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Flange Connections for Different Pipe Types

flange connections

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Flanges you select for your piping system must match the pipe material, size, pressure class, facing type, and service conditions to ensure mechanical integrity and standard compatibility. Most metallic pipe systems in the U.S. reference ASME B16.5 (NPS ½–24) and ASME B16.47 (NPS 26–60); waterworks systems commonly reference AWWA C207 (steel) and AWWA C228 (stainless). These standards govern geometry and drilling that ensure interchangeability and safe bolting. Choosing the correct and compatible flange for your system helps prevent leaks, overstress, or galvanic corrosion between dissimilar metals.

Key Concepts

1. Pressure class & temperature:

ANSI/ASME B16.5 uses Classes 150–2500 (temperatures and materials determine the allowable working pressure). AWWA classes are different and oriented to waterworks service.

2. Facing & gaskets:

Raised Face (RF) and Ring-Type Joint (RTJ) are common on steel/stainless systems; gray-iron systems are typically flat-faced and paired with full-face gaskets.

3. Bolt patterns:

Even for plastic piping systems, manufacturers follow the ANSI/ASME B16.5 Class 150 bolt pattern, so flanges physically mate with standard drilling (while pressure ratings are limited by the plastic).

4. Large diameters:

Over NPS 24, steel flanges follow ANSI/ASME B16.47 (Series A/B).

Compatibility Table: Pipe Material vs. Flange Standards

Pipe Material

Typical Flange / Fitting Standard(s)

Common Pressure / Size Range

Key Compatibility Notes

Carbon steel pipe

ASME B16.5 (½–24 in); ASME B16.47 (>24 in). Common flange grades: ASTM A36, Q235, ASTM A105

Classes 150–2500 depending on P–T; NPS ½–60.

Selected flange class must match design pressure–temperature. Raised Face (RF) and Ring-Type Joint (RTJ) facings are standard. Material grade ensures ductility and toughness for low-temp service.

Stainless steel pipe

ASME B16.5 / B16.47. Materials: 304, 304L, 316, 316L, High Carbon Variants (304H, 316H), Dual Grades (304/304L, 316/316L)

Classes 150–2500; NPS ½–60.

Resistant to corrosion; RTJ faces often used for high-pressure. It is important to ensure gasket materials compatible with media.

Ductile-iron / Gray-iron pipe & fittings (waterworks)

AWWA C110-21 (ductile-iron and gray-iron fittings with flanged or mechanical joints, 3–48 in); AWWA C115 (flanged DI pipe); AWWA C207 (steel flanges for waterworks).

C110 fittings: 3–48 in; flanged DI pipe up to 64 in; typical 250 psi working pressure (350 psi for ≤24 in flanged DI).

C110 fittings cover flanged & mechanical joints; drilling patterns often match ASME B16.1 Class 125. Flat-face flanges are standard; use full-face gaskets. It is essential to confirm alignment with valves/pumps (steel or cast iron).

Gray cast iron pipe / valves

ASME B16.1 (Classes 25, 125, 250). 

NPS per B16.1; pressure ratings per class.

Flat-face flanges must be paired with full-face gaskets. And raised-face steel flanges should not be connected directly to cast-iron flat faces due to the risk of cracking.

Copper / copper-alloy systems

ASME B16.24 (cast copper-alloy flanges/fittings/valves, Classes 150–2500). 

NPS sizes per B16.24; Classes 150–2500.

Typically used in seawater, cooling, or HVAC service. when joined to carbon steel in wet environments, the potential for galvanic corrosion should be considered.

Stainless steel (waterworks service)

AWWA C228 (stainless steel flange joints for water, 2–72 in). 

Sizes 2–72 in; pressure ranges per C228.

These flanges are designed for potable water and wastewater applications. Bolt drilling may differ and should be verified when connecting to ASME flanges. 

PVC / CPVC pressure pipe

Manufacturer flanges machined to ASME B16.5 Class 150 bolt pattern. Standards: ASTM D4024, D1784 (material), F437/F439 (fittings).

Typically rated ~150 psi at 73°F (lower at higher temperatures).

The bolt pattern is compatible with Class 150 steel flanges, but the joint pressure rating is limited by the plastic material. Flat-face flanges with full-face gaskets are typically used, while RTJ facings are not suitable.

HDPE pressure pipe

AWWA C906 pipe; connections via stub-end + steel backing ring drilled to ASME B16.5 Class 150; or proprietary flange adapters. 

Pressure rating depends on pipe DR (e.g., DR9 ≈ 250 psi at 73°F).

HDPE pipe connects to metal flanges through flange adapters or backing rings, typically drilled to the ASME Class 150 bolt pattern. Compatibility should be verified against AWWA C906 specifications.

Practical Guidance for Choosing Flange Connections:

1. Pipe material defines the standard.

Steel and stainless systems generally follow ASME B16.5/B16.47. Waterworks systems are guided by AWWA standards, while plastic pipes typically require adapters or backing rings to mate with metallic flanges.

2. Flange pressure class reflects the limiting component.

A joint takes on the rating of its weakest element. For example, when steel flanges connect to PVC or HDPE systems, the plastic dictates the overall capacity.

3. Facing type aligns with material and service.

Raised Face (RF) and Ring-Type Joint (RTJ) are common in higher-pressure metallic systems, while Flat Face (FF) is typical for cast iron, ductile iron, and plastic to avoid overstressing brittle materials.

4. Service environment influences material choice.

Copper-alloy and stainless flanges are often used in seawater or HVAC applications. Galvanic corrosion should always be considered when dissimilar metals are paired.

5. Bolt drilling must be compatible.

Even when sizes are the same, ANSI/ASME and AWWA standards may differ in drilling patterns, so alignment should be verified. Read our blog on “Bolt Hole Patterns and Compatibility Between AWWA Flange Classes”

6. Pressure and temperature limits are flange and pipe dependent.

For plastics, allowable limits are governed by the pipe’s dimension ratio and thermal properties rather than the strength of steel backing rings.

Conclusion

Selecting correct flange connections comes down to aligning pipe material, standards, and service conditions into a safe and reliable joint. A clear understanding of these factors simplifies design choices and reduces the risk of costly issues in the field.
Our sales team at API can answer any questions about your flange requirements. Depending on its application, they can also recommend alternative products that may be better suited for your system. We also provide custom flange solutions tailored to your project specifications. Get connected with a dedicated sales representative today! Or call us: 503.692.3800