Is thermal expansion and contraction of HDPE pipe a problem?
No. All pipes expand and contract with change in temperature. The key is management of the resultant thermal strain. As with all materials, expansion and contraction must be taken into consideration when designing a HDPE piping system. Buried pipelines usually do not move due to soil friction. However, thermal effects must be considered for above grade applications. The unrestrained coefficient of thermal expansion for HDPE pipe is approximately 9×10-5 in/in/o F. Information regarding thermal calculations for restrained and unrestrained above-ground and slip-lined pipelines can be found in PPI’s Handbook of Polyethylene Pipe, 2 nd ed.drinking water service.
After HDPE has been buried and allowed to relax, will the pipe continue to expand and contract a great amount with temperature variations?
No. When HDPE pipe is buried, the temperature of the system becomes much more stable than an above ground pipeline and therefore will exhibit far less dimensional change. In most systems, buried HDPE pipe does not move after it is buried.
Why is there a difference in pressure rating (PR) and working pressure rating (WPR) when comparing HDPE pipe and PVC pipe?
The term pressure rating (PR) refers to the static pressure rating of the pipe, calculated from the hydrostatic design basis (HDB) with an appropriate design factor (DF) and is for a pipeline with no flow. However, all municipal water systems involve flowing water. For example, HDPE (PE4710) DR 17 pipe has a static pressure rating for water of 125 psig. The working pressure rating (WPR) is based on actual system requirements and is a dynamic pressure rating, that is, a pressure rating for pipe with flowing water. The WPR includes an allowance for water hammer surge pressures. At a daily recurring average flow surge velocity of 5 fps and at 80o F, the working pressure rating of HDPE (PE4710) DR 17 pipe is also125 psig; similarly, the working pressure rating for PVC DR 18 is 120 psig per AWWA C900-07, Equation 4. As such, PE 4710 has a higher working pressure rating than PVC at these common conditions Also, based on AWWA C900-07, Example B.2 modified with 5 fps recurring surge velocity, the estimated number of cycles to failure for the DR18 PVC pipe is less than 1 million cycles and the Fatigue Life is about 20 years which is less than half of the 50 year Design Life that was required in the example; in addition, the assumed 55 cycles per day may not be adequate; assuming 1 surge cycle every 15 minutes (96 cycles per day) and the 5 fps recurring surge velocity results in a Fatigue Life of about 11 years. On the other hand, IGN 4-37-02, “Design Against Surge and Fatigue Conditions for Thermoplastic Pipes”, can be used to show that under the same conditions, an HDPE DR17 pipe has a fatigue life of 10,000,000 cycles or in excess of 100 years. This striking difference is due to HDPE’s toughness and fatigue resistance.
Where can I find engineering properties such as the modulus and tensile strength values for HDPE pipes?
Engineering data for HDPE pipes may be found in Chapter 3 of the PPI’s Handbook* of Polyethylene Pipe, 2nd ed.
Do I have to be concerned with the long-term effects of creep?
All plastic materials, including HDPE and PVC, are subject to creep. Proper design, such as using the long-term modulus of the material where appropriate, accounts for creep effects.
Is there a flow loss in HDPE pipe due to the inner bead resulting from the butt fusion process?
The fusion bead has very little effect on the flow as it is basically rounded and protrudes very little on the inside surface of the pipe Secondly, the Hazen-Williams C-factor of 150 takes into account the inner bead. Field tests confirm that a 150 C- factor used in the Hazen- Williams equation properly calculates actual flow and that the bead is of no hydraulic significance for either pressure or flow. The Hazen-Williams Friction Factor, C, for PE pipe was determined in a hydraulics laboratory using heat fusion joined lengths of pipe with the inner bead present.
Will sunlight adversely affect HDPE pipe?
Sunlight is not a concern if the black pipe is used. Carbon black, utilised in most all HDPE pipe is the most effective ultraviolet stabiliser and therefore, black is the recommended pipe colour for exposed long term service or storage. Pipe of this colour will provide decades of outdoor use similar to that of black power-line cable jacketing. HDPE pipe produced in non-black colours may also be supplied for outdoor exposure (storage and use) but its life expectancy is much less and is usually specified for a particular time period. Questions on this topic should be referred to the pipe manufacturer.
