9.6.5. Plastic pipes


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Fig. 9.17: An assortment of plastic shaped parts and fittings
There are plastic pipes for pipe systems from various manufacturers and in various materials. There are also polyamide pipes for high pressures and polyethylene pipes for large diameters. This means that there are plastic pipes with the appropriate properties for almost every area of application. For this reason it is difficult to generally apply information about sizes, operating pressures and temperatures.

Advantages

Because plastic pipes do not corrode, there is no need for any protective surface material. They are up to 80 % lighter than steel. This simplifies installation and there are fewer demands placed on the pipefittings.

The inside surface is very smooth. Flow resistance is low (low pressure loss) and deposits such as calcium and rust etc. have practically no chance to build up. Plastic pipes are usually harmless from a toxicological and hygienic standpoint.

PVC pipe systems and the like have a large number of shaped parts and fittings available for them. Installation is very easy. The pipe sections are fitted together and given an airtight seal with special adhesive. No special knowledge is necessary for installation. Pressure loss and leakage is generally very low in plastic piping.

Disadvantages

The low-cost PVC pipe systems have a maximum operating pressure of only 12.5 bar at 25° C. It must also be carefully noted that the maximum operating pressure of these plastic pipes drops heavily if the temperature is increased. For this reason plastic pipes may not be used in the hot areas of a
compressor station and must be protected from direct sunlight.

Plastic pipes have large coefficients of linear expansion and their mechanical stability is not particularly high.

Resistance to certain condensates and types of oil is not always guaranteed with some plastics. The composition of condensates in the network must therefore be checked beforehand.

Plastic pipes are not made in large quantities for high pressures or large diameters. This makes them expensive and the number of shaped parts available is limited. An experienced plastic welder is needed to install these pipes.