Longitudinal Weld: Everything you want to Know.

Longitudinal Weld Meaning

During welding of piping and pressure vessels, not all welds are created equal. While a Circumferential (Girth) Weld joins two sections together, the Longitudinal Weld is what turns a flat steel plate into a pipe or a vessel shell. Because of the physics of internal pressure, this seam bears the highest stress of any joint in the system. It is the “spine” of the vessel, and its integrity dictates the safety factor of the entire asset.

This guide covers everything from the Hoop Stress theory to ASME Joint Categories and NDT inspection requirements.

A longitudinal weld is a type of welding joint in which two edges of a rolled material are welded along the lengthwise direction of the part. This type of weld is often used to join sections of pipe or tubing or shell.

Longitudinal weld or Longitudinal seam both are identical terms used in fabrication industries to point out the weld made along the length of a pipe or shell.

Longitudinal welds are very critical as they save tons of money by allowing the fabrication of pipes of larger diameter by welding. They are accompanied usually by 100% NDT to ensure the welds are of high quality and integrity.

What is longitudinal seam welding?

The term “longitudinal” refers to the axis of the weld, which is parallel to the length of the workpiece. A longitudinal weld (often called a “Long Seam”) runs parallel to the main axis of a pipe or vessel.

In contrast, a transverse weld is perpendicular to the length of the workpiece. longitudinal weld seam orientation is parallel to the length of the part as shown in below figure, highlighted by green line.

Longitudinal welding can be performed using various methods, including gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), and flux-cored arc welding (FCAW).

Features of Longitudinal welds:

• Orientation: It creates the cylindrical shape.
• Material: Usually the same grade as the base metal (e.g., A106 Gr B, API 5L X65).
• Purpose: Containment. It must withstand the full force of the internal fluid or gas trying to expand the cylinder.

The Physics: Why Long Seams Fail First (Hoop Stress)

To understand the importance of the longitudinal weld, you must understand the stress distribution in a pressurized cylinder.

When a vessel is pressurized, the internal force pushes outwards in all directions. However, the material resists this force differently along two axes:

1. Longitudinal Stress (sigma_L): The force trying to pull the cylinder apart end-to-end (acting on the Girth Weld).
2. Hoop Stress (sigma_h): The force trying to split the cylinder open along its length (acting on the Longitudinal Weld).

The Critical Formula

According to thin-walled pressure vessel theory:

sigma_h = (P x D)/ 2t

sigma_L = (P x D)/ 4t

• P: Internal Pressure
• D: Diameter
• t: Wall Thickness

The Engineering Reality: Hoop Stress is exactly TWICE the magnitude of Longitudinal Stress (sigma_h = 2 sigma_L).

This means the longitudinal weld is always under double the load of the circumferential weld. If a vessel bursts due to over-pressure, it will almost always fail along the long seam, splitting open like a zipper.

ASME Code Classification: Category A

In the ASME Boiler and Pressure Vessel Code (BPVC) Section VIII, Division 1, welds are defined by their location in the vessel, known as Joint Categories.

• Category A: Longitudinal welded joints within the main shell, communicating chambers, transitions in diameter, or nozzles.
• Category B: Circumferential welded joints connecting shell sections or heads.

Because Category A welds bear the highest stress (Hoop Stress), ASME assigns them the strictest testing requirements. The “penalty” for not testing this weld is severe:

Joint Efficiency (E)

The Joint Efficiency factor (E) is a multiplier in the wall thickness calculation.

• Full Radiography (RT-1): E = 1.0. You can utilize 100% of the material’s strength.
• Spot Radiography (RT-3): E = 0.85. The material is considered only 85% strong, forcing you to increase wall thickness by ~18% to compensate.
• No Radiography: E = 0.70. You must increase wall thickness by ~43%.

Cost Implication: Failing to X-Ray a longitudinal seam can force you to buy significantly thicker steel plate, often costing far more than the NDT itself.

Manufacturing Methods: LSAW vs. ERW vs. Spiral

How the longitudinal seam is created determines its risk profile.

A. Submerged Arc Welding (LSAW)

Used for heavy-wall pressure vessels and large-diameter pipes.

• Process: Plates are formed into a “U” then an “O” shape (U-O-E method). A single longitudinal pass is welded internally and externally using Submerged Arc Welding (SAW).
• Defect Risk: Centerline solidification cracking (due to the depth of the weld pool) and slag inclusions.

