ANSI flanges connect pipes, valves, and equipment using standardized dimensions, bolt patterns, and pressure classes. In practice, “ANSI flange” is a common industry label for flanges manufactured to ASME B16.5 and ASME B16.47 dimensional rules. Standardization matters because flange failures are rarely “random” — they usually trace back to one of four controllable items: (1) wrong standard (ASME vs EN/DIN), (2) wrong pressure class, (3) wrong facing/gasket combination, or (4) poor joint assembly (alignment/bolt load).
Engineering note: If your project references ASME B16.5/B16.47, the safest workflow is “standard first, material second, assembly third.” Start with the governing standard and design code, then confirm material group and pressure-temperature rating, then apply proven bolted-joint assembly practice (e.g., ASME PCC-1). ASME B16.5 scope and content and ASME B16.47 scope and series define what is interchangeable and what is not.
- Compatibility across systems (bolt circle, bolt holes, facing geometry).
- Reliable installation when paired with correct gasket + controlled bolt load (not “tight as possible”).
- Traceable selection: NPS + class + facing + material + standard = a joint you can verify and audit.
Quick answer for most buyers/engineers: choose the standard (B16.5 or B16.47), confirm NPS, select pressure class by pressure-temperature rating (not by “psi name”), pick facing to match gasket and equipment, then verify bolt pattern and assembly method.
| Selection checkpoint | What to verify on drawings / datasheet |
|---|---|
| Standard | ASME B16.5 (typical up to NPS 24) or ASME B16.47 (large diameter, Series A/B). Do not mix with EN 1092-1 bolt patterns unless you use an engineered adapter. |
| Size definition | NPS (nominal), pipe schedule, and bore requirement (especially for flow/erosion service). |
| Pressure class | Class 150–2500 is a pressure-temperature rating system dependent on material group; verify at design temperature. |
| Facing + gasket | RF/FF/RTJ must match gasket type and mating equipment limits (cast iron, lined pipe, etc.). |
| Bolting + assembly | Bolt grade, lubrication, tightening sequence, target bolt stress/torque method, and joint alignment practice. |
ANSI Flanges Overview
What Are ANSI Flanges
ANSI flanges are standardized pipe connectors designed for compatibility and safety.
On drawings, you will often see “ANSI flange” used as shorthand for ASME/ANSI dimensional standards. For most industrial piping, that means ASME B16.5 for common sizes and classes, and ASME B16.47 for large-diameter steel flanges. The critical point is not the label — it is whether the flange you buy matches the referenced standard, pressure class, and facing geometry, because that is what controls interchangeability and bolting layout.
Here is what ASME B16.5 standardization gives you in practice:
| Characteristic | ANSI Flanges Description |
|---|---|
| Versatility | Used across oil & gas, chemical, utilities, water, HVAC, and sanitary services (material/facing selection drives suitability). |
| Standardized Specifications | Dimensional rules and pressure-temperature ratings are defined in ASME B16.5 (and large diameter rules in ASME B16.47). |
| Pressure Ratings | Class system (150–2500) is a pressure-temperature rating by material group — not a single “psi number.” |
| Dimensional Specifications | Standard OD, bolt circle, bolt holes, thicknesses/hubs, facing details by NPS + Class. |
| Bolt Hole Patterns | Recognized bolt patterns that must match the mating flange (including Series A vs Series B differences for large diameter). |
| Material Specifications | Material is governed by ASTM/ASME material specs (forgings/plates/castings) and project requirements (corrosion, temperature, code). |
Role in Piping Systems
ANSI flanges make assembly, maintenance, and upgrades simple — if the joint is assembled correctly.
You can connect or disconnect sections of pipe by bolting flanges together. This modular approach is why flanged joints show up around valves, pumps, strainers, control stations, and equipment nozzles. The tradeoff is that a flanged joint is sensitive to bolt load and alignment: a flange can be dimensionally “correct” and still leak if the gasket is damaged, the faces are misaligned, or bolt load is uneven.
- Good bolted-joint assembly practice (ASME PCC-1) focuses on alignment, controlled tightening, and repeatability — the same basics that prevent most startup leaks.
