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Pipe stiffness explained: PVC and Ductile Iron

By PVC Pipe Association

Pipe stiffness is the measure of a flexible pipe’s resistance to deflection (ovalisation) under load. A flexible pipe is any pipe that can deflect 2% without suffering damage. Both Ductile Iron (DI) and PVC meet this requirement and are considered flexible pipes. Some specifications require DI pipe due to its ‘strength’ in resisting external loads. While this was true in the era of thick-walled DI class pipe, the thinner walls of DI Pressure class pipe have made PVC the stronger option.

Deflection limits – advantages of PVC

PVC pipe is able to deflect 30% before experiencing failure (by reverse curvature). AWWA C605 (pressure pipe) and ASTM D3034/F679 (gravity pipe) limit PVC pipe deflection to 71⁄2%, providing a 4:1 safety factor.

Conversely, the linings used significantly limit DI pipe deflections:

  • Cement-mortar (gravity sewer and pressure water pipe) – maximum allowable deflection is 3% due to potential cracking of the lining. Safety factor is 2:1.
  • Flexible lining (gravity sewer and forcemain sewer pipe) – maximum allowable deflection is 5% due to potential damage to the lining. Safety factor is 2:1.

Conclusion: DI pipe fails at a lower deflection than PVC pipe and DI has a lower safety factor against failure.

Comparison of pipe stiffness

Pipe stiffness (PS) is calculated using the equation PS = 6.7 EI /r3where:

      E = modulus of elasticity (psi)

      I = moment of inertia (in4/in)

      r = mean diameter (in)

Pipe geometry is used to calculate I and r. Modulus values are E = 400 000 psi for PVC and 240 000 psi for DI.

Figure 1 compares pipes of about the same pressure class (PC), namely DI PC250 pipe and PVC PC235 pipe.

Figure 1


<note: please change figure heading to sentence case>

The figure shows that PVC pipe has a higher pipe stiffness throughout the size range and PVC’s advantage grows as pipe size increases.

In terms of the numbers:

• For the 14” size, PVC has 1.7 times DI’s pipe stiffness.

• For 24” pipe, PVC has 2.6 times the stiffness.

• For the 36” size, PVC is 3.4 times as stiff.

Conventional wisdom is not always correct, for the following reasons:

  • PVC PC235 pipe has more pipe stiffness (and therefore will deflect less) than DI PC250 pipe, and
  • PVC pipe can safely withstand about five times as much deflection as cement-mortar lined DI pipe.

In this example, PVC pipe is the ‘stronger’ pipe.

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