What Is 3LPE Anti-Corrosion Coated Line Pipe?
3LPE, also written 3PE, is a three-layer polyethylene external coating applied to steel line pipe to isolate the steel from soil, water and stray current. The coating is built up in three bonded layers: a fusion bonded epoxy primer that bonds chemically to the blast cleaned steel surface, a copolymer adhesive layer that bridges the epoxy and the outer jacket, and a polyethylene topcoat that provides the mechanical strength and weathering resistance of the system.
Each layer has a defined job. The epoxy layer delivers electrical insulation and chemical stability against the electrolyte in the soil, the adhesive layer locks the epoxy and the polyethylene together through chemical bonding so that the layers cannot be peeled apart, and the polyethylene jacket resists handling damage, soil stress and ultraviolet exposure during storage and installation. Used together on an API 5L pipe body, they form a coating that survives trenching, rock impingement and decades of service without losing adhesion.
Coating Structure
| Layer | Function | Notes |
|---|---|---|
| Epoxy primer, first layer | Chemical bond to steel, electrical insulation, barrier to electrolyte | Applied by electrostatic spray to the thickness required by the coating specification while the pipe is at coating temperature |
| Copolymer adhesive, second layer | Bonds the epoxy primer to the polyethylene jacket | Applied immediately after the primer, before the epoxy has cured |
| Polyethylene topcoat, third layer | Mechanical protection, abrasion and impact resistance, weathering resistance | Extruded over the adhesive to the minimum total coating thickness required for the pipe diameter |
Minimum Total Coating Thickness by Nominal Diameter
| Nominal diameter | General type, minimum | Reinforced type, minimum |
|---|---|---|
| DN 100 and below | 1.8 mm | 2.5 mm |
| DN 100 to DN 250 | 2.0 mm | 2.7 mm |
| DN 250 to DN 500 | 2.2 mm | 2.9 mm |
| DN 500 to DN 800 | 2.5 mm | 3.2 mm |
| DN 800 and above | 3.0 mm | 3.7 mm |
The reinforced thickness class is selected for rocky ground, for pipe that will be pulled through a bored crossing, or for a line that will be handled several times before it is laid. The industry practice for coating thickness and inspection is set out in pipeline coating standards such as GB/T 23257 and ISO 21809-1, which define the acceptance tests as well as the thickness values.
Pipe Body Specification
| Item | Value |
|---|---|
| Line pipe standard | API 5L, with additional national standards available on request |
| Outside diameter | 25 mm to 3000 mm |
| Wall thickness | Schedule 5S to schedule 160 and heavier |
| Length | Up to 12 m, or cut lengths to order |
| Ends | Beveled for welding, or threaded where required |
| Coating type | Three layer polyethylene, single layer epoxy and other systems available |
Applications
Buried oil and gas transmission and gathering lines
Water transmission mains and cooling water pipelines
City gas distribution networks
Pipe crossings installed by boring or thrusting under roads and rivers
Offshore and coastal lines where an abrasion resistant coating is needed
Coating Inspection and Field Handling
Holiday detection over the full length confirms that the coating is continuous and free of pinholes.
Peel strength and cathodic disbondment tests confirm adhesion and resistance to disbonding at coating defects.
Impact, indentation and flexibility tests demonstrate that the jacket will survive handling and bending at the lay temperature.
Cutbacks at the pipe ends are kept clean and square so that the field joint coating bonds correctly to the parent coating.
Pipes are lifted with belt slings rather than hooks, and are stacked on padded timber with the coating clear of the ground.
FAQ
Q: What does 3LPE stand for?
It describes a three layer polyethylene coating, made up of a fusion bonded epoxy primer, a copolymer adhesive layer and an outer polyethylene jacket applied over a steel pipe.
Q: Why is a three layer coating used instead of epoxy alone?
A single epoxy layer gives good adhesion and insulation but is easily damaged in handling and in rocky ground. The polyethylene jacket absorbs that mechanical damage while the epoxy keeps the bond to the steel.
Q: How is the coating thickness selected?
The pipe nominal diameter sets the minimum total thickness, and the installation conditions decide whether the general or the reinforced class is used, as shown in the table above.
Q: What pipe standards are used for the steel body?
The body is normally API 5L line pipe, and the coating system is applied and inspected to the requirements of the pipeline coating standard specified for the project.
Q: How are the field joints coated?
The mill coating is cut back at both ends to leave bare steel for welding. After welding, the joint is blast cleaned and coated with a compatible heat shrink sleeve or a spray applied epoxy system.
Q: How should coated pipe be stored?
Stack it on timber or sand bags with the coating clear of the ground, keep it out of standing water, use belt slings for lifting, and avoid long exposure to direct sunlight where the specification limits ultraviolet exposure.
