When pulling-in curved piping, the sum of the created individual extensions with pulling and bending of 3 to 4 % must not be exceeded.
If an extension of 2 % is permitted during pulling-in, then the permissible extension caused by bending in the region of the curve is still approx. 1 to 2 %.
As a criterion for the determination of permissible bending radii, one must view, on the one hand, the kinking, for small relationships of pipe wall thicknesses to diameter (thus small pressure levels) and the edge fibre extension, on the other hand, for larger relationships (thus higher pressure levels). For calculating the bending radius the following equations can be utilised as approximations:
Bending radius to kinking
Bending radius to extension
Taking the two criteria into account, the permissible bending radii are obtained in accordance with (Table 18.104.22.168.22.214.171.124-1).
At temperatures around 0 °C, the above-mentioned bending radii must be increased by a factor of 2.5. Between 0° and 20° C the respective bending radii can be determined by linear interpolation [Bröms87].
In curves, the force F increases in front of the curve by the factor eμβ to the required tensile force Fβ behind the curve
The permissible pull-in length for a curve is then determined by: