Comparison of metal-plastic and polypropylene pipes

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Met­al-plas­tic (met­al-poly­mer) and polypropy­lene pipes (PP or PP) do not rust, do not impair water qual­i­ty, they are sim­ply cut and mount­ed, deposits do not linger on their inner sur­face due to their smooth­ness. There­fore, this type of prod­uct is often used to install heat­ing sys­tems, water sup­ply. It is dif­fi­cult for a sim­ple buy­er to dis­tin­guish them by their appear­ance, with­out under­stand­ing the mark­ings, but PP prod­ucts are lighter and stiffer (they do not bend). What is bet­ter to buy — met­al-plas­tic or polypropy­lene pipes?

Fig.1 Plas­tic tubes

Met­al-poly­mer

Met­al-plas­tic is a mate­r­i­al con­sist­ing of a com­bi­na­tion of lay­ers of ther­mo­plas­tic poly­mers and alu­minum foil. Met­al pro­vides rigid­i­ty, plas­tic — anti-cor­ro­sion resis­tance, dirt-repel­lent prop­er­ties, attrac­tive appear­ance. Ther­mo­plas­tics are capa­ble of repeat­ed­ly soft­en­ing and becom­ing rigid again when heat­ed and cooled with­out loss of prop­er­ties. This prop­er­ty allows them to be used in the man­u­fac­ture of pipes for hot water sup­ply or heat­ing sys­tems.

In met­al-poly­mer pipes, an alu­minum lay­er is locat­ed between two lay­ers of cross-linked poly­eth­yl­ene (PE‑X or PE-RT grades). Alu­minum pieces are weld­ed “over­lap”. To con­nect the met­al and plas­tic lay­ers into a sin­gle whole, heat-resis­tant glue is used.

Rice. 2 Met­al-plas­tic tube device

The main char­ac­ter­is­tics of met­al-plas­tic pipes:

  • oper­at­ing tem­per­a­ture — +75°С;
  • max­i­mum allow­able tem­per­a­ture (short-term) — +110 °С;
  • oper­at­ing pres­sure at +75°C — 1MPa, at +20°C ‑2.5MPa;
  • ther­mal con­duc­tiv­i­ty coef­fi­cient — from 0.39 to 0.45 W/mK;
  • the ser­vice life of hot water sup­ply sys­tems is from 25, cold — from 50 years (obser­vance of the oper­at­ing mode is manda­to­ry);
  • diam­e­ter — 16, 20, 26, 32 mm (larg­er prod­ucts are rarely found on sale).

Polypropylene

Polypropy­lene (PP‑R, PPRC grades) is anoth­er ther­mo­plas­tic poly­mer. In terms of strength, hard­ness, tubes made from it are ahead of met­al-poly­mer ones, and in appear­ance they are not infe­ri­or.

PP pipes are of two types:

  • sin­gle-lay­er (with­out rein­force­ment) — only from polypropy­lene;
  • three-lay­er (with rein­force­ment) — a lay­er of PP, a lay­er of fiber­glass or alu­minum foil, anoth­er lay­er of polypropy­lene.

Main tech­ni­cal char­ac­ter­is­tics of polypropy­lene tubes:

  • oper­at­ing tem­per­a­ture — +75°С;
  • melt­ing tem­per­a­ture — +146 ° С;
  • oper­at­ing pres­sure at +75°C — 1MPa, at +25°C — 10, 16, 20 or 25 bar to choose from (look at the mark­ing, 1MPA = 10 bar);
  • ther­mal con­duc­tiv­i­ty coef­fi­cient (depend­ing on the diam­e­ter) — from 0.15 to 0.23 W / mK;
  • the ser­vice life of hot water pipelines is from 25, cold — from 50 years (obser­vance of the oper­at­ing mode is manda­to­ry);
  • diam­e­ter — 16, 20, 25, 32, 40, 50, 63 and 76 mm.
Rice. 3 Polypropy­lene tube device

Similarities

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Gen­er­al dis­ad­van­tages of pipelines made of polypropy­lene and met­al-plas­tic pipes:

  1. Large coef­fi­cient of lin­ear expan­sion. The length of the PP pipeline can increase by sev­er­al cen­time­ters due to expan­sion under the influ­ence of high tem­per­a­ture (met­al-poly­mer pipes elon­gate less). This prob­lem is solved by installing com­pen­sators — con­nect­ing parts in the form of loops. The prop­er­ty of polypropy­lene and met­al-plas­tic to length­en must be tak­en into account when design­ing.
  2. Con­nec­tions of pipe seg­ments are one-piece. This often cre­ates prob­lems when repairs are need­ed.
  3. Flam­ma­bil­i­ty. In case of fire, after the pipe burns out, a strong draft is cre­at­ed through­out the ris­er. A fire can engulf an entire house very quick­ly. There­fore, at the points of tran­si­tion through the ceil­ings, fire cou­plings are installed on the pipes.
  4. With an instan­ta­neous pres­sure surge result­ing from a sud­den stop in the move­ment of water (water ham­mer), the pipe delam­i­nates. It is for this rea­son that it is rec­om­mend­ed to use met­al pipes to con­nect to a cen­tral­ized DHW sys­tem.
  5. Fire-fight­ing water sup­ply sys­tems can­not be installed from met­al-poly­mer and PP pipes.

