PPI TN-28-2014 Guide to Differences in Pressure Rating PE Water Pipe using the ASTM PPI and ISO Methods.pdf

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1、 Guide to Differences in Pressure Rating PE Water Pipe using the ASTM/PPI and ISO Methods TN-28/2014 Foreword This technical note was developed and published with the technical help and financial support of the members of the PPI (Plastics Pipe Institute, Inc.). The members have shown their interest

2、 in quality products by assisting independent standards-making and user organizations in the development of standards, and also by developing reports on an industry-wide basis to help engineers, code officials, specifying groups, and users. The purpose of this technical note is to provide important

3、information available to PPI on describing the differences in the calculated design pressure for water piping applications using the ASTM/PPI and ISO pressure-rating methods. These descriptions are based on discussions with several internationally recognized technical experts in the plastic pipe ind

4、ustry. More detailed information on its purpose and use is provided in the document itself. This note has been prepared by PPI as a service of the industry. The information in this report is offered in good faith and believed to be accurate at the time of its preparation, but is offered “as is” with

5、out any express or implied warranty, including WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Any reference to or testing of a particular proprietary product should not be construed as an endorsement by PPI, which does not endorse the proprietary products or processes of any man

6、ufacturer. Industry members offer the information in this note for consideration in fulfilling their own compliance responsibilities. PPI assumes no responsibility for compliance with applicable laws and regulations. PPI intends to revise this note from time to time, in response to comments and sugg

7、estions from users of the note. Please send suggestions for improvements to the address below. Information on other publications can be obtained by contacting PPI directly or visiting the web site. The initial publication of this note was in 2003 with a first revision in 2013. The Plastics Pipe Inst

8、itute, Inc. www.plasticpipe.org March 2014 Copyright 2014 Plastic Pipe Institute, Inc. 2 GUIDE TO DIFFERENCES IN PRESSURE RATING PE WATER PIPE BETWEEN THE ASTM/PPI AND ISO METHODS 1.0 Scope and Purpose This technical note illustrates differences between the North American ASTM/PPI and ISO methods fo

9、r pressure rating polyethylene (PE) water distribution pipe. This note focuses specifically on pressure rating of PE water pipe because different recommended operating pressures are obtained for the same polyethylene pipe depending on which method is utilized. This technical note provides general in

10、formation, background and guidance for comparing ASTM/PPI and ISO-based pressure-rating methods for polyethylene water pipe. It is not an in depth technical comparison of ASTM/PPI and ISO long term stress rating methods. Other documents such as PPI Technical Note 7 (TN-7) and Technical Report 9 (TR-

11、9) and various published technical papers serve this purpose. This note focuses specifically on how the long-term stress ratings determined under the ASTM/PPI and the ISO systems are applied to determine pressure ratings for a polyethylene pressure pipe using these two widely recognized industry pro

12、tocols. 2.0 Introduction The North American thermoplastic pipe compound rating methodology uses ASTM standards and PPI policies and procedures to provide design stress ratings that are used to determine polyethylene pipe pressure ratings for various end use applications such as water distribution an

13、d transmission. A similar but different rating method, based on ISO standards, is used in other parts of the world. Potential for confusion exists within the engineering community when these two rating methods are compared because the same polyethylene pipe can appear to have different pressure rati

14、ngs by one method or the other. Intuitively, the same pipe installed in the same way, operated under the same conditions, would be expected to have the same pressure rating and, hence, the same service lifetime regardless of the rating method used. In the case of PE water pipe, the pressure rating a

15、ssociated with a specific size and DR can appear to be quite different depending on which standards system is used to rate it. This technical note helps to explain the perceived and real differences between these two rating methods. 3 3.0 Background The ASTM/PPI and ISO methods for pressure rating p

16、olyethylene water piping developed separately in North America and Europe over nearly half a century. Both methods are technically correct, but when viewed on the surface appear to produce different pressure ratings for the same pipe. Around 1980, higher strength high density polyethylene materials

17、were introduced in North America and Europe. Under the ASTM/PPI system, these HDPE materials were characterized as PE 3408, and under the ISO system these materials were first characterized as PE 80 and later in 1988 as PE 100 as a result of further improvements in these materials. These nomenclatur

