What Is a Pipe and Pipe Class? Complete Engineering Guide (ASME B31.3)

What is a pipe?

In plant engineering, a pipe is a pressure-retaining tubular component with a round cross-section that moves a fluid – liquid, gas, steam, two-phase mixture or slurry – from one point to another. Unlike a tube, a pipe is ordered by a nominal size and a wall designation rather than by its exact dimensions. Two numbers therefore describe any steel pipe: the Nominal Pipe Size (NPS), or its metric equivalent DN, and the schedule.

The NPS fixes the outside diameter so that every flange, fitting and valve of that size fits every pipe of that size, whatever its wall thickness. Pressure capacity is then adjusted by changing the wall – thicker walls move inward, reducing the bore. This simple rule is what allows thousands of components from different manufacturers to be assembled into one leak-tight system.

Pipes are often confused with tubes. The difference matters on purchase orders, because the same “2-inch” description means a different outside diameter for a pipe (60.3 mm) than for a tube (50.8 mm).

Seamless vs welded pipe

How a pipe is made determines its strength consistency, its cost and where the code lets you use it. Seamless pipe is produced by piercing a heated solid billet and drawing it over a mandrel, so there is no longitudinal weld. Welded pipe starts as flat coil or plate that is formed into a cylinder and joined along a straight or spiral seam. For a deeper look at the mills themselves, see the Aienginear guide to the seamless pipe manufacturing process.

In practice, many carbon steel pipe classes call for seamless pipe up to NPS 16 or 24 and allow longitudinally welded pipe with 100% radiographed seams above that, where seamless becomes scarce and expensive. ERW/HFW is widely accepted for utilities and lower-risk services when the specification permits it.

NPS, OD and pipe schedule explained

For NPS ⅛ to NPS 12 the outside diameter is larger than the nominal number – an NPS 2 pipe measures 60.3 mm (2.375 in) across the outside. From NPS 14 upward the OD in inches equals the NPS. The metric DN designation is a label too: DN 50 corresponds to NPS 2, not to a 50 mm diameter.

The schedule (Sch 5 to Sch 160 in ASME B36.10M) is a wall-thickness designation. Historically it approximated 1000 × P/S, but today it is simply a table lookup. The older weight classes – STD, XS and XXS – still appear in specifications, and they match Sch 40 and Sch 80 only over part of the size range. Use the Aienginear pipe sizes and schedule chart for the full matrix.

How wall thickness is calculated (ASME B31.3)

A pipe class does not pick schedules by habit. For straight pipe under internal pressure, ASME B31.3 para. 304.1.2 gives the pressure design thickness, valid when t < D/6:

where P is internal design gauge pressure, D is outside diameter, S is the allowable stress at design temperature (B31.3 Table A-1), E is the longitudinal weld quality factor, W is the weld joint strength reduction factor, Y is a temperature coefficient (0.4 for ferritic steel below about 482 °C), and c is the sum of corrosion, erosion and thread/groove allowances.

Worked example

NPS 6 ASTM A106 Gr B seamless, design 40 barg (4.0 MPa) at 200 °C, corrosion allowance 3.0 mm, S = 138 MPa, E = 1.0, W = 1.0, Y = 0.4, D = 168.3 mm:

• t = (4.0 × 168.3) / [2 × (138 × 1.0 × 1.0 + 4.0 × 0.4)] = 673.2 / 279.2 = 2.41 mm
• t_m = 2.41 + 3.0 = 5.41 mm
• With 12.5% mill under-tolerance: 5.41 / 0.875 = 6.18 mm minimum nominal wall
• Next schedule: Sch 40 = 7.11 mm ✔ (acceptable)

What is a pipe class?

A pipe class – also called a piping class, piping material class or line class – is a controlled document within the project’s Piping Material Specification (PMS). It defines exactly which components may be used together for one combination of fluid, pressure, temperature and corrosion. If the pipe is the brick, the pipe class is the building code for the entire wall.

Without pipe classes, every designer would select components line by line, procurement would buy hundreds of near-duplicate items, and site teams would have no reliable way to verify that a gasket or bolt matches its flange. A well-built class removes that variability: designers model with it, the material take-off is generated from it, buyers order from it, and inspectors check installed materials against it.

What a pipe class sheet contains

Formats differ between owners and EPC contractors, but a complete pipe class sheet always answers the same questions. Use the table below as a review checklist when you receive a specification for comment.

Pipe class codes and line numbers

Each class is given a short alphanumeric code so it can be written on drawings. That code is then embedded in every line number on the P&ID, line list and isometric. When a pipefitter reads A1B on an isometric, the specification behind it tells them the flange rating, material and corrosion allowance without opening another document.

