Master the Art: How to Read a Piping Class Sheet Like a Pro

Q: Why do some Pipe Classes have 'Sour Service' written on them?

Sour Service indicates the fluid contains Hydrogen Sulfide (H₂S). These classes require materials compliant with NACE MR0175/ISO 15156 standards to prevent Hydrogen-Induced Cracking (HIC) and Sulfide Stress Cracking (SSC).

Q: Is the Pipe Class code the same across all companies?

No. While the logic (Rating-Material-Service) is similar, every project or owner has a unique coding system. You must always refer to the 'Legend' or 'General Instructions' within that project's Piping Material Specification (PMS).

Q: How do I choose between two different classes for the same line?

Always verify the Pressure-Temperature (P-T) rating. Select the class that safely covers the maximum possible pressure and temperature the line might experience, including upset conditions or steam-out procedures.

II JAY SHRI KRISHNA II

Hello Piping Enthusiasts!

In our previous discussion, we explored the foundational role of the Piping Material Specification (PMS). We learned that the PMS is the "law" of a piping project, ensuring safety and standardization. But as a piping engineer or designer, knowing the theory is only half the battle. The real work happens when you open the document and face the Piping Class Sheet.

📘 Table of Contents

What is a Piping Class?
The Header: General Data & Service Conditions
Understanding the Pressure-Temperature Rating
Decoding the Component Tables (Pipe, Fittings, Valves)
Stainless Steel vs Carbon Steel class differences
The Branch Table: The Roadmap for Connections
The Power of "General Notes"
Frequently Asked Questions (FAQ)
Conclusion

3D engineering illustration showing a piping class sheet with piping layout and design tools

Understanding piping class sheets used in industrial piping design

At first glance, a class sheet looks like a dense, intimidating matrix of numbers, codes and abbreviations. However, once you learn the "alphabet" of these sheets, they become the most helpful tool in your arsenal. Today, we are going to perform a deep dive into How to Read a Piping Class Sheet, breaking down every column and row so you can specify components with 100% accuracy.

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What is a Piping Class?

Before we look at the sheet, let’s define the term. A Piping Class (or Pipe Class) is a specific sub-category within the PMS. Each class is usually identified by an alphanumeric code (e.g., A1A, B1B or 150-CS). This code tells you three critical things instantly:

Pressure Rating: (e.g., 150#, 300#, 600#).

Material: (e.g., Carbon Steel, Stainless Steel, Alloy Steel).

Corrosion Allowance: (e.g., 1.5mm or 3.0mm).

Think of a Class Sheet as a "Shopping List." If a line is designated as Class A1A, the engineer is prohibited from using any material not explicitly listed on that specific A1A sheet.

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The Header: General Data & Service Conditions

The top section of the sheet provides the "envelope" of the design. You will typically find:

Design Code: Usually ASME B31.3 (Process Piping) or ASME B31.1 (Power Piping).

Base Material: Such as ASTM A106 Gr. B for Carbon Steel.

Corrosion Allowance: The thickness added to the pipe to account for metal loss over time.

Service: Some companies list the intended service, such as "Sour Service" or "Utility Water."

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Understanding the Pressure-Temperature (P-T) Rating

This is perhaps the most critical table on the sheet. Metals weaken as they get hotter. Therefore, a pipe that can handle 20 bar at 30 $°C$ might only be safe for 15 bar at 200 $°C$ .

When reading the P-T table:

Locate your Operating Temperature.

Move across to find the Maximum Allowable Pressure.

Ensure your process conditions fall below these limits. If your pressure is too high for the temperature, you must move to a higher class (e.g., from 150# to 300#).

Branch table and pressure-temperature rating in a piping class sheet

The Branch Table: The Roadmap for Connections

At the bottom or on a separate page, you will find the Branch Table. This is a grid where the "Header Size" is on one axis and the "Branch Size" is on the other.
If you are connecting a 4" pipe (Header) to a 2" pipe (Branch), the table will show a symbol like "WEL" (Weldolet), "TE" (Reducing Tee), or "SOC" (Sockolet).
Standard Tee (T): Used when the header and branch are the same size.
Reducing Tee (RT): Used for specific size reductions allowed by the code.
Olets: Used when a small branch is taken from a much larger header.

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Decoding the Component Tables (Pipe, Fittings, Valves)

The bulk of the sheet is divided into rows for each component. Let's look at the columns:

1. Item Description

This identifies the specific component—be it an elbow, tee, flange or valve—ensuring the correct part is ordered for the specific application.

2. Size Range

You will see entries like 1/2" to 2" or 3" to 24". Different sizes often have different specifications. For example, small-bore piping (up to 2") might use Socket Weld fittings, while large-bore (3" and up) uses Butt Weld.

3. Material Specification (ASTM/ASME)

This is where the symbols come in. You will see codes like:

ASTM A106 Gr. B: Seamless Carbon Steel Pipe.

ASTM A234 WPB: Wrought Carbon Steel fittings.

ASTM A105: Forged Carbon Steel for flanges and small fittings.

4. Schedule/Thickness

It will specify "Sch 40," "Sch 80," "STD," or "XS." This must match the stress calculations for that class.

5. End Connections

BW: Butt Weld.

SW: Socket Weld.

SCRD: Threaded/Screwed.

RF: Raised Face (for flanges).

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Stainless Steel vs Carbon Steel class differences

When selecting a piping class, the choice between Carbon Steel (CS) and Stainless Steel (SS) is the most common decision an engineer faces. While they may look similar on a 3D model, their Piping Class Sheets are worlds apart in terms of pressure handling, corrosion resistance, and cost.

