Why Leak-Before-Break Ensures Safety in Critical Systems

In the PVP industry, safety is paramount. Whether it's in nuclear power plants, pipelines carrying hazardous materials, or aircraft structures, ensuring the integrity and sustainability of critical systems is of utmost importance. One method that plays a crucial role in guaranteeing this safety is the Leak Before Break (LBB) analysis.

What Is LBB?

Leak Before Break (LBB) analysis is a proactive approach engineers use to prevent catastrophic failures in structures or systems that handle hazardous materials or substances under pressure. As the name suggests, the objective is to identify potential leaks before they evolve into full-blown ruptures.

How Does LBB Work?

Fundamentally, engineers use LBB to assess the structural integrity of a system to determine if it can tolerate small leaks without leading to catastrophic failure. Engineers employ sophisticated techniques such as finite element analysis (FEA), fracture mechanics, and stress analyses to evaluate the behavior of materials under varying conditions.

LBB can be a useful tool because the operating pressure may be well below the burst pressure for the geometry. Further, leak behavior should be highly considered since not all leaks cause a significant pressure drop, and where secondary containment is available, leaks may not escape into the environment, pose any risk to personal or require immediate shut-down of the operating plant.

How to Calculate LBB

Thru-wall Cracks

When an existing crack becomes thru-wall, the thru-wall crack stays in place and a detectable leak remains. When an existing crack becomes thru-wall, if the stress or stress intensity (KI or J) at the crack is excessive, then a break or an opening of the crack occurs. If the pressure drops and the crack expands, a rupture will ensue.

To prevent rupturing in any shell model, opened cracks should be specified and J integral values calculated for geometry's crack corners. Engineers should compare resulting nonlinear J results against these allowable J values. Additionally, residual stresses at welds can be conservatively included either as welded or if post-weld heat treated.

The method described in the following section utilizes the virtual crack displacement method to compute the nonlinear J integral for a thru-wall crack. This will give more accurate results for thru-wall cracks that can be evaluated using thick-walled shell theory.

Calculating LBB

The first step in a LBB calculation is to add two times the thickness to the measured crack length and then put the 2(c+t) crack length in the geometry (c is the half-crack length).

For normal stress on a thru-wall crack: KI = 1.12 x Sy x ( π(c+t) )0.5.?        

Sy = Yield stress + 10ksi c = half length of the thru-wall crack t = wall thickness.?        

After the nonlinear J value is calculated, the engineer should:

1. Calculate KI = (JE)0.5. E = modulus of elasticity.
2. Compute KI + H x 1.27 x 1.1 x 1.12 x S’y x (π x (crack length + thickness))0.5.?
3. H = 1 if NO PWHT, H=0.2 if PWHT; S’y = flow stress; CS=Sy + 10ksi.
4. Compare the value computed in Step 2 with the allowable fracture toughness like displayed in the figure below.
5. Ensure that no tensile overload exists in the shell if the full depth crack is present. Note: The elastic-plastic large rotation solution should converge.

Applications of LBB

In PRG software, LBB calculations can be automatically determined in any shell geometries with thru-wall cracks to indicate if pin-hole leaking occurs before any significant rupture of the pressurized component. Axisymmetric and brick calculations are then determined to support linear or nonlinear fitness for service. K(I) for linear and J for nonlinear. From this, the linear and nonlinear pressure fatigue is then determined on nozzles and olets.

The software also identifies cracks existing in brick geometries (that are partial thru wall or embedded). When the cracks are defined J (or KI, KII and KIII for elastic solutions), the software then automatically calculates those values for each converged load case.

J integral/flaw detection results are then converted into a readable and downloadable report.

Sustaining Equipment Life & Preventing Ruptures

LBB analysis stands as a cornerstone method in ensuring the safety and reliability of critical systems across various industries. By proactively identifying potential failure points and implementing preventive measures, engineers can mitigate risks and prevent catastrophic incidents. As technology advances and our understanding of materials and structural mechanics improves, LBB analysis continues to evolve, contributing to safer and more resilient infrastructure worldwide.

To learn more about automated LBB in PRG software, visit paulin.com.