- You can replace or modify parts without cutting pipes, which reduces outage time.
- Flanges create planned access points for inspection, cleaning, and equipment swaps.
Field example (maintenance leak on restart): A Class 300 RF joint passed hydrotest, then seeped after the first hot run. Root cause was uneven bolt load from a rushed, single-pass tightening sequence and dry bolts. Fix: clean/lubricate threads/nut bearing surfaces, tighten in a cross pattern in multiple passes, and verify gap consistency. Prevention: specify a controlled assembly method (PCC-1 style) for critical joints.
Importance of Standardization
Standardization ensures safety, compatibility, and efficiency.
When you use ASME/ANSI standardized flanges, you can verify fit-up with measurable parameters: bolt circle diameter, bolt hole diameter/count, facing type, and thickness/hub geometry. That auditability is what makes procurement and quality control realistic in multi-vendor projects.
Where standardization fails in the field is usually “mixed standards.” The most common mismatch is bolting layout (EN/DIN PN pattern vs ASME class pattern). Even when the nominal size looks similar, the bolt circle and bolt-hole count can be different, and “making it fit” often creates gasket damage or eccentric loading.
Field example (bolt pattern mismatch): A contractor tried to mate an EN 1092-1 PN flange to an ASME Class flange on a water skid. Several bolts aligned, the rest did not. Root cause: different bolt circles/holes by standard. Fix: install the correct matching flange or an engineered adapter spool. Prevention: lock the governing flange standard in the project MTO and inspection checklist before purchasing.
Types of Flanges
Weld Neck Flanges
Weld neck flanges are preferred for high integrity joints — high pressure/temperature, high cyclic loads, or high consequence service.
The tapered hub improves stress distribution and helps resist bending moments from piping loads. In real plants, weld neck is commonly selected for main process lines, compressor discharge, high-temperature steam headers, and anywhere vibration/cycling makes joint stability a priority. If your system sees thermal cycling, vibration, or frequent pressure transients, weld neck is usually the first flange type to evaluate.
Slip-On Flanges
Slip-on flanges are widely used for low to moderate pressure service where installation speed and cost matter.
You slide the flange over the pipe and apply fillet welds. Slip-on is common in utility services (cooling water, firewater, low-pressure air) and non-critical systems. The engineering check to apply is not “will it fit,” but “is this service sensitive to fatigue or vibration.” In vibrating services, repeated stress can concentrate at the fillet weld area, so weld neck may be more reliable long-term.
Blind Flanges
Blind flanges seal the end of a piping system or vessel.
You use blind flanges for isolation, future tie-ins, hydrotest boundaries, and safe maintenance. Because a blind flange sees full end load, correct class, correct bolting, and correct gasket selection matter more than “general service.” If the system may be opened later, specify corrosion protection on the facing and protect the gasket seating surface during storage.
Socket Weld Flanges
Socket weld flanges are used on small-bore, high-pressure lines where a compact weld detail is needed.
They are commonly applied on instrument lines and small utility/process connections (often NPS 2 and below; some specifications allow up to NPS 4 depending on code/service). A key engineering note for stainless systems is crevice/under-deposit risk at the socket gap in chloride-bearing service — if the medium is aggressive or hygiene is critical, a butt-weld alternative may be easier to inspect and maintain.
Threaded Flanges
Threaded flanges let you connect pipes without welding, mainly on small-bore lines where hot work is restricted.
You screw the pipe into the flange. Threaded joints can be practical for temporary hook-ups, low-temperature services, and locations with strict hot-work controls. The limitation is vibration and cyclic movement: if supports are poor or the line shakes, threads can loosen over time. In higher consequence services, engineers often add a seal weld (as a leakage barrier, not as a strength upgrade) when allowed by code and procedure.
Lap Joint Flanges
Lap joint flanges are chosen where frequent disassembly is expected and corrosion-resistant stub ends are beneficial.
You use these flanges with a stub end. The flange can rotate for bolt alignment, which helps on complex skids and tight equipment rooms. In sanitary or corrosive service, you can use a higher-alloy stub end while keeping the backing flange material economical, as long as the system design accepts that configuration.
