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The pit length can be determined using the following formula as per [DWA-M 143-13]: (Formula: Die Baugrubeläange) lG : Length of the insertion pit in m hG : Depth of the pit (Pipe invert depth) [m] R : Minimum bending radius in m (manufacturer specified) (Image: Example design of an insertion pit for the insertion of plastic pipelines in a reference to [DWA-M143-13]) |
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The length of the insertion pit is determined by several factors, the most relevant being the minimum radius of curvature RK [m] of the new pipeline. Plastic pipelines are distorted during the bending process in such a way that the outside bend of the pipeline (1) is elongated and the inside bend in compressed (2). Therefore, the strain limits of the pipeline must be taken into account during the insertion process, and a distortion or stress analysis … |
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The determination of the elongation strain is carried out by means of the maximum tensile stress σz. For this, the determination of the σz is necessary at both the bursting head (1) as well as at the old pipe (2). The larger value is used for the elongation strain determination. (Image: Static pipe bursting - Pulling-in of the new pipeline) |
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The DVS guideline 2205-1 specifies elongation limits for various plastic materials free from residual stress (isotropic material). The limits for the safe installation conditions (short-term, γ = 1.4) are given in the following table. (Table: Elongation limits of various plastics [DVSM2205]) For PE pipelines, the total outer bend elongation (from pulling and bending) may not exceed ε = 3 %. Due to the risk of buckling during compression, the max εK… |
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The limiting values specified in the following table apply for PE-HD pipelines (materials free from residual stress (isotropic material)). Other materials require further verification. (Table: Limiting values for PE-HD) 1. Diameter / wall thickness ratio (dL,a, dL,i [mm] Outside-/Inside diameter of the pipeline / liner). 2. Permissible bending radius. 3. Permissible compression reduced by for 3 % for stability purposes. 4. Permissible longitudinal tension … |
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(Table: Material properties of plastics (excerpt from DWA-A 143-2)) Legend: 2 ) The figures are calculated values that are determined from deformation measurements of liners. 3 ) Compressive stresses can also be the decisive factor particularly for thin walled liners. 4 ) Tested in accordance with DIN 54852 (4- point bending creep test) , test procedure according to DIN 53457 , test specimen manufactured according to DIN 16776-2 5 ) Higher values … |
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(Image: Pipeline pulling force tension) (Image: Pipeline pulling force angle) The force required for the pulling in of the pipeline is determined by:
(Image: Static pipe bursting pulling … |
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Tensile force from the friction between the new pipeline and the old pipeline underground and above ground (coefficient μG) Equation 1: (Formula: Zugkraft aus Reibung des Rohrstranges im Altrohr Untergrund und auf dem Gelände)
Tensile force from the friction against the guide rollers (rolling friction, coefficient μR) Equation 2: (Formula: Zugkraft aus Reibung … |
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Interpolation between the curves is allowed. Pipeline weight: ḡL = AQ • YL [kN/m] A1 = (A1/ ḡL) • ḡL (→ Table: Equation variables) of mathematical variables (Image: Chart A1 / 6 bearing forces of PE pipes SDR 21 (SN 8) on the old pipe(A1) and the edge of the pit (A2)) |
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Permissible tensile force for the pulling-in of HD-PE and PP pipelines . (Table: Maximum permissible tensile forces for pipes made of PE 80) |
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The maximum tensile force (Z) is exhibited at the bursting head (1), but without a bending moment. With the weld factor αw, the net cross section AQ,n (after deduction of the screw holes) and Ez≥ Edis the tensile stress calculated: Equation 5: (Formula: Ermittlung der maximalen Zugspannung σZ) (→ Table: Equation variables) of mathematical variables (Image: Inside view of the pulling head with bolt connections) (Image: Pulling head with bolt connections … |
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At the old pipe (2) the determination of the σZ is based on the tensile force (Z), area of the cross section of the new pipe, the bending moments and the moment of resistance of the cross-section of the new pipe: Equation 6: (Formula: Ermittlung der maximalen Zugspannung σZ Am Altrohr) with (Formula: Wq) With full wall systems (Formula: Wq bei vollwandsystemen)(Table: Equation variables) |
