Significant cost savings should be expected over traditional pipe materials (steel, fiberglass, PVC) within the same design parameter. Savings accrue both on the initial product cost and by improved installation efficiency.
4.1 Ease of Handling and Installation
Polyethylene weighs less than one-fourth of comparable steel pipe. Significant savings can be realized in transportation, handling and installation.
Pipe diameters up to 6" IPS can be coiled for ease of handling and storing. Polyethylene® is quickly and easily joined and installed by using the heat fusion method or special polyethylene mechanical fittings. Heat fusion produces a solid, leak proof joint as strong as the pipe itself.
4.2 Water Hammer and Pressure Surge
The momentum, or inertia, of a moving column of liquid has inherent characteristics that must be dealt with in a piping system. When liquid flow is stopped suddenly, as by a quick closing valve, the inertia is converted into a shock wave or high-pressure surge. The faster the liquid velocity and the longer the line, the greater the shock loads. Controlled dissipation of this energy is a design feature and a significant advantage of polyethylene pipe.
4.4 Obstacles to Change
Journal articles and reports have some consensus as to why PVC has made such strong inroads in the water and sewer markets, and why HDPE, a viable alternative resin, has had difficulty in the market place.
According to an Environment Canada report, “In the case of HDPE, one reason for the low market share is the different marketing strategies initially employed for PVC and HDPE. Though HDPE has always been a competing plastic, with a longer history of use in pipe than PVC, the initial target markets for HDPE pipe suppliers were industrial settings, such as the chemical process industries, and the mining sector. In contrast, PVC pipe suppliers, who also sold ductile iron pipe, targeted municipal infrastructure pipe markets. As a result, municipal design engineers and contractors are more familiar working with PVC pipe, and seldom specify or design HDPE systems.” A trade journal article reporting on the difficulty in breaking into the market reports, “Even though polyethylene pipe has gained AWWA acceptance, U.S. water utilities and the engineers who design water systems have been slow to consider it as an alternative to the products they know and have used for years. Why risk trying something that – to the potable water industry- is new? (Underground Construction, June 2000).
One of the great barriers to changing the PVC water and sewer market is the resistance to change. Most engineers, contractors and public works officials have been working with the same materials for a long time, have become very familiar with their characteristics and are not anxious to take on a new material with different characteristics.
4.5 Heat Fused Joints
HDPE pipe can be heat fused together to form a joint that is as strong or stronger than the pipe itself and is leak free.
- This eliminates the potential leak points every 10-20 feet as found with PVC and Ductile Iron bell and spigot connections.
- The Life Cycle Cost of HDPE pipe differs from other pipe materials because the "allowable water leakage" is zero rather than typical leakage rates of 10 to 20% for PVC and Ductile Iron.
- HDPE pipe fused joints are self-restraining and costly thrust restraints or thrust blocks are not required.
- HDPE pipe's fused joints simply do not leak, eliminating infiltration and infiltration problems experienced with alternate pipe joints.
4.6 Flexible and Fatigue Resistant
- HDPE pipe can be bent to a radius 25 times the nominal pipe diameter (Example: 12" HDPE can be cold formed in the field to a 25ft radius). This can eliminate many fittings required for directional changes in a piping system where fittings and thrust blocks or restraints are required with alternate materials.
- The flexibility of HDPE pressure pipe makes it well suited for dynamic soils including areas prone to earthquake.
- HDPE pressure pipe can accept repetitive pressure surges that significantly exceed the static pressure rating of the pipe.
4.7 Construction Advantages
The combination of flexibility and leak free joints allow for unique and cost effective types of installation methods that the rigid PVC and Ductile Iron pipes cannot use with bell and spigot connections. These alternate installation methods (Horizontal Directional Drilling, Pipe Bursting, Slip lining, Plow and Plant, Submerged or Floating Pipe, etc.) can save considerable time and money in most potable water applications.
