Glossary

Local stress analysis

A local stress analysis examines individual highly stressed areas – a nozzle, a branch, a welded attachment – in more detail than the global beam model of the piping system can.

Also: Local stress evaluation · Nozzle stress analysis · Detailed stress analysis

Also known as

Local stress evaluation
Nozzle stress analysis
Detailed stress analysis

What the beam model cannot see

A pipe stress analysis represents a piping system as beams. The model delivers internal forces, displacements and the stresses in the pipe cross-section – reliably and quickly. What it does not know is the shell: how the wall of a pipe or vessel deforms locally under an applied load and where it is highly stressed as a result.

This is where local stress analysis comes in. It examines a single location – a nozzle, a branch, a welded support attachment – with a model that represents the wall as a shell, and assesses the stresses found there.

When it is required

A local investigation is not routine. It answers a specific question that the global model leaves open.

Nozzle loads from the stress analysis exceed the allowable values of the equipment manufacturer, and the nozzle is to be justified rather than the piping rerouted.
Support attachments such as trunnions, lugs or brackets are welded directly to the pipe and introduce large forces.
A branch or special component lies outside the range of validity of the stress intensification factors in the code.
Thin-walled pipe of large diameter ovalises under load more than the beam model represents.
A location is at risk of fatigue and is to be assessed more precisely than with blanket factors.

Methods: analytical or finite element

Analytical methods exist for standard cases. The bulletins of the Welding Research Council – WRC 107 and its successor WRC 537, together with WRC 297 – are widely used for loads on nozzles and attachments on cylindrical and spherical shells. EN 13445-3 contains its own rules for local loads on nozzles. These methods are fast but valid only within defined geometric limits.

Outside those limits, finite element analysis is used: the affected area is built as a shell model and the loads from the pipe stress analysis are applied at its boundary. What matters is that load cases, signs and coordinate system are transferred from the global model without contradiction.

How the results are assessed

A shell model shows very high, tightly confined stresses at junctions. Comparing them with the allowable stress for internal pressure would be wrong. The codes therefore divide stresses into categories – membrane and bending, primary and secondary, plus peak stresses – and set a separate limit for each.

Primary membrane stresses are tightly limited because they can lead to plastic collapse. Secondary and peak stresses are assessed through the stress range and fatigue. Assigning stresses to a category is the most demanding part of the work – and the part where assessments most often differ.

Common mistakes

The calculation itself is rarely the problem – the assumptions are.

The model boundary lies too close to the location examined and stiffens it artificially.
Loads are taken from a single load case although another one governs.
The mesh at the junction is too coarse to capture the stress gradient.
Numerical peaks at a model edge are assessed as membrane stress – or ignored altogether.
Internal pressure is missing from the local model although it acts together with the external loads.

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Overview

Three ways to justify a single location

Method	Suitable for	Limits
Beam model with stress intensification factor	Standard bends, tees and branches within the scope of the code	Knows no shell deformation and no welded attachments
Analytical method (WRC bulletins, EN 13445-3)	Nozzles and attachments on cylindrical and spherical shells	Valid only within defined geometric limits
Finite element shell model	Special components, large thin-walled pipe, fatigue assessment	Effort; the result depends on boundary conditions and assessment

Frequently asked questions

Frequently asked questions about Local stress analysis

Does a local stress analysis replace the pipe stress analysis?

No. It builds on it: the loads used to examine the location come from the global model. Without a pipe stress analysis, the local analysis has no input.

Does every nozzle need a local analysis?

No. If the nozzle loads are below the allowable values of the manufacturer or the project specification, that is the justification. Local analysis is the route for the cases where this cannot be achieved.

Which is better: justifying the nozzle or rerouting the line?

It depends on the project stage. Early in design, an additional support or more flexibility in the routing is usually the shorter route. Once the equipment is installed, a calculation is often the only option that avoids modification.

Does the local analysis feed back into the pipe stress analysis?

It can. A shell is more flexible than the rigid connection often assumed for a nozzle in the beam model. Taking this flexibility into account reduces the calculated nozzle loads in many cases.

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