SMLS carbon steel pipe sizes

What Does SMLS Stand For?

SMLS stands for Seamless, commonly used in terms such as SMLS pipe or seamless steel pipe.

In the steel pipe industry, SMLS identifies pipes manufactured without any longitudinal weld seam, ensuring a monolithic structure formed from a solid steel billet through heating, piercing, rolling, or extrusion.

Unlike welded pipes, seamless pipes do not contain fusion lines, making them structurally uniform and mechanically reliable under demanding service conditions.

What Is an SMLS Pipe?

A seamless steel pipe is a hollow tubular product manufactured by:

Heating a solid billet (carbon steel, alloy steel, stainless steel)

Piercing to form a hollow shell

Rolling or extruding to achieve final dimensions

Producing a pipe body with no weld seams

Because the pipe wall is continuous and homogeneous, SMLS pipes are widely specified in:

High-pressure systems

High-temperature pipelines

Oil & gas transmission

Power plants

Petrochemical processing

Why Is SMLS Explicitly Specified?

In industrial procurement and engineering specifications, pipe type must be clearly defined.

There are two primary manufacturing categories:

SMLS (Seamless) – typically used for high-pressure and critical applications

Welded Pipe – generally used for medium to low-pressure systems

Specifying SMLS in contracts ensures:

No weld seam risk

Improved pressure resistance

Better fatigue performance

Compliance with strict safety standards

In technical documents, the abbreviation frequently appears in product descriptions and marking systems to eliminate ambiguity.

Common Technical Combinations

You will typically see SMLS combined with grade or standard identifiers, such as:

SMLS Pipe

SMLS Carbon Steel Pipe

ASTM A106 Gr.B SMLS

Seamless (SMLS) Tube

This helps distinguish the product from ERW, LSAW, or SSAW welded pipe types.

SMLS Carbon Steel Pipe Sizes

Pipe dimensions are defined by three primary parameters:

Outer Diameter (OD)

Wall Thickness (WT)

Length

The most authoritative dimensional standard globally is:
ASME B36.10M – Welded and Seamless Rolled Steel Pipes

This standard covers NPS 1/8" to NPS 48" and defines OD, wall thickness, and theoretical weight.

1. Outer Diameter (OD) Standards

According to ASME B36.10M, seamless pipe outer diameters follow a standardized NPS-based series.

Small Diameter Series

10.3 mm (1/8")

13.7 mm (1/4")

17.1 mm (3/8")

21.3 mm (1/2")

26.7 mm (3/4")

33.4 mm (1")

42.2 mm (1-1/4")

48.3 mm (1-1/2")

60.3 mm (2")

Medium Diameter Series

88.9 mm (3")

114.3 mm (4")

141.3 mm (5")

168.3 mm (6")

219.1 mm (8")

273.0 mm (10")

323.8 mm (12")

Large Diameter Series

355.6 mm (14")

406.4 mm (16")

457.2 mm (18")

508 mm (20")

610 mm (24")

Up to NPS 48"

Outer diameter values are fixed per standard and not arbitrarily customized.

2. Wall Thickness (Schedule System)

Wall thickness is typically expressed using the Schedule (SCH) system:

Common schedules include:

SCH 10

SCH 20

SCH 30

SCH 40 (STD)

SCH 60

SCH 80 (XS)

SCH 100

SCH 120

SCH 140

SCH 160

XXS

Schedule Designations

STD → Standard weight (generally equivalent to SCH 40)

XS → Extra strong (generally equivalent to SCH 80)

XXS → Double extra strong

Important Engineering Rule

The wall thickness of a schedule increases with pipe diameter.

Example:

NPS 2" (60.3 mm) SCH 40 → 3.91 mm

NPS 8" (219.1 mm) SCH 40 → 8.18 mm

Therefore, SCH 40 does not equal one fixed thickness; it varies depending on NPS.

3. Length of SMLS Pipes

Seamless pipes are supplied in three typical length categories:

Random Length

Delivered within a standard range (typically 3–12 meters).

Fixed Length

Delivered according to contractual requirement (e.g., 6 m, 12 m).

Multiple Length

Delivered in integer multiples of a specified length with machining allowance.

FAQ

What does SMLS mean in piping?

SMLS means seamless. It refers to a steel pipe manufactured without a weld seam.

Is SMLS stronger than welded pipe?

In high-pressure and critical applications, seamless pipes generally offer better structural integrity because they do not contain weld seams.

Which standard defines SMLS pipe dimensions?

ASME B36.10M defines the dimensional requirements for welded and seamless steel pipes.

What determines SMLS pipe size?

Size is defined by outer diameter (OD), wall thickness (Schedule), and length.

Summary

SMLS pipe refers to seamless steel pipe manufactured without weld seams. It is widely used in high-pressure, high-temperature, and safety-critical industries. Sizes follow ASME B36.10M standards, and wall thickness is designated by Schedule numbers (SCH 10–SCH 160, XS, XXS).

When specifying industrial piping, clearly stating SMLS ensures the procurement of monolithic, high-integrity steel pipe suitable for demanding engineering applications.

Related: carbon steel pipe sizes