Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)

Internal Pressure in Flexible Pipes

If the inserted pipe without annular space is used as a pressure pipe, then the pipe including pipe joints and fittings must be so sized that it can absorb the internal pressure during the desired lifespan - in the municipal field at least 50 years.

This is valid also for the case that the annular space is grouted and the grouting material, including the old pipe still cannot absorb the internal pressure. In this case, the inserted plastics pipes must not possess grooves that are too deep. According to DIN 8075 [DIN8075c], small flat longitudinal grooves are permissible as long as the wall thickness remains within the permissible deviation (Abschnitt 5.3.2.1).

For a given internal pressure (Image 5.3.3.7.5.2-1), safety factor and external pressure of the pipe there is valid for the calculation of the wall thickness:

image
Image 5.3.3.7.5.2-1:  Loading of the pipe due to internal pressure pi [FI-Hoech] [Koch85]

The fundamental effective stress in the calculation is taken from the time stability diagram (Image 5.3.3.7.5.2-2) (Image 5.3.3.7.5.2-3) with reference to the operating temperature and operating time. In accordance with the respective operating conditions, the effective stresses (minimum values) determined from the time stability diagram are provided with safety values.

image
Image 5.3.3.7.5.2-2:  Time stability of pipes [FI-Hoech] [Koch85] - Of HD-PE (Hostalen GM 5010 T2)
image
Image 5.3.3.7.5.2-3:  Time stability of pipes [FI-Hoech] [Koch85] - Of PP (Hostalen PPH 2222)

The presence of certain chemicals e.g. in the sewage with simultaneous effect of mechanical loading, can reduce the time stability strength in comparison with water. This reduction is given by the so-called resistance factor (with reference to the stresses with equal operation duration as stressing factor and with reference to the loading time period with the same stress as the time factor). The stress factor is generally of interest in the calculation of the pipe wall thickness of a pipeline consisting of HD-PE pipes. Stress factors of pressure pipes of HD-PE for various chemicals can be taken from (Table 5.3.3.7.5.2-1).

Table 5.3.3.7.5.2-1: 

Stress referenced resistance factors for PE-HD pipes [FI-Hoech] [Koch85]

Medium l) Concentration [%] Temperatur [°C] Stress
[N⁄mm2]
Stress factor
fCRσ
Sewage from a cellulose factory M 100 80 4 to 2 0.95
Sewage from a chemical factory M 100 80 4 to 2 0.75
Sewage from a milk processor M 100 80 4 to 2 0.73
Water (H2O) A 100 80 4 to 2 1.0
Water with wetting agent M 2 80 4 to 2 0.6
Various cleansers M 80 4 to 3 0.3 to 1.0
1) A = Anorganic substance, M = Mixture of anorganic and organic substance

Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)