Understanding Sizing Systems: DN vs. NPS
In the industrial piping and valve industry, standard sizes are typically cataloged using two main terminology systems depending on the engineering standards applied to the pipeline:
1. DN (Nominal Diameter / Diamètre Nominal)
The **DN** system is the metric-based designation governed by European and international standards such as **EN ISO 6708**. It denotes the nominal internal size of a pipe or valve in millimeters. While it relates to the internal diameter of standard-thickness pipes, it is important to remember that DN is a **dimensionless numeric designation** and does not correspond to an exact measurable diameter unless specified by the schedule.
2. NPS (Nominal Pipe Size)
The **NPS** system is the imperial-based designation governed by North American standards, including **ASME B36.10M** (for carbon and alloy pipes) and **ASME B36.19M** (for stainless steel pipes). Like DN, NPS indicates a standardized, nominal reference size.
Important Sizing Concept:
For pipe sizes below NPS 14" (DN 350), neither the DN nor the NPS represents the exact physical dimensions of the pipe. For instance, an NPS 2" (DN 50) pipe has a physical outer diameter (OD) of **2.375 inches (60.3 mm)**. The actual inside diameter (ID) will vary based on the wall thickness schedule (e.g., Schedule 40 or Schedule 80), while the OD remains fixed to ensure fitment of flanges and valves.
The Mathematical vs. Commercial Rounding Rule
To convert valve and pipe sizes from millimeters (DN) to inches (NPS), divide the millimeter size by \(25.4\). In the piping industry, sizes are typically rounded to standardized fractions or whole inches.
Because valves and pipes must mate with standard fittings, standard commercial sizes are rounded to standard imperial units. For example:
- DN 15 mathematically divides to \(15 / 25.4 = 0.590"\). The closest standard commercial size is 1/2" (NPS 0.5).
- DN 50 mathematically divides to \(50 / 25.4 = 1.968"\). The standard commercial size is 2" (NPS 2.0).
The "Rule of 25" for Large Sizing
For sizes larger than 6 inches (DN 150), valve sizes usually follow exact inch measurements using a simple factor-of-25 scale:
This rule makes conversions instantaneous for larger industrial systems:
- 10" NPS is converted to DN 250 (10 × 25 = 250 mm).
- 12" NPS is converted to DN 300 (12 × 25 = 300 mm).
- 24" NPS is converted to DN 600 (24 × 25 = 600 mm).
For a complete breakdown of commercial sizing and exact metrics, you can refer to comprehensive resources like the Gemini Valve DN to NPS Guide. At Supreme Valves, we engineer gate, globe, ball, and butterfly valves conforming to both API (ASME NPS-based) and European EN (DN-based) standards.