Sofia Martinez
Polyethylene (PE) plastic water lines have been used for drinking water supply since the 1950s. They are classified by their material and come in different types, such as PEX (cross-linked polyethylene), PVC (polyvinyl chloride), and LLDPE (linear low-density polyethylene). Polyethylene tubing is known for its excellent pressure and temperature ratings and resistance to environmental stress cracking. It is commonly used for potable water applications and is manufactured with FDA-approved materials. When choosing a polyethylene water line, it is important to consider the chemical compatibility, temperature rating, and pressure requirements for your specific use case.
| Characteristics | Values |
|---|---|
| Plastic type | Polyethylene (PE), Polyvinyl Chloride (PVC), Cross-Linked Polyethylene (PEX), Linear Low-Density Polyethylene (LLDPE) |
| Use | Drinking water supply, food and beverage applications, pneumatic compressed air applications |
| Advantages | Excellent pressure and temperature ratings, high durability, strength and burst resistance, flexibility, outstanding resistance to environmental stress cracking |
| Testing | Taste, odour, appearance of water, growth of aquatic microorganisms |
| Standards | NSF-approved for potable water, FDA-approved materials, NSF Standard 51 and 61 certified |
| Pipe size | 3/4", 1" CTS SDR-9, 1/2" to 1 1/2" (inner diameter) |
| Fittings | Lead-free bronze pack joint fittings, compression fittings |
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What You'll Learn
Polyethylene pipes are safe for drinking water
Polyethylene pipes have been used for drinking water supply since the 1950s. The plastic pipes are tested rigorously, and the industry has taken responsibility for ensuring that the products used do not adversely affect water quality. The tests undertaken on polyethylene (PE) pipes cover taste, odour, appearance of water, and the growth of aquatic microorganisms. This is a more extensive range of tests than those applied to traditional pipe materials, such as metals and cement.
PE pipes are lightweight, flexible, and easy to install. They are also highly resistant to corrosion and abrasion, which helps to reduce leaks in the system. This makes them a cost-effective and low-maintenance option for water distribution. PE pipes are manufactured with FDA-approved materials and are NSF Standard 51 and 61 certified. Linear low-density polyethylene (LLDPE) is the most common type of plastic water pipe and is perfect for potable water. LLDPE offers excellent pressure and temperature ratings and is highly resistant to environmental stress cracking.
However, there are some disadvantages to PE pipes. They are not as strong as pipes made from concrete or metal, so they may leak or break if exposed to high water pressure or ground movement. Some people also express concerns about the safety of all plastics, including the potential presence of BPA and estrogen-mimicking chemicals.
Overall, polyethylene pipes are considered safe for drinking water and are widely used for water distribution to homes and businesses. They offer several advantages over traditional pipe materials, including reduced leakage, low maintenance, and cost-effectiveness.
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Polyethylene vs PVC
Polyethylene and polyvinyl chloride (PVC) are both plastics that are formed through radical polymerization. However, they have distinct differences in terms of their reaction conditions, structures, properties, and applications.
Structures and Properties:
Polyethylene is a homopolymer, meaning it is made up of only one type of monomer unit, ethylene (CH2=CH2). It is a large chain of carbon atoms with hydrogen atoms attached and no other elements such as chlorine, oxygen, or any other elements. Polyethylene's properties depend on its type. Low-density polyethylene (LDPE) is softer and more malleable, while high-density polyethylene (HDPE) is sturdier. Ultra-high molecular weight polyethylene (UHMWPE) exhibits exceptional strength.
On the other hand, PVC consists of subunits of vinyl chloride, with each monomer having a pair of double-bonded carbons, one of which has a chlorine atom attached. PVC always has the same basic structure, and it is characterized by long chains of carbon atoms, with every other carbon atom having a chlorine atom attached. PVC is known for its tensile strength, making it sturdy and reliable. It is also highly resistant to weathering, including UV radiation, water, and extreme temperatures. Additionally, PVC is more fire-resistant than polyethylene due to the release of chlorine atoms during combustion, which inhibits the process.
Applications:
Polyethylene's versatility lends itself to a wide range of applications. LDPE is commonly used for plastic films, shopping bags, and marine and industrial wrap. HDPE is used for large containers, plastic milk jugs, and children's toys. UHMWPE is found in bulletproof vests and ice skating rinks. Polyethylene is also used in plumbing, with HDPE being rated for direct burial in water service lines.
PVC is one of the most popular plastics in plumbing and is used for pipes, window frames, and doors. It is also used for garden hoses, raincoats, vinyl leather bags, and tarpaulins (tarps). In its rigid form, PVC is favoured in construction for its durability and resistance to environmental elements. PVC is generally less expensive than other plastic materials, making it a popular choice for budget-conscious projects.
Comparison:
While both materials offer unique advantages, PVC stands out for its tensile strength, weather resistance, and affordability. Polyethylene, on the other hand, excels in flexibility, impact resistance, and chemical resistance. When choosing between the two, considerations such as the specific application, required strength, and exposure to environmental factors come into play.
