Anti-Corrosion Coated Steel Pipe

3PE, 3PP and FBE Compared

FBE, or fusion bonded epoxy, is a single layer of epoxy powder applied electrostatically to a heated pipe surface, where it melts and cures into a hard, tightly bonded film typically 300 to 500 microns thick. It is the most widely used external coating for buried pipelines, offers excellent adhesion and high temperature resistance, but is relatively thin and can be damaged by handling. Three layer polyethylene, 3PE, consists of an FBE primer, a copolymer adhesive layer and an outer high density polyethylene jacket, giving total thickness typically 2.0 to 3.7 mm. It combines the adhesion of FBE with mechanical toughness, and is the preferred coating for large diameter transmission lines that must survive handling, rocky backfill and construction traffic. 3PP substitutes polypropylene for the outer layer and is chosen where higher operating temperatures exceed the capability of polyethylene.

How to Specify a Coated Pipe

A coated pipe specification must define four things: the coating system and standard, the coating thickness, the cutback length at the pipe ends, and the field joint coating method. Cutback is the length of bare pipe left at each end so that the tie-in weld can be made on uncoated steel; a typical cutback is 100 to 150 mm. The field joint is then re-coated on site, and the specification should state whether this uses heat shrink sleeve, FBE, tape wrap or a moulded sleeve. For water lines the internal surface is often lined as well, either with epoxy for flow and corrosion control or with cement mortar for potable water. Coating test reports covering thickness, holiday detection, adhesion and cathodic disbondment are provided with each production run.

Coating Names and Pipe Sizes

3PE and 3LPE are two names for the same coating – three-layer polyethylene – and both appear on enquiries, so a 3LPE coated pipe and a 3PE coated pipe are the same article. 3PP is the polypropylene version, chosen where the pipe runs hotter, and FBE is the single-layer alternative; an FBE coated pipe is a different product with the different layer stack compared above. Coated pipe is commonly supplied from 2 in (60.3 mm) up to 48 in (1219 mm) outside diameter, and the coating is applied to pipe that has already passed its dimensional and weld inspection, so the line pipe underneath can be API 5L, ASTM A53 or ASTM A106. Specify the coating system, the film thickness, the standard it is applied to, and whether the ends are left bare for field welding.

Coated pipe: the soil and the service choose the coating

Coating is applied over a base pipe that is already specified, so the coating question is separate from the pipe question. What decides it is where the line is buried, what the soil and the groundwater are like, whether the line is pulled or laid, and what operating temperature the coating has to survive.

The two numbers to fix in the specification are the dry film thickness and the holiday test. FBE is specified at 300–500 µm; 3LPE as a total thickness of 2.0–3.7 mm — an FBE primer of roughly 150–250 µm, then an adhesive layer and a polyethylene outer. Holiday detection is run at high voltage on every coated length, with the voltage set from the coating thickness per the coating standard, to find pinholes that a thickness gauge alone will miss. Adhesion, peel and cathodic disbondment are run on coupons from the production run and reported on the coating certificate.

The three layers and what each one does

Three-layer polyethylene is a system, not a single coating. Each layer has one job, and a failure in any one of them shows up as a coating failure years later.

Source: coating structure per DIN 30670 and ISO 21809-1. Total film thickness depends on pipe diameter and the coating class written in the project specification — it is not chosen from the pipe size alone.

One consequence worth stating on the order: because the thickness is set by the project class rather than by the pipe size, two quotations for the same pipe diameter can legitimately differ. Ask which coating class and which total thickness the price is based on.

What is inspected before the pipe leaves the coating plant

Source: test families per ISO 21809-1 and DIN 30670. The nominated test, temperature and acceptance value must come from the project coating specification; confirm them before the coating order is placed.

Field joints are where 3PE pipelines usually fail

The mainline coating is applied in a plant under controlled conditions. The girth weld area is coated in the field, by a different crew, often in worse weather. That is where water finds its way in.

Three details decide the outcome:

• Compatibility. The field-joint material has to be qualified against the plant-applied 3PE system. A heat-shrink sleeve that is compatible with one polyethylene topcoat is not automatically compatible with another.
• Cutback length. The coating is cut back from each pipe end at the coating plant, so the cutback has to match the field-joint sleeve width. Fix the cutback with the field-joint supplier before coating starts, not after the pipe is coated.
• Surface preparation at the joint. The field joint is the only part of the line where the steel is prepared outside a coating plant; its surface preparation standard should be written into the construction specification.

Which pipe can be coated, and what to state on the enquiry

• Base pipe type: seamless, ERW, LSAW or SSAW.
• Outside diameter and wall thickness, or the pipe schedule.
• Coating system and class: 3PE (3LPE), 3PP, or FBE, with the standard (DIN 30670, ISO 21809-1 or the project specification).
• Required total film thickness, or the coating class from which it follows.
• Pipe end condition and cutback length.
• Inspection requirement: coating plant test reports, and whether third-party witnessing is required.
• Quantity, length per joint, and destination port.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the API 5L grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example API 5L X65 pipe. The coating goes on pipe that has already passed its dimensional and weld inspection, so these are the base pipe requirements; the coating system is specified separately on top of them.

Source: specified minimum yield and tensile strength for each grade, per API 5L / ISO 3183, Table 7. The standard is the governing document.

• Depends on base pipe
• Grade B to X80

Download Anti-Corrosion Coated Steel Pipe data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

Where to go next

• 3PE vs FBE coating — which system suits which soil, temperature and installation method.
• API 5L LSAW welded steel pipe — the base pipe most often used for large-diameter coated lines.
• Quality and inspection — how the coating inspection fits into the pre-shipment checks.

Related reading: 3PE vs FBE coating, CRA clad and lined pipe and steel pipe vs tube.