What does nominal size do?
A DN or NPS marking does not mean a ruler will show the same outside diameter. These designations identify nominal sizes in a piping system. Mating flanges, valves and fittings are selected within their own standardised connection system. Nominal size is useful in purchasing, but can be incomplete without the applicable dimensional standard.
When replacing a machine pipe, record measured outside diameter, wall and existing markings together. Ordering from an approximate inch-to-millimetre conversion can create fit problems if the original system is unknown. Treat nominal identification and measurement as separate information that cross-checks the request.
Why is schedule not one fixed wall?
Read schedule with nominal size in the applicable dimension table. The same schedule does not give one millimetre wall at every nominal diameter. Instead of writing only thin wall or SCH, specify the complete schedule and, where possible, the required wall dimension. Inside diameter depends on outside diameter and wall; define any internal-clearance requirement for flow or an inserted component.
Pipe, tube and manufacturing route
Process pipe, heat-exchanger tube and a mechanical hollow section can use different dimensional and acceptance approaches. Welded or seamless describes how the product is manufactured. A route name does not establish universal superiority for every application. Assess the specified product standard, material grade, inspection and finish together.
Stainless 316L and titanium Grade 2 can be considered in similar tube geometry, but welding, cleanliness and product specifications differ. A dimensional standard for one product family does not automatically define another material’s delivery conditions. Build the enquiry around actual service and the project specification.
A short check before sending
Collecting the following information on one line or drawing makes quotations easier to compare. List each size and quantity separately. For cut lengths, include length tolerance and end preparation; mill lengths and assembly-ready components are different scopes.
- Grade, product specification and any existing pipe markings.
- DN/NPS together with outside diameter, wall or complete schedule.
- Welded/seamless route, length, quantity, ends and finish.
- Medium, temperature, design conditions and required documents.