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PROPERTIES OF PIPELINES MADE OF POLYETHYLENE

INTRODUCTION

             This analyze of properties of pipelines made of polyethylene has a goal to show the possibility of using the pipes made of polyethylene as a relatively new material in construction of water supply systems. This analyze should prove the economical and technical justify for use of this modern pipes instead pipes made of classic materials such as iron-cast, steel, PVC and polyester.

            Most of classic materials are theoretically and practically researched, therefore, as an etalon will be taken only the pipes made of PVC, which have most advantages of all classic material.

            Analyze is made for the object that is already projected and his construction is coming, and if the real principles for comparation are seatled, then this analyze has widhter meaning, i.e, it will be valid for all pipeline systems.

            In technical terms, the following will be analized:

• Geometry and weight properties of pipes
• Construction works for mouinting of pipes
• Mounting of pipelines made of PE
• Fitings for connection and forming of network
• Examination
• Hydraulic properties of pipelin

 Development of polyethylene for production of pipes

             Polyethylene as a material for building of pipelines is in use for more then 40 years. In the beginning it was used low density polyethylene with MRS (minimal required strength) of 3,2 Mpa. The next improvements of the material increased the MRS to 4,0 Mpa. This type of polyethylene is still used in some countries for indoor installation and in irrigation, but only in small diameters and low pressure.

            In the late ’50 was introduced high density polyethylene with MRS of 6,3 Mpa. The new material was able for construction of pipelines with decreased wall thickness of pipes, which spread the use of PE pipes on many fields. Permissible operating pressure of pipes and fittings made of HDPE (classified as a PE 63), was limited on 5,0 Mpa.

            On the end of ’70, the second generation of high density polyethylene was introduced, with MRS of 8,0 Mpa and was signed as a PE 80. That allows to increase permissible operating pressure to 6,3 Mpa, but national standards of most countries keeped the previous limit of 5,0 Mpa. Later was introduced the medium density polyethylene with MRS of 8,0 Mpa. It’s basic advantige was that it allows a production of more flexibility pipelines with improved long term mechanical properties. Today, most of pipelines are made of this two types of polyethylene: HDPE/MDPE (signed as PE 80).

            Further reseach result with introduction of new types of polyethylene, including the modified MDPE. It allows the transport of fluids with temperature higher than 20 ⁰C. Permissible operating pressure for this sub-type of polyethylene was also 5,0 Mpa, but the operating temperature was increased to 50 ⁰C.

            Finally, for some time and due to development, the new generation of HDPE polyethylene is introduced. This material is signed as a PE 100, the MRS of this material is 10 Mpa, and the permissible operating pressure is limited on 8,0 Mpa. The increasement of permissible operating pressure from 5,0 to 8,0 Mpa theoretically gives difference of 33%, which can be used to decrease the thickness of the wall of pipes and fittings for the same operating pressures, or to increase the operating pressure for the same diameters of pipes. The use of PE 100 soon will be taken in concederation in the next revision of ISO standards as well as new Europian standard for polyethylene pipes for water and gas transport (CEN).

MECHANICAL AND EXPLOATATION PROPERTIES

             The basic classification of PE pipes is made upon MRS (minimal required strength) of polyethylene. That means the highest inside pressure that pipe can hold without danger of breakage for period of 50 years and operating temperature of 20 ⁰C. Namelly, with convention it is accepted the life time of polyethylene is limited to 50 years for operating temperature of 20 ⁰C. Theoretically, the life time of pipelines made of, for exemple PE 80, for the same operating temperature is 15.000 years. In order to speed up the process of examination, PE pipes are examinedon a higher temperature (60 and 80 ⁰C). The results achived are extrapolated for the operating temperature of 20 ⁰C and the period of 60 years. All procedure of examination of long term mechanical properties is defined in ISO standards. Maximum permissible working pressure in PE pipelines of polyethylene class, SDR classification of pipes and conditions of exploatation SDR (coefficient of dimensions) is relation between outside diameter and  the pipe wall (D/E). This relation is standardize and regarding of his value the existing PE pipes are divated in few groups as it’s shown in

 Table 1.

 Table No.1

In table 1 is shown the connection between standard class of polyethylene, operating pressure and serie of polyethylene pipes.

             The wide use of polyethylene as a construction material for water supply pipelines is because of following properties:

1. First and probably the most significant factor is its flexibility. It allows some advantages as:

Ø     Supply of PE pipes in coil in large length which decrease the number of fittings needed for conection and decrease the time needed for mounting.

Ø     Fitness for use in sismeic areas, and resistance of stress caused of heavy trucks.

Ø     Bending (with radius R=20D) or crossing of existing installations without need of fittings.

Ø     Resistance of corrosion.

Ø     Life time of more than 50 years.

