Polypropylene is produced by polymerization of propylene. His varieties:
- Homopolymer (PP-H);
- Block copolymer (PP-B);
- Random Copolymer (PP-RC).
One of the main indicators of the quality of pipes – a long-term resistance to stress, both by pressure and temperature. It should be noted a pronounced temperature-time dependence of the strength of polymer pipes. For cross-linked polyethylene pipe, this dependence is a standardized form:
s – the circumferential stress (hoop stress – in U.S. dollars. literature), MPa;
t – temperature, K;
T – duration of life or exploitation chacov;
A = -105,8616;
B = 57895,49;
C = -24,7997;
D = -18506,15.
The values of these coefficients established in the international standard ISO 15875 “Piping for hot and cold water XLPE. For the PP will be not one but two equations, which is associated with the presence of a transition from plastic to brittle type of fracture at elevated temperatures and timing of operation (ISO 15874):
A = -55.725;
B = 25502,2;
C = 6,39;
D = -9484,11;
A1 = 19,98; B1 = 9507; C1 = -4,11.
Shows the “allowable circumferential stress depending on length of service” for cross-linked polyethylene (PEX a) and polypropylene (PP-C) at an operating temperature of 95 ° C. We see that the rate of decrease in strength with increasing temperature in tubes of polypropylene is much higher than that of XLPE. Compare, then PEX and PPRC, taking in both cases, the pipe-type PN20 (type PN40, for example, means by definition that the pipe can withstand the pressure of 40 atm at 20 ° C for 50 years) with different external diameters of 20 and 110 mm compare the thickness of the walls: / Fig. /
The thickness of the walls of PEX-pipes is less than for PP-pipes.
Let us now compare Pipe PN20 allowable working pressure at different temperatures, but at the same general condition, life expectancy – 50 years: / Fig. /
From this comparison shows that the PEX pipes can withstand high pressures at high temperatures than PPRC-pipe. Installation of polypropylene pipes in underground laying by straight sections, which require special technology for the connection (welding, soldering, adhesive). Severe stress arising in PP-pipes require the installation of compensating devices. All these shortcomings are deprived of pipelines from PEX, because they are flexible and samokompensiruyuschiesya tracks, installation of which is carried out with the use of bays. Replacement of steel pipes to PEX in the city wins prior to the change in propylene, as in this case requires especially virtuoso laying tracks, keeping unchanged the current structure of other communications.
If we consider the trends of various plastic pipes in USA, then, again, PEX leads (Fig. 8 / Fig. /). It should be noted that in several regions of Canada have experience in the practical application of plastic pipes in foam insulation and hydro proof sheath made of polyethylene.
Production Association TVEL mastered the production of pre-insulated pipes in foam insulation with a protective layer of polyethylene pipe and the bearing of cross-linked polyethylene. To date, the maximum amount of work on laying of new and replacement of old mains produced in association Texas. Also performed work in St. Petersburg, Surgut and other cities.
It is interesting to note an example of the replacement of worn-out heating, paved channel method in Seattle. Dilapidated pipes were dismantled and replaced with a plastic pipe was placed 50 m and a diameter of carrier pipe 110 mm in the existing concrete box. During the hydraulic tests, he was in good condition. At the time of the pressure and temperature (7 atm, 50 ° C), the pipeline did not show any signs of strain.