Pressure-temperature ratings

A pipe class has a design envelope, not a single design point. Its upper boundary is usually set by the flange rating for the class material group: as temperature rises, allowable stress falls and so does the permitted pressure. Valves are rated to the same tables through ASME B16.34. Any line assigned to the class must have every design pressure-temperature case inside that envelope.

The ratings show why a Class 150 carbon steel class is rarely used above about 300 °C – the permitted pressure has already halved. Bolting must also suit the temperature; check flange make-up loads with the bolt torque flange calculator.

ASME B31.3 fluid service categories

The fluid service category written in the class header controls how much examination and testing the welds receive and which joint types are permitted. Assigning the wrong category is one of the most expensive errors in a piping specification because it is discovered late – usually at inspection.

Material selection by service

Material selection starts with the corrosion study and the minimum and maximum design temperatures. The table gives proven starting combinations; each row is a family of compatible ASTM specifications so that pipe, fittings and flanges share similar chemistry and weldability. For detailed grade guidance see the Aienginear guides to carbon steel, alloy steel, stainless steel, non-ferrous and non-metallic piping materials.

Sample pipe class sheet (A1B)

The example below shows how the pieces come together for a common utility and hydrocarbon class. Every schedule was checked against the class design pressure, so you can follow the logic from the thickness equation to the final selection.

Branch tables

A branch table tells the designer which component to use wherever one line tees off another. The choice balances reinforcement, stress intensification, inspection access and cost: equal and reducing tees for large branches, integrally reinforced outlets such as weldolets for small branches on large headers, and sockolets for small-bore take-offs.

How to develop a pipe class: 8 steps

1. Collect design data. Gather fluid, design pressure and temperature (including upset and minimum temperatures), flow regime, and contaminants such as H₂S, CO₂, chlorides or oxygen from the process line list.
2. Set the design code and fluid service. Confirm whether ASME B31.3, B31.1 or another code governs and assign the fluid service category (Normal, D, M, High Pressure, Elevated Temperature, High Purity).
3. Select the base material and corrosion allowance. Use the corrosion study to choose the material family and a corrosion allowance that covers the design life, including erosion where velocities are high.
4. Choose the pressure rating. Select the lowest ASME B16.5 / B16.47 class whose rating, for the chosen material group, covers every design pressure-temperature point the class must serve.
5. Calculate pipe wall thickness by size. Apply the B31.3 pressure design equation, add allowances, correct for mill tolerance and pick the next standard schedule; check mechanical strength for small bore.
6. Specify fittings, flanges, gaskets and bolting. Pick compatible standards, materials, facings and gasket types, keeping the galvanic and temperature compatibility of each item in mind.
7. Define valves and the branch table. Assign valve types and data-sheet codes by size range and build the header-branch matrix that satisfies reinforcement rules.
8. Add notes, review and issue. Add NDE, PWHT, impact test, hardness and test-pressure notes; obtain multi-discipline review and issue the class under revision control.

Pipe vs pipe class vs pressure class

Common pipe class mistakes and best practices

• Confusing STD with Sch 40 above NPS 10 – state the exact designation in the class.
• Ignoring minimum design temperature – A106 Gr B and A105 need impact-tested alternatives (A333 Gr 6, A350 LF2) for low-temperature or auto-refrigeration cases.
• Mixing gasket and bolting combinations – spiral-wound gaskets need enough bolt load; low-strength bolts on high-rated flanges can cause leaks.
• Too many classes – each new class multiplies inventory. Merge classes where the envelope allows and the cost penalty is small.
• Unmarked specification breaks – show every break on P&IDs and isometrics, and apply the stricter class at the joint.
• Not revising the class after process changes – a new design case or contaminant (e.g. H₂S) must be checked against the class envelope before construction.
• Skipping positive material identification – on alloy classes, PMI at receipt and after fabrication prevents mix-ups. See pipe inspection methods.

Before field hydrotesting, verify the test pressure for each class with the hydrotest pressure calculator, and plan supports with the pipe support spacing calculator.

Applicable standards and codes

Frequently asked questions

Test your knowledge: pipe and pipe class quiz

Ten questions based on this guide. Select an answer to see the explanation instantly.

Reviewed and updated: 10 October 2026 by Muhammad Raza, founder of Aienginear. Code references checked against ASME B31.3-2024, ASME B31.1-2024, ASME B16.5-2025 and ASME B36.10M-2022. This article is educational; always follow the specification and code edition approved for your project. More guides: Aienginear piping guides.