In a pipeline system, “inch” refers to the nominal diameter, often abbreviated as DN. For example, a 2 inch pipe uses this nominal diameter to standardize the dimensions of pipes and fittings (such as elbows and valves), but it doesn’t specify the inside or outside diameter. This nominal diameter simplifies the identification and specification of parts, ensuring components can be used together in the same pipeline system.
Why Are Nominal Diameters Necessary?
Nominal diameters are crucial for several reasons:
- Unification: Without standardization, different manufacturers would have unique dimensions for valves and elbows, making it hard to find compatible parts.
- Communication: Engineers and construction workers would face difficulties in communicating component dimensions, increasing the risk of errors.
- Standardized Production: Without nominal diameters, manufacturers would produce parts of varying sizes, disrupting the industrial system.
To solve these issues, nominal diameters (such as 1 inch for DN25 and 2 inches for DN50) were introduced, ensuring parts fit together despite slight measurement variations. While inches and DN are often used together, they are not always mathematically equivalent.
Pipe Sizes and Standards
For steel pipes, the DN refers to the outside diameter (OD), but the OD size may differ according to standards. For example:
- According to American standards (ASTM A53, ASTM A795), the OD size for a 2-1/2 inch (DN65) steel pipe is 73 mm.
- According to European standards (BS1387, EN10255), the OD size is 76.1 mm for the same pipe size.
How Do I Know the Inner Diameter of a Steel Pipe Fitting?
The inner diameter (ID) can be calculated based on the OD and wall thickness. The following example shows pipe sizes and thicknesses according to US standards:
| Size | OD Size (mm) | SCH 10 Thickness | SCH 40 Thickness | STD Thickness |
|---|---|---|---|---|
| 1/2” / DN15 | 21.3 | 2.11 | 2.77 | 2.77 |
| 3/4” / DN20 | 26.7 | 2.11 | 2.87 | 2.87 |
| 1” / DN25 | 33.4 | 2.77 | 3.38 | 3.38 |
| 1-1/4” / DN32 | 42.2 | 2.77 | 3.56 | 3.56 |
| 2” / DN50 | 60.3 | 2.77 | 3.91 | 3.91 |
To calculate the ID of a 2″ (DN50) steel pipe:
- For Sch 10 steel pipes, ID = 60.3 mm – 2.77 mm = 28.76 mm
- For Sch 40 steel pipes, ID = 60.3 mm – 3.91 mm = 28.19 mm
This shows that pipes with different wall thicknesses have different internal diameters. Therefore, the ID of a 2-inch pipe depends on the pipe’s wall thickness.
Can Pipes with Different Inside Diameters Be Connected?
Yes, pipes with the same nominal diameter (DN) can generally be connected using standardized fittings, regardless of their actual inside diameters. If the DN differs (e.g., DN50 to DN80), special procedures are required.
How to Connect Pipes with Different Inside Diameters?
Using a Reducer: The most common method is to use reducers. There are two main types:
- Concentric Reducers: These are used for vertical pipes to avoid air pockets.
- Eccentric Reducers: Used for horizontal pipes to avoid fluid pockets or maintain consistent pipe bottom elevations.
Key Considerations When Connecting Pipes
- Fluid Resistance and Pressure Loss: Connecting pipes of different sizes can lead to significant pressure loss due to sudden changes in flow rate, which increases turbulence.
- Cavitation: Poorly designed connections, especially from large to small pipes, can cause rapid flow velocity that leads to cavitation and pipe damage.
- Deposition and Clogging: Using concentric reducers in horizontal pipes can lead to the accumulation of solid particles, potentially causing clogging.
Fluid Tech Group Services
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