Preparation to Blast Cleaning :

Prior to blast cleaning, the each pipe shall be inspected by the followings;

• Pipe identification control
  • Pipe shall be accepted as PE coated pipe condition

• The internal surface control
  • The pipe internal surface shall be inspected and cleaned to remove all traces of oil, grease and loose deposits. Any visible trace of oil or grease shall be removed using an appropriate non-oily and non-toxic product.

• If pipes have been in contact with marine or industrial atmosphere, they shall be cleaned before abrasive blasting to remove any contamination. Cleaning shall be carried out using an adapted process.

Surface Preparation :

The pipe surface shall be grit/shot blasted to ensure the SA 2 1/2 degree of cleanliness. The finished pipe surface shall be completely removed all traces of rust and shall be provided a surface average roughness (R_Z) of  50 to 100 microns.

Pipes shall be heated to remove any surface moisture and to ensure more than 3⁰C above the ambient dew point. 

The ambient relative humidity shall not exceed 85 % as measured using a hydrometer and digital thermometer.

Immediately after abrasive blasting, all dust and traces of abrasives shall be removed from the pipe surface by brushing and vacuum cleaning.

Each cleaned pipe shall be inspected for surface defects. Any defect shall be removed.

Ambient Control :

The ambient temperature and ambient relative humidity shall be measured using 

a hydrometer  and digital thermometer.

The ambient relative humidity and ambient temperature shall be recorded to the shift report.

The grit-blasting is not permitted when the ambient relative humidity exceeds 85 %.

The metal temperature shall be 3ºC above the ambient dew point temperature.

Any pipe which has not been coated before degradation of its surface preparation shall be completely re-blasted before coating.

PE Test and Inspection :

The below tests and inspections shall be performed on blast cleaned pipe:

• Surface profile :

Two methods  can be used in measuring surface roughness measurements.          

                    Method 1- By visual comparison gauge

                    Method 2- By a digital profile measuring device.

       Standard grid and shot comparator is compared to the pipe surface. The coinciding segment of the comparator shall be decided.

Surface roughness measured by a digital profile measurement device. Surface roughness shall comply with Rz=50-100 µm

The calibration of equipment shall be performed by reference  blocks.               

• Surface cleaning degree :

Standard comparison pictures are used to decide the degree of the surface cleanliness. Surface quality degree shall be Sa 2 ½.

• Dust  control :

Residual dust shall comply with acceptable categories of ISO 8502-3 and

specification

• Visual inspection :

The surface defects which appearance effect of grit-blasting or other reason shall be checked  by visual inspection. In case of any grinding or mechanical reworks, the wall thickness shall be ultrasonically examined and  remaining wall thickness shall be  met minimum specified requirements.

Pipe external surface shall be cleaned of all dust and foreign matter using clean dry compressed air or vacuum cleaning. Residual dust shall comply with categories of ISO 8502-3 and specification.

Heating for Coating Application :

The pipes shall be coated on a continuous coating line without any risk of deterioration (external pipe wall, bevels, coating) during the various heating, powder spraying, curing and quenching operations.

The pipes shall be heated for coating application using induction coils.

The  pipe temperature shall be according to the manufacturers recommendations. The temperature  shall be measured continuously  by the control equipment which shall be  pyrometer, thermocouples, infra-red sensors, or equivalent proper methods.

No chemical treatment of the surface of steel using chromates, phosphates or any other chemical is allowed to improve adhesion. Required adhesion shall be obtained only by using suitable fusion bonded epoxy primer and surface preparation.

PE Coating :

PE coating is consist of ;

1.      layer : electro statically applied fusion bonded epoxy primer

2.      layer : extruded adhesive wrapping

3.      layer : extruded polyethylene wrapping

Application :

• Fusion bonded epoxy primer :

Pulverized epoxy powder which is electro static (-) loaded is separated on to

the rotating pipe, having a (+) electro static load and it bonds and fuses on the

heated  pipe surface forming a uniform plastic layer which, when cool, firmly

adherence to the pipe.

The thickness of the primer coat shall be between 60 to 150 µm

• Adhesive :

Adhesive is applied while an endless extruded tape (foil) is wrapped all around the circumference of the treated steel pipe, so that the single foils weld together to homogeneous coating with the specified wall thickness.

Hot applied adhesive tape is pressed onto pipe surface by silicon roller to get an adhesion between epoxy and adhesive layer.

The copolymer adhesive thickness shall be 150 t0 250 µm

• Polyethylene :

High density polyethylene is applied by side extrusion wrapping method under controlled temperature and presses by silicon roller to form uniform coating thickness.

The 3 layer polyethylene total thickness shall be minimum 3 mm or according to specification.

• Cooling :

When wrapping is completed, the tube shall be sprayed with water in order to cool it down 60 ±10ºC. 

70 ºC is the maximum value authorized for personnel safety.

Holiday Test :

Holiday testing is done for all the pipes just after the cooling.

High voltage testing system is stable while pipe is moving axially and full length of the pipe is tested. The brushes may be circumferential or longitudinal depending upon the movement of the pipe provided that 100 % holiday detection coverage is performed.

Automatic spray marking is used for identification of faulty areas such as pinholes and flaws. 

Testing voltage is 25 kV.

The calibration of holiday detector shall be  checked at twice per shift with a 1.0 mm through drilled hole on the coated surface.

Production Test and Inspection :

Application area :

The PE coated pipes shall be test and inspected to conform to the specified requirements.

The test and inspection list: