The Topeka Capital-Journal recently published a story on the large number of water main breaks that the city has endured in recent years. Topeka’s water system relies on 870 miles of pipe. Since the 1940s, Topeka has diversified its system by adding ductile iron pipe and polyvinyl chloride (PVC) pipe. Today, the system is 39% cast iron pipe, 27% ductile iron pipe, and 25% PVC pipe (figure 1).


Percentage of Pipe Materials Used

Figure 1: Topeka's system of pipes is about 39% cast iron, 27% ductile iron, and 25% PVC, with the remaining made of various other materials.

Of the city’s water main breaks, approximately 66.8% were from cast iron pipes, 18.4% were from ductile iron pipes, and a mere 4.5% were from PVC pipes (figure 2).

Percentage of Pipe Breaks by Material

Figure 2: In Topeka, 66.8% of breaks occur from cast iron pipes, 18.4% from ductile iron pipes, and 4.5% from PVC pipes.

In Topeka, all iron pipes have a failure rate that is more than 18x the failure rate of PVC (figure 3). Ductile iron pipe devotees (and their lobbyists) will tell you that ductile iron is a great improvement over cast iron. And yet, ductile iron pipe still fails at more than 4x the rate of PVC pipe (figure 4).

Pipe Breaks: Iron V. PVC (%)

Figure 3: All iron pipe accounts for more than 18x as many breaks as PVC pipe.

Pipe Breaks: Ductile Iron v. PVC (%)

Figure 4: Ductile iron pipe accounts for more than 4x as many breaks as PVC pipe despite the fact that ductile iron pipe makes up almost the same share of the system (27%) as PVC pipe (25%).

In fact, Topeka has seen a significant number of ductile iron pipes grossly underperforming their expected life span:

Ductile iron pipe should have a lifespan of 100 years, but [Deputy Director of Utilities Braxton Copley] said the city has seen a large number of breaks in pipes that are only 40 to 50 years old.
— Topeka Capital-Journal

The ductile iron pipe industry wants the public to believe that its product is more reliable than PVC but the facts show otherwise. As demonstrated by this case study, PVC is clearly the more durable material.