A layout that ignores thermal growth pushes the problem to the stress engineer, who then has to add loops where there is no room for them. This lesson shows how to leave that room on the rack from the first sketch.

What you should be able to do

• Estimate free thermal growth of a rack run before any stress model exists.
• Decide where expansion loops go and which line sits on the outside of a loop nest.
• Place anchors and guides so each loop takes the growth of its own run.

Estimating growth

Free growth is ΔL = α × L × ΔT. For carbon steel a useful working figure is about 1.2 mm per metre for every 100 °C of temperature rise. A 60 m run going from 20 °C to 220 °C grows about 144 mm. Take exact values from the thermal expansion table in the design code for the project.

Placing loops on a rack

• Group the hot lines at one edge of the rack so their loops can nest and share one set of loop supports.
• Put the largest, hottest line on the outside of the nest. It needs the biggest loop, and smaller loops fit inside it.
• Raise the loop above the rack tier so cold lines can pass underneath without being rerouted.
• Put an anchor about midway between loops, and guides on both sides of each loop, so growth is directed into the loop.
• Check that loop legs do not block access ways, cable trays or future tie-in space agreed in Module 3.

When a formal analysis is not required

ASME B31.3 para. 319.4.1 lists the cases where a formal flexibility analysis may be skipped. Layout engineers use it as a first screen, never as a replacement for the stress engineer's review. Use the study assistant to walk through the clause.