This content provides a sample pipe three layer polyethylene coating specification and highlights important inspection and testing requirements.
Prior to start of regular production, a Procedure Qualification trial for maximum pipe diameter available, for each coating material combination, and for each plant, to establish that plant, materials and coating procedures resulting quality of end product confirming to the properties of the material, relevant standards, specifications and material manufacturer’s recommendations. 6 test pipes shall be selected for Procedure Qualification. Out of 6 test pipes, 1 pipe partly coated with epoxy and partly coated with both epoxy and adhesive layers shall be included, remaining 5 test pipes shall be having all three layers. The applied coating system shall comply to properties of coating system.
1.1 Qualification Procedure
1.1.1 Pipe pre-heating
The temperature variation due to in-coming pipe temperature, line speed variation, interruptions, etc. shall be established and recorded during PQT. Proper functioning of pipe temperature monitoring and recording system shall also be demonstrated during PQT.
1.1.2 Surface Preparation
This qualification shall be performed through a visual inspection & check of the degree of cleanliness.
1.1.3 Coating application
Pre-heating pipe surface temperature prior to epoxy powder application temperature, line speed, temperature of co-polymer adhesive and Polyethylene shall be established during PQT and these values shall be complied with during regular production.
2.1 Tests on pipe coated partly with epoxy and partly with epoxy & adhesive
a. Epoxy Layer Thickness
Epoxy layer thickness shall be checked at every one meter of spacing at 3, 6, 9 and 12 O’ clock positions. The thickness shall be 150 μm min.
b. Adhesive Layer Thickness
Adhesive layer thickness shall be checked at every one meter of spacing at 3, 6, 9 and 12 O’ clock positions. The thickness shall be 150 μm min.
c. Adhesion Test
Adhesion of FBE shall also be separately determined at ambient temperature at two locations by the “St Andrews Cross” method i.e. by cutting two straight lines through the epoxy layer with a sharp knife. The incisions shall intersect at an angle of 30º/15º. The epoxy coating shall resist disbondment from the steel when attempts are made to flick/lift coating from the 30° angles with a sharp knife.
d. Thermal Analysis
Thermal Analysis Test shall be carried out on one pipe as specified in CSA Z.245.20 or any specified standard by client.
2.2 Tests on coated pipes with all three layers
a. Peel Resistance
Five test pipes shall be selected for Peel Resistance tests. On each of the selected pipes, two Peel strength test shall be performed each specified temperature i.e. one at each end of the pipe at 50ºC ± 2ºC and 80°C ± 2°C as per client specification. None of these samples shall fail. Acceptance criteria shall be minimum 25N/cm at 50°C ± 2°C and 20N/cm at 80°C ± 2°C.
b. Impact Strength
Five test pipes shall be selected for impact strength test and the test shall meet the specified requirements. Acceptance criteria shall be no holiday at 11 J at 23°C ± 2°C as per of client Specification.
c. Indentation Hardness
Five samples for 23ºC ± 2ºC and 80ºC ± 2ºC from Five pipes shall be taken. If any one of the samples fails to satisfy the requirements, then the test shall be repeated on four more samples. In this case, none of the samples shall fail. Acceptance criteria shall be as per Client specification.
d. Elongation at Failure
Six samples each from Five coated pipe shall be tested and the test shall comply the specified requirement. Only one sample may fail. Acceptance criteria shall be 300% minimum as per client specification.
e. Cathodic Disbondment Test
Two tests shall be carried out for the total lot of test pipes having all three layers. One test shall be carried out at 65°C for 48hrs. & another test shall be carried out at 80°C for 48 hrs.
f. Holiday Inspection
All The Pipes shall be subjected to holiday inspection. The voltage shall be 25KV as per client specification. This test must be witnessed by assigned coating inspection company.
g. Coating Thickness Measurement
All pipes shall be subjected to coating thickness measurements. The acceptance criteria shall be 2.9 mm minimum as per client specification.
2.3 Inspection of all Test pipes
Pipes shall be subjected to the following inspections:
a. Surface Cleanliness, immediately after abrasive blast cleaning.
b. Visual Inspection of finished coating, cut back dimension, internal/external cleanliness, end sealing and bevel inspection.
Regular / Commercial Production shall be commenced after getting approval from the Customer/Customer’s representative.
Following tests shall be performed at the frequency as defined there during regular production.