Tip: The right flange type depends on pressure/temperature, cyclic loading, corrosion mechanism, and how often the line will be opened. “Easy to install” is rarely the best single criterion for long-term reliability.
Flange Applications and Market Dynamics
| Flange Type | Key Applications | Engineering Note (selection driver) |
|---|---|---|
| Weld Neck | Oil and gas, power generation, chemical processing | Best for cyclic loads, high temperature/pressure, and high consequence service where stress distribution matters. |
| Slip-On | Water treatment, HVAC, general industrial piping | Common for utility service; verify fatigue/vibration risk and welding quality control. |
| Socket Weld | Small diameter, high-pressure piping systems | Compact for small-bore; check crevice risk and code limitations for service. |
| Threaded | Disassembly or fire hazard areas | Useful when welding is restricted; confirm vibration control and leakage risk management. |
| Blind | Sealing ends, maintenance | Full end load; bolt load control and gasket selection are critical for leak-free isolation. |
| Lap Joint | Frequent inspection or cleaning | Rotatable alignment; good for frequent teardown and corrosion-resistant stub-end strategy. |
Differences in Flange Dimensions
Flange dimensions control compatibility and load capacity — and they change with both NPS and pressure class.
In ASME systems, two flanges with the same NPS but different classes will often have different outside diameter, thickness, bolt circle, and bolt-hole details. That means you cannot assume “NPS matches = flange matches.” For large diameter, ASME B16.47 introduces Series A and Series B, which are not universally interchangeable due to different bolt patterns and geometry.
| Flange Type | Pipe Size Range | Pressure Classes |
|---|---|---|
| Slip-on | Commonly supplied in NPS 1/2″ to 24″ (verify by class and availability per ASME tables) | Varies by NPS and flange type; verify in ASME B16.5 dimension tables. |
| Blind | NPS 1/2″ to 24″ is common in ASME B16.5 scope | Classes per ASME B16.5; confirm rating at design temperature. |
| Lap joint | Typically within ASME B16.5 size scope for common process lines | Classes per ASME B16.5; check stub-end compatibility. |
| Socket weld | Small-bore applications (often NPS 2 and below; project-dependent) | Classes per ASME B16.5 where socket weld is listed; verify service limits. |
| Weld neck | Common across ASME size scope; also large diameter under ASME B16.47 | Wide class coverage depending on standard; confirm Series A/B for large sizes. |
| Threaded | Small-bore applications where threads are allowed by code/service | Classes per ASME B16.5 where threaded is listed; confirm leakage risk controls. |
You should always match flange type, standard series (if applicable), and dimensions to your piping system for safe and efficient operation — especially bolt circle and facing details.
ANSI Flange Dimensions
Understanding flange dimensions is essential for safe and efficient piping systems. Each dimension affects fit-up, bolt load distribution, and sealing reliability. In practical QA/QC, you do not “check one dimension” — you check a set: NPS + class + facing + bolt circle/holes + thickness/hub, then verify marking and material traceability. Sunhy uses CNC machining and inspection controls to keep flange geometry repeatable.
Nominal Pipe Size (NPS)
NPS determines the basic flange size and compatibility.
Nominal Pipe Size (NPS) is the primary size designation you use to match flanges to pipes and fittings. ASME B16.5 covers NPS sizes from 1/2 to 24 inches in its scope, and ASME B16.47 applies to large diameter flanges (commonly NPS 26 and above). ASME B16.5 selection should always be paired with pipe schedule/bore requirements, because the pipe’s actual OD/ID and the required flange bore can affect flow behavior and erosion risk in high-velocity service.
| NPS Size | Description |
|---|---|
| NPS 1/2 to NPS 24 | Common scope for ASME B16.5 flanges, enabling interchangeability when standard + class + facing match. |
| Dimensions | Change with NPS and class, affecting flange OD, bolt circle, bolt holes, and thickness/hub geometry. |
Tip: Always check NPS and pressure class together. Two NPS-matched flanges can still be non-mating if their class or standard series differs.
