Why did the ductile iron pipe industry change to pressure classes in 1991, and did the change reduce safety factors?

The answer to the last part of the question is a resounding NO. The safety factors were not reduced, and we will discuss that issue in more detail later. As to the first part of the question, a reasonably short answer might be to make the product more "user friendly," to use one of today's buzz words. But, let's look at the change in more depth. The history of ductile iron pipe as we know it today is fairly short compared to its predecessor cast iron pipe, which has centuries of history. But the growth and acceptance of this product since 1965, when the first design standard was published, is phenomenal. The reason for such growth can only be attributed to the strength, toughness, and versatility of the pipe. And throughout the 40-plus year history, as with most manufactured products, change has been continual. As an example, ASA 21.50-1965, the first ductile iron pipe design standard, contained 6 thickness classes, and only two trench types. The classes of pipe were numbered 1-6, and the trench types were A and B. Three additional trench types were added in 1976 to give the installer more flexibility. As experience and knowledge of the product grew, and manufacturing methods improved, changes in design parameters as well as designations to define the product occurred. In 1976, designations of the thickness classes were changed from Thickness Classes 1-6 to Thickness Classes 51-56 and Class 50 was added. So, with a long history of changes to the standard, the change to pressure classes in 1991 should not have been a surprise. This change was merely a response to a need in the marketplace. In today's market, nearly all piping materials used for water and wastewater service are classified by a pressure rating. This is a very obvious advantage, since a pressure rating tells a user or engineer something about the product he needs to specify. Neither of the old thickness class systems that were previously used for ductile iron pipe told the user anything about the pipe. They were merely reference numbers used to designate standard metal thickness classes. As far as safety factors are concerned, as stated earlier, there was absolutely no decrease in safety factors involved in the new pressure class designation. There were no changes in design equations, design parameters, safety factors, or design requirements in C150/A21.50. Nor were there any changes in manufacturing requirements in C151/A21.51. The conservatism that has always been the guiding principle of ductile iron pipe design remains identical to what it has been for more than 20 years. It is interesting to note that the Ductile Iron Pipe Research Association compiled a partial list of organizations that have specified/used minimum pressure class ductile iron pipe since the pressure classes were adopted in 1991. The list, by no means comprehensive, includes both American and Canadian cities, counties, utilities, and engineering firms. It shows that more than 1,400 organizations have specified min imum pressure class ductile iron pipe in the past six years. Such overwhelming use of minimum pressure class pipe attests not only to the acceptance of the pressure class designation, but also demonstrates the need in the marketplace for a lower pressure rated ductile iron pipe. You, the customer, are provided proven assurance that all AMERICAN Pressure Class Ductile Iron pipe will do the job. Every single pipe is still high-pressure hydrostatically tested at AMERICAN in accordance with ANSI/AWWA C151/A21.51. You get even more assurance with all 30-inch and larger ductile iron pipe from AMERICAN because it is all subjected to a special, even-higher pressure hydrostatic test, according to the following schedule: Pipe Size (in.) Minimum Pressure Class Minimum Required AMERICAN Proof Test Pressure (PSI*) Rating Actually Proven By Factory Test on Every Pipe** 30 150 669 4.5 36 150 625 4.2 42 150 580 3.9 48 150 570 3.8 54 150 558 3.7 60 150 552 3.7 64 150 537 3.6 *Contact AMERICAN if higher proof test capabilities are desired. **Proof Test Pressure divided by Pressure Class. Safety factors vs. yield or tensile strength are even higher. In summary, although ductile iron pipe now has a different designation, it is still the most conservatively designed, versatile, and long lasting piping product on the market for water and wastewater.

Ben Helton, Technical Director, AMERICAN Cast Iron Pipe Company