Pipe Schedule and Wall Thickness in Construction Specifications

[edit] Introduction

Pipe schedules are widely used in construction specifications to describe the wall thickness series of steel pipe. However, a schedule number is not a direct measurement and should not be treated as a fixed wall thickness. The actual wall thickness depends on both the nominal pipe size (NPS) and the selected schedule.

This distinction is important in building services, fire protection, district heating and cooling networks, industrial utilities and other construction systems. If a specification lists only 'Schedule 40 pipe' without confirming the nominal pipe size, material standard and service requirements, contractors and suppliers may interpret the requirement differently.

[edit] Pipe schedule and wall thickness

A pipe schedule is a standardised wall thickness designation. Common designations include Schedule 10, Schedule 20, Schedule 40, Schedule 80, Schedule 160 and XXS (Double Extra Strong). The schedule number identifies a wall thickness series; it is not itself a wall thickness expressed in millimetres or inches.

For example, Schedule 40 pipe does not have one universal wall thickness. An NPS 2 Schedule 40 pipe has a different wall thickness from an NPS 6 Schedule 40 pipe. The schedule must therefore always be considered together with the nominal pipe size and the applicable dimensional standard.

[edit] Relationship between NPS, outside diameter and wall thickness

Nominal pipe size is a designation rather than an exact measured diameter. For most pipe sizes, the NPS value does not equal the actual outside diameter. Instead:

• The nominal pipe size identifies the pipe size series.
• The outside diameter is fixed for a given nominal pipe size.
• The schedule determines the wall thickness.
• The internal diameter is calculated as the outside diameter minus twice the wall thickness, subject to manufacturing tolerances.

As the schedule increases, the wall thickness generally increases and the internal diameter decreases. This affects flow area, pressure loss, pipe weight, support loading, welding preparation, corrosion allowance and the compatibility of fittings and valves.

[edit] Example schedule dimensions

The table demonstrates why a schedule number cannot be converted into a single wall thickness value. The wall thickness should always be verified against the relevant dimensional standard for the specified nominal pipe size and material.

[edit] Material standards

Carbon steel and alloy steel pipe dimensions are commonly specified in accordance with ASME B36.10M, while stainless steel pipe dimensions are commonly specified in accordance with ASME B36.19M. The available schedule designations are not identical for every material family. For example, stainless steel pipe commonly uses schedule designations such as 5S, 10S, 40S and 80S.

ISO 6708 defines the DN (nominal diameter) designation used in metric piping systems. Although DN and NPS are broadly equivalent for coordination purposes, neither represents the measured outside diameter or internal diameter of a pipe. The applicable dimensional standard should always be consulted.

[edit] Information to include in a construction specification

A pipe specification should normally identify more than the schedule designation. Depending on the application, it may include:

• Service or system.
• Material specification and grade.
• Nominal pipe size or size range.
• Pipe schedule or specified minimum wall thickness.
• Manufacturing method, where relevant.
• End preparation and jointing method.
• Corrosion allowance, where applicable.
• Internal and external coating or lining requirements.
• Design pressure and design temperature.
• Inspection, testing and certification requirements.
• Applicable dimensional standard and permitted tolerances.

Where hydraulic calculations depend on the internal diameter, the designer should verify the actual bore corresponding to the selected outside diameter and wall thickness. It should not be assumed that the nominal pipe size is equal to the internal diameter.

[edit] Coordination and procurement

Several recurring problems arise when pipe schedules are specified without sufficient supporting information:

• Drawings specify an NPS or DN value while the material schedule lists only an outside diameter.
• A specification states Schedule 40 without identifying whether the pipe is carbon steel or stainless steel.
• Hydraulic calculations assume an internal diameter that does not correspond to the selected schedule.
• Pipes, fittings and valves are specified using incompatible pressure classes or dimensional standards.
• A procurement document specifies a nominal schedule when the design requires a minimum wall thickness after allowing for corrosion.
• A substitution to a heavier schedule is accepted without assessing its effects on bore, weight, support loads, welding procedures and component compatibility.

Before procurement, the design team, contractor and supplier should ensure that the drawings, pipe class, bill of quantities and material requisition consistently specify the same nominal size, schedule, material standard, grade and end preparation.

Where discrepancies exist, they should be resolved through the project change control or technical query process rather than by assuming that a heavier schedule is automatically acceptable. Although a thicker wall may increase mechanical strength or corrosion allowance, it may also reduce flow capacity, increase weight and require changes to fabrication or installation.

[edit] Summary

A pipe schedule is a standardised wall thickness series rather than a direct wall thickness measurement. It only has meaning when used in conjunction with the nominal pipe size, material standard and intended service. Construction specifications should provide sufficient information for designers, contractors and suppliers to identify the required outside diameter, wall thickness and internal diameter without ambiguity.

[edit] Related articles on Designing Buildings

• Pipe
• Pipework
• Building services
• Mechanical services
• Fire suppression systems
• District heating
• Specification
• Performance specification
• Bill of quantities
• Construction specification

[edit] External references

• ASME B36.10M, Welded and Seamless Wrought Steel Pipe.
• ASME B36.19M, Stainless Steel Pipe.
• ISO 6708, Pipework components — Definition and selection of DN (nominal size).