18、es for HDPE compounds are similar, but reflect differences in the measurement systems (inch-pound vs. metric) and the stress rating methodologies. In both nomenclatures, PE is the abbreviation for polyethylene. In PE 3408, the 34 indicates cell classification values for density and slow crack growth

19、 in accordance with ASTM D3350, which is a standard for identifying PE piping compounds in North America. The 08 is the materials hydrostatic design stress, HDS, for water at 73F in hundreds of psi with tens and units dropped; that is 08 = 800 psi. HDS is the materials hydrostatic design basis, HDB,

20、 per ASTM D2837 and PPI TR-3 at 73F multiplied by a design factor, DF for water at 73F. The HDB is the categorized intercept of the average long term hydrostatic strength at 100,000 hours (11 years). For PE 3408, the ASTM D2837 HDB is 1600 psi and the PPI TR-3 DF is 0.50, which yields a HDS of 800 p

21、si for water at 73F. For PE 100, the 100 represents a 10 MPa or 100 bar minimum required strength, MRS, at 20C in accordance with ISO 9080. MRS is the categorized intercept of the lower predictive limit (LPL) of the materials long term hydrostatic strength at 438,300 hours (50 years) at 20C (68F) pe

22、r ISO 12161. HDB per ASTM D2837 and MRS per ISO 9080/ISO 12162 are both predicted long- term strength ratings for the PE compound, but are different points at different temperatures and different extrapolated times. The matter is somewhat further complicated as requirements and properties for pressu

23、re rated PE compounds may overlap and may be the same or different under the two systems. For example, currently, some PE 4710 compounds can be rated as PE 100 or PE 80 under the ISO protocol, and some PE 100 compounds can be rated as either PE 4710 or PE 3408 when evaluated within the context of th

24、e ASTM/PPI protocol. Listings of recommended long-term hydrostatic strengths for PE materials that are rated under the ASTM/PPI and ISO systems are published in PPI TR-4, which is available from www.plasticpipe.org.11Note - As of this writing, commercial factors have resulted in the replacement of P

25、E 3408 compounds by designated as PE 3608 or PE 4710. PE 3408 is used in this discussion because updates to some North American water standards have not yet been completed. Although not reflected in ISO standards, the PE 100 Association identifies PE 100+ as PE 100 with enhanced performance properti

26、es. 4 4.0 The ASTM/PPI Method The ASTM/PPI pressure rating method was the first of its kind and was initially published in 1962. It uses a categorized long-term hydrostatic strength value, the Hydrostatic Design Basis (HDB), that is reduced by a design factor (DF) to determine an allowable stress (h

27、ydrostatic design stress, HDS) for water pipe pressure rating. The HDB is developed by extensive hydrostatic testing of PE pipe samples and mathematical analysis of the stress-rupture data in accordance with ASTM D2837 and the policies of PPI TR-3. The Hydrostatic Stress Board (HSB), an independent

28、assembly of plastics experts within the Plastics Pipe Institute (PPI), reviews data submitted by manufacturers, and then recommends HDB, DF and HDS values for the material to the manufacturer. PPI publishes recommended HDB and HDS values in PPI TR-4, which is available at www.plasticpipe.org. The ca

29、tegorization of HDB values in accordance with ASTM D2837 is replicated here as Table 1 for ease of reference. Table 1: Hydrostatic Design Basis Categories per ASTM D2837 Range of Calculated LTHS Values Hydrostatic Design Basis (HDB) psi (MPa) psi (MPa) 190 1) to derate the pipe as opposed to multipl

30、ying the design stress by a design factor (1) as is done in the ASTM/PPI method. The net effect is the same, to reduce the maximum design stress of the pipe. Other ISO application product standards may apply higher or 10 additional design coefficients to further reduce the design pressure based on o

31、ther application specific variables. Note: A significant difference is that this ISO recommended design coefficient only takes into consideration the variations due to extrusion and processing and static water pressure. When using the ISO method, it is the responsibility of the design engineer or go

32、verning authority to determine the actual application conditions and apply additional design coefficients required for the specific application. As defined in ISO 12162, the MRS is the categorized value of the LPL (lower predictive level) at 20C (68F) and 50 years. For any other temperature or any o

33、ther time, ISO 12162 defines the categorized value of that LPL as the CRS (categorized required strength). This is the methodology used by the ISO system to account for the effect of temperature or the effect of time on the LPL value. The pressure rating at the desired temperature or desired time is

