Krah pipes, made of Polyethylene PE100 in typical black colour, provide a perfect weather and UV resistance. If a Polyethylene pipeline is planned for an application in exposed conditions the external loads and influences have to be considered for the design.
Typical applications are:
• Mining industry
• Sewage and water pipelines
• Hydropower plants (headrace, penstock)
• Industrial applications
For exposed installations we mainly differ between industrial pipelines installed in clamps and free installed pipelines on the ground. In the following abstract we concentrate on free installed pipelines under exposed conditions. Often we receive inquiries regarding the planning of Polyethylene pipes for such exposed conditions. People ask for installation rules and what has to be considered in general.
What are the reasons to install a pipeline exposed on the ground without trench/backfilling and without covering? Often the installation area has very difficult accessibility for vehicles and equipment, either because of natural environment or infrastructure. Sometimes the soil conditions are very challenging and sometimes it is only because the effort is much less and the pipeline is a temporary solution only. And why are plastic pipes and especially Polyethylene pipes/Krah pipes so popular for exposed installed pipes? The main reason is the unbeatable corrosion and erosion resistance and the durability of high quality Polyethylene materials! Further the homogenous joining procedure by Electrofusion or butt-fusion guarantees a leakage-free pipe system. The flexibility is often another important argument, because the line is following the environment!
What are the difficulties, if Polyethylene pipes will be installed in exposed conditions?
Firstly we have to consider, that typical black PE 100 provides already an outstanding UV resistance due to the carbon black content. All the used modern PE 100 raw materials are ex works well compounded with all needed stabilizers and ingredients to guarantee a problem-free usage. The carbon black is well distributed in the Polyethylene matrix and absorbs the UV radiation – it is an very effective agent against oxidation and molecule-chain degradation! The minimum content should be more than 2 % – most raw materials guarantee al value of 2,25 % according to ISO 6964, carbon black dispersion ≤ 3 according to ISO 18553.
A typical indicator for the proof of Oxidation stability of black PE 100 is the OIT testing according to DIN EN ISO 728 (OIT = Oxidation Induction Time). For non black Polyethylene other stabiliszers e.g. HALS have to be used (HALS = Hindered Amine Light Stabiliser), but then has to be considered that OIT testing is not an adequate testing method.
Outside installed pipes are exposed to all weather conditions during the seasons. Polyethylene is generally insensitive to cold weather and provides even at minus temperature a sufficient impact stability. For the design the minimum and the maximum temperatures have to be considered and also the temperature of the transported fluid can have an effect. By temperature analysis we find the maximum difference of t emperature and also the related time period for the change. The shorter the time period, the higher the E-modulus/creep modulus and the higher the occuring forces!Any temperature change leads to a change of length and so far the pipeline is installed between fix-points, stress will occure by the inhibited temperature change. At free installations, the change in length will be normally compensated by flexible legs or compensators. If stress occurs because of inhibited change in length the stress has to be combined with other stress-factors of inner pressure, bending etc.
Mostly there is not only one solution and the engineers have to find out and to evaluate what is the best choice of installation for installer, contractor, operator and at the end for the owner!
For illustration and clarification a typical load case is calculated in the table on page 22 and the used formulae and the results are explained briefly: From the results in the table we get the information that the axial forces due to temperature change are much lower for profiled pipes in comparison to solid wall pipes. The forces can be handled much easier!
Note: Also internal pressure can have an effect to the length and the forces have to be considered for fix installed pipelines.
Regarding axial buckling stability the roundness of the pipe is very important, because it significantly affects the axial moment of inertia. For this point we have to take into account that solid wall pipes suffer under their high dead weight load, a vertical deformation happens very fast and affects the stability. But Krah pipes with a profiled wall pipe have a low dead weight load in combination with a high stiffness. The result is a very low deformation and minimum reduction of stability.
The axial buckling stability becomes important if the change in length gets inhibited and the pipe is anchored by fix points at both pipe ends. The critical buckling length should be calculated under consideration of adequate safety factor. By using partially embankment or guide bearings it can be ensured that the free, non-guided length will not be exceeded.
To reduce deformation is also very important for the joining technology and efficient installation at site! A Krah pipe with a profiled wall stays round and can be joined easily by integrated Electrofusion-socket or if requested by butt-fusion. But if a standard solid wall pipe gets deformed by dead weight and sun radiation, it becomes very difficult to re-round the pipe for joining and welding!
If other thermoplastic materials or reinforced thermoplastics are used, the linear expansion coefficient is different:
|Material||PE 100, black||PP-H, grey||PE-GF (MRS20), black|
|Linear expansion coefficient [1/K]||0,18||0,16||0,05|
Further important things to know regarding exposed pipes:
Polyethylene is a thermoplastic material and by nature flamable, classified acc. to DIN4102 in class B2. But Krah pipes can be equipped either with non-flammable or with hardly flammable layer (B1) on outer surface! The production procedure of Krah pipes provide all possibilities to integrate such insensitive layer!
Avoiding of unwanted tapping is an important point for non buried water pipelines. Krah pipes can be designed as double wall pipe and the ringspace between inner and outer wall can be monitored by sensors. If someone starts tapping and stealing water, the sensor will give an alarm!
Pipes installed outside should have a sufficient protection against mechanical damages. Polyethylene is well-known for high impact resistance even at low temperatures. The profiled wall structure of Krah pipes is another safety factor to avoid damages. For critical application the pipes can be designed as double wall pipes!
Krah pipes are applicable for exposed conditions and provide many advantages against other pipe systems! The smart profile design allows a tailormade design for all internal and external loads, to guarantee a most efficient and durable pipe system!
Stephan Füllgrabe, Plaspitec GmbH
for Krah Pipes Germany