Reading a line class — the document that has already decided

Four topics in, you know what a material grade means, how schedules work, what a flange class is and how a branch is reinforced. Here is the uncomfortable part: on a real project you will almost never choose any of them.

They were chosen once, by a piping materials engineer, and written into a document that is then applied to thousands of lines. Learning to read that document is the single most useful piping skill there is.

What a line class is

A line class — also called a piping material specification or PMS — is a specification for one service, over one pressure–temperature range, covering every component that will ever be welded or bolted into that line:

pipe · fittings · branch connections · flanges · blinds · gaskets · bolting · valves · corrosion allowance · permitted joint types

One sheet per class. Pick the class and every decision below it is made.

What a class sheet actually contains

Filter Worked example — line class A1A

14 rows

Component	Size range	Specification	Detail
Service	—	General hydrocarbon, non-sour	Design 19.6 bar g at 38 °C, falling per ASME B16.5 Class 150
Corrosion allowance	—	3.0 mm	Included in the wall thickness calculation
Pipe	NPS 1/2 – 1 1/2	ASTM A106 Gr B, SCH 80, SMLS	Heavier wall than pressure requires, for mechanical strength
Pipe	NPS 2 – 24	ASTM A106 Gr B, SCH 40, SMLS	Bevelled ends to ASME B16.25
Fittings, butt weld	NPS 2 – 24	ASTM A234 WPB, ASME B16.9	Wall to match the connecting pipe. Long radius elbows.
Fittings, socket weld	NPS 1/2 – 1 1/2	ASTM A105, Class 3000, ASME B16.11	Vents, drains and instrument connections only
Branch connections	All	Per branch table	Equal and reducing tees down to 1:2; weldolets below
Flanges	NPS 1/2 – 24	ASTM A105, Class 150, RF, ASME B16.5	Weld neck, bore to match pipe. Slip-on not permitted.
Blinds	NPS 1/2 – 24	ASTM A105, Class 150, RF	Spectacle blinds at isolation points
Gaskets	NPS 1/2 – 24	Spiral wound, 316 winding, graphite filler, CS outer ring	ASME B16.20, to suit Class 150 RF
Bolting	All	Studs ASTM A193 B7, nuts ASTM A194 2H	Fully threaded studs, two nuts per stud
Gate valve	NPS 2 – 24	ASTM A216 WCB body, 13Cr trim, API 600	Flanged, Class 150 RF, OS&Y rising stem
Ball valve	NPS 1/2 – 8	ASTM A216 WCB body, 316 ball, API 6D	Full bore, fire safe to API 607
Check valve	NPS 2 – 24	ASTM A216 WCB body, 13Cr trim, API 594	Swing type, flanged

This is what a line class sheet actually contains: one row per component family, each naming a material specification and the size range it applies over. Read across a row and you know exactly what to buy and what a fabricator will be checked against.

Read down that table and notice how much it settles. It answers “which gasket?” and “can I use a slip-on flange?” and “what wall thickness on a ¾ inch vent?” — questions that would otherwise be asked, and answered differently, hundreds of times.

Notice too the small-bore row: SCH 80 for NPS 1½ and below, where the pressure would happily allow SCH 40. That is the mechanical strength rule from pipe sizes and schedules, written into the class so nobody has to remember it.

Decoding a line number

Every line on a P&ID carries a number. A common form:

6"-P-1502-A1A-IH

Part	Meaning
6"	Nominal size, NPS 6
P	Service code — P for process, S steam, CW cooling water, N nitrogen, FG flare gas
1502	Sequential line number, usually carrying the unit or area number
A1A	Line class
IH	Insulation code — here, heat conservation

The exact format varies between contractors, and the project’s drawing legend defines it. But the line class is always in there, and it is the part that tells you what the line is made of.

Decoding the class itself

Class codes are usually structured too, though the scheme is project-specific. A widespread pattern is rating · material · variant:

• A1A — A = Class 150, 1 = carbon steel, A = standard variant
• D2C — D = Class 600, 2 = stainless steel, C = a specific variant

Some contractors use 1CS1 or CS150 instead. Do not guess the scheme. The PMS index lists every class with its rating, material and service, and that index is the document to read first when you join a project.

How a class is chosen

The piping materials engineer works from four inputs:

1. The fluid — what it is, and what it does to metal. This drives the material, and it is where sour service and NACE enter.
2. Design pressure and temperature — which fixes the flange class, via the pressure–temperature tables in flanges and pressure classes.
3. Corrosion allowance — extra wall deliberately sacrificed over the design life.
4. Client and project standards — many operators impose their own specifications on top of the code, and those usually win.

The output is a class covering a range, not a point. A Class 150 carbon steel class covers every line in that service up to the Class 150 rating curve — which is why one class serves hundreds of lines.

Spec breaks: where classes meet

A single physical run of pipe often changes class partway along. The point where it changes is a spec break, and it is marked on the P&ID.

Spec breaks happen at:

• Vessel and equipment nozzles — where the vessel’s specification takes over
• Battery limits — where responsibility passes between contractors or operators
• A change in design conditions — downstream of a pressure reducing station, the pressure is lower, so a lighter class is justified
• A change in material — where a corrosive stream is diluted or neutralised

When nothing fits

Occasionally a line genuinely falls outside every existing class — an unusual fluid, a temperature at the edge of a range, a one-off vendor package.

The wrong response is to pick the nearest class and note the difference on the isometric. That note will be missed by procurement, by the fabricator, or by the person who does a repair in eight years’ time.

The right response is one of:

• Raise a deviation — a controlled, approved and recorded departure for that line.
• Request a new class from the piping materials engineer, if the case will recur.

Both leave a traceable record. That is the entire point of a controlled document.

What the class does not cover

Worth knowing, because it is where the gaps get missed:

• Wall thickness calculation for a specific line — the class gives a standard schedule; an unusual pressure, temperature or external load may still need a check to ASME B31.3.
• Pipe supports and spring hangers — a separate specification.
• Insulation and painting — separate specifications, coded on the line number.
• Specialty items — steam traps, strainers, sight glasses, expansion joints, usually a separate list.
• Vendor package piping — arrives built to the vendor’s own standard, and reconciling it with the plant class is a known source of disputes.

What to take away

• A line class fixes every component for one service over one pressure–temperature range. Pick the class, and the decisions are made.
• The line number on the P&ID carries the class. Learn the project’s numbering scheme from the drawing legend, not by guessing.
• Read the PMS index first when you join a project — it is the fastest orientation there is.
• At a spec break, the component belongs to the higher specification side.
• When nothing fits, raise a deviation or request a new class. Never improvise, and never assume a heavier class is automatically safer.