Outside Diameter (O.D.)
O.D. defines the total width of the flange and affects gasket fit, bolt access, and equipment clearance.
The outside diameter is measured edge-to-edge across the flange. Correct O.D. ensures the flange clears nearby structures, provides room for bolt tightening tools, and supports a gasket seating area appropriate for the chosen facing. In skid packages, O.D. errors show up as “we can’t get the wrench in” and “the flange hits the frame,” not only as leakage — so it is worth checking early during layout.
Inside Diameter (I.D.)
I.D. controls flow profile at the joint and can influence pressure drop, turbulence, and erosion in high-velocity service.
The inside diameter is the bore opening in the flange. In many services, minor bore mismatch is acceptable; in erosive/dirty service (slurry, sand-laden fluids) or high-velocity steam/gas, a sharp step can create local turbulence and accelerated wear at the pipe-to-flange transition. If your P&ID indicates high velocity or solids, treat flange bore as a functional requirement, not a “nice-to-have.”
- Flow behavior: a smooth bore transition reduces turbulence and noise in gas/steam lines.
- Sealing reliability: correct bore alignment helps prevent gasket edge exposure and blowout risk in some configurations.
- Integrity over time: bore steps can become erosion points; once the flange bore erodes, gasket load distribution can also degrade.
Field example (unexpected erosion at a flanged joint): A high-velocity wet gas line developed localized thinning near a slip-on flange bore after months of operation. Root cause was a bore mismatch creating a sharp internal ledge. Fix: replace with correctly bored flange/spool and remove the internal step. Prevention: specify bore tolerance/finish for high-velocity or erosive services and verify at incoming inspection.
Bolt Hole and Circle
Bolt hole patterns and bolt circle diameters control whether the joint can be assembled and whether bolt load is evenly distributed.
ASME standards define the number, size, and spacing of bolt holes for each NPS and pressure class. You need the correct bolt pattern to avoid forced alignment, which can damage gaskets and introduce bending into bolts. The bolt circle is the diameter of the circle passing through the centers of the bolt holes.
| NPS (inches) | Flange Diameter (in) | No. of Bolts | Bolt Diameter (in) | Bolt Hole (in) | Bolt Circle (in) |
|---|---|---|---|---|---|
| 1/2 | 3-1/2 | 4 | 1/2 | 0.62 | 2-3/8 |
| 1 | 4-1/4 | 4 | 1/2 | 0.62 | 3-1/8 |
| 2 | 6 | 4 | 5/8 | 0.75 | 4-3/4 |
| 4 | 9 | 8 | 5/8 | 0.75 | 7-1/2 |
| 6 | 11 | 8 | 3/4 | 0.88 | 9-1/2 |
| 12 | 19 | 12 | 7/8 | 1 | 17 |
| 24 | 32 | 20 | 1-1/4 | 1.38 | 29-1/2 |
Note: If bolts “almost” fit, do not ream holes on pressure boundary flanges as a shortcut unless the design authority approves it. Forced fit frequently turns into gasket damage and chronic leakage.
Flange Thickness
Flange thickness contributes to bending resistance, gasket seating stability, and pressure capability — but it is not the only strength variable.
Thicker flanges are generally associated with higher pressure classes, but the pressure-temperature rating is tied to material group and design temperature as well. Do not compare classes across different materials as if they are identical. If your line has significant external loads (heavy valves, thermal expansion forces, misalignment), thickness and hub geometry become just as important as internal pressure.
- Higher classes usually increase thickness and bolting requirements.
- External loads can cause flange rotation, which reduces gasket compression at the ID and increases leakage risk.
- For critical joints, specify alignment tolerances and assembly method, not only flange class.
Flange Face Types
Flange face type controls sealing mechanics, gasket selection, and mating limitations.
