For the design of pipes, the flexibility and bending characteristics are an important factor, in terms of consideration for the correct installation of the pipe system.  For buried pipe applications, the “field-bending” can help to reduce the quantity of needed bends and elbows. The allowed minimum bending radius depends on the pipe wall structure, jointing method, environmental conditions (temperature, sun radiation) and duration of the bending process. For marine application, the bending process is a must for the sinking process and one of the decisive advantage of thermoplastics in comparison to rigid pipe materials.For the design of pipes, the flexibility and bending characteristics are an important factor, in terms of consideration for the correct installation of the pipe system.  For buried pipe applications, the “field-bending” can help to reduce the quantity of needed bends and elbows. The allowed minimum bending radius depends on the pipe wall structure, jointing method, environmental conditions (temperature, sun radiation) and duration of the bending process. For marine application, the bending process is a must for the sinking process and one of the decisive advantage of thermoplastics in comparison to rigid pipe materials.

Determination of bending radius
For determination of the bending radius the strain of the outer fiber and the buckling resistance of the pipe have to be considered separately. For the design all elements of the pipe string, incl. jointing, have to be verified for the bending process. The permitted strain of outer fiber mainly depends on the load period, but typical for Polyethylene and Polypropylene is to define a strain-limit for bending of 2,5% (adequate safety included). 

OD =     Outer diameter [mm], for profiled pipes with inner wall only: OD= ID + 2 * e1
ℇ    =      Strain of outer fiber [%]
Rℇ =       bending radius, strain related [mm

The bending radius to avoid buckling depends on pipe stiffness and wall structure and is for solid wall pipes and closed profile pipes (CPR) calculated as follows:

ID  =       Inner diameter [mm]
e_equ =    equivalent wall thickness, for solid wall pipes: wall thickness
S_FF  =     Spiral factor [-], for solid wall = 1, for profiled wall ca. 0,8
R_B  =     Minimum bending radius, buckling related [mm]

The spiral factor SFF considers the helical profile around the pipe and the distance between the profiles. It should be noted that all equations assume a circular round pipe without ovality, what is especially important for more heavy solid wall pipes with higher weight/stiffness-ratio. In case an additional safety factor can be considered.   The minimum bending radius R_min must be bigger than R_B and R_ℇ  ! The final determination depends further on material characteristic (flexural modulus, creep-factor etc.), the environmental conditions (temperature, sun radiation, wind etc.) and how the bending load is applied. But the below-mentioned values of a bending radius are typical.

The relation between eSDR respectively SDR and SN values can be described as follows:

eSDR =     equivalent SDR class for profiled pipes
E =           short term flexural modulus [N/ mm²]
SN =        stiffness value/class acc. to ISO 9969 [kN/m²] Author: Dipl.-Ing. Stephan Füllgrabe,Krah Pipes GmbH & Co. KG