Comparison Summary Table

Feature	Carbon Steel Class (e.g., A1A)	Stainless Steel Class (e.g., E1A)
Common ASTM Pipe	A106 Gr. B (Seamless)	A312 TP316L / 304L
Corrosion Allowance	High (1.5mm - 6.0mm)	Nil to Low (0.0mm - 0.5mm)
Typical Schedules	Sch 40, 80, 160	Sch 5S, 10S, 40S
Low Temp Limit	$-29^\circ C$	$-196^\circ C$ °C (Cryogenic)
High Temp Limit	Up to $425^\circ C$ °C	Up to $815^\circ C$ °C (varies by grade)
Cost	Baseline ( $1\times$ )	Expensive ( $3\times$ to $5\times$ )

Here is a detailed breakdown of the differences you will encounter when reading CS vs. SS class sheets.

1. Corrosion Allowance (CA)

One of the first things you’ll notice in the header of the class sheet is the Corrosion Allowance.

Carbon Steel Class: Typically has a CA of 1.5mm, 3.0mm, or even 6.0mm. Carbon steel is susceptible to uniform oxidation, which is why we 'over-design' the wall thickness—building in a sacrificial layer to ensure the pipe meets its 20–25 year design life.

Stainless Steel Class: Usually has a 0.0mm or 0.5mm CA. Because stainless steel forms a passive chromium-oxide layer that resists rust, we don't need to add extra "sacrificial" metal thickness.

2. Pressure-Temperature (P-T) Scaling

The way these materials react to heat is a major differentiator in the P-T charts.

Carbon Steel (e.g., A106-B): Excellent at ambient and moderate temperatures. However, brittle fracture may occur at very low temperatures (usually below −29°C) and oxidation/scaling at very high temperatures (above 425°C).

Stainless Steel (e.g., A312-TP316L): While SS has a lower "yield strength" than CS at room temperature (meaning a 300# SS flange might actually have a slightly lower pressure rating than a 300# CS flange at 38°C), it excels in cryogenic conditions (down to -196°C) and maintains integrity at much higher temperatures where CS would fail.

3. Schedule and Wall Thickness

In your class sheet's "Pipe" row, look at the Schedule column.

Carbon Steel: You will commonly see Sch 40, Sch 80, or Sch XS. These are standard thicknesses designed to handle high pressure plus the heavy corrosion allowance mentioned above.

Stainless Steel: You will frequently see the letter "S" after the number, such as Sch 5S or Sch 10S. Because SS is expensive and doesn't corrode, engineers use these "Thin Wall" schedules to save weight and reduce project costs.

4. Branch Connections (The Branch Table)

The way you join pipes differs due to weldability and cost.

CS Branch Table: Often allows for "Stub-ins" (where the branch pipe is welded directly into a hole cut in the header) for low-pressure utility services to save on fitting costs.

SS Branch Table: Almost always mandates the use of Tees or Reinforced Fittings (Olets). Stainless steel is more susceptible to "heat tint" and warping during welding, so using manufactured fittings ensures the structural integrity of the joint.

5. Post-Weld Heat Treatment (PWHT)

Check the "General Notes" section of the class sheet.

Carbon Steel: Often requires PWHT (stress relieving in a furnace or with heating blankets) if the wall thickness exceeds a certain limit (usually $>19mm$ 19mm or 38mm depending on the code) to prevent cracked welds.

Stainless Steel: Generally prohibits standard PWHT. Heating austenitic stainless steel (like 304L/316L) to the wrong temperature can cause "sensitization," where the chromium precipitates out, making the pipe more likely to corrode at the weld.

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The Power of "General Notes"

Never skip the notes! They often contain "hidden" requirements that aren't in the table. Common notes include:

"All valves must be Fire Safe tested to API 607."

"Gaskets shall be Spiral Wound with SS316 filler."

"Galvanizing required for all utility lines."

A mistake in reading the notes can lead to a massive failure during the procurement or inspection phase.

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Frequently Asked Questions (FAQ)

1. What does it mean if a component is "Not Listed" in a Pipe Class?

If a component isn't on the sheet, you cannot use it without a formal "Spec Deviation" or "Technical Query (TQ)." It usually means that component is not compatible with the pressure or material of that class.

2. Why do some Pipe Classes have "Sour Service" written on them?

Sour Service means the fluid contains Hydrogen Sulfide (H₂S). These classes require special materials (like NACE MR0175 compliance) to prevent Stress Corrosion Cracking.

3. Is the Pipe Class code the same across all companies?

No. While many companies use similar logic, every project or owner has their own coding system. Always refer to the "Legend" or "General Instructions" of your specific project PMS.

4. How do I choose between two different classes for the same line?

You must check the P-T rating. Always pick the class that covers the highest possible pressure and temperature the line might face during upsets or steam-out.

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Conclusion:

Reading a Piping Class Sheet is like mastering the shopping list of a safe industrial plant. By understanding the header, the P-T ratings, the ASTM material codes, and the branch table, you ensure that every elbow, bolt and valve is perfectly suited for its environment.

If you want to see how these class-specified components are laid out in the physical space of a plant, be sure to check out our existing series on Piping GA (General Arrangement) Drawings.

If this guide helped you, bookmark it or share it with fellow piping engineers.

How to Read a Piping Class Sheet: A Practical Engineering Guide

Master the Art: How to Read a Piping Class Sheet Like a Pro

Hello Piping Enthusiasts!

📘 Table of Contents