When should a pipe stress analysis be carried out – and when is good engineering practice enough?

A full pipe stress analysis is not necessarily required, particularly for simple and easily understood piping systems.

Typical examples are: short pipes inside buildings,
small to medium nominal sizes, fluids of low density such as air or nitrogen, small temperature differences between installation and operation, low external loads, proven pipe supports, defined and sufficiently small support spacings, no sensitive pump or equipment connections.

Piping on pipe racks can also be built without a detailed pipe stress analysis if, for example, support spacings are standardised, piping classes are known and thermal loading is low.

“Good engineering practice” by no means implies design without technical consideration. Support span tables, simple strength calculations, manufacturers’ specifications, codes and standards and proven design details can constitute suitable verification for such systems.

What matters is that it can be assessed transparently why no further system calculation is required.

The more a pipe is influenced by external loads or imposed movements, the more difficult a reliable assessment based on experience alone becomes.

Thermal expansion

Changes in temperature lead to changes in the length of the pipe. If these movements are restrained by anchors, guides, friction or equipment connections, forces and moments arise.

Particularly with: long pipes, steam and hot water lines, high operating temperatures, low temperatures, frequently changing operating conditions, a pipe stress analysis is therefore useful.

Wind and earthquake

On outdoor piping, wind loads can cause considerable lateral forces. They become more critical in particular with: large nominal sizes, large support spacings, high pipe racks, thick insulation, free-standing lines.

It is not only the nominal pipe size itself that must be considered. A heavily insulated small pipe can present a surprisingly large area to the wind because of its much larger outside diameter.

Trace heating can also play an indirect role, since it influences the governing pipe temperature and thus the thermal expansion.

For seismic loading, consideration should always be given to how the dynamic loads are transferred via the pipe, supports and anchors into the supporting structure.

Large support spacings, heavy lines and large nominal sizes

As the distance between two pipe supports increases, bending moments and deflections rise.

Relevant here are not only pipe diameter and wall thickness but also, for example: weight of the fluid, valves, insulation, attachments, concentrated loads.

On large-diameter piping, local deformations of the pipe wall can additionally become relevant and may require a more detailed finite element analysis.

A particularly important reason for a pipe stress analysis is determining the forces and moments at connected components.

Typical interfaces are: pumps, compressors, vessels, heat exchangers, columns, valves, special equipment.

The nozzle loads occurring there result from the interaction of the entire system. Dead weight, thermal expansion, friction at pipe supports, anchors and external loads can reinforce or reduce one another.

A pipe stress analysis makes it possible to determine these forces unambiguously and to compare them with allowable values from manufacturers or codes.

More than just a strength verification

Even if a pipe could theoretically be designed in accordance with proven engineering practice, a pipe stress analysis can make sense from a project point of view.

A calculation report documents, for example: design basis, temperatures and pressures, load cases, support concept, anchors and guides, displacements, support loads, nozzle loads,

stress verifications.

This produces a transparent technical document for client, operator, structural steelwork and equipment manufacturer.

Particularly on larger projects this is an important mark of quality. Design and calculation can be checked independently of each other and thus support the four-eyes principle.