Summary:
- Flange standards govern the dimensional and pressure-temperature rating requirements for pipe flanges used across virtually every industrial piping system — and choosing between ANSI/ASME, DIN, EN, and JIS standards has implications for interchangeability, cost, availability, and compliance that extend far beyond the initial specification decision.
- Pressure Class (ASME) and Pressure Nominal (PN) ratings are fundamentally different systems that cannot be directly equated without reference to specific material, temperature, and bore combinations — a misunderstanding that regularly causes specification errors and procurement headaches.
- Understanding the full picture of flange standards — face types, bore configurations, gasket selection, bolting requirements, and installation practice — is essential for anyone specifying valve connections or managing piping systems in industrial environments.
There’s a moment that every process engineer experiences at least once: you have a valve from one supplier, pipe from another, and a flange from a third, and they absolutely should all be compatible because they’re all “150 pound flanges” — and yet the bolt holes don’t line up. Welcome to the world of flange standards. It’s a world where “compatible” means something very precise and the price of not understanding the detail is a very awkward conversation with the client. This guide will make sure that conversation never has to happen.
The Main Flange Standard Families
Industrial flange standards fall into several major families, each with its own dimensional system, pressure-temperature rating method, and geographic dominance:
ASME B16.5 governs pipe flanges and flanged fittings for NPS ½ through NPS 24 (nominal pipe sizes ½ inch through 24 inches) in Pressure Classes 150, 300, 600, 900, 1500, and 2500. This is the dominant standard in North America, the oil and gas industry globally, and much of the Middle East and Asia-Pacific. ASME B16.5 defines flange dimensions (bolt circle, number of bolts, OD, facing dimensions), allowable materials, and pressure-temperature (P-T) ratings for each material group.
ASME B16.47 extends ASME B16.5 to larger diameters — NPS 26 through NPS 60 — in two series: Series A (formerly MSS SP-44) and Series B (formerly API 605). The two series are not interchangeable, even within the same NPS and Class — their bolt circle diameters and bolt hole patterns differ. This catches out engineers who assume all “ASME” large-bore flanges are the same.
EN 1092-1 (previously DIN 2501 in Germany) is the European standard, using the PN (Pressure Nominal) rating system — PN2.5, PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160, PN250, PN320, and PN400. EN 1092-1 is dominant in Europe, across much of Africa, and in projects engineered to European standards. The standard covers dimensions for each PN and DN (nominal diameter) combination.
JIS B2220 is the Japanese Industrial Standard for pipe flanges, widely used in Japan and Japanese-engineered projects across Asia. JIS flanges use a similar PN-type rating system but have different dimensions from EN flanges — they are not interchangeable with EN or ASME flanges even at nominally similar pressure ratings.
API 6A and API 6D are industry-specific standards from the American Petroleum Institute. API 6A covers wellhead and Christmas tree equipment and introduces flanges with much higher pressure ratings than ASME B16.5 Class 2500 — up to 20,000 psi (1380 bar). API 6D covers pipeline valves and uses ASME B16.5 dimensional data but with specific additional requirements.
ASME Pressure Classes Explained
The ASME Pressure Class system is widely misunderstood. The Class number (150, 300, 600, etc.) is NOT a direct pressure rating in psi. It is a group designation that, combined with the flange material group and the operating temperature, gives the allowable pressure rating from tables in ASME B16.5.
The pressure rating of a Class 150 flange in carbon steel (A105 or A216 WCB) at ambient temperature (up to 38°C) is 19.6 bar (285 psi). At 260°C, that same Class 150 carbon steel flange is rated at only 13.1 bar (190 psi). At 450°C, it drops to 9.3 bar (135 psi).
This temperature de-rating is critical and is one of the most common sources of engineering error. Engineers specify Class 150 flanges for “low pressure” service without checking the temperature de-rating, then find that their high-temperature system exceeds the allowable pressure for Class 150 at operating temperature. The fix is either to upgrade to Class 300 or to specify a higher-strength material group for the Class 150 flanges — a higher material group can maintain a higher pressure rating at elevated temperature.
