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Key takeaways
- A piping class (line class) is a pre-approved recipe: one material, one pressure–temperature envelope, one corrosion allowance and a fixed list of components.
- The class name in the line number tells you everything you need to buy and build that line.
- Spec breaks on the P&ID mark where one recipe ends and another begins; they often need a flange or a valve.
What a piping class is
A piping material specification (PMS) is a set of piping classes. Each class is a recipe that has already been engineered and approved, so designers don’t have to choose materials line by line. A typical class covers:
- Design envelope: the maximum pressure at each temperature, usually set by the flange rating.
- Base material: for example carbon steel to ASTM A106 Gr. B or 22Cr duplex.
- Corrosion allowance: for example 3 mm.
- Component table: pipe, fittings, flanges, gaskets, bolting and valves, with the standard, material grade and wall thickness for every size.
- Branch table: which branch connection to use (tee, weldolet, sockolet, reinforced) for each header and branch size.
- Notes: PWHT, impact testing, NDE extent, special service such as sour or caustic.
Reading a line number
A typical line number looks like this: 6"-P-1023-A1A-H
Here 6″ is the size, P the fluid code (process), 1023 the sequence number, A1A the piping class, and H the insulation code (heat conservation). Projects vary the order, but the class is always there. Look up A1A in the PMS and you know the flange rating (class 150), the material and every component you need.
How classes are named
Many contractors use a letter-number-letter scheme. The first letter often shows the flange rating (A = class 150, B = class 300), the number the material, and the last letter the corrosion allowance or service. There is no universal standard, so always read the legend at the front of the PMS.
Spec breaks
When a line changes class, the P&ID shows a spec break symbol. Common reasons are a pressure change after a control valve or relief valve, a change of material from carbon steel to stainless steel, or a move from process to utility service. Each spec break needs a physical detail: a flange, a valve, or a dissimilar metal weld or insulating joint.
Onshore
Onshore plants often use a large number of carbon steel classes with different corrosion allowances, because weight is not a concern and carbon steel is cheap.
Offshore
Offshore projects tend to use fewer, higher-alloy classes (duplex, 6Mo, CuNi, GRE) to cut weight and maintenance. NORSOK L-001 gives a standard set of classes that many North Sea operators adopt.
What to take back to your desk
When you get a new project, read the PMS legend before you read a single isometric. Ten minutes there will save you hours of confused take-offs.
Educational content. Always design to your project specification.
