Comparing Polyethylene Pipe Material Designations

For engineers who work internationally, it is especially important to know unit conversions and the differences between the basics like Celsius and Fahrenheit. In the field of engineered plastics, and polyethylene pipe specifically, there are a few material designations worth memorizing. Much like their Imperial and Metric counterparts, some of these material designations are more common in the regions where North American standards are utilized than they are in Europe or those regions where alternative standards are used. From a global perspective, pipe grade polyethylene materials are testing and classified in accordance with two widely recognized standards systems, ASTM International (ASTM) and the International Organization for Standardization (ISO). The pervasive and sustained use of these two standards systems has culminated in two predominant polyethylene pipe material designations that are widely used around the world. These are: PE4710 within those jurisdictions recognizing the ASTM standards systems and PE100 within those utilizing the ISO systems of standards. Knowing the similarities and differences between these designations is essential for proper product specification, procurement, and use.

Converting the information behind these designations is a bit more complicated than converting Celsius to Fahrenheit. The information that follows is provided as a refresher on these globally recognized polyethylene pipe material designations in an attempt to assist engineers or designers in learning about the similarities and differences between PE 100 and PE4710.

ISO versus ASTM methods of standardization

First, it’s important to understand that PE 100 is a polyethylene pipe grade material designation within ISO that defines a different set of criteria when compared with PE4710, which is an ASTM pipe material designation. PE 100 defines the product as being made with polyethylene (PE) that maintains a minimum required strength (MRS) rating of 10 MPa (multiplied by 10) over a 50-year period (statistically projected) at 20 degrees Celsius. This ISO designation does not qualify other product properties like density, slow crack growth (SCG) or rapid crack resistance (RCR). Therefore, one may  find PE 100 pipe materials that have other technical or physical properties with significant differences. Some PE 100 possess superior SCG or RCP. To really understand the differences between competing PE 100 products, it is essential that the designer carefully review each product’s specification or technical data sheet

Unlike the ISO designation, ASTM classifies the PE pipe grade material on the basis of density, slow crack growth resistance, and hydrostatic design stress (HDS). The last three parts are all measured according to specific ASTM testing conditions. PE4710, for example, means that the product is made of polyethylene with a “4” rated density, “7” rated slow crack growth class, and has a 1,000-psi maximum recommended HDS with a 0.63 design factor at 23 degrees Celsius in accordance with ASTM D3350 and the stress rating policies in the Plastics Pipe Institute’s PPI-TR-3.Pipe grade polyethylene materials with the PE4710 designation describe a pipe grade material that meets exceptionally high performance requirements.

While it is true that some PE 100 may be produced of sufficient quality and tested to meet the requirements of PE4710, all of today’s commercially available PE4710 meet the ISO requirements for PE 100 designation.

Different designations and differing capabilities

While MRS and HDS are similar means of categorizing long-term hydrostatic strength ratings, there are several significant differences that should be kept in mind. First, MRS is measured using ISO 9080 and 12162, which sets testing conditions at 20 degrees Celsius. Meanwhile, HDS is measured using ASTM D2837, and is tested at 23 degrees Celsius.

Additionally, the manner in which these two terms, HDB and MRS, are used to determine a pressure rating for specific pipe profile differ as well. Consider the following comparison of the pressure rating equations under the ASTM and ISO standards systems.

ASTM Pressure Rating Equation:

PR = (2 x HDB x DF)/ (DR-1)

Where:            

• PR = Pressure rating for water in psig at 73⁰ F (23⁰ C)
• HDB = Hydrostatic design basis in psig at 73⁰ F (23⁰ C)
• DF = Design factor for PE4710 in accordance with PPI TR-3 = 0.63
• DR = Dimension ratio (or SDR) = Nominal OD/minimum wall thickness

ISO Pressure Rating Equation

PN = 20 x MRS/ (C_min x (DR-1))

Where:            

• PN = Nominal pressure rating for water in bar at 20⁰C
• MRS = Minimum required strength for water in bar at 20⁰C
• C_min = Design coefficient for water in accordance with ISO 12162 = 1.25
• DR = Dimension ratio (or SDR) = Nominal OD/minimum wall thickness

As has been indicated, these equations differ by their use of the two terms, HDB and MRS, and the temperatures at which these are determined. However, they also differ in another very important respect.  The ASTM methodology applies one design factor, DF, for water that is intended to consider all aspects of the use of this material in piping applications. This includes variability in laboratory testing and variability in construction or operation. The design coefficient, C_min, of ISO is not intended to address variability in construction or operation in the field. ISO encourages the application of additional service factors where construction or operational factors warrant.

As can be seen from Table 1, the actual pressure rating calculation at their respective standard temperatures yield different results. This is a characteristic of the differences in the way the two predominant standards systems (ASTM and ISO) determine the long-term hydrostatic strength of the PE material and the manner in which the categorized strengths (HDB or MRS) in each system are applied to determine the pressure rating for a specific PE pipe.

Table 1. Comparative Polyethylene Pipe Pressure Ratings (PR) for PE4710 and PE 100 in Water Services

Notes:

• (I) Calculated in accordance with ASTM F714 and ASTM D3035
• (II) Calculated in accordance with ISO 4427

The ASTM method results in a more conservative pressure rating for the same pipe dimension ratio (DR) as compared to the ISO methodology as shown in Table 1. The ~15% difference between the pressure ratings for PE4710 as compared to the pressure rating for PE100 pipe DR’s is associated with essentially two points:

1. The different temperatures at which the two methodologies stress rate the polyethylene pipe material (23⁰C versus 20⁰C), and
2. The ISO design coefficient (1.25) used in the pressure rating equation does not take into consideration variability in construction or field operation while the ASTM design factor (0.63) used in pressure rating equation is intended to give consideration to these two variables.

There are additional factors within the two respective stress-rating protocol that also contribute to the differences in pressure ratings. For additional information on these factors the reader is referred to the references cited.