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3Layer Polyethylene Coating-Part-02

3Layer Polyethylene Coating Application.

After external surface preparation the pipes shall be heated in Induction heating oven to obtain a pipe temperature as per raw material manufacturer’s recommendation.Required pipe temperature shall be maintained as it enters the epoxy coating chamber .Temperature of the pipes shall be continuously monitored and recorded using suitable calibrated instrument such as pre-calibrated contact type thermometers with probe or infrared thermometers.Temperature measuring instruments calibration shall be as per approved ITP. The temperature of pipe shall be recorded as per approved ITP.

Epoxy Powder Application.

The fresh Epoxy Powder shall be fed in Epoxy Tank set on the high voltage as recommended by 3LPE powder manufacturers .Preheated pipes shall be rotated through epoxy cabin fitted with electro statically charged epoxy spray guns in such a way which provides as per approved ITP. Epoxy thickness shall be checked by a partially coated pipe depicting sufficient length of 3LPE only. The compressed air used for spraying shall be free from moisture and other contaminations. Nos. of guns, flowing air and atomising air pressure to assist proper spray of 3LPE  powder  shall  be  established  and  maintained to  maintain  a proper epoxy layer thickness throughout the shift.Epoxy layer thickness shall be as per manufacturer’s recommendation.

The use of recycled coating powder shall be limited to the manufacturer recommendation. The reclaimed epoxy powder shall not be more than 10%. All recycled powder shall be subject to magnetic and particle sizing screening prior to re-use and shall be continuously blended into the virgin material. If 3LPE powder is recycled, then the daily sample pipe & CD testing shall be prepared under the same conditions.

Adhesive Applcation.

After epoxy coating, the extruded adhesive layer shall be extruded after epoxy powder application.These parameters shall be established during the Procedure Qualification. The adhesive film temperature is as per raw material recommendation . The application of adhesive layer shall be applied as per manufacturer’s recommendations.

Inter-coat time and distance between last epoxy guns to start of adhesive applications at a temperature range shall also be established. Entrapment of air or void formation along the adhesive overlap shall be avoided by forcing the coating on to the pipe during application using silicon pressure rollers. The thickness shall be measured as per specification using magnetic thickness gauges. Adhesive layer thickness shall be minimum 200 microns. The adhesive temperature shall be adjusted as recommended by Raw material manufacturer.

3Layer Polyethylene Coating Application.

PE application parameters shall be established during the Procedure Qualification .The P.E thickness is maintained according to required thickness. The total 3LPE coating thickness shall be minimum 3.0 mm. The polyethylene film temperature is as per raw material recommendation. to provide the specified thickness and good quality coating. Entrapment of air or void formation along the P.E. overlap shall be avoided by forcing the coating on to the pipe during application using silicon pressure roller.

3Layer polyethylene coating
3Layer polyethylene coating

Cooling of Pipes.

After coating, the rotating pipe shall be conveyed through cooling tunnel for quenching.The temperature of water at the inlet of cooling tunnel shall be cool enough to avoid the mechanical damages during handling followed by other inspection stages.The temperature of pipe after cooling shall be as per approved ITP, so that subsequent inspection and testing shall be carried out. Same water shall be re­ circulated though cooling towers. Temperature of pipe after cooling shall be checked frequency as per approved ITP.

Coated Pipes Inspection.

Immediately after cooling, each coated pipe shall be visually checked for imperfections and irregularities of the coating. The coating shall be free of defects such as pinholes, blister,cuts, wrinkles, scratches, engravings, swellings, air inclusion, tears voids or any other irregularities. The coating shall be natural color and gloss, smooth and uniform, and shall be with no dust or other particulate inclusions. In addition to these inside surface of the pipe shall be free of foreign materials such as shots or grits, dust etc.

Thickness Inspection.

The thickness gauge shall be calibrated at least once per shift in accordance with the instruction specified by the DFT gauge manufacturer. The coating DFT shall be measured at 12 locations per pipe equally spaced along the length of the pipe. At each location, the thickness shall be measured at four positions, equally spaced around the circumference of the pipe. At least 12 spot readings in accordance with SSPC PA2. The average of the 12 readings shall be within the specified DFT range as per approve ITP.At any one-meter length of the coated pipe, the maximum allowed tolerance shall not be less than 10% of the specified thickness range. However the thinner area shall not cover more than 5 cm2 at no more than five (5) locations in any pipe length.The required total thickness may be reduced by max. Of 10% on the weld seam for saw welded pipe. The frequency of thickness check shall be as per approved ITP.

Pipe End Cleaning (Cut-Back)

At the pipe ends, the coating shall be cut back over a length 125 mm to 150 mm. The coating edge shall be shaped to form a bevel angle s3o·. After completion, coated pipe shall be kicked out on the testing conveyor where the cut-back portion shall be measured and accepted pipes shall be conveyed further for holiday test and subsequently for the final inspection skid. The Exposed pipe surface at the cut back area of each pipe shall be coated for atmospheric corrosion  protection using Zinc primer.

