3 Layer Coating Process Description

Line control and loading

     The bare pipes that arrive from the storage area are loaded onto the inbound rack by the help of an overhead crane.

     While on the rack, the pipes are controlled one by one.

     The pipes are also checked against any traces of grease, oil, or damage.


Surface preparation and sandblasting

     Prior to the sandblasting stage, the pipes are pre-heated in a gas furnace with a capacity of 600.000 kcal per hour in order to eliminate any residual moisture and meet the parameters provided in the sandblasting specification.

     To obtain the specified degree of roughness and surface cleanliness, the pipes are then sandblasted with an angular-shaped metal grit.

     .The sandblasting has three 30 kW impellers and the cabin, which is insulated and soundproofed, has a tank for filtering and recycling dusts.


Intermediate rack and inspection

     One by one, all blasted pipes are checked in the intermediate rack. Following the quality control instructions, the surface of the pipes is checked for the level of blasting, cleanliness and roughness.

     The pipes are then loaded onto the inbound coating conveyor and joined by means of alloy or plastic connectors with masking tape on top in order to form one single pipe on the coating process line.

High pressure washing and pre-heating

     To avoid all possible residual of dust, before being introduced in the induction coil, all joined pipes are washed with hot pressurized water and dried.

     The pipes are then pre-heated by means of an induction heating coil of 1000 kW, with the purpose of raising the surface temperature of the steel to the level required for a correct application of the epoxy powder (primer).

     The temperature of each pipe is constantly monitored by an optical pyrometer and by Tempilstick.


Application of triple layer coating

     For the application of the first layer, the epoxy powder, the pipes are introduced in a spray primer booth that has 29 electrostatically charged spray guns positioned all around the surface of the pipes in three lines. They ensure that the primer is applied in an uniform way and as thickly as required.The correct air flow and electrostatic charge of each spray gun is automatically controlled and regulated.

     The two subsequent layers, the adhesive and the top coat (polyethylene or polypropylene), are applied with a single extrusion head composed of two central blocks that distribute the products on the pipe in a uniform manner. The adhesive and the top coat, originally in granule, are melted by means of electrical resistance inside two extruders with a screw of 100mm for the adhesive and 160mm for the top coat. They are pushed inside the O-Die head from where they go out through circular lips. They are compressed all around the surface of the pipes by means of a vacuum system.


     The coated pipes are immediately immerged in a water cooling tank that reduces their temperature in order to avoid any possible flaws due to the softening of the coating.

     The cooling tunnel is 25m long and can contain 200 cubic meters of water per hour all around the surface of the coated pipes.



     After exiting the cooling tunnel, the pipes are separated and the connector is manually removed. The separated pipes pass then through the Holiday-Detector devices that control the continuity of the coating on the entire surface of the pipes by applying a 25kW tension along the entire circumference.

     This control is conducted continuously on the entire surface of the pipes. In case of some pin-hole or of some discontinuity point in the coating, an electrical discharge will appear and the Holiday-Detector will advise the operator with an acoustic signal.


     While in the Holiday-Detector area, each pipe is marked on its entire length by means of an automatic programmable machine that stamps the coating with a continuous text in ink. All information related to the steel and coating characteristics and clients or project references are marked on the pipes.



     Two machines with proper suction systems and filters remove 100 - 150 mm length of coating from the pipe ends using abrasive steel brushes that also form the appropriate level at the extremity of the coating.

Final control and storage

     At this point, all pipes are fully controlled in the control rotating station. If required, the cut-back is protected by temporary paint and both ends are fitted with end cups. With the help of the overhead crane, the pipes are removed from the final rack inside the hangar for storage purposes.