The term PE3408 has become synonymous with tough, durable polyethylene piping systems. The evolution of this material designation dates back to the late 1970's and it's market introduction has provided a backdrop against which innovative installation techniques such as slip-lining, pipe bursting and directional drilling have become highly developed and widely commercialized. Taken together and combined with other market factors we now see PE3408 piping products used in demanding applications such as natural gas distribution, potable water, industrial and mining pipe, sewer force mains and other critical applications where a tough, ductile material is needed to assure long-term performance.
The expanding success of polyethylene piping systems has continued to be the focus of additional industrial research and development and today the industry is preparing to commercialize a new level of technical performance for these high quality piping products. These new piping materials will be referred to as PE4710 and they hold the potential for continued growth in the use of HDPE piping within the construction industry. In this discussion, we will briefly examine the nature of the improvement that has been made in these piping products and the overall impact that these products will have in the use of these modern piping systems.
In order to understand the significance of the PE4710 designation and the potential impact of its forth-coming commercialization, one must first develop an understanding of the PE3408 designation and its use in the design of polyethylene piping systems. The term PE3408 is based on the standard thermoplastics pipe material designation code defined in ASTM F412 and has been referenced extensively within the North American piping industry since the early 1980's.(2)
The standard thermoplastic code consists of an abbreviation for the type of plastic followed by Arabic numerals, which describe the short-term properties for that material in accordance with applicable ASTM standards. The final two numerals in the term refer to the hydrostatic design stress (HDS) for that material in units of 100 psi with any decimal figures dropped.
|PE||PE designation refers to polyethylene|
|3||Density cell class 3 per D3350, 0.941 - 0.947 gr/cc|
|4||SCG cell class 4 per D3350, PENT value > 100 Hours|
|08||800 psi hydrostatic design stress for water at 73° F|
As shown in Figure II, the term PE3408 clearly identifies the piping product as a polyethylene grade P34 with a density cell class of 3 in accordance with D3350, a slow crack growth cell class of 4 also in accordance with D3350, and an 800 psi maximum hydrostatic design stress at 23°C (73°F) as recommended by the Plastics Pipe Institute.(3)
In the mid-1080's, extensive research was undertaken on an international scale to further improve on the recognized performance capabilities of the then-current polyethylene piping systems. The culmination of this research was the introduction of a new level of performance in polyethylene pipe which was designated as PE100 in accordance with the ISO standards systems prevalent throughout the world outside of North America. These piping materials were characterized by significant improvements in resistance to slow crack growth, outstanding resistance to rapid crack propagation and a higher degree of linearity to the regression analysis by which the stress ratings of these materials are established.(4)
Piping materials with the same performance properties as the ISO-recognized PE100 will soon be commercialized within the North American market with a standard thermoplastics piping material designation code of PE4710. As shown in Figure III, this piping material designation provides a basis for recognition and identification of the higher performance capability of these new piping products.
|PE||PE designation refers to polyethylene|
|4||Density cell class 4 per D3350, 0.948 - 0.955 gr/cc|
|7||SCG cell class 7 per D3350, PENT value > 500 Hours|
|10||1000 psi hydrostatic design stress for water at 73° F|
These new polyethylene piping products bring with them some clear improvements in performance capability. Specifically, these are: a) improved slow crack growth resistance, b) improvement resistance to rapid crack propagation, and c) improved long-term hydrostatic strength.
Slow crack growth (SCG) is a principle mode of failure for most plastics products in service. It relates to the concentration of stress around or in close proximity to regions of inhomogeneity in the polymer matrix from which the product is produced. These inhomogeneities may include such things as extremely sharp or deep gouges resulting from job site handling and/or installation, certain angular configurations associated with molding or joining of plastics parts or irregularities within the molded or extruded parts such as occlusions, contaminants or concentrations of pigments.
Historically, the PE3408 quality of polyethylene piping has provided outstanding resistance to failure due to slow crack growth. However, the new PE4710 piping products are produced from a unique molecular configuration which affords a step-change enhancement in resistance to slow crack growth resistance. See Figure V.(5) This step-change enhancement in the slow crack resistance of the PE4710 piping products is the basis for the higher 500 hour PENT test requirement as opposed to the 100-hour level of performance typically associated with the PE3408 type of polyethylene piping products.
To the installer or the owner, these improvements in slow crack growth resistance provide a higher degree of confidence in the long-term serviceability of these new PE4710 piping products.
