The success of a pipe product is due not only to its material
composition but also to its proper design. The design must consider and
respond to many adverse influences. The primary design considerations
for ductile iron pipe are as follows:
1. Trench Load (ANSI/AWWA C150/A21.50)
The consideration includes the basic static earth load, the dynamic
truck load together with the force of the resulting impact upon pipe
structure. To resist this influence a design ring bending stress of
48,000 psi is utilized.
2. Internal Pressure (ANSI/AWWA C150/A21.50)
Here the concern is with the intended working pressure of the
pipeline together with an additional surge allowance of 100 psi. These
falues combine to provide the required design standard. In response to
this requirement, a tensile strength of 42,000 psi is utilized.
3. Laying Conditions (AWWA C-600)
Once the above criteria has been determined, it is necessary to
decide on how the pipeline shall be bedded and backfilled in the field.
There are five standard methods, each offering its particular design
value.
The above is intended to provide a general understanding of the design environment of an underground pressure pipe product.
4. Field Cutting (AWWA C-600)
Beveling of the spigot end should be adequately performed to assure
proper installation.
The various pipe manufacturers recommend that under no circumstances
should large diameter pipe (14"+) be cut unless the pipe is identified
for field cutting. Out of round barrels are common throughout the
industry on ductile iron pipe. During manufacturing spigot ends may be
rounded to facilitate field assembly.
Cutting of all diameters of pipe within 24" of the bell face may
result in outside diameters too large to join into the bell of another
pipe, fitting, valve or accessory. Maximum diameters of the initial 24"
of pipe barrel from face of bell cannot be assured due to tapering of
the pipe molds. This tapering is necessary in order to facilitate
extraction of the pipe from the molds during the manufacturing process.
5. Conductivity
Push Joint and Mechanical Joint gaskets act as insulators.
Therefore, means must be provided to conduct electricity where
electrical thawing, cathodic protection or a grounding system is
required. There are several methods to provide electrical conductivity:
copper conductivity straps, bronze wedges, etc. Contact your NAPF
member for information.
