Nominal size and schedule
Nominal pipe size is a designation, not a measurement. For a given nominal size the outside diameter is fixed, and the schedule number sets the wall thickness. A heavier schedule therefore has the same outside diameter and a smaller bore — which is why a heavier wall reduces flow area rather than increasing the pipe’s external size.
This matters in two ways. The outside diameter fixes what fittings and flanges will fit, so it must match. The schedule sets the pressure the pipe will hold and how much corrosion allowance it carries, so it must be specified alongside the size. "Four inch pipe" is not a specification.
| Schedule | Relative wall | Typical application |
|---|---|---|
| SCH 10 | Light | Low-pressure stainless lines, drainage, vent service |
| SCH 40 / STD | Standard | General water, air and process service — the common default |
| SCH 80 / XS | Heavy | Higher pressure, threaded connections, added corrosion allowance |
| SCH 160 / XXS | Very heavy | High-pressure and severe service lines |
Material selection
Material follows the fluid, the temperature and the pressure — and, for buried or outdoor runs, the external environment as well.
- Carbon steel — general industrial service for water, steam, air, oil and non-corrosive process fluids
- Stainless steel — corrosive process fluids, hygienic service and higher temperatures
- Galvanised steel — water and general service where external corrosion protection is wanted
- PVC and CPVC — chemical service and cold water; CPVC extends the temperature range
- HDPE — buried water mains, effluent and chemical service, joined by fusion welding
- Lined pipe — aggressive chemistry carried in a steel pressure envelope with a chemically resistant liner
Fittings and how the line is joined
The joining method affects cost, maintainability and where leaks appear. Welded joints are permanent and leak-free; flanged joints allow disassembly; threaded joints are quick in small bore but are a common leak point under vibration.
- Butt weld fittings give the strongest, smoothest joint and suit permanent process lines
- Socket weld suits small-bore high-pressure service
- Threaded fittings are practical up to about 2 inches on low-criticality service
- Flanged connections are used wherever equipment must be removed for maintenance
- Grooved couplings allow rapid assembly and some angular movement
- Compression and push-fit systems suit plastic pipe and small-bore instrument lines
Flanges — get the standard right
Flanges from different standards can share a nominal size and still not bolt together, because the bolt circle, bolt count and hole size differ. Confirm the standard, the class, the facing type and the flange style before ordering.
- The standard and pressure class must match the existing line
- Facing — raised face, flat face or ring type joint — must match on both halves
- Style — weld neck, slip-on, blind, socket weld or lap joint — depends on the service and pressure
- Gasket material must suit the fluid and temperature, and bolting must suit the class
What to include in a pipe and fittings enquiry
- Nominal size and schedule or wall thickness
- Material grade and specification
- Fitting types and quantities, with angles and reduction sizes where relevant
- Flange standard, class, facing and style
- Design pressure, design temperature and the fluid carried
- Whether the line is buried, outdoor or insulated
- Total length or piece count, delivery city and required date