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Linear low-density polyethylene (LLDPE)
LLDPE has a density of 0.916-0.940 g/cm3 and unique rheological or melt flow properties. It is less shear-sensitive than LDPE due to its narrower molecular weight distribution and shorter chain branching. LLDPE has a higher viscosity during extrusion, making it harder to process, but it does not strain harden when elongated like LDPE. Its lower shear sensitivity allows for faster stress relaxation during extrusion, and its physical properties are susceptible to changes in blow-up ratios. LLDPE also has a lower viscosity during melt extension at all strain rates.
LLDPE is the most common type of plastic water pipe and is perfect for potable water, food and beverage applications, and pneumatic compressed air applications. It offers excellent pressure and temperature ratings and is resistant to environmental stress cracking. LLDPE is more robust than traditional low or medium-density polyethylene and is recommended for use with cold and intermittent hot water. It is also flexible, protecting it from bumps, and is manufactured with FDA-approved materials.
LLDPE is non-reactive and can be stored without the risk of off-gassing. It does not have any adverse effects on human health when in direct contact with skin. However, inhalation of the raw material's dust can irritate the respiratory system. LLDPE is not environmentally friendly, as it is difficult to recycle and makes up a large percentage of environmental pollution. However, it can be recycled into new items such as industrial bins, composite lumber, flooring, and new plastic films.
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Cross-linked polyethylene (PEX)
Cross-linked polyethylene, commonly abbreviated as PEX, XPE, or XLPE, is a form of polyethylene with cross-links. It is predominantly used in building services pipework systems, hydronic radiant heating and cooling systems, and domestic water piping. PEX is used as an alternative to polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), or copper tubing for residential water pipes. PEX tubing is made of cross-linked polyethylene for extra strength and is suitable for hot or cold drinking water. The size of the tubing is typically larger in diameter, ranging from 1/2" to 1 1/2", and is based on the inner diameter (ID) or nominal/CTS (copper tube size).
PEX is an excellent choice for plumbing due to its high thermal stability and resistance to environmental stress cracking. It displays improved creep behaviour, maintaining high strength and hardness at very high temperatures compared to thermoplastic polyethylene. The cross-linking in the polymer structure enhances its chemical, thermal, and mechanical properties, making it ideal for hot and cold potable water delivery.
The process of cross-linking can occur during or after the extrusion of the tubing, and the degree of cross-linking is essential. According to ASTM standards, the peroxide linking process for XLPE should result in polymeric interlinks greater than 60% but less than 90% of the total PE resin. A higher degree of cross-linking may lead to material brittleness and stress cracking, while a lower degree may result in poorer physical properties.
PEX pipe is a popular choice for household plumbing, with the entire distribution system, from the service line to the faucet, constructed with PEX. It is also used in other applications, such as natural gas, offshore oil, chemical transportation, and sewage and slurry transportation.
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Rigorous testing of polyethylene pipes
Polyethylene (PE) pipes are widely used for water lines due to their durability, low cost, and excellent pressure and temperature ratings. However, rigorous testing is necessary to ensure their long-term safety and performance.
One common test method is the Cyclic Pennsylvania edge-notch tensile test (CPENT), which was developed to assess the integrity of in-service PE pipes. This method allows for direct testing of unaltered material sectioned from the pipe wall, providing results in a short timeframe. CPENT results are combined with oxidation induction time (OIT) data to determine the remaining life of pipes, which is influenced by their service location and time in use.
Another test method is the long-term hydrostatic strength test, which is used to predict the lifetime performance of pipelines. However, this method does not account for material aging, a critical factor in the failure of PE pipes. Other mechanical tests and performance models are also employed to study defect growth and predict long-term performance.
Ultrasonic testing is a specific inspection method for PE pipes, although its usage is restricted to welded sections. The challenge of developing adhesion-based repair techniques due to PE's low surface energy further emphasizes the importance of early defect detection.
In summary, rigorous testing of polyethylene pipes involves various methods to assess their integrity, predict performance, and detect defects. These tests are essential for ensuring the safe and efficient operation of PE pipes, which are commonly used for water lines and gas distribution networks.
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Frequently asked questions
Polyethylene is a common material for water pipes and tubing. It is used for drinking water supply and connects water filters, countertop faucets, ice machines, and under-sink water filters.
Polyethylene tubing offers excellent pressure and temperature ratings and has outstanding resistance to environmental stress cracking. It is also flexible, protecting it from bumps under the counter.
Polyethylene plastic water lines are typically used for potable water applications, such as drinking water supply and food and beverage applications.
There are two main types of polyethylene plastic water lines: High-Density Polyethylene (HDPE) and Linear Low-Density Polyethylene (LLDPE).
The size of the polyethylene plastic water line depends on the inner diameter (ID) of the tubing, also called nominal or CTS (copper tube size). The right size also depends on the application's chemical compatibility, temperature rating, and fluid pressure.
