What Is a Reducing Flange?
A reducing flange is a single flange body with two different bore diameters that allows connection between pipes of different sizes. The larger bore matches the larger pipe diameter, while the smaller bore matches the smaller pipe. The design is typically eccentric, meaning the top of both bores are aligned to prevent air or vapor accumulation at the high point of the piping system. This eccentric design is particularly important in liquid services where vapor pockets could cause flow problems or corrosion issues.
Design and Eccentric Bore Geometry
Reducing flanges are designed per ASME B16.5 with specific bore reduction ratios. The eccentric bore keeps the top surface of both bores at the same elevation, which prevents air pockets in horizontal liquid piping. The bolt pattern matches the larger flange size, ensuring compatibility with the larger pipe's flange. The facing on the large end is typically raised face, and the flange thickness must satisfy the pressure class requirements based on the larger flange dimensions. The compact design significantly reduces the overall piping assembly length compared to a conventional flange-plus-reducer arrangement.
| Large End NPS | Small End NPS | Flange OD (in) | Thickness Class 150 (in) | Number of Bolts |
|---|---|---|---|---|
| 4 | 3 | 9.00 | 0.94 | 8 |
| 6 | 4 | 11.00 | 1.00 | 8 |
| 8 | 6 | 13.50 | 1.12 | 8 |
| 10 | 8 | 16.00 | 1.19 | 12 |
| 12 | 10 | 19.00 | 1.25 | 12 |
| 14 | 12 | 21.00 | 1.38 | 12 |
Pressure Ratings
The pressure rating of a reducing flange is based on the smaller bore size and the pressure class designation. The flange must meet the full system design pressure on both the large and small bore sides. Reducing flanges are available primarily in Class 150 through 600. Higher classes may require special design and are less commonly available from stock. Some derating may apply based on the bore transition geometry, so it is important to verify the pressure rating for the specific size reduction required.
Materials
Reducing flanges are available in the same materials as standard ASME B16.5 flanges. ASTM A105 carbon steel is standard for general service. ASTM A182 F304 and F316 stainless steels are specified for corrosive environments. ASTM A350 LF2 low-temperature material is available for cold service applications. Alloy grades are available for high-temperature service but may have longer lead times. The material must meet both bore size requirements and the applicable ASME material specification for the full flange cross-section.
Applications
Reducing flanges are commonly used at pump suction and discharge nozzles where the pump inlet and outlet sizes differ from the connecting piping. Equipment connections with different inlet and outlet sizes benefit from the compact design. Piping size transitions in tight spaces where a conventional flange-plus-reducer assembly would not fit are ideal applications. Systems where eccentric reduction prevents vapor accumulation, such as pump suction lines, are a primary application. Pressure vessel connections with stepped nozzle sizes also commonly use reducing flanges.
Reducing Flange vs Flange + Reducer
The single-component design of a reducing flange saves significant space compared to a separate flange and concentric reducer assembly. Fewer welded joints reduce the number of potential leak points in the system and eliminate the need for one circumferential weld. The overall cost is typically lower than purchasing two separate components. However, reducing flanges are more difficult to align than separate flanged reducers because the orientation matters. They are also less flexible for future piping modifications; a flange-reducer combination allows the reducer to be replaced independently if needed.
For most equipment connection applications where space is limited and the size reduction is known, reducing flanges provide an elegant and cost-effective solution. The key takeaway is to use reducing flanges when you need a compact, leak-resistant size transition at equipment connections where the space savings justify the reduced flexibility.
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