“When Welds Are Too Close: What Happens When You Break the Code or Standard Rule — And How to Engineer a Way out."

What really happens when welds are placed too close — and the 5t rule is ignored (ASME B31.3 requirement)?

In one of our recent brownfield projects, a seemingly small deviation from ASME B31.3 led to plastic deformation, stress concentration, and a costly redesign.

In this post, I’m sharing the full story — what went wrong, how we diagnosed it with nonlinear FEA, and how we engineered a practical and verifiable safeguard to fix it.

🔹 For full context, check out these two companion articles — highly recommended to read in parallel:

• Minimum Distance Between Welds – Technical Requirements Across International Standards
• Safeguarding in ASME B31.3 – Going Beyond the Code with Confidence

📌 The Problem: Weld Spacing Violation on a 6” Line in a Brownfield Retrofit

In a brownfield upgrade project inside an aging gas dehydration unit, we had to install a new 6” Sch. 80 CS line to re-route glycol return flow. Due to physical limitations of the existing structure — including structural steel, cable trays, and a nearby vessel — the spool design left only ~18 mm of straight pipe between two girth welds.

According to ASME B31.3, the recommended minimum distance between adjacent butt welds is 5 times the wall thickness (in this case: 5 × 10.97 mm ≈ 55 mm), unless engineering justification is provided.

Unfortunately, in this case:

• The fit-up was approved based solely on dimensional feasibility.
• There was no finite element evaluation, no local PWHT, and no hardness verification.
• The justification given was: “It passed the hydrotest.”

But hydrotest doesn’t account for long-term cyclic thermal loading or residual stress interaction in overlapping HAZs.

⚠️ The Result: Localized Deformation & Stress Hotspots After Startup

Roughly 3–4 weeks into commissioning:

• A slight ovality appeared near the downstream girth weld.
• Infrared thermography during shutdown showed heat concentration between the two welds.
• Ultrasonic shear wave testing confirmed out-of-roundness and wall thinning.

Key contributing factors:

• HAZ overlap from adjacent welds
• Residual stresses locked in due to proximity and lack of PWHT
• Localized cyclic thermal expansion and constraint
• Progressive plastic deformation due to material fatigue

🔬 The Diagnosis: Nonlinear FEA in ANSYS Reveals the Root Cause

The pipe section was modeled in ANSYS using:

• Nonlinear material properties (temp-dependent)
• Boundary conditions reflecting actual support and connection constraints
• Thermal cycling from 30°C to 140°C
• Internal pressure
• Mesh refinement near the weld area

Results showed:

• A localized von Mises stress spike between the welds
• Plastic strain >0.2% in thermal cycles
• Predicted deformation consistent with field data

FEA confirmed ratcheting behavior due to weld proximity and repeated expansion in an overstressed zone.

💪 The Fix: Engineering Safeguards in Action

Once the failure mechanism was identified, a multi-step safeguarding strategy was implemented. Each action was carefully engineered, verified, and documented:

1. Reinforcement Pad Installation

• A custom non-circular reinforcement pad was designed (elliptical profile) to fit within the constrained geometry.
• To achieve this, a minor rerouting of adjacent cable trays and a support frame was carried out with formal approval from the operations team. This allowed sufficient clearance to implement the pad.
• Precision welding was performed under a tailored WPS, followed by RT and MPI.
• Purpose: redistribute stress and increase local stiffness to reduce deformation risk.

2. Local Post-Weld Heat Treatment (PWHT)

• Local PWHT was conducted using resistance heating and monitored thermocouples.
• Parameters aligned with ASME Section VIII Div. 1 for the specific wall thickness.
• Purpose: relieve residual stresses and improve HAZ toughness.

3. Updated FEA with Revised Conditions

• A new FEA model was developed reflecting the pad installation and heat treatment effects.
• Results showed decreased stress concentration and elimination of plastic strain zones.
• Purpose: confirm design adequacy and stress relief effectiveness.

4. Engineering Justification & Documentation

• A technical deviation dossier was compiled, including:

- Stress reports

- Modified isometric drawings

- Weld maps and NDT

- PWHT logs and thermal graphs

• Purpose: provide engineering traceability, assurance, and compliance.

5. Formal Client Approval Process

• The safeguard strategy and justification package were submitted for client review.
• After technical walkthroughs and risk discussions, the deviation was formally accepted — with a commitment to monitor the location during service.

🧠 The Lesson: Safeguarding Is a Must, Not a Choice

This was not simply about spacing. It was a case where field constraints pushed us outside the code envelope — and safeguarding became the only acceptable path forward.

ASME B31.3 allows for flexibility, but only if adequate safeguards are in place. As explained in detail in this article, safeguarding is the provision of protective measures to minimize the risk of accidental damage to the piping or to minimize the harmful consequences of possible piping failure.

It is a concept that works well in the context of the B31.3 Section because the owner has overall responsibility for all aspects (design and operation) of the piping system. This differs from the much more limited scope of responsibilities in ASME B31.1 and ASME BPVC, Section VIII, Division 1.

Designers should note that, based on B31.3, the owner also has the ability to effectively specify and implement safeguarding provisions. For example, ASME B31.3 permits the use of certain components, joining methods, and procedures when appropriate safeguards are provided. One classic example: brazed joints are normally prohibited in flammable/toxic service unless safeguarded (para. 317.2).

In our case, the safeguarding strategy was not just a fix — it was a structured engineering response to a real-world constraint. Without it, the risk of failure remained unacceptably high.

Whenever you are forced to go below code minimums, you must: ✅ Analyze the failure modes ✅ Design appropriate safeguards ✅ Justify and document the changes ✅ Obtain formal client approval

This mindset ensures integrity — not just compliance.

📣 Final Thought

Codes give us the minimums. Projects often force us below them. What matters is how we respond.

Have you ever engineered your way out of a code violation? Share your experience.

Let’s learn, build safer systems, and lead with engineering judgment.

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