A bi-metal pipe in the bimetallic composite family, mechanically lined pipe (MLP) gets less attention than clad pipe but moves more meters in real projects. From subsea flowlines off Brazil to municipal water networks in China, MLP is the workhorse answer to one question: how do you get a corrosion-proof bore at carbon-steel prices? This guide explains how MLP is made, where it works, where it fails, and how to specify it.
1. What Is Mechanically Lined Pipe?
MLP is a bimetallic pipe built from two separate components:
- Outer pipe (carrier): carbon or low-alloy steel per API 5L — carries pressure, tension, and external loads.
- Inner liner: 1–4 mm of corrosion-resistant alloy (304/316L stainless, duplex, or nickel alloy) — faces the medium.
- The interface: an interference fit. The liner is forced against the inner wall of the carrier pipe and held there by residual contact pressure — not by a weld or metallurgical bond.
Because the expensive alloy is only a thin liner, MLP costs 40–60% less than solid CRA pipe and 10–30% less than metallurgically clad pipe. That cost advantage explains its dominance in long-distance, moderate-pressure service.
2. How MLP Is Manufactured: Three Methods
| Method | Process | Interface Pressure | Typical Products |
|---|---|---|---|
| Hydroforming / hydraulic expansion | Liner tube is expanded inside the carrier pipe with high-pressure water, then the pressure is released and the liner springs back into tight contact | Uniform, controllable | Line pipe DN50–DN1500 |
| Thermal shrink-fit | Liner is cooled or carrier is heated for assembly; differential thermal contraction creates the fit | Uniform, moderate | Smaller diameters, fittings |
| Mechanical draw / roller expansion | Liner is pulled or rolled into the carrier pipe | Lower, less uniform | Low-pressure water pipe |
ZONX uses controlled mechanical expansion for AQUACORE™ water-grade lined pipe, with end-facing and connection options (threaded, grooved, flanged) suited to building and municipal installation.
3. Where MLP Works — and Where It Doesn't
The interference fit is MLP's economic advantage and its engineering ceiling. Bond strength at the interface is under 5 MPa — roughly 3% of what a metallurgical bond delivers. That single number explains the whole selection logic:
| Service Condition | MLP Suitability | Why |
|---|---|---|
| Water distribution, ≤ 2.5 MPa, stable temperature | ✅ Excellent | Static fit is stable; cost advantage is decisive |
| Onshore gathering, moderate cycling | ✅ Good with design margin | Axial strain must stay below liner buckling threshold |
| Pipe bending, reeling, tight radius | ⚠️ Limited | Axial compression wrinkles the liner |
| Rapid depressurization / vacuum | ⚠️ Risky | External pressure can buckle the thin liner inward |
| Thermal or pressure cycling | ❌ Poor | Cyclic slip degrades the fit; crevice corrosion risk grows |
| Sour high-pressure / subsea critical | ❌ Use clad pipe | Bond integrity is a qualification gate — see selection guide |
4. Liner Wrinkling, Collapse, and Galling: The Three Failure Modes
Every MLP risk traces back to the same physics: the liner is thin, and the fit that holds it can be overcome. Three documented failure modes dominate:
- Liner wrinkling: axial compression from bending, reeling, or lateral pipeline buckling forces the liner into sinusoidal folds. Wrinkles restrict flow, trap solids, and become corrosion initiation sites. Mitigation: minimum bend radius limits (typically ≥ 40D), reel-lay only with qualified anti-wrinkle designs, and axial strain checks.
- Liner collapse: external pressure exceeding the liner's buckling resistance — during vacuum commissioning, rapid depressurization with subsequent cooling, or hydrostatic pressure at depth if the annulus floods. Mitigation: hydrostatic collapse calculations per DNV-OS-F101, sealed ends, and pressure management procedures.
- Galling and crevice corrosion: if the liner slips or the annulus takes on fluid through an end seal breach, the crevice behind the liner becomes a corrosion cell. Mitigation: end-seal weld quality control, and periodic inspection on critical lines.
None of these failure modes is exotic — all three are managed by specification. The mistake is treating MLP as a drop-in substitute for clad pipe rather than a different product with its own qualification envelope.
5. MLP vs Clad Pipe: The Decision in One Table
| Dimension | MLP | Metallurgically Clad |
|---|---|---|
| Bond | Interference fit, < 5 MPa | Metallurgical, ≥ 140 MPa (API 5LD) |
| Relative cost | Lowest | +10–30% over MLP |
| Bending / reeling | Restricted | Unrestricted (qualified) |
| Thermal cycling | Not recommended | Full range |
| Vacuum / rapid depressurization | Collapse risk | No liner to collapse |
| Field welding | End-seal welding required | Standard transition welds |
| Best fit | Water, moderate-pressure stable service | Sour, high-pressure, cycling, subsea |
The full 12-dimension comparison, including solid CRA, is on our comparison page.
6. Specifying MLP: What to Put in the Purchase Order
- Carrier pipe: API 5L grade, OD, wall thickness, PSL level.
- Liner: alloy grade, thickness, surface finish.
- Fitting method: hydroformed / shrink-fit; minimum interface contact pressure if specified.
- End connections: weld overlay ends, flange overlay, or mechanical joints — seal strategy is part of the spec, not an afterthought.
- Testing: hydrostatic test of the composite pipe, liner collapse margin calculation, UT spot checks on bond, and end-seal PT/RT.
- Documentation: MTRs for both carrier and liner, collapse calculations, and welding procedures for end seals.
7. Conclusion
MLP earns its market share honestly: for water and moderate-pressure service, it is the cheapest reliable way to put a corrosion-proof bore in the ground. Its limits are equally honest — the mechanical fit that saves money cannot survive bending, cycling, or vacuum the way a metallurgical bond can. Specify within the envelope and MLP delivers decades of service; specify outside it and liner wrinkling writes the post-mortem.
ZONX manufactures both sides of this decision — AQUACORE™ mechanically lined pipe for water and building service, and DURACORE™ metallurgically clad pipe for everything the liner can't handle. Tell us the medium, pressure, and temperature, and we'll tell you which one your project actually needs.