On this page
Key takeaways
- Thermal growth is ΔL = α × L × ΔT. A 100 m carbon steel line heated by 200 °C grows by about 240 mm.
- Expansion loops on the rack are simple and reliable; they need space and extra pipe.
- Expansion joints (bellows) save space but add anchors, pressure thrust and a weak point, so most hydrocarbon systems avoid them.
The scenario
A hot oil line runs 100 m along the main pipe rack of a refinery unit at an operating temperature of 220 °C. Installation temperature is about 20 °C. How much does it grow, and where does that growth go?
Calculating growth
Thermal growth is:
ΔL = α × L × ΔT
For carbon steel, α is about 12 × 10⁻⁶ per °C in this range (use the code table value for the actual temperature).
ΔL ≈ 12 × 10⁻⁶ × 100 000 mm × 200 °C ≈ 240 mm
If both ends were fixed, the pipe would try to push 240 mm into its anchors, producing enormous forces and stresses. The growth needs somewhere to go.
Option 1: expansion loop
A loop is a U-shape added to the line, usually turned up and over the rack or horizontally out to the side at a different elevation. The loop’s two legs bend to absorb growth from both directions.
The standard arrangement:
- an anchor at each end of a section,
- guides close to the loop and along the run so the pipe grows straight along the rack,
- the loop in the middle so it takes half the growth from each side.
Loops are robust: no moving parts, no extra leak points, and no special maintenance. Their costs are rack space (hot lines are grouped so their loops can be nested together), extra pipe and fittings, and extra pressure drop.
Option 2: expansion joint
A metal bellows expansion joint absorbs movement in a short length. It is attractive where space is tight, but it brings complications:
- Pressure thrust: an unrestrained bellows produces a force equal to pressure times the bellows effective area, which the main anchors must resist.
- Strict guiding close to the joint to avoid buckling (squirm).
- A thin flexible element that is more vulnerable to corrosion, fatigue and damage than the pipe.
- Inspection and maintenance over the plant’s life.
Many owner specifications prohibit or discourage bellows in hydrocarbon service and accept them only for large, low-pressure lines such as flue gas, steam exhaust or some cooling water services.
Which one here?
For a 100 m hot oil line on a rack, the usual answer is two sections with anchors and one loop each, or one central loop if the stress analysis shows the legs are long enough. The stress engineer sizes the loop legs so displacement stresses stay within the allowable range and the anchor loads are acceptable to the rack design.
Code reference
ASME B31.3, para. 319 Flexibility analysis and displacement stress range.
EJMA Standards Design of metallic bellows expansion joints.
What to take back to your desk
On any long hot line, calculate the growth by hand first. If the number is more than a few tens of millimetres, plan the anchors, guides and loop locations with the rack designer before routing anything else.
Educational content. Always design to your project specification.