ANSI/ASME systems commonly use Raised Face (RF), Flat Face (FF), and Ring Type Joint (RTJ). The “best” face is the one that matches your service and mating equipment.
| Flange Face Type | Typical Applications |
|---|---|
| Raised Face (RF) | General process piping; commonly paired with spiral wound or sheet gaskets when assembly control is good. |
| Flat Face (FF) | Equipment/nozzles with flange-strength limits (common in some cast iron or fragile mating flanges); full-face gaskets are often used. |
| Ring Type Joint (RTJ) | High-pressure/high-temperature service where a metal ring gasket and machined groove provide robust sealing (refineries, drilling, critical hydrocarbon service). |
Field example (cast iron pump flange damage): A steel RF flange was tightened against a flat-face cast iron pump nozzle without proper facing compatibility. Root cause was concentrated contact stress at the raised face leading to flange stress in the cast iron. Fix: use a compatible flat-face mating arrangement (or approved spacer/adapter) and gasket suited to the equipment manufacturer’s limits. Prevention: confirm facing compatibility any time you connect steel piping to cast components.
Sunhy’s Precision and Quality Control
Sunhy ensures every flange meets strict dimensional requirements expected on engineered projects. In practice, that means you should be able to verify what you receive:
- CNC machining control to keep OD/ID, bolt circle, bolt holes, and facing geometry consistent.
- Incoming/outgoing dimensional checks with documented inspection points (OD, thickness, bolt circle, hole count/diameter, facing type).
- Traceability support (heat number/MTR) and optional PMI/NDT when specified in the PO/ITP.
You protect schedule risk when flange fit-up is predictable — especially in shutdown work where rework time is limited.
Flange Dimension Standards
ANSI/ASME B16.5
B16.5 covers flanges for the most common piping sizes and classes.
ASME B16.5 is the baseline reference for dimensional requirements and pressure-temperature ratings for many industrial flanges and flanged fittings. If your project specifies B16.5, treat it as the “source of truth” for bolt patterns, facing types, and dimensional interchangeability within its scope.
ANSI/ASME B16.47
B16.47 covers large-diameter steel flanges and introduces Series A and Series B.
Large diameter flange selection is not only “bigger B16.5.” Series A and Series B can differ in bolt patterns and geometry, so you must confirm which series is referenced on the drawing before purchasing. If you replace one series with the other without design approval, bolts may not align or bolt loads may change.
| Flange Type | ASME B16.5 (<NPS 26) | ASME B16.47 Series A (≥NPS 26) | ASME B16.47 Series B (≥NPS 26) |
|---|---|---|---|
| Weld Neck | Classes within B16.5 scope | Large diameter scope; series-specific | Large diameter scope; series-specific |
| Slip on | Classes within B16.5 scope | – | – |
| Blind | Classes within B16.5 scope | Large diameter scope; series-specific | Large diameter scope; series-specific |
| Lap Joint | Classes within B16.5 scope | – | – |
| Socket Weld | Classes within B16.5 scope | – | – |
| Threaded | Classes within B16.5 scope | – | – |
| Ring Joint Face | RTJ facing per B16.5 scope | RTJ facing options per series | RTJ facing options per series |
Note: For waterworks, you may also see AWWA flange standards used on specific systems. If the project is AWWA-based, do not assume ASME bolt patterns will match without verification.
Pressure Classes (150–2500)
Pressure classes are pressure-temperature ratings that depend on material group and temperature.
Engineers select class by checking the rated pressure at the design temperature, then adding margin for transients where required by the design code and project rules. The same class number can represent different allowable pressures for different materials at the same temperature. Do not size class using ambient pressure only if the system will run hot.
Example pressure-temperature ratings tables show the expected trend: as temperature increases, allowable pressure decreases. The simplified values below are provided as a typical reference range (final selection must use the official ASME tables referenced by your project):
| Temperature (°C) | 150 | 300 | 400 | 600 | 900 | 1500 | 2500 |
|---|---|---|---|---|---|---|---|
| -29 | 15.9 | 41.4 | 55.2 | 82.7 | 124.1 | 206.8 | 344.7 |
| 100 | 13.3 | 34.8 | 46.4 | 69.6 | 104.4 | 173.9 | 289.9 |
| 300 | 10 | 26.1 | 34.8 | 52.1 | 78.2 | 130.3 | 217.2 |
| 450 | 4.6 | 23.4 | 31.2 | 46.8 | 70.2 | 117.1 | 195.1 |
Tip: When service is hot or cyclic, verify both pressure-temperature rating and joint assembly method. Many “mystery leaks” are assembly/control issues, not flange geometry issues.
