In addition to the established wall thickness design, there were also additional considerations given to the wall thickness which included a 0.05&#; &#; 0.09&#; (depending on diameter) casting tolerance and a 0.08&#; service allowance. The casting tolerance would account for any minor deviations in wall thickness during the manufacture.
The service allowance was included to help mitigate any minor corrosion that may take place in cast iron pipe. With the introduction of polyethylene encasement, this &#;corrosion allowance&#; is no longer needed, but it is still included in the overall design (as an added factor of safety).
Industry experts knew that the design of the stronger ductile iron pipe was extremely conservative and believed that the pipe could be consistently manufactured with a thinner wall.
So, in , the design criteria were revisited, and testing determined the addition of &#;CL 0&#; to the original thickness class designation was needed. However, many of the folks involved in this decision questioned whether a class 0 pipe would be accepted by end users.
Therefore, the number &#;5&#; was added to the front of the class designation and the CL50 &#; CL56 designations which still exist today were born.
You are probably thinking, that&#;s great and all, but why is there now another class consideration utilizing a pressure class rating system?
In , DIPRA member companies decided to include a pressure class rating system for the following two reasons:
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Now that we&#;ve had a little history lesson, you probably want to get to the meat of the discussion and learn how to determine the correct wall thickness to specify for a given scenario.
To understand what class of pipe you should specify, you must understand that ductile iron pipe is a flexible conduit (unlike its cast iron predecessors). Because it is a flexible conduit, as is PVC, HDPE and steel conduits, the pipe can deform and re-form to its original shape as forces are applied to, and removed from, the pipe wall.
Two factors to consider when deciding what wall thickness, and therefore correlating pressure or thickness class, for your specific need:
Ductile iron pipe is the only material that considers external loading when determining the wall thickness needed for a specific pipe diameter. All others simply rely on the internal pressure of the fluid.
While there are many formulas that will help you determine the pipe wall needed for your project, McWane Ductile has developed a tool to help minimize the effort and alleviate the frustration - the &#;Thickness Design Calculator&#; as part of thePocket Engineer. If you are not familiar with the McWane Pocket Engineer, you can download here at MD Pocket Engineer. This tool has many other useful design calculators as well as access to technical information.
By utilizing the PE, you can easily input design criteria such as pipe diameter, maximum internal pressure, depth of cover, wheel load and the weight of the soil.
Once you input this information, it will yield results that tell you exactly what thickness of pipe and the correlating class of pipe that your application requires. The results are broken down by the five DIPRA / AWWA recognized trench types. It will also let you know if the internal pressure or external loading (bending stress) govern the design.
Contact us to discuss your requirements of ductile iron valves. Our experienced sales team can help you identify the options that best suit your needs.