Holiday Detection.

Each pipe shall be inspected for holidays over 100% of its coated surface using a high voltage DC detector. The Holiday Detector shall be low pulse DC full circle electronic detector with audible alarm and precise voltage control. One end of the pipe shall be connected to the earth. If any defect is found, the moving conveyor shall be stopped by operator and shall mark the defect for repair. The results shall consist of the number of holidays detected. The holiday test is carried out on the external pipe surface which shall be free of water. The test voltage shall be 25 KV & the travel speed will be 300-mm/sec max. The test voltage shall be checked at least once per shift by Jeep Meter. Full length of each pipe shall be subjected to holiday testing to ensure defect free coating.

Inspection and Testing (Laboratory & Field).

X- Cut Adhesion Test.

The x-cut adhesion test of cure 3LPE coating shall be checked by inserting the blade of a utility knife under the coating at the x­ cut.The test shall be carried out on the trail pipe.Procedure: insert the blade of a utility knife under the coating at the x-cut using a levering action, chip off the coating.Definite resistance to levering action.The frequency of x- cut test shall be as per approved ITP.

Degree of Cure.

Sample shall be taken from Pipe & test shall be conducted in accordance with CAN I CSA Z245.20 I ISO 21809-1 to determine the Glass transition temperature differential (Tg: +3I-2″C) and % curing minimum 95%. Acceptance criteria are as per epoxy manufacturer’s recommendation. The frequency of degree of cure test shall be as per approved ITP.

Cathodic Disbondment Test.

Cathodic Disbondment (Make  Coesfeld,Caltech, nova instrument) This test shall be performed as per CAN I CSA Z245.21-ISO 21809-1.

  • 28 Days CD test shall be conducted during PQT.Maximum radius of disbandment shall be 15 mm at 65″C & -1.5V Frequency: as per approved ITP
  • 24 Hours CD test shall be conducted during PQT & production.Maximum radius of disbandment shall be 7 mm at 65″C & -3.5V Frequency as per approved ITP.

Residual Magnetism.

Five pipes per shift shall be checked for residual magnetism and shall not be more than 25 gauss.

Impect Strength.

A coated pipe shall be impact tested in accordance with the procedures specified DIN 30670. Tests shall be performed at temperature s as per approved ITP & Impact Energy shall be as per approved ITP of coating thickness. Test shall be conducted at 30 equidistant along the length of the sample and then impact area shall be checked at high voltage i.e. 25 K.V.The acceptance criteria for Impact test for 30 impact points, the detector should not detect any single of defects. The Frequency of impact test shall be as per approved ITP.

Bond/Peel Strength Test.

In this test,the force applied for peeling is determined by peeling a fixed dimension strip from surface of coated pipe.Test method shall be as per DIN 30670. The frequency of test for cut back portions shall be all pipes peel test shall be performed for each specified temperature in PQT (three test each temp.). The acceptance criteria shall be as per ITP.The Frequency of peel test shall be as per approved ITP.

IN Process Degradation of PE.

Sample shall be taken from Pipe & test shall be carried out as per ISO 1133.Acceptance criteria shall be as per approved ITP for raw material & applied material.The Frequency of in process degradation of PE test shall be as per approved ITP.

Flexibility Test.

Sample shall be taken from Pipe & test shall be carried out as per CAN I CSA Z245.21-ISO 21809-1 Acceptance criteria shall be as per approved ITP.The Frequency of flexibility test shall be as per approved ITP.

Shore-D Hardness.

The test shall be carried out as per ISO 868.Acceptance criteria shall be as per approved ITP. The Frequency of shore D hardness test shall be as per approved ITP.

Coating Porosity of 3LPE.

The test shall be carried out as per NACE RP-0394.Acceptance criteria shall be as per approved ITP.The Frequency of coating porosity of 3LPE test shall be as per approved ITP.

Indentation Test.

The test shall be performed on a cut from the pipe on strips of polyethylene removed from the pipe surface as per DIN 30670/150 21809-1. A total weight of 2.5 kg shall be loaded on the sample. Acceptance criteria- minimum indentation depth exhibited after testing shall not exceed the value as per approved ITP.The Frequency of indentation test shall be as per approved ITP.

Elongation at Break Test-Only PE Coated Pipes.

This test shall be performed on a coupon cut from the pipe or on strips of polyethylene removed from the pipe surface as per DIN 30670/ ISO 21809-1. Acceptance criteria of percentage elongation shall be as per approved ITP.The Frequency of elongation test shall be as per approved ITP.

Marking/Stenciling.

The original marking from the pipe mill shall be retained inside the pipe. Marking shall be paint stenciled on outside surface of each pipe at one end as per approved ITP.

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