Reading Flange Dimension Tables
You read flange dimension tables to match the right flange to your pipe, pressure class, and facing.
Use a verification approach that mirrors how leaks are prevented in the field:
- Confirm the governing standard (B16.5 vs B16.47; Series A vs B for large sizes).
- Confirm NPS + class, then verify OD, bolt circle, bolt hole count/diameter, and thickness/hub geometry.
- Confirm facing type (RF/FF/RTJ) and gasket type compatibility.
- Confirm assembly method requirements for critical joints (controlled tightening sequence, lubrication, and inspection points).
Standardized dimensions help you swap parts and control quality — but only if you treat “standard” as a controlled project requirement, not a label on a quote.
Remember: If you cannot verify the flange standard + class + facing from documentation or markings, do not assume interchangeability.
Flange Connections in Industry
Applications in Oil, Gas, and Chemical
You find ansi flanges most often in oil and gas, petrochemical, and chemical processing plants.
These industries require joints that can handle pressure/temperature combinations, vibration, and strict leak-control expectations. On higher consequence hydrocarbon service, engineers often prefer weld neck with RF or RTJ depending on class and plant standard, then enforce controlled joint assembly and inspection.
- Flanges connect pipes, valves, and equipment across long runs and modular skids.
- In chemical plants, material selection (316L, duplex, nickel alloy) is often driven by chloride, acid, or caustic exposure risk.
- Flanges support planned isolation points for shutdowns and maintenance, reducing cutting/welding work on live units.
Field example (corrosion mechanism mismatch): A coastal chemical utility line used 304 stainless flanges on a chloride-bearing service. After operating exposure, cracking initiated under tensile stress at the joint area. Root cause: chloride stress corrosion cracking susceptibility in austenitic stainless under certain temperature/stress conditions (risk depends on chloride level, temperature, residual stress, and oxygen). Fix: upgrade material (often 316L/duplex/nickel alloy per process engineering) and control residual stress/crevices. Prevention: perform a corrosion mechanism review during material selection, not after leaks start.
Use in Food Processing and Waterworks
You depend on flanges for clean, reliable connections in food processing and water treatment.
These industries require corrosion resistance and maintainability. In food plants, flanged joints may be opened for cleaning; in waterworks, large diameters and site assembly constraints matter as much as corrosion resistance.
- In food processing, you use stainless steel flanges to connect pipes carrying liquids, steam, and CIP chemicals.
- Flanges let you disassemble and clean equipment, but gasket choice must tolerate temperature and cleaning chemicals.
- Water treatment facilities use flanges to join pipes on pumps, tanks, and distribution headers where planned maintenance is expected.
| Industry | Typical Flange Use |
|---|---|
| Food Processing | Corrosion-resistant, cleanable connections; verify gasket compatibility with cleaning chemicals and temperature cycling. |
| Waterworks | Leak-proof joints in treatment facilities; verify standard (ASME vs AWWA/EN) and bolt pattern before procurement. |
Power Plants and Mechanical Systems
You rely on ansi flanges to keep power plants and mechanical systems running safely.
Steam and thermal cycling create joint movement. If bolt load relaxes, leakage can occur at temperature even when the joint was tight at ambient. In those systems, controlled tightening, correct bolt material, and gasket selection matter as much as flange class.
- In power generation, you use flanges in boiler feed, steam distribution, cooling water, and auxiliary piping.
- Mechanical systems in buildings and factories use flanges for maintainability and replacement of rotating equipment.
- Standardization supports planned spares and predictable outage work.
Tip: For joints that see thermal cycling, treat bolt lubrication, tightening method, and re-torque policy (where allowed) as part of the engineering design, not a “field detail.”
Selecting ANSI Flanges
Assessing Application Needs
You must match flange selection to your system’s specific requirements.
