material specification and grade.
2. Establish design pressure and design temperature, not merely normal operating conditions.
3. Confirm the applicable dimensional and rating standard and project-required edition.
4. Find the rating for that material group, class, and temperature in the official table.
5. Check the governing design code, project specification, loads, corrosion requirements, and service restrictions.
6. Verify the mating flange, facing, gasket, bolting, and complete joint arrangement.

Do not calculate or guess a pressure rating from the class number alone.

Moving to a higher class does not automatically solve the specification.

For replacement work, treat a marking such as “2 inch, 150” as a starting clue rather than a complete identification. The standard, type, face, dimensions, material, bore, mating component, and service basis still require confirmation.

Material and facing choices: narrow the field without guessing

Material selection begins with the process and environment, not with whichever grade is available. Relevant inputs can include:

• Fluid composition and concentration
• Chlorides and other contaminants
• Design and upset temperatures
• Expected corrosion or erosion mechanisms
• Fabrication and welding requirements
• Cleaning chemicals and external exposure
• Material pairing and contamination controls
• Governing code and owner specification

This article’s specific grade comparison is limited to common stainless options. It is not a selection guide for carbon steel, low-alloy steel, duplex stainless, or nickel alloys.

304L and 316L are common stainless flange grades in supplier catalogs. One supplier describes 316 as containing molybdenum and generally offering greater corrosion resistance than 304. That supplier comparison is useful for initial screening, not final service approval.

Fluid chemistry, chloride concentration, temperature, fabrication, contamination, specific corrosion mechanisms, and project requirements can change the result. General supplier recommendations about elevated-temperature grades should not be converted into universal design rules.

Common facing configurations include:

• Raised face (RF)
• Flat face (FF)
• Ring-type joint (RTJ)

The facing must be compatible with the mating flange and specified gasket arrangement. The applicable specification may also control surface condition and finish. Bolt holes lining up does not, by itself, establish a compatible joint.

Gasket and bolting selection also requires more than flange type. Process conditions, facing geometry, surface condition, joint design, assembly method, and project requirements must all be considered. Lubricant and tightening requirements belong in the approved assembly procedure rather than being inferred from size and class.

A steel flange specification and purchasing checklist

A request such as “price a two-inch stainless 150 flange” leaves major decisions unresolved. Suppliers commonly ask for size, class, facing, and grade, but a project-ready purchase description usually needs more.

Use this copy-ready template:

Nominal pipe size: Pressure class: Flange type: Facing and required finish: Material specification and grade: Dimensional standard: Project-required standard edition: B16.47 series, where applicable: Bore or matching pipe schedule, where applicable: Quantity: Design and service requirements: Mating-component details: Inspection and testing requirements: Material test report requirements: Heat-number traceability requirements: Marking, preservation and packaging requirements: Required submittals and approvals: Delivery requirements:

Here is a format example, not an approved design:

NPS 2, Class 150, raised-face slip-on flange, ASTM A182 Grade F316L, ASME B16.5 edition required by project, bore matched to specified pipe, material test report and heat traceability required.

The responsible designer must still confirm that the flange type, material, facing, class, bore, and joint arrangement suit the service and governing requirements.

Keep stable technical requirements separate from volatile commercial information. Price, stock, shipping status, and lead time should be confirmed for the exact manufacturer, product, quantity, documentation package, and delivery location when the order is placed.

For replacement work, run a structured audit:

• Confirm the governing dimensional standard and required edition.
• Confirm the B16.47 series where applicable.
• Verify class and pressure-temperature suitability.
• Identify the flange type and attachment method.
• Check the facing and face condition.
• Verify dimensions, bolt pattern, and bore.
• Confirm the material specification and grade.
• Verify the matching pipe schedule where applicable.
• Check the mating flange.
• Confirm gasket and bolting compatibility.
• Resolve missing or illegible traceability before installation.

A catalog match is not enough when the legacy component’s standard, material, or service basis is unknown.

Receiving and assembly checks before the joint enters service

Receiving inspection should compare the delivered flange with the purchase order, approved submittals, drawings, and required documentation. Check:

• Manufacturer identification
• Nominal pipe size and class
• Flange type and facing
• Material specification and grade
• Heat number where applicable
• Material test reports and certificates
• Dimensions, bore, bolt holes, and bolt circle
• Required standard and series
• Face condition and shipping damage
• Preservation and traceability
• Specified inspection or test records

Supplier documentation indicates that flange identification may include the manufacturer, nominal size, material grade, and heat number where applicable, but those markings must agree with the purchase requirements and supporting records (PipingNow).

At assembly, crews should follow the approved project procedure rather than a generic internet torque chart. At a high level:

1. Clean and inspect flange faces, gasket-contact surfaces, fasteners, nuts, and related components.
2. Verify acceptable alignment and fit-up without using the bolting to force an unapproved condition into place.
3. Confirm the specified gasket and bolting against the work package.
4. Install the gasket and fasteners as directed.
5. Apply the specified lubricant when the approved procedure requires it.
6. Use the controlled tightening method and sequence in that procedure.
7. Complete required pre-assembly, in-process, and final quality checks.
8. Record the required joint and inspection information.

These topics align with ASME’s bolted-flange joint assembly training, which covers component identification, cleaning, inspection, alignment, gasket and bolt installation, lubrication, tightening methods, and quality assurance. This article deliberately provides no torque values, bolt loads, gasket prescriptions, or universal tightening sequence; those details depend on the engineered joint and approved procedure.

Before a pressure-boundary joint enters service, crews should have the approved procedure, applicable standards and editions, manufacturer documentation, and required inspection release. Ratings, loads, substitutions, damaged faces, unknown legacy components, and nonstandard fit-up require qualified engineering resolution.

Can ASME and EN, DIN, or ISO flanges be bolted together?

Do not assume they can.

Verify both component standards and editions, dimensions, materials, facings, gasket arrangement, bolting, pressure-temperature ratings, and governing-code acceptance. Any proposed cross-standard connection or field discrepancy should be resolved through a project-approved engineered design, not by modifying holes or forcing the components together.

Do flange markings prove that a steel flange is compliant?

No. Markings may identify a manufacturer, nominal size, class, material grade, heat number, or claimed standard, but the stamp alone does not prove material identity or conformity.

Compliance requires the marking to agree with the purchase order, dimensional inspection, material test report, heat traceability, required certificates, and specified inspection or testing. Missing, inconsistent, altered, or unreadable identification is a hold point, not a reason to infer compliance from appearance.

Is an ASME bolted-flange microcredential the same as assembler qualification?

No. A microcredential documents completion of a defined training and evaluation pathway. It is not automatically equivalent to practical qualification under an employer, owner, code, or project program.

ASME states that its on-demand pathway requires completion of the online material and a passing score of at least 75%. It also says registrations for its former Bolting Specialist Qualification Program stopped in 2023 because PCC-1—2022 no longer aligned with that program (ASME). Determine separately whether the job requires a practical examination, employer authorization, documented experience, refresher training, or another qualification route.

The field-ready rule is simple: never order or replace a steel flange from size and class alone. Build the specification across the complete chain—service conditions, governing code, project-required standard edition, flange type, class, facing, material, bore, mating components, gasket and bolting requirements, documentation, and approved assembly procedure. Stop for qualified engineering review whenever those inputs are missing or uncertain.