It is not rec­om­mend­ed to install pipes made of PP and met­al-plas­tic out­doors. Polypropy­lene is unsta­ble to ultra­vi­o­let radi­a­tion, and the met­al poly­mer may burst when frozen.

Dif­fer­ences

The main dif­fer­ences between met­al-poly­mer and polypropy­lene pipes:

  1. The method of con­nect­ing pipeline parts.
  2. Price.
  3. Appli­ca­tion areas.

Mounting

Man­u­fac­tur­ers do not rec­om­mend bend­ing polypropy­lene tubes. This can only be done by heat­ing them to a tem­per­a­ture at which the strength of the walls decreas­es. They are sold in 4 m whips and cut into pieces of the desired length. Parts of the pipeline are con­nect­ed using spe­cial fit­tings, elbows, tees, cou­plings, also made of polypropy­lene.

There are two tech­nolo­gies for installing polypropy­lene pipes:

  1. Sol­der­ing (weld­ing) with a spe­cial weld­ing machine (sol­der­ing iron for plas­tic pipes). A piece of tube and a fit­ting are heat­ed until melt­ing begins and are pressed into each oth­er. Cool­ing polypropy­lene hard­ens again, cre­at­ing a per­ma­nent con­nec­tion. Instal­la­tion only seems sim­ple, but in the absence of expe­ri­ence it is impos­si­ble to achieve the per­fect qual­i­ty of the joints. There is a high risk of over­ex­pos­ing the sol­der­ing iron or vice ver­sa under­heat­ing the joint. There­fore, polypropy­lene pipes are not rec­om­mend­ed to be installed: under a con­crete screed; under­ground; inside the walls (con­cealed instal­la­tion).
  2. Cold weld­ing (only when installing the pipeline of the cold water sup­ply sys­tem). To con­nect pipes by this method, a spe­cial aggres­sive glue is used.
Fig.4 Weld­ing of polypropy­lene pipes

Detach­able con­nec­tions on polypropy­lene pipelines (for valves and taps) are obtained using spe­cial fit­tings with a tran­si­tion to an exter­nal or inter­nal thread. Their dis­ad­van­tage is their large size, some­times it is incon­ve­nient that they take up a lot of space.

Met­al-poly­mer pipes are rel­a­tive­ly easy to bend. They are sold in coils (coils) 50–200 m long. Spe­cial col­let and crimp fit­tings are used to assem­ble a met­al-plas­tic pipeline. Cor­ner fit­tings are com­mer­cial­ly avail­able, but they are not nec­es­sary due to the flex­i­bil­i­ty of the tubes. For instal­la­tion, sim­ple tools are need­ed — for one-piece con­nec­tions press tongs, for detach­able open-end wrench­es.

Rice. 5 Assem­bly of the pipeline from met­al-plas­tic

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In terms of instal­la­tion com­plex­i­ty, met­al-plas­tic pipes sig­nif­i­cant­ly out­per­form polypropy­lene pipes.

Application

Polypropy­lene pipes are suit­able for installing a sew­er or ven­ti­la­tion sys­tem, but met­al-plas­tic is not.

PP pipes are not rec­om­mend­ed for instal­la­tion of floor heat­ing cir­cuits. There are sev­er­al rea­sons:

  • low coef­fi­cient of ther­mal con­duc­tiv­i­ty — more ener­gy will be need­ed for heat­ing;
  • ele­ments of the con­tour of the warm floor, usu­al­ly laid in the form of loops — PP pipes have to be cut and weld­ed at a right angle, there is a high risk of obtain­ing unre­li­able joints, it will be impos­si­ble to solve the prob­lem with leak­age under a con­crete screed;
  • it is also impos­si­ble to install expan­sion loops under the screed, which are nec­es­sary to elim­i­nate the prob­lems asso­ci­at­ed with elon­gat­ing the tubes.

The only plus of using PP pipes is cheap­ness. On the con­trary, met­al-plas­tic pipes are one of the best options for installing a “warm floor” sys­tem. It is only nec­es­sary to choose high-qual­i­ty prod­ucts from a reli­able man­u­fac­tur­er that are not sub­ject to delam­i­na­tion. In order not to be mis­tak­en, you can pur­chase a small piece of pipe and heat it up to 100 ° C.

Rice. 6 Under­floor heat­ing from met­al-plas­tic pipes

Price

A run­ning meter of a polypropy­lene pipe costs almost three times cheap­er than a met­al-plas­tic one. Pipelines from PP also win at the cost of instal­la­tion. The price of work on their instal­la­tion is from 150 to 5 dol­lars / lin­ear meter. m. Instal­la­tion of met­al-plas­tic costs from 5 dol­lars / lin­ear meter.

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