Start with the design code and operating envelope: pressure, temperature, medium, cyclic loading, corrosion mechanism, and how often the joint will be opened. Then select flange type, size, class, facing, and material. This is also where you lock in the governing standard to avoid bolt pattern surprises later.
| Factor | Description |
|---|---|
| Types | Choose from weld neck, slip-on, blind, lap joint, socket weld, or threaded flanges based on load/cycle/maintenance profile. |
| Size | Confirm NPS, pipe schedule/bore, and dimensional table requirements (OD/bolt circle/holes/thickness). |
| Thickness | Supports bending resistance and gasket stability; do not treat thickness alone as “pressure rating.” |
| Bolt holes | Number, size, and bolt circle must match the mating flange; verify series for large diameter. |
| Standards | Lock ASME B16.5 vs B16.47 (Series A/B) or other standard (EN/DIN/AWWA) per project spec. |
| Pressure class | Select by pressure-temperature rating at design temperature for the correct material group. |
| Materials | Match corrosion mechanism and temperature to material (stainless/duplex/nickel alloy/carbon steel) per process and code requirements. |
Tip: If a joint has high consequence leakage (hydrocarbon, steam, toxic), treat flange selection and assembly requirements as one package: flange type + facing + gasket + bolting + controlled tightening method.
Pressure and Temperature Considerations
You need to select flanges that withstand your system’s pressure and temperature.
For safe selection, use a defined method rather than guesswork:
- Identify maximum operating pressure and design temperature (including upset cases if the code requires).
- Use the referenced standard’s pressure-temperature rating tables for the correct material group.
- Account for transients (water hammer, compressor surge, thermal cycling) as required by the design code/project rules.
Note: Always verify pressure-temperature rating at the design temperature, not only at ambient. Many under-classed flanges look fine in commissioning, then leak or distort in hot operation.
Material Selection
You must pick the right material for your environment and fluid type.
Material choice affects corrosion resistance, strength, and service life. Use process information (chlorides, acids, caustics, temperature) to determine corrosion mechanism risk, then match material accordingly. For stainless selection, pay attention to chloride SCC and crevice corrosion risk zones (gasket interfaces, sockets, deposits), not just “stainless = corrosion resistant.”
| Material | Advantages |
|---|---|
| Carbon Steel | Cost-effective and strong for many services; corrosion allowance/coatings may be required. |
| Alloy Steel | Improved elevated-temperature strength; used in some high-temperature/power services. |
| Stainless Steel | Corrosion resistance for food/chemical/marine; grade selection matters (e.g., 316L vs 304 in chloride exposure). |
| Cast Iron | Used in specific equipment/nozzle standards and low-pressure services; check facing/mating limitations carefully. |
| Aluminum | Lightweight for special services; verify temperature/strength limits and galvanic compatibility. |
| PVC | Corrosion resistant for specific chemical services; limited by temperature/pressure and mechanical strength. |
Sunhy specializes in stainless steel flanges where corrosion resistance and dimensional control are primary requirements. For aggressive media, confirm alloy grade, heat treatment condition, and inspection requirements in the PO/ITP.
Step-by-Step Selection Guide
You should follow a clear process to select the correct ANSI flange.
- Define service and risk. Pressure, temperature, medium, cyclic loads, consequence of leakage, and maintenance frequency.
- Confirm the governing standard. ASME B16.5 or B16.47 (Series A/B), or other specified standard.
- Select pressure class by design temperature. Use pressure-temperature ratings for the correct material group, and apply project/code margin rules.
- Choose facing + gasket as a pair. RF/FF/RTJ selection must match gasket type and mating equipment limits.
- Define bolting and assembly method. Bolt grade, lubrication, tightening sequence, and inspection hold points for critical joints.
Assembly checkpoint that prevents repeat leaks: Confirm flange face condition (no dents/spiral scratches), gasket is correct and undamaged, bolts are clean/lubed, alignment is within tolerance, then tighten in multiple passes using a cross pattern. If your project allows, align the method with ASME PCC-1 guidance.
Choosing the correct flange dimensions and pressure class is essential for safety and efficiency.