Material groups in ASME B16.5 range from Group 1.1 (standard carbon steel, the lowest) through Group 3.3 (high-alloy nickel steels). Higher group numbers generally equate to higher pressure-temperature ratings for the same Class designation. The ASME B16.5 P-T rating tables must be consulted for each specific material group and Class to confirm adequacy.
PN Rating vs. ASME Class: Why They Don’t Directly Convert
One of the most persistent sources of confusion in international piping work is the attempted direct equivalence of PN ratings and ASME Classes. You’ll see tables published that say “PN40 ≈ Class 300” or “PN16 ≈ Class 150.” These tables are misleading. Here’s why:
PN (Pression Nominale or Pressure Nominal) is a designation for pressure rating at ambient temperature in bar. PN40 means the flange is rated for 40 bar at ambient temperature (nominally around 20°C). But the pressure-temperature relationship for PN flanges, defined in EN 1092-1, is different from the ASME system. The permitted operating pressure at elevated temperatures follows a different de-rating curve.
Furthermore, the dimensional standards are different. A PN16 EN 1092-1 flange and a Class 150 ASME B16.5 flange in DN50/NPS2 have different bolt circle diameters, different bolt hole counts (typically), and different face dimensions. They are NOT interchangeable, even though they’re at similar nominal pressure ratings. You cannot bolt an EN flange directly to an ASME flange and expect a leak-free joint — the faces won’t align correctly and the bolt circles may not match.
The only correct approach to interfacing between EN and ASME piping standards is to use an adapter flange that matches both dimensional standards, or to establish a defined interface point where one standard ends and the other begins, with both sides of the interface following their respective standards at that point.
Flange Face Types and Their Applications
The flange face is the machined surface on which the gasket sits and the seal is made. Different face types suit different applications and are not interchangeable without modification.
Raised Face (RF) is the most common face type for ASME B16.5 Class 150 and Class 300 flanges. The raised face has a defined height (1.6mm for Class 150 and 300, 6.3mm for Class 600 and above) and standard spiral-wound gasket seating surface finish. RF flanges are used with spiral wound, flexible graphite, or compressed fibre gaskets. The vast majority of industrial process piping uses RF flanges.
Flat Face (FF) is used where the mating flange (typically a cast iron or non-metallic flange) cannot accept the concentration of bolt load that a raised face creates. Flat face flanges mate with full-face gaskets that extend to the bolt holes. You must use flat face on both flanges when mating to cast iron flanges — installing a raised face steel flange against a flat face cast iron flange will crack the cast iron due to the concentrated bolt load.
Ring Type Joint (RTJ) uses a metal ring gasket seated in a groove machined into the flange face. RTJ provides superior sealing under high pressure and temperature, and is the standard for Class 600 and above in high-pressure hydrocarbon and gas service. The ring gasket is available in soft iron, 304SS, 316SS, Inconel, and other materials depending on the service. RTJ flanges must be mated with RTJ flanges — they cannot be mixed with RF or FF flanges.
Tongue and Groove (TG) and Male and Female (MF) are self-aligning face types where one flange has a raised tongue or male face and the mating flange has a groove or female face. They provide inherent gasket alignment and good sealing, and are used in high-pressure applications, particularly for heat exchanger and vessel nozzle connections. They must be mated in matching pairs.
Flange Bore and Schedule Compatibility
A critical specification point that often gets missed: the bore of a flange must match the internal bore of the adjacent pipe. A flange with a bore sized for Schedule 40 pipe installed on Schedule 80 pipe will have a step discontinuity at the weld that creates turbulence, erosion, and in some services, a significant flow obstruction or dead zone that traps corrosion product.
ASME B16.5 flanges are available with bores machined to suit standard pipe schedules. When ordering flanges, specify the bore schedule — “NPS 4, Class 600, ASME B16.5, RF, A105, bore to suit Schedule 80.” The manufacturer will machine the bore to the correct ID for Schedule 80 pipe at NPS 4 (approximately 97mm ID). Don’t leave the bore unspecified and assume the flange will come correctly bored for your application.
Gasket Selection for Flanged Joints
The gasket is the sealing element of the flanged joint, and its selection is at least as important as the flange specification itself. The wrong gasket will leak; the right gasket, properly installed, will seal reliably for years.