B. Electric Resistance Welding (ERW)

Used for standard pipe up to 24 inches.

• Process: High-frequency current heats the edges of the strip, which are then forged together. No filler metal is added.
• Defect Risk: “Hook Cracks” and Lack of Fusion at the bond line. Historic failures in older ERW pipelines have made this a focus area for integrity management.

C. Spiral (Helical) Welding (HSAW)

Often used for water transmission and piling.

• The Stress Trick: By winding the steel coil at an angle, the weld seam enters the pipe diagonally.
• Advantage: The stress acting perpendicular to the weld is less than the full Hoop Stress, effectively increasing the safety factor of the seam itself.

Peaking & Banding: The Geometric Killers

A longitudinal weld must be perfectly round. If the rolling process leaves a “flat spot” at the weld (Peaking) or creates a localized “roof-top” profile, it introduces a massive Stress Concentration Factor (SCF).

• Peaking: When the weld joint is pointy (like a roof). Under pressure, the vessel tries to “round out,” creating bending moments at the toe of the weld. This leads to fatigue cracking.
• Banding: When the weld joint is flat.
• ASME Tolerance: Section VIII, Div 1, UG-80 strictly limits out-of-roundness to 1% of the diameter to prevent these secondary bending stresses.

5. Inspection Strategy (NDT)

Because a failure here is catastrophic, the inspection strategy for long seams is aggressive.

1. Ultrasonic Testing (UT) – Angle Beam: Essential for thick-wall vessels (>25mm). RT often misses “Lack of Side Wall Fusion” in steep bevels (e.g., J-preps). UT is mandatory to detect planar flaws oriented along the weld axis.
2. Radiography (RT): The standard for checking volumetric defects (slag, porosity). For longitudinal seams, source placement is critical to avoid geometric unsharpness (U_g).
3. TOFD (Time of Flight Diffraction): Ideally suited for long seams as it provides rapid scanning with accurate depth sizing of defects.

Summary

Feature	Longitudinal Weld	Circumferential Weld
Stress Type	Hoop Stress (sigma_h)	Longitudinal Stress (sigma_L)
Stress Magnitude	Maximum (100%)	Half (50%)
ASME Category	Category A	Category B
Failure Mode	Rupture / Fish-mouth split	Leaking / Snap
Criticality	High	Medium

What are the types of Welding seams?

A welding seam is a point where two edges of the material are welded together. There are three main types of welding seams:

1. Longitudinal,
2. Circumferential, and
3. Helical.

Longitudinal weld seams are created when two edges of the material are welded together along their length. This type of seam is often used for making large-diameter pipes and shells for vessels & tanks.

Circumferential seams are created when two edges of rolled pipe/ shell are welded together around their circumference. This type of seam is often used for cylindrical tanks and vessels. In pipe welding, a Circumferential seam is called Girth Weld.

Helical weld seams are created when two edges of the metal are welded together in a spiral pattern. This type of seam is often used for pipes and tubes.

Which weld is Stronger, a longitudinal or a transverse Weld?

Longitudinal weld joint falls under Category A as per ASME Section VIII Div. 1. Longitudinal welds are typically stronger than transverse welds because they experience less stress during use.

Longitudinal weld joint efficiency

Longitudinal weld joint efficiency is 0.7 when only visual inspection is carried out. One way to improve the efficiency of longitudinal weld joints is to use spot radiography.

A joint efficiency of 0.7 means the strength of the weld joint is 70% of the base material strength. For example, if the base metal strength is 100Ksi, a weld joint with a 0.7 joint efficiency will have a strength of 70 Ksi.

By adding radiography, the joint efficiency for Longitudinal welds is increased from 0.7% to 0.85%.

A joint efficiency of 0.85 means the strength of the weld joint is 85% of the base material strength. For example, if the base metal strength is 100Ksi, a weld joint with a 0.85 joint efficiency will have a strength of 85 Ksi.

Longitudinal weld vs Transverse weld

A longitudinal weld is a weld made along the length of a workpiece, while a transverse weld is a weld made across the width of a workpiece. Each type of weld has its own advantages and disadvantages.

Longitudinal welds are stronger than transverse welds because they have less surface area exposed to potential failure. This makes them ideal for joining together two pieces of metal that will be under high stress.

Transverse welds are not as strong as longitudinal welds, but they are essential to fabricate higher-length vessels, tanks, columns and reactors for industries.

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