To provide a high quality pipe system, all Krah pipes have to pass the following minimum quality control requirements. In case of inside pressure applications, additional tests can be applied.
Additional to the own, in-house quality control a third-party, random inspection is recommended, to ensure a high quality product – the Krah Pipe System. The third-party quality control should take place twice a year. The company´s internal Quality Assurance department should provide a form for the checking. All samples checked shall be considered representative for the whole production. Raw material tests should be carried out before adding the product to the production cycle. Undamaged / non affected samples can be sold afterwards. Those tests made in Krah-Production Plants are exceeding the requirements of the international standards like DIN16961, ASTM F894, etc.
A. Tests with finished products
The following test, are carried out on produced pipes/fittings/manholes – the product is affected due to test-procedure and can´t be used anymore. These tests are made once per months and when there are major changes of the raw material supplier (change of supplier).
1. Stiffness class test according to ISO9969, to determine the pipe at deflection
Three pipe sections with a length equal to 2xDN (but not longer than 1m) should be deformed with a constant speed (constant speed method). The result of the applied force will provide the nominal stiffness of the pipe, called SN.
2. Ring stiffness test, according to DIN 16961, to determine the pipe
Three pipe sections with a length equal t2 x DN (but not longer than 1m) should be loaded with a test force for 24hr (constant load method). The test force is the result of a formula applying the ring-stiffness. The test force is calculated according to DIN16961. If the deformation after 24hr is less than 3% the applied test force (expected ring stiffness) was correct and the pipe fulfils the requirements. This test can be carried out with the equipment shown in photo 1.
The applied creep-module should be minimum according to the given table in the DIN16961, part 2. But according to the raw-material supplier higher values can be used.
|Photo 2: A pipe under deflection of 30°C||Photo 3: A pipe under deflection of 30°C|
3. Deflection test, according to DIN EN ISO13968
The defection test will deform the pipe by 30%, and the test specimen will be visually checked for failures like:
- Break on the inside wall or the inside layer
- Break on the pipe wall
- Delamination of pipe wall
- Break of the sample
- Direction change of the bending of the sample (bum)
This test is very important to show the flexibility of the pipe. After the deflection the pipe should go back to
the original design for up to 95%. The test is done with the universal testing machine, and is basically carried out according to ISO9969, just with a maximum deformation of 30%.
If the pipe should be conform to ASTM F894, this test is called flattening test and the maximum deflection of the pipe should be 40% instead of 30%. This test shows the safety of a pipe against overload, especially to show the resistance against ground movements – e.g. earthquakes. Only good materials will fulfil this test. The report should contain photos of the test.
4. Tensile strength, according to DIN EN ISO 527 for test specimen in axial or circumferential direction
This standard lays down the general principles for determining the tensile properties of plastics under specified conditions. The methods are used to study the tensile deformation behaviour of test specimens and to
determine tensile strength, tensile modulus and other aspects of tensile/strain relationship under specified conditions.
Photo 4: Preparing the test specimen from a pipe
|Photo 5: Tensile test of a prepared test specimen||
Photo 6: Test samples
The result of this test will give you information over the following points:
- Tensile stress
- Yield stress
- Tensile strength
- Tensile stress at x % elongation
- Tensile modulus
- Elongation of the tensile strength
- Nominal elongation
The following tests are raw material tests, they have to be carried out and passed before using the raw material for production. These tests have to be carried out with „every“ raw-material before the material is used. In case the material is supplied with a silo truck, this test has to be carried out before unloading the truck. If the material does not comply with the requirements it should be sent back. As an additional reference the supplied data-sheet of the raw-material supplier is given, for example to find out the “carbon black content”. In case that the pipe producer is using master batch an additional special test should be carried out.
5. Melt-flow rate
The melt-flow-rate (MFR) shall be determined according to DIN EN ISO 1133, using a nominal mass of 5,0 kg at 190°C for Polyethylene high density. PE should have a MFR of 0,2 – 0,5 g/10min.
6. Density Test
Performed according to DIN EN ISO 1183-1. Generally, the immersion method
(procedure A) is used to determine the density. However, the determination of density with the liquid pycnometer (procedure B) can also be used. The density (crystallinity) determines the flow behaviour, processing properties and quality of the later pipes. Density ρ belongs to the substance constants and is a physical quantity that characterises the substance of which a body consists. Its SI unit is kg/m3. It is determined solely by the material of the body in question and is completely independent of its size and shape. While the specific gravity describes the ratio of the weight force to volume, the mass in density is in proportion to the volume.
C. Product tests
The following test are product tests, the samples will not be damaged and can be used afterwards if they pass the tests. If a product (pipe/fitting/manhole) will
not pass the tests the product should be marked „QC failed“ and should be stored separately. These tests have to be done after each pipe produced and a target-performance comparison should be carried out.
|Photo 7: Device for testing of the melt flow rate (5)||Photo 8: Device for moisture analyzing||Photo 9: Device for density determination|
7. Moisture Test
To avoid bubbles and blisters in the pipe wall it is important to know the moisture content of the raw material.
The maximum moisture inside the material should be 3%, to have a secure and safe production of pipes.
8. Surface finish - inner and outer surface
All pipes and fittings shall have a smooth or profiled outside surface. The inner surface should always be smooth, due to the necessary hydraulic value. Minor sinks and fluctuations in the pipe wall are permissible, provided that the minimum wall thickness is maintained throughout.
9. Product dimensions
The dimensions of pipes and fittings shall be measured using suitable measuring devices. The measurements should be carried out according to EN ISO 3126. And the measurements have to be done 24hr after the production of the pipe. The results of pipe length, inside diameter, length of socket, length of spigot, height of the profile and the weight of the pipe/fitting/manhole should be noted down for each pipe.
|Photo 10: A staff member measuring out the pipe profile||Photo 11: A staff member measuring out the pipe profile|
10. Colour of the pipe
The individual extrusion layers of pipes and fittings shall be throughout uniformly coloured.
11. Checking of the correct marking
According to several standards, different minimum markings should be seen on the pipe.
12. Checking of the QC-Report of the used raw material
Here a final check is carried out, to prove that a proper raw-material check was done, before using this raw material.
Test of jointing: In case a „not-welded“ joint system is used, the joint has to be tested against „root penetration“. All welded joints (like the Krah integrated electro-fusion“) are „resistant against root penetration“.
The results of the internal control testing shall be recorded, and, as far as possible, statistically evaluated. Records shall be kept for at least five years, and submitted to the inspection body on request. If the pipe / fitting passed all test a quality certificate according to DIN EN 10204 can be made. Three different classification of quality certificates are available: 2.1, 2.2 and 3.1 (a sample pipe should be produced, to carry out the „damaging“ test according to A).