What is the life expectancy of HDPE pipe in water applications?
Many installations of HDPE pipe in water applications are already reaching 50 years of successful service. 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.
Will HDPE pipe float in water?
Yes, HDPE pipe, due to its density being slightly less than water, will float even when full of water. When it is desired to ensure flotation of the line, various forms of collars, saddles, and strap-on flotation devices are available. For underwater anchored pipeline installations, it is important to specify the proper weights and spacing of the weights. Screw-anchors are a practical alternative. Whenever possible, an underwater pipeline should be installed in a trench with protective crushed rock cover. Refer to Chapter 10 of the PPI Handbook of Polyethylene Pipe, 2nd ed.
What is the max-min temperature range across which HDPE pipe for water pressure applications may be used?
HDPE pipe’s typical operating temperature range is from -40o F (-400C) to 140o F (60oC) although some products may be pressure rated for service as high as 180o F (82oC). Since water freezes below 32o F (00C) the practical lower temperature limit for water is 32o F (0oC). Consult with the pipe producer for information on applications.
When using HDPE pipe, will the pipe deliver the same flow rate as the modestly larger ID ductile-iron pipe with the same outside diameter?
The inside surface of HDPE pipe is devoid of any 6 roughness which places it in the “smooth pipe” category, a category that results in the lowest resistance to fluid flow. For water applications, HDPE pipe’s Hazen and Williams C factor for design is 150 and does not change over time. In contrast, the C factor for iron pipe and other traditional piping products declines dramatically over time due to corrosion and tuberculation or biological build-–up. In view of these advantages, it is often possible to utilize HDPE pipe of smaller inside diameter than Ductile Iron pipe, and still achieve or exceed the project’s required flow parameters. A detailed examination of the flow computations is encouraged. For flow factors and hydraulic design equations refer to Chapter 6 of PPI’s Handbook of Polyethylene Pipe, 2nd ed.
What is the maximum water pressure rating for HDPE pipe?
The maximum rating depends on several factors, the material designation code from which the pipe is made, the DR of the pipe, and the design operating temperature of the application.
What is the safe peak pressure (surge plus pumping) for HDPE pipe?
AWWA C901 defines two types of surge pressure, recurring and occasional. The safe peak pressure or allowed total pressure for HDPE pipe is 1.5 times the pipe’s pressure rating for recurring surge, and 2.0 times the pipe’s pressure rating for occasional surge. For instance a DR 11 PE 4710 has a pressure rating of 200 psig at 80o F and can safely handle total pressure during recurring surge of 300 psig and total pressure during an occasional surge of 400 psig. Refer to the answer under Q. 22 for additional data.
What is the maximum flow velocity for HDPE Pipe?
In a pumped system the maximum operating velocity is limited by the surge pressure capacity of the pipe. The Plastics Pipe Institute’s Handbook of Polyethylene Pipe states that “if surge is not a consideration, water flow velocities exceeding 25 feet per second may be acceptable.”
How does surge pressure in HDPE pipe compare with DI or PVC pipe?
Surge pressures in HDPE pipe are significantly lower than in DI pipe and lower than PVC pipe due to the lower value of dynamic modulus for HDPE. For example, a velocity change of 5 fps would cause a 51 psig surge in 7 HDPE DR 17 pipe, a 87 psig surge in PVC DR 18 pipe, and a 262 psig surge in DI Class 350 lined pipe. Lower surge pressures often means longer life for pumps and valves in an HDPE pipeline, as well as lower pressure class pipes.
How does HDPE pipe’s capacity for recurring surge pressures (fatigue) compare to other pipes?
