Common tests for polyethylene pipe production (inspection)
Melt flow index
MFR (ISO 1133)
In this test, the melt flow rate of the material is measured at a constant temperature and time, so that the results can be used to determine how the material behaves inside the extruder. This test is performed on raw materials (to confirm the quality of the material) and also on the product. The MFR value of the product should not differ by more than 2% from the MFR of the raw material. According to the Gas Company standard, this value is 0.2 ≤ MFR ≤ 0.7.
Density determination(ISO 1183)
The density of the raw materials and the density of the product are determined by flotation using a precise balance and a fluid with a certain density. In addition, the density number of the product is an indicator of the quality of the production process. According to the National Iranian Gas Company standard, this value is ≥945 kg/m3.
Determination of carbon percentage
Carbon Black Content (ASTM D 1603)
The amount of carbon in the raw materials and the final product is determined. For this purpose, a certain amount of the raw materials is pyrolyzed in a furnace at a temperature of 800 degrees Celsius under the influence of pure nitrogen gas, and the remaining carbon is burned under the influence of oxygen to determine the percentage of ash (additives) in the raw sample.
The permissible percentage of carbon in polyethylene pipe is 2 to 2.5 percent by weight and must be evenly distributed throughout it.
In areas where carbon accumulation exceeds the allowable percentage, stress concentration will occur and the pipe will become vulnerable. If the carbon content is less than the allowable limit, the pipe's strength against the sun's ultraviolet rays will decrease.
Tensile test( BS2782-EN63-ASTM D 2412)
Using specialized laboratory equipment, the mechanical properties of polyethylene pipes, including maximum strength against external load, the amount of change in length at the point of rupture, determination of the coefficient of elasticity, and the amount of scratching under the effect of three-point loads, can be measured. According to the results of the above test, it is possible to examine how the product performs under operating conditions. According to the Iranian National Gas Standard, this value is ≥450%.
Hydrostatic pressure test
(EN 921)
The above test is performed to check the strength of the product against hydrostatic pressure. In this test, pipe samples are immersed in a water basin at a temperature of 20°C for 100 hours, 80°C for 165 hours, or 80°C for 1000 hours, depending on the pipe size and type of raw material, and are then subjected to a constant internal pressure.
The appearance of any defect in the samples (bursting, swelling, localized swelling, leakage, and hairline cracks) means that the product is rejected.
Bursting pressure test
(ASTM D 1599)
In this test, pipe samples are floated in a pool with a constant temperature of 23 degrees Celsius and then subjected to increasing internal pressure, causing them to swell and then burst within 60 to 70 seconds.
A pipe that is manufactured with high-quality materials and the correct process undergoes plastic deformation and swells, and then bursts in the form of a beak tip. In this case, the fracture cross-section is perpendicular to the longitudinal axis of the pipe.
A pipe that bursts or cracks without inflation is unusable.
Thermal reversal test
reversion (ISO 2505) Longitudinal
In this test, samples approximately 30 cm long are placed in an oven with hot air circulation (110+2) degrees Celsius for one to three hours (depending on the thickness of the pipe wall). After cooling, the length of the pipe will be less than the initial state at normal temperature. This behavior in installed pipes can lead to changes in the roundness of the pipe. Therefore, with the above test, the allowable length changes (up to 3%) are examined in the laboratory.
Experiment O.I.T
This test is performed on raw materials and the purpose of this test is to determine the thermal stability of the materials. According to the aforementioned standard, the degradation time of polyethylene should not be less than 20 minutes.
25min < ۲۱۰c
50min > ۲۰۰c
Compression test
(ISO-4437) و (EN-12106)
In this test, samples of 8 times the nominal diameter of the pipe are prepared and placed in zero-degree water for 10 hours. After the pipe is compressed by the SQUEEZER device, it is returned to its original state for 1 hour by the RE-Rounding device. Then, by closing both ends of the pipe with a cap, it is placed under pressure (for PE80, pressure of 4 MPa and for PE100, pressure equivalent to 5 MPa) for 1000 hours at a temperature of 80 degrees Celsius. According to the above standard, the smallest gap should not be created in the pipe.