Insulation Economics: How Much Lagging Pays
Insulating a bare steam line is among the fastest paybacks in a plant. Beyond the first layer, each extra millimeter saves less than the last — and somewhere the saving stops paying for the material. That point is the economic thickness.
You'll learn
- Calculate the cost of a bare steam line
- See diminishing returns with thickness
- Understand economic thickness
- Know what insulation is often left off
Before this: Heat loss from pipes, Expansion, supports and insulation, Cost of Steam
Bare pipe is expensive
A bare DN80 line at 10.54 kg/cm²(g) in 30 °C air loses about 909 W per meter. Over 100 m that condenses 164 kg of steam an hour, every hour the line is live — about ₹11,15,312 a year of coal.
Just 50 mm of insulation cuts that to about 83 W/m, 91% less.
Diminishing returns
| Insulation | Heat loss, W/m | Steam condensed, kg/h | Coal cost, ₹/year |
|---|---|---|---|
| Bare | 909 | 164.1 | 11,15,312 |
| 25 mm | 125 | 22.6 | 1,53,348 |
| 50 mm | 83 | 14.9 | 1,01,334 |
| 75 mm | 65 | 11.8 | 80,111 |
| 100 mm | 56 | 10.1 | 68,366 |
Heat loss from natural convection and radiation for the bare pipe, and conduction at an installed conductivity of 0.07 W/(m·K) for mineral wool. Coal at indicative price, calorific value and efficiency.
The first 25 mm does most of the work. Each further 25 mm saves less than the step before, while the cost of material and cladding keeps rising with diameter.
Economic thickness, and the parts left bare
The economic thickness is where the total of insulation cost (annualized over its life) and heat-loss cost is lowest. Add one more step of thickness only if its extra annual saving exceeds its extra annualized cost. Take the material and erection cost from a real quotation; the saving column above, recomputed for your line, does the rest.
- Valves, flanges, strainers and PRV stations are often left bare after maintenance. Removable insulation jackets let them be lagged and still opened.
- Wet insulation conducts heat far better than dry; damaged cladding that lets rain in quietly ruins the lagging beneath.
- Condensate lines and feed tanks are hot too, and worth lagging.
Key takeaways
- A bare steam line wastes a large amount of heat continuously.
- The first layer of insulation saves most of it; further thickness saves less each step.
- Economic thickness balances annualized insulation cost against heat loss.
- Lag valves and flanges with removable jackets, and keep cladding watertight.