HDPE has exceptional capacity for handling recurring surge pressures. For example, in AWWA standards recurring surge pressure must be subtracted from PVC pipe’s Pressure Class whereas PE has resistance up to 150% of its Pressure Class. Marshall and Brogden report on the cyclical fatigue strength of PVC and HDPE and their report shows, at a cyclical stress range of 10 MPa (1450 psi) some PVC pipes failed at approximately 400,000 cycles whereas HDPE pipe reaches 10,000,000 million cycles before failure.
What are the safe maximum and minimum burial depths for HDPE pipe?
Safe burial depths vary and should be calculated. In lieu of calculations, AWWA states that for an embedment soil with an E’ of 1000 psi and no surface water, HDPE pipes with DR’s ranging from 7.3 to 21 can be safely buried from a depth of 2 ft to 25 ft where no traffic load is present and from 3 ft to 25 ft where H20 live load is present. However, most HDPE pipes can be buried to deeper depths, e.g. HDPE leachate collection pipe in landfills often have cover depths in excess of a hundred feet. Equations for calculating burial depth can be found in Chapter 6 of PPI‘s Handbook of Polyethylene Pipe, 2nd ed.
Is HDPE pipe suitable for use under railroads?
While HDPE pipe is structurally capable of direct burial under railroads it is not recommended under a railroad mainline unless it is encased for safety reasons. In fact, many if not most railroad specifications require that pressurized pipes located beneath rail lines must be encased, and this requirement applies regardless of pipe material. However, uncased direct burial may be considered for use under rail lines located within plant yards.
Are thrust blocks required with HDPE pipelines?
No. HDPE pipe and fittings joined by heat fusion are self-restrained in all applications, and therefore do not require thrust blocks, provided the entire system is fused. Thrust blocks may be required in cases where special gasketed mechanical fittings are used. This may be necessary to prevent separation of the gasketed joint just as it is required for gasketed PVC and ductile iron pipe in pressure applications. Detailed discussion of this topic can be found in Chapter 9 of PPI’s Handbook of Polyethylene Pipe, 2nd ed. Publications available from the pipe manufacturer may also cover this subject.
Can pipe-bursting technology be used on ductile-iron and cast-iron pipes?
Yes, pipe bursting has proven to be a very valuable means of pipeline rehabilitation. The pipeline rehabilitation industry has adopted “pipe bursting” as the name of the process of rupturing brittle pipes (e.g., cast iron, clay, etc). A variation employed for ductile materials (i.e., ductile iron, steel, etc.), is called “pipe splitting”, which uses cutters to achieve enlargement of the host pipe. Preparation of the host pipe to receive the HDPE pipe is accomplished with a cone shaped head in both bursting and splitting processes. Refer to PPI Handbook of Polyethylene Pipe, 2nd ed., Chp 16.
How does the impact strength of HDPE compare with other pipes?
HDPE is a ductile material and has exceptional impact strength. HDPE’s superior impact strength provides a piping system that is near impervious to impact damage and to damage from improper tapping. In the real world, engineers understand that pipes must be tough and resist impact and handling damage. HDPE pipes are field tested and proven to be impact tough.
Will the presence of hydrocarbons in soils in which HDPE pipe is buried affect the pipe or permeate through its wall into the water stream?
AWWA has addressed concerns regarding hydrocarbon permeation by including a permeation statement in all of its pipe standards including standards for polyethylene (PE) (C901-08 p. 6, C906-07 p. 6), polyvinyl chloride (PVC) (C900-07 p. 6, C905-97 p. 3), steel (C200-05, p. 7), ductile Iron (C110-03, p. 3), and others. Hydrocarbons do not degrade polyethylene but can diffuse through the wall of HDPE pipe in areas of gross contamination. The 8 exterior contact may affect saddle fusion connections, thus, after HDPE pipes have been exposed to grossly contaminated soils, mechanical connections may be preferred. In addition, while measures need to be taken to limit the impact of hydrocarbon permeation, the vast majority of HDPE water pipe installations will never be impacted by this problem.
If you haven’t seen one and want to, let me know on +61 (0)3 89753888, rob@worldpoly.com, or +61 (0)418356959.
Need help with poly welding time and pressure calculations? Download our butt welding app!