Spiral wound gaskets (SWG) are the standard for ASME B16.5 raised face flanges in Class 300 and above, and widely used in Class 150 for elevated temperature and pressure applications. They consist of a V-shaped metallic strip wound with a soft filler (typically flexible graphite or PTFE) in alternating layers. The metal winding provides resilience and recovery; the filler provides sealability. SWGs are available with inner and outer rings — the outer centering ring locates the gasket concentrically in the bolt circle; the inner ring prevents inward buckling under high bolt loads.
Compressed non-asbestos fibre (CNAF) gaskets are the standard for lower pressure Class 150 flanges in non-critical service. They’re flat sheet gaskets cut to the raised face dimensions, made from aramid, carbon, or glass fibre with a rubber binder. They’re cheap, widely available, and suitable for water, steam, and mild chemical service. They should NOT be used in high-temperature, high-pressure, or critical hydrocarbon service — use SWG instead.
RTJ ring gaskets for Ring Type Joint flanges are available in several cross-sectional shapes: oval ring (R type) and octagonal ring (R and RX types). Octagonal rings have flat seating surfaces that provide more contact area than oval rings and are generally preferred for new installations. RTJ rings must be specified in a material softer than the flange face material to achieve sealing — a 316SS ring in a 316SS RTJ groove will gall and not seal. Specify soft iron, 304SS, or 410SS rings in 316SS or higher alloy RTJ grooves.
Bolting Requirements for Flanged Joints
Flanged joint integrity depends on achieving the correct bolt load. Insufficient bolt load means the gasket is not adequately compressed and will leak; excessive bolt load can crush the gasket (particularly CNAF types) or overload the flange, causing bowing or cracking.
ASME B16.5 specifies the stud bolt sizes and quantities for each flange class and nominal size. The standard bolting material is ASTM A193 B7 studs with A194 2H heavy hex nuts for ambient temperature service up to around 425°C. For low-temperature service (cryogenic applications), A320 L7 studs with A194 4 nuts provide adequate toughness at low temperatures. For high-temperature service above 425°C, higher alloy bolting (B16, B7M, B8 grades) may be required.
Bolting torque must be specified and followed. Torque specifications are provided by gasket manufacturers based on the flange dimensions, gasket type, and target bolt load. Using a torque wrench (not air-impact tools) to achieve consistent bolt loading is mandatory for critical service flanges. Torque should be applied in a cross-pattern (opposite bolts tightened sequentially) to ensure even gasket compression, typically requiring three passes to reach the target torque.
Large Bore Flange Standards: ASME B16.47 Series A vs. Series B
For piping above NPS 24 (DN600), the choice between ASME B16.47 Series A and Series B flanges is a significant decision that must be made consistently across the project. The two series are not interchangeable:
Series A (originally MSS SP-44) is the standard used in general process piping and the chemical industry for large diameter systems. It tends to have larger bolt circles and more bolts than Series B.
Series B (originally API 605) is common in pipeline applications and transmission system piping. It uses fewer, larger bolts than Series A for the same nominal size and Class.
A fundamental rule: never mix Series A and Series B flanges in the same system. They cannot be bolted together. Project specifications should state which series is required and enforce it throughout procurement.
Flange Specification Checklist
Before finalising any flange specification, confirm the following:
□ Standard: ASME B16.5, B16.47 Series A or B, EN 1092-1, JIS B2220, API 6A
□ Nominal Size (NPS or DN)
□ Pressure Class (ASME) or PN (EN)
□ Facing Type: RF, FF, RTJ, TG, MF
□ Material (body and any hard-facing): A105, A182 F316L, Duplex, etc.
□ Bore/Schedule: to match adjacent pipe
□ Mating flange standard confirmed compatible
□ P-T rating verified at operating temperature for specified material group
□ Gasket type and material specified
□ Bolting material and torque specification included
□ Corrosion protection for external surfaces specified
□ NACE MR0175 compliance if required for sour service
Flange standards are not glamorous, but getting them right is what separates a project that installs cleanly and commissions first time from one that generates a stream of NCRs, procurement re-orders, and very red-faced conversations in the engineering office. Master the detail here and it becomes second nature — the kind of unconscious competence that marks out the experienced piping engineer from the rest.