Not every pipeline gets buried. Some run above ground. Some need a smooth inner surface for water flow. Others carry hot fluids through permafrost or city heating networks – requiring both corrosion protection and thermal insulation.
This guide covers liquid-applied paints (internal & external) and pre-insulated pipes (anticorrosion + foam + outer casing).
Internal & External Liquid Spray Painting
Liquid coatings are for when you don't need a thick extruded layer – internal flow improvement, above-ground exposure, or repairs.
Internal Flow Coatings (Liquid Epoxy)
Why coat inside? Bare steel rusts, tubercles build up, flow drops. A smooth internal coating boosts Hazen-Williams C-factor above 150 (vs 80–100 for uncoated steel) – lower pumping cost for decades. Also prevents microbial growth and meets potable standards.
Typical material: High-build liquid epoxy, 400–1000 µm. Polyurethane or ceramic-filled epoxy for special cases.
How we apply it (short version):
Internal blast to SA 2.5 (venturi head or buggy).
Coat either with a centrifugal spinning head (pulled through) or airless spray on a traveling carriage.
Cure – ambient (8–24h) or forced hot air (1–2h).
Holiday test with low-voltage sponge or spark tester. Repair pinholes.
When to use:
Water transmission mains (large dia where TPEP is too expensive)
Firewater lines, RO feed lines, rehabilitation projects
Limitation: Liquid takes time to cure. Internal FBE (like TPEP's inner layer) cures in seconds.
External Atmospheric Coatings (Above-Ground)
Pipes on racks, bridges, or marine risers don't need 3 mm of extruded plastic. They need UV, salt, and weather resistance.
Typical three-coat system:
| Layer | Material | Thickness | Job |
|---|---|---|---|
| Primer | Zinc-rich epoxy | 50–75 µm | Sacrificial protection at cut edges |
| Mid | Epoxy MIO | 150–200 µm | Barrier, stiffness |
| Top | Aliphatic polyurethane | 50–75 µm | UV & color retention |
Application:
Blast SA 2.5 → spray primer → spray intermediate → spray topcoat (airless). Cure ambient or forced.
Field lessons:
Epoxy topcoats alone fail under UV – always use polyurethane or polyurea for outdoor.
Coastal areas: bump total thickness to 500–600 µm.
Keep repair cans on site for touch-up after welding.
Where used: Refinery above-ground lines, bridge crossings, marine risers, tank exteriors.
Pre-Insulated Pipes (Anticorrosion + Thermal Insulation)
For hot fluids or frozen ground – standard solution: three layers.
Steel pipe (already coated with FBE/3PE/liquid epoxy)
Thermal insulation – usually polyurethane (PUR) or polyisocyanurate (PIR) foam
Outer casing – HDPE or steel
Materials Quick Guide
| Insulation | Max Temp | Use |
|---|---|---|
| PUR | 120°C | District heating, hot water |
| PIR | 140°C | Higher temp heating |
| Mineral wool | 400°C+ | Steam (needs sealed jacket) |
Outer casing: HDPE (cheap, tough, max 60°C casing temp) or steel (high mechanical/fire resistance, needs its own coating).
Manufacturing – Two Ways
Pipe-in-pipe (batch):
Insert coated steel into HDPE casing → spacers keep concentric → inject liquid PUR foam → expands and bonds → trim ends for welding.
Continuous foaming (camel-back):
For small diameters, long runs – HDPE extruded around steel while foam injected simultaneously. Faster but less flexible.
Key Quality Checks
| Test | Pass criteria |
|---|---|
| Foam density | 60–80 kg/m³ |
| Closed cells | ≥90% |
| Compressive strength | ≥0.3 MPa at 10% deflection |
| Thermal conductivity | ≤0.028 W/(m·K) at 50°C |
| Shear bond | ≥0.12 MPa (DIN 30670) |
| Casing holiday | 25 kV spark – no failure |
Field Joints – Where Most Failures Happen
Pipe ends come bare for welding. After welding, you must restore both anticorrosion and insulation in the field.
Typical field joint:
Clean & blast weld area.
Apply liquid epoxy or heat-shrink sleeve for corrosion.
Mount a mold, inject PUR foam.
Cover with heat-shrink HDPE casing.
Watch out: If this joint fails, water wicks in, steel corrodes, insulation lost. Always ask the supplier for their field joint training and inspection protocol.
When to Choose Pre-Insulated
District heating (hot water) – standard PUR+HDPE.
Permafrost oil lines – prevent ground thaw. Often steel casing.
Deepwater subsea flowlines – hydrate/wax prevention. Multi-layer foam.
High-temp steam (200–400°C) – mineral wool + stainless steel jacket.
Cost Reality
Factory-insulated pipe costs more upfront than bare pipe + field insulation. But total installed cost is lower for any project over a few hundred meters – because field insulation is slow, weather-dependent, and labor-intensive.
Quick Selection Guide
| Application | Recommended | Why |
|---|---|---|
| Buried water line, ambient | 3PE or TPEP (not liquid) | Liquid not durable underground |
| Internal flow only | Internal liquid epoxy | Cheap, smooth |
| Above-ground, mild climate | Zinc epoxy + polyurethane topcoat | UV resistant |
| Above-ground, marine splash | Glass flake epoxy + PU topcoat | Salt barrier |
| District heating (hot water) | Pre-insulated: steel+PUR+HDPE | Standard |
| Steam line (150°C+) | Pre-insulated: steel+PIR/mineral wool+steel casing | High temp |
| Repair/temporary | Liquid epoxy (brush/spray) | Field-applied |
Bottom Line
Liquid coatings are simple and flexible – use inside pipes for flow, and above-ground for weather. But don't bury them in rock.
Pre-insulated pipes solve heat loss. Buy factory-made, test it, and pay extra attention to field joints – that's where 90% of problems live.
At LEFIN STEEL, we make both. Send us your fluid temperature, soil conditions, and budget – we'll tell you what works.
Previous part of this series: *How to Pick Epoxy-Polyolefin Coatings for Steel Pipelines (FBE, 2FBE, 2/3PE, 3PP, TPEP)*.
