Heat a steel pipe from a 30 °C morning to 185 °C working temperature and every metre of it grows by about 1.9 mm. Trivial — until you own a hundred-metre main, which is now 190 mm longer than the pipework, anchors and branch connections it was welded to at ambient. That growth cannot be argued with; it can only be directed. Directing it is the whole craft of expansion and support design, and getting it wrong is how flanges weep, guides seize and nozzles crack years before their time.
The arithmetic of growth
Carbon steel grows roughly 1.2 mm per metre per 100 °C. The practice plant's mains at 185 °C, from a 30 °C start:
growth ≈ 1.2 × (185 − 30)/100 ≈ 1.9 mm per metre — 190 mm on a 100 m run
Do this sum for every long straight run in the plant. Any run whose growth has nowhere deliberate to go is storing the difference as force — and thermal force in restrained steel is measured in tonnes, applied every start-up, relieved every shutdown, cycling the welds through fatigue the designer never intended.
Anchors and guides: deciding where the movement goes
An anchor is a support that permits no movement — it divides the pipe into expansion sections and gives each one a fixed end to grow away from. Guides permit movement along the axis only, keeping the growing pipe straight so it arrives at its expansion device travelling in the right direction (an unguided pipe under compression bows sideways instead — and a bowed pipe pools condensate, feeding lesson C4's problem). The pattern for each section is: one anchor, guides at intervals along the run, and one device that absorbs the growth. Plan it per section, on the drawing, before steel is cut; expansion design retrofitted after the first hot alignment problem costs five times as much.
Absorbing it: the loop and the bellows
The expansion loop — a U of pipe, fabricated from four bends — is the robust answer: no moving parts, no seals, nothing to inspect but paint, a service life equal to the pipe's. Its cost is space (a DN150 loop for a long run is metres on a side) and a little extra pressure drop. Where the run allows it, changes of direction do the same job free: an L-shaped route lets each leg flex to absorb the other's growth. The bellows joint — a corrugated stainless element absorbing growth in its own length — wins where space forbids a loop. It demands respect in exchange: accurate guiding either side (a bellows offered misaligned pipe fails early), anchors sized for the pressure thrust it transmits, and a place on the inspection schedule, because a bellows is a wearing part in a way a loop never is. The honest rule: loops where there is room, bellows where there is not, and never a bellows to save the cost of proper supports — it transfers that cost to the anchors with interest.
Supports: the spacing that stops the sag
Between the engineered points, the pipe still needs carrying at intervals that prevent sag — because a sagged span pools condensate between supports, and lesson C4 explained what pooled water becomes. Working spans for steel steam pipe run from about 2.5 m at DN25 through 3.5 m at DN50 to 6 m or more at DN150 — closer wherever valves or heavy fittings load the span. Supports on a hot line must also allow the movement the design directs: rollers or slide plates on long runs, spring hangers where vertical movement occurs. A rigid clamp bolted tight on a run designed to slide is a guide turned accidental anchor — and the force finds the next weakest flange.
- Walk the hot line and look for the signs of fighting steel: scored slide plates, bent guide brackets, weeping flanges near bends, lagging crushed at supports.
- Find every bellows and get it onto the inspection schedule with its design movement noted. An unlisted bellows is a countdown.
- Sight along long runs for sag; re-pack or add a support the week you see it — the pooled water is already forming.
- After any re-routing or new branch, re-ask the expansion question for the whole section: one added anchor point can redirect the growth of forty metres of pipe.
- Steel grows ~1.2 mm/m per 100 °C — 190 mm on the practice plant's 100 m main. The growth is non-negotiable; only its direction is yours.
- Anchors divide, guides direct, one device per section absorbs. Design it per section, on the drawing.
- Loops where there is room — fit-and-forget. Bellows where there is not — guided, anchored, inspected.
- Support spans tight enough that nothing sags: a sagged span is a condensate pocket you didn't order.
District-heating cities are the world's largest expansion-design exercise: under Moscow, Copenhagen and Seoul run thousands of kilometres of hot mains, every metre growing and shrinking with the season, absorbed by loops and compensators buried in concrete galleries their designers will never revisit. The discipline they standardised — fixed points on the drawing first, movement budgeted per section, every device reachable for inspection — is exactly the method for your hundred metres, minus the city on top.