scope:

This Technical Report (TR) describes key features of the two pressure rating methods for plastic piping compounds used in gas pressure piping applications:

1. ASTM D2837, the North American pressure rating method, called the HDB (hydrostatic design basis), first published in 1969, and

2. ISO 9080 and ISO 12162, the international pressure rating method, called the MRS (minimum required strength), first introduced in the 1970's.

This technical report first discusses the differences in the pressure ratings determined by these two methods for gas distribution and gas gathering applications. This technical report then reviews changes made to the design factors during the past few years to several industry standards, such as ASTM, CSA, and API. This technical report is not an endorsement for either of these two pressure-rating methods for gas applications. PPI does not recommend one method or the other - both are technically sound methods used by the industry. This technical report simply provides a historical overview and current status of these two pressure-rating methods.

It is important to keep in mind that when calculating a design pressure (P) for a particular plastic pipe made from a certain material, this design pressure is dependent on the particular application that the pipe is used for and also dependent on regional or national codes and regulations for selecting the design factor. As stated in PPI TN-28, recommended design factors are based on an engineering risk assessment that incorporates two groups of variables; testing quality and application requirements. The design pressure for a PE or PEX pipe is calculated for a given application and pipe material with consideration to the risk. The design factor (DF) used with the HDB in the ASTM equation is generally regulated for certain jurisdictions with consideration for various risks. Gas engineers select the design factor within the confines of the regulations governing their jurisdiction. The resulting design pressure considers those risks calculated within the minimum safety standards of local regulations. The design pressure in ISO using the MRS equation calculates, with the minimum design coefficient (C), the maximum design pressure allowed. Local regulations may specify a lower design pressure. Gas engineers select the appropriate design coefficient for their application. Using these two design equations, the design pressure for a given pipe may be different using HDB and DF (under certain jurisdictions) than when calculated with MRS and C "for the same pipe". When reviewing this Technical Report, the reader must keep in mind that one jurisdiction can view risk differently than another jurisdiction for a given application.