34、 obtained by using the Equation 2 as above, except that the CRS value is substituted for the MRS value. This methodology is very accurate as it uses the same 90 data points for ISO 9080 to calculate the CRS that were used to calculate the MRS. To account for different temperatures, one could also us

35、e temperature design factors as an alternative to using the CRS. However, just as using the elevated temperature HDB is more accurate than a temperature design factor within the ASTM/PPI methodology, using the CRS is considered more accurate than a temperature design factor within the ISO methodolog

36、y. 6.0 Conclusions The above discussion and examples illustrate the differences between ASTM/PPI and ISO pressure-rating methodologies. This difference in pressure rating is attributed, in large measure, to the ASTM/PPI methods more conservative design factor that provides for normally anticipated p

37、ipe manufacturing, handling and installation, and operation factors. The design coefficient used in the ISO method accounts only for normally anticipated pipe manufacturing factors. In the ISO method the design engineer is responsible for ascertaining additional handling and installation, and operat

38、ion factors and increasing the 1.25 design coefficient for the application. ISO 12162 provides some guidance in the application of additional design coefficients. Additional differences in the two methods relate to the determination of long-term strength by HDB or MRS. Both methods apply various ass

39、umptions, requirements, temperature basis and mathematical treatment of the data to arrive at a different categorized long-term strength forecast. These inherent differences will result in a different design basis. For more information on the 11 differences in the long-term stress rating methods und

40、er ASTM/PPI and ISO, the reader is referred to PPIs TN-7. In practice, the ISO methodology reflects maximum pressure ratings and the designer should address reduction factors for the application that are not in the minimum 1.25 design coefficient. For the ASTM/PPI methodology used in North America a

41、dditional pressure reduction factors would generally only be considered for extremely severe applications. At this point, a DR 11 ASTM/PPI PE 4710 pipe is rated for 200 psig (14 bar) water service at 73F/23C, but a DR 11 PE 100 pipe under the ISO system is rated for 230 psig (16 bar) for water servi

42、ce at 20C/68F. The appearance is that the same DR water pipe at about the same service temperature has a 15% difference in allowable service pressure as PE 4710 or PE 100, but the actual difference is that the rating methods set different allowable design stress and are at different temperatures. Th

43、e performance potential of a single PE piping material is still the same whether it is designated a PE4710 or PE100. However, the pressure-rating methods under the two systems are different. Both are valid when used within the context of each respective system. Taking one element of one system and c

44、ombining it with elements of the other system may lead to erroneous results. In conclusion, this document provides some guidance in comparing the two rating methods for PE pipes in water applications, and why each may appear to result a slightly different maximum pressure rating. It is ultimately th

45、e responsibility of the system designer to determine the operating and service conditions for a particular water system and how to address those needs using either of these pressure rating systems. 12 References: PPI TN-7, “The Nature of Hydrostatic Stress Rupture Curves”, Plastics Pipe Institute, I

46、rving, TX, 2005. PPI TR-9, “Recommended Design Factors and Design Coefficients for Thermoplastic Pressure Pipe”, Plastics Pipe Institute, Irving, TX, 2002 PPI TN-41, “High Performance PE Materials for Water Piping Applications”, Plastics Pipe Institute, Irving, TX, 2007 ASTM D2837, “Standard Test Me

47、thod for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials or Pressure Design Basis for Thermoplastic Pipe Products”, ASTM International, West Conshohocken, 2011. PPI TR-3, “Policies and Procedure for Developing Hydrostatic Design Basis (HDB), Pressure Design Basis (PDB), Strength

48、Design Basis (SDB) and Minimum Required Strength (MRS) Ratings for Thermoplastic Piping Materials or Pipe”, Plastics Pipe Institute , Irving, TX, 2012 ISO 9080, “Plastics Piping and Ducting Systems-Determination of the Long-Term Hydrostatic Strength of Thermoplastics Materials in Pipe Form by Extrap

49、olation”, International Organization for Standardization (ISO), Geneva, Switzerland, 2012 ISO 12162, “Thermoplastics Materials for Pipes and Fittings for Pressure Applications-Classification, Designation and Design Coefficient”, International Organization for Standardization (ISO), Geneva, Switzerland, 2009 13 Appendix I Commonly Accepted Conversion Factors 1 MPa = 145 psi (stress) 1 bar = 14.5 psig (pressure) 14