- Polyethylene pipe is produced in straight lengths up to 50 foot long and coiled in diameters up through 6". Coiled lengths over 1000ft are available depending on size providing low cost installations..
- Polyethylene is about one-eighth the density of steel, it does not require the use of heavy lifting equipment for installation.
4.8 Cost Effective, Long Term and Permanent
- Polyethylene pipe installations are cost effective and have long term cost advantages due to its physical properties, leak free joints and reduced maintenance costs.
- The polyethylene pipe industry estimates a service life for HDPE pipe to conservatively be 50-100 years. This relates to savings in replacement costs for generations to come.
4.9 Corrosion and Chemical Resistant
HDPE pipe will not corrode tuberculation or support biological growth.
- HDPE pipe has superb chemical resistance and is the material of choice in harsh chemical environments
- Many plastic pipes share the advantages of corrosion and chemical resistance over traditional metal pipes, but HDPE pipe uniquely combines these attributes with the aforementioned advantages of heat fused joints, flexibility and fatigue resistance.
- It is much easier to handle and install HDPE pipe vs. the heavier, rigid metallic or concrete pipe segments, allowing for huge cost advantages in the construction process.
- Polyethylene pipe is better able to structurally withstand an impact than PVC pipe, especially in cold weather installations when other pipes are more prone to cracks and breaks.Hazen Williams C Factor is 150 and doesn't change over time
- HDPE pipe has a smooth ID that does not corrode or tuberculation and maintains its flow capability over time.
- The C Factor of Ductile Iron pipe is dramatically reduced over time due to corrosion and/or tuberculation.
Engineering and Design Water and Sewer Layout
Though it may not be obvious, pipelines are designed to follow the natural topographic variation and mimic natural drainage flow to take advantage of gravity. It is important that pipelines not be too deep to make excavation and installation prohibitively expensive, but also that the pipeline not be so shallow that it is difficult to service the building.
4.11 Pressure Pipe
In pipes, fluids are transported under two conditions; pressure or gravity flow. These two methods are important as they have implications on pipe material selection and layout. In pressure pipe, fluids are moved through the pipe by a pump, and as a result are put under pressure. Water pipes are usually pressure systems, as lakes, rivers, reservoirs, and other bodies from where water is obtained are typically located at elevations below where the water needs to be delivered. Because water does not flow against gravity, water delivery to apartment buildings, and upper floors of houses needs to be pumped, and hence is under pressure. Because the fluid is under pressure, joints for pressure systems must be rather “strong” or tight, otherwise as the pressure wave hits the joint, the pipe will fall apart, or create unacceptable leaks. Most codes for pressure pipe have what is called an allowable leakage rate. This means that the code allows pipes to have a small degree of leakage. Pipes, which are seamless, such as HDPE, will have no leakage, even though it may be allowed by code. Though wastewater is typically designed for gravity flow, there are occasions where wastewater is also pumped. Usually this occurs where the topography is very flat, or where there is a hill along the pipe route.
4.12 Gravity Pipe
As implied, gravity flow uses gravity to transport the liquid. Wastewater is typically conveyed in a gravity system. A major concern with gravity systems is what is called Inflow and Infiltration (I & I) where water enters cracks or leaky joints. This is a concern for a variety of reasons. First, it can cause added expense to the wastewater plant because the plant is “forced” to treat excess water that does not require treatment. Associated with this excess dilute loading is poor wastewater treatment and unneeded treatment expense. Secondly, it can cause overflows at the wastewater plant or in the downstream conveyance system during rainstorms when pipe capacity is exceeded. This is one reason why during heavy rains wastewater plants may not function and that there are overflows. It is not necessarily because of poor system design but because breakdown of old pipe or poor construction have allowed for I&I. When a pipe collection system passes through an area of high water table, outside I&I can be problematic. The water table puts pressure on the gravity pipe, and it is continuously forced into the collection system. A
standard for sewage water
infiltration allowance is 200 gallons per inch diameter per mile per day. US