Most engineering materials are susceptible to failure by rapid crack propagation or RCP. RCP refers to the formation of a rapidly accelerating crack that results when a material placed in service is exposed to a sudden and rapid increase in stress such as that typically associated with an impact. Typical examples of impact might include a strike by a back-hoe or construction equipment or impact from a fallen boulder or other object. The susceptibility of plastics materials to this type of failure mode is generally antagonized by very low service temperatures. It should be noted that in these types of failures in HDPE piping systems are extremely rare. Design practices, applications standards and installation guidelines are in place throughout the North American industry that minimize the potential for failure due to RCP. (6) To this end, PE3408 piping products in use today provide a substantial level of resistance to this type of failure mode when designed, installed and operated in accordance with these industry-established standards and specifications.
Notwithstanding the capabilities of the current PE3408 polyethylene piping products, the technological improvements of the new PE4710 products provide an even higher level of resistance to RCP. In either full-scale or S4 type of testing, these new piping products bring a new level of resistance to this type of failure phenomena. See Figures VI. To the end-user of polyethylene piping systems this means another level of confidence that the long-term performance of these systems will remain unaffected by sudden changes in the operation of the piping system, whether that be unforeseen impact or sudden decrease in the service temperature.
Since the mid 1950's, the long-term hydrostatics strength of plastics piping has been determined on the basis of stress-regression analysis of pipe specimens placed on hydrostatic test. The test protocol has been standardized in the North American market place in the form of ASTM D2837 and within the international community with the form of ISO 9080. (7),(8) In both standardized test methods, pipe specimens are placed on hydrostatic test in a controlled environment under specific conditions of stress and temperature. The specimens are monitored and data is gathered as pipe specimens fail over time. The data generated is then analyzed using the regression analysis algorithm inherent to each of these standards.
Within North America, the long-term hydrostatic strength (LTHS) determined through this analysis is then categorized into one of a series of hydrostatic design basis or HDB. The HDB is then reduced by a service factor for the specific type of application to obtain a hydrostatic design stress (HDS) which is then used to pressure rate the pipe profile. In the case of PE3408 piping products, the HDB for these products is typically 1600 psi. For water related applications intended to operate at 73°F, the HDB is then multiplied by a service factor of 0.50 to establish a hydrostatic design stress of 800 psi; hence, the "8" in PE3408. A standardized mathematical relationship referred to as the ISO equation is then utilized to establish a recommended pressure rating for a specific pipe profile such as an DR 11.
|S = 2 x HDB x DF (DR-1)|
|Where:||S = Internal Pressure or Hoop Stress, psi
HDB = Hydrostatic Design Basis, psi
DF = Design Factor
0.50 for PE3408 with water at 73°F
DR = Dimension Ratio
A task group operating under the jurisdiction of the Hydrostatic Stress Board of the Plastics Pipe Institute (PPI) conducted extensive studies on the nature of the regression analyses characteristic of the PE3408 and PE4710 piping materials. They determined that the stress rupture curves generally associated with the PE4710 type of piping materials provided a higher degree of linearity over time at standardized testing temperatures and a higher degree of correlation with the resulting regression projection for the long-term strength of these materials.
These findings regarding the linearity of the stress-rupture curves for the new PE4710 materials combined with significant improvements in the technical performance (resistance to SCG and RCP) of the PE4710 materials as compared to the PE3408, have provided the industry with a basis for modifying the service factor for the use of these newer materials in water-related applications at service conditions of 73° F. Historically, the North American plastics piping industry has applied a service factor of 0.50 to the PE3408 piping products whereas the PE4710 will utilize a service factor of 0.63. The net result of this advancement is shown in Table I below.
|Hydrostatic Design Basis, HDB (psi)||1600||1600|
|Hydrostatic Design Stress, HDS (psi)||800||1000|
|Pressure Rating for DR 11 pipe (psi)||160||200|
The introduction of the new PE4710 polyethylene piping products holds tremendous potential for the continued expansion in the use of HDPE piping systems. As a direct result of the unique molecular structure of the polyethylene resins from which these piping products are made, we can identify specific benefits to be realized by the designer, installer or the owner of the piping system. Briefly, these are:
High density polyethylene pipe has proven to be a highly successful construction material for modern piping installations. Since its inception in the mid 1950's polyethylene has been utilized for a broad array of piping applications such as pressure-rated potable water distribution, natural gas distribution, force main applications, oil and gas collection, marine outfalls and many others. The expanding use of HDPE pipe has been the result of continual research and improvement in resin, pipe and installation technologies. Through these sustained efforts the role of HDPE pipe as a tough, durable piping product offering distinct advantages for various piping installations has continued to grow.
Recent research and development has culminated in a new level of technical performance for polyethylene which is now being introduced to the North American market place under the material designation PE4710. Originally commercialized as PE100 within the European HDPE pipe market, these new piping products are now being introduced to the North American market as PE4710 products offering unique technical and operational benefits. The recognized improvement in resistance to slow crack growth and rapid crack propagation combined with the higher design factor afforded these new products positions HDPE piping products for an even larger role within the pipeline construction and operation industries.
McElroy PR and Marketing Department