Ø     No change of taste or smelt of the water

Ø     Abrasion resistance

Ø     Possibility of upgrade the existing pipelines with polyethylene.

2. The fact that polyethylene has electrical and chemical resistance, eliminate the corrosion as a one of most reasons for destruction of pipelines made of classic material. That results with longer life time, without additional cost for keeping.
3. Low weight of polyetyhylene make the mounting of pipeline much easier and with low costs. At the same time the polyethylene is string enough to hang on the ruff handling, which is important during the reconstruction of existing pipelines.

 Because of its low resistance to UV-rays, it usually used for underground pipelines or indoor pipelines which are not exposed to UV rays. Exception is so called “black polyethylene which have 2-3% of carbonate. Carbonate is a UV stabilizator, which allows upground installation of black PE pipes. For increasement of UV resistance of other pipes (yellow, blue etc.) it is needed to add UV stabilizator (carbonate).

      The operating temperature of fluids in PE pipelines is from – 30 ⁰C to + 40 ⁰C, but its needed to corigate the life time with real value of temperature. Exeption is modified MDPE which can hold the temperature of 50 ⁰C for period of 50 years. Maximum allowed short term temperature is 80 ⁰C. The use of PE pipes is spread to many fields such as water supply, irrigation systems, in chemical industry for transport of chemical agresive fluids, wastage water, gas transport and for some time for transport of liquid fuels.

      Because it is lightproofness, polyethylene keep all properties of clean water, no change in taste or smelt of the water and stops the development of bacterias. The operating pressure is increased to 16 bar. The introduction of PE 100 allows the pipe with same diameter to increase the pressure up to 32 bar. All this allows PE pipes to be competitive alternative to Steel pipelines with operating pressure of 16 and 20 bar. The pipes are produced with geometric and weight properties according the following standard: ISO 4427.

             According to this characteristics of material, it compare with this characteristics of PVC.

Comparison schedule – example:

             Mentioned characteristics are for 10 atm. pressures.

Building of PE pipe lines

            Pipe systems is lying down in a ditch with width for PVC d=0.4m but for the mounting it has to be minimum width d=0.7m. In this case the worker go in to the ditch and mount the pipe system.

            PE – 80 and PE – 100 pipe systems are mounted on the top of the ditch and after that they should be pulled in to the ditch. Length of the PE pipe system for F 90mm can be 100m, and for smaller dimension can be 1000m. Width of the ditch is d=0.4m.

Savings on PE pipelines with reducing excavations is from 30 to 50%. The height of the ditch is 0.80m above the pipeline to prevent the pipe from next excavations near to pipeline.

It’s good to mentioned that pipelines from PE has better isolation properties so the height of the ditch can be reduced up to 0.60m without danger of blowing out the pipeline. In extreme low temperatures the material is stable up to –30 degree Celsius. 

Mounting of the pipeline can be done in a tree waves:

            1.     Electrofusion welding

2.     Butt welding

3.     Heating element but welding

First wave of welding we use only for gas distribution.

Second wave of welding is using for dimension from F 63mm up to F630mm and you must use special tools and equipment and special team of workers. We has this tools, equipment and team for mounting the pipelines.

Generally bends, elbows end tees (heating element) are used for changes in direction.

            You have two types of elements:

-        Elements from PE material used for PE pipelines.

-        Elements from PE, PP and combination PE – iron for PVC pipelines.

 Those kinds of elements are accepted in the European union like materials with long lifetime and ecological good for use, and the European famous companies make them and we have an excellent cooperation with them.

              Operating pressure

Checking examination for pressure is the same like the PVC pipes.

            PVC pipes are examine:

-        Working pressure highest up for 30%;

-        For example F63mm working pressure 10 atm. Its examine on 13atm. pressure.

 PE pipes are examine like this:

-        Working pressure highest up for 200-300%;

-        For example F63mm working pressure 10 atm. it’s examine on 24 atm.

 Healthy and ecological justificable of PE pipes

PE material, which is use for PE pipes production in its composition, hasn’t any hard metals, so PE pipes are physiological nontoxic.

With respect to its composition PE complies with the relevant foodstuff regulations (according to ONORM B 5014, part 1, BGA, KTW guidelines).

            Contrasting with PVC in lot of countries (Germany, France, Austria, Swiss) is limited or totally forbidden because of presence of Pb like warmth stabilization.

PE pipes are also ecological justificable because it can be reciclated and use for production for other products.

For More Information Contact:

KONTI HIDROPLAST
INDUSTRISKA BB GEVGELIJA MACEDONIA
Tel: ++ 389 34 212 064
FAX: ++ 389 34 212 064
Internet: hidropl@mt.net.mk

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Last modified: October 18, 2000    designed by  CENTRAL BUSINESS ADMINISTRATION (kompjuterski programi i veb dizajn)