You reduce leaks, rework, and downtime by selecting the correct standard + class + facing, then controlling the bolt load during assembly. Sunhy’s manufacturing and inspection controls support reliable fit-up when the engineering inputs are correctly defined.
Precise ANSI (ASME) flange dimensions keep your piping systems safe and efficient.
You gain long-term reliability when you choose standardized flanges and verify them against the project’s referenced standard.
- Reduce incompatibilities by locking the flange standard and series early in the project.
- Improve joint reliability by matching facing + gasket + bolting + assembly method to service conditions.
- Use pressure-temperature ratings at design temperature, not ambient assumptions.
| Standard | What It Covers |
|---|---|
| ASME B16.5 | Flange dimensions, tolerances, and pressure-temperature ratings within its size scope. |
| ASME B16.47 | Large diameter steel flanges (Series A/B) with series-specific dimensional rules. |
| ASME B31.3 | Process piping design rules that govern how flanged joints are applied in process plants. |
| ASME B31.1 | Power piping design rules for power/steam systems. |
You should consult your project specifications, use the correct dimension tables, and apply proven assembly practice. For technical support or custom flange solutions, contact Sunhy.
FAQ
What is the significance of the Outside Diameter (O.D.) in ANSI flanges?
O.D. ensures compatibility with pipes, bolt access, and equipment clearances.
You use O.D. to confirm the flange will physically fit the installation space and allow tool access for bolting. Incorrect O.D. often causes installation delays even before leakage becomes a concern.
Which dimensions should you check when selecting an ANSI flange?
You must check NPS, O.D., bore (I.D.), bolt circle, bolt holes, thickness/hub geometry, and face type.
Use documentation and markings to verify standard + class + facing, then confirm the critical dimensions for fit-up and sealing.
| Dimension | Why It Matters |
|---|---|
| NPS | Matches nominal piping size designation |
| O.D. | Ensures fit and tool clearance |
| I.D. / Bore | Controls flow profile and erosion risk |
| Bolt Circle | Determines bolt alignment and interchangeability |
| Thickness/Hub | Affects flange rotation resistance and gasket stability |
| Face Type | Determines gasket selection and sealing mechanics |
How do bolt holes affect flange performance?
Bolt holes control bolt load distribution and whether the joint can be assembled without forced fit.
Wrong hole count/diameter/bolt circle causes misalignment, uneven bolt load, and higher leakage risk. For large diameter, confirm Series A vs Series B before ordering.
Do ANSI flanges have different pressure ratings?
Yes. ANSI (ASME) flanges use pressure classes (150–2500) that change allowable pressure with temperature and material group.
You select class by checking the pressure-temperature rating at the design temperature for the correct material group, then applying project/code margin requirements.
Why should you choose stainless steel flanges for your system?
Stainless steel resists corrosion and reduces maintenance when the grade matches the corrosion mechanism.
In sanitary or corrosive environments, correctly selected stainless (often 316L or duplex depending on chlorides/temperature) can reduce downtime from corrosion-related leaks. Grade selection still matters — “stainless” is not one material.
What is the safest way to prevent flange leaks during commissioning?
Control alignment, gasket condition, and bolt load — then tighten in multiple cross-pattern passes with clean, lubricated bolts.
Many commissioning leaks are caused by uneven bolt load or damaged gaskets, not by wrong flange dimensions. For critical joints, align the tightening method to a controlled bolted-joint practice such as ASME PCC-1.
Can you mix ASME (ANSI) flanges with EN/DIN flanges if NPS/DN look similar?
Not safely without verification — bolt circles and bolt-hole patterns often differ.
If you must connect different standards, use an engineered adapter spool or transition piece approved by the design authority, and verify face compatibility and gasket coverage.
What flange surface finish should you check before installing a spiral wound gasket?
Check that the facing is clean, undamaged, and within a typical spiral-wound surface finish range.
In many engineering practices, spiral wound gaskets are installed against a controlled serrated finish (typical ranges are commonly published by bolted-joint/gasket references; always follow the gasket manufacturer and project specification).