3.1 Test on Coated Pipes
a. Visual Inspection
The resultant coating shall have uniform gloss and appearance and be free from air bubbles, wrinkles, irregularities, discontinuities, separation between layers of PE/Adhesive, etc.
In addition inside surface of the pipe shall also be visually inspected before coating for presence of any foreign material or shots and grit (free or embedded/sticking to pipe inside surface) using fled light from one end.
b. Coating Thickness
The coating thickness shall be measured by using a non-destructive gauge and at least 12 measurements (three 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) shall be taken over the pipe length of each pipe.
Any joint of pipe with a measured coating thickness less than 2.9 mm at any location shall be rejected and re-processed.
The thickness of Epoxy and Adhesive layers shall be checked at the beginning of each shift.
Minimum thickness of Epoxy layer shall be 150 microns.
Minimum thickness of Adhesive layer shall be 150 microns.
Minimum thickness of all three layers shall be 2.9 mm
The thickness gauge shall be calibrated at least once per shift. Calibration shall be carried out on a steel plate of a similar thickness to the pipe wall by means of plastic shims of the same thickness as the nominal coating system
c. Holiday Detection
After completing the thickness check and brushing of cut back ends, each coated pipe length shall be checked over 100% of coated surface by means of a holiday detector. The pipe shall be conveyed to the outlet conveyor where the pipe shall move through a holiday ring connected to a high voltage unit. The holiday detector shall be a low pulse D.C. full circle electrode detector with audible alarm and precise voltage control and the voltage for inspection shall be 25 KV. If any defect is found, the operator shall mark the defect area for repair. The holiday detector shall be calibrated at least once every four hours of production. Detection speed shall be found in conjunction with the voltage. Maximum speed will not exceed 300 mm/second.
d. Peel Resistance Test
The Peel strength is the force required to peel a strip of coating from the pipe under specified conditions. Two samples (One at each cut back portion of the pipe) shall be tested at a temperature of (50 ± 2)ºC and (80 ± 2)ºC. The mean force applied shall be at least 25 N/cm of test piece width at (50 ± 2)ºC and at least 20 N/cm of test piece width at (80 ± 2)ºC. Peel Tests shall be carried out by a dynamometer as per the client specification. The strip width shall be between (20 to 50 mm). The frequency of test shall be one in 100 Pipes.
e. Impact Strength
The impact strength is defined as the impact energy, coatings can withstand under specified conditions. Under this test, the coated pipe shall be subjected to withstand 10 impacts located equidistant along the length of coated pipe, without any electrical breakdown when tested at 25 KV. For this test, 25 mm diameter of sphere shall be dropped on the pipe from one-meter height. The falling weight shall be 11Joules per 2.9 mm of coating thickness at 23 ± 2ºC. This falling weight shall be guided by using a Pipe to minimize friction and so that the direction of fall is normal to the pipe surface. The pipe shall be subjected to impact 10 times and the distance between two points of impact shall be at least 30 mm. immediately after testing, the test area shall be checked for holiday at 25 KV. No electrical break down should occur and the pipe shall be rejected if any holiday is noted on the test area.
The frequency of test shall one in 100 pipes.
f. Indentation Test
The indentation hardness is a measure of the resistance of the coating to penetration of a test pipe under specified conditions. Under this test, the resistance of the polyethylene to penetration of an indenter under 2.5 Kg. load shall be determined at 23 ± 2ºC and 80 ± 2ºC. Acceptance criteria shall be as per table 3-4 of client specification. Two samples shall be taken for each temperature, from the cut back portion of pipe. The adhesive shall be removed from the test samples and shall be abraded the reverse side of the samples to obtain 2.0 mm thickness. After conditioning the samples in a water bath (indentation tank) for one hour at the test temperature, the indenter (without additional weight) shall be carefully lowered on the test sample, and the zero value set at the penetro meter and the additional weight shall be attached to the indenter immediately. After 24 hours, the depth of penetration shall read from the penetro meter. The indentation depth shall not exceed 0.2 mm at 23 ± 2ºC and 0.3 mm at 80 ± 2ºC.
The frequency of test shall be one pipe per 100 pipes.
g. Thermal Analysis
Thermal Analysis Test Shall be carried out on one pipe per shift as per client specification.
h. Cathodic Disbondment Test
Cathodic Disbondment Test shall be carried out on one pipe per 200 Pipes or Minimum one pipe per unit of production initially. Test carried out for 48 hrs. @80° C duration as per client specification.
Damages occurring to the pipe coating during above tests shall be repaired in accordance with approved coating repair procedure.
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