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The tensile force Z [kN] produces a tensile stress σZ in the pipeline, which must be less than the allowable tensile stress. (Formula: Rohr Zugspannung σZ) The allowable stress is dependent on
Only winches capable of measuring and recording the pulling force should be used. |
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The compressive stress at the old pipe (2) is calculated as follows: Equation 7: (Formula: Die Druckspannung am Altrohr) (Table: Equation variables) |
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The tensile stress from equation 5 and equation 6 is used to calculate the expansion, and the compressive stress from equation 7 is used for the compression. Equation 8: (Formula: Berechnung der Stauchung εz) Equation 9: (Formula: Berechnung der Stauchung εd) (Table: Equation variables) |
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(Image: Tabellenicon) Taking into account any later occurring axial expansion due to internal pressure in non-pressure pipes or bending strains in curved pipelines, the expansion εzshould not exceed a value of 2.0 % during the pulling-in. If an expansion of 2 % is allowed during the pulling-in, the expansion in the bends is also to be a further 1 to 2 %. |
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The new pipeline is fed through a support bracket located at the top edge of the insertion pit into the old host pipe with sufficient clearance (due to the expansion). In general, it is recommended to shorten the insertion pit by utilising additional roller supports, at a distance I3 from the edge of the trench, to increase the clearance Δh3 [mm] above ground level. Equation 10: (Formula: minlG) For PE pipes SDR 33, 26, 21 and 17 without additional … |
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The minimum insertion pit dimensions are determined using the following parameters [DWA-A 143-2:2015] :
(Image: Pipeline and insertion pit parameters (sufficient … |
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Basis for the calculation model The required insertion pit length lG is determined for a unrestrained pipeline, i.e. freely stored at the top edge of the pit, insertion with clearance between expansion and the new pipeline (i.e. Ᾱ1 = 0). lGresults from the following conditions:
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Interpolation between the curves is allowed. (Formula: minlG) (→ Table: Equation variables) of mathematical variables (Image: Diagram for the required pit length lG for PE pipes SDR 21 (SN 8) during the insertion into the old pipeline (clearance Δh / dL,a)) |
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Diagrams for the determination of the pit length for PE pipes SDR 33, 26, 21 and 17 (Image: Required insertion pit length lG for PE pipes SDR 33 (SN 2) pulled into a host pipe (clearance Δh / dL,a) [DWA-M 143-2 draft]) (Image: Required insertion pit length lG for PE pipes SDR 26 (SN 4) pulled into a host pipe (clearance Δh / dL,a) [DWA-M 143-2 draft]) (Image: Required insertion pit length lG for PE pipes SDR 21 (SN 8) pulled into a host pipe (clearance … |
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If the insertion pit is excavated along its entire length, pipelines can be pulled in two directions. (Image: Insertion pit for the pulling in of two pipelines) |
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The maximum strain of σ = 21 N/mm2 (Short Term) applies to PE pipelines before buckling damage occurs, which is limited to (γ = 1.4) max σ ≅ 15 N/mm2 during installation. This strain includes the short-term module Eσ=15 = 500 N/mm2 and the elongation max ε = 3 % according to the DVS 2205-1 guideline. In order to prevent the buckling of the pipeline during the pulling-in procedure, the radius of curvature is to be determined as per equation 11: Equation … |
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To the max. RKbelongs the maximum allowable strain εK, with: Equation 12: (Formula: die maximal zulässige Dehnung εK) Due to the buckling danger, the strain max σKmust be reduced accordingly (see table on the next slide). The associated modulus of elasticity can be approximated by interpolation: Equation 13: (Formula: E-Modul) (Table: Equation variables) |
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In addition, the variability of stresses and thus the effective modulus of elasticity Em [N/mm2] of the pipeline over the pit length lG , and the pipe diameter are taken into account as follows: Equation 14: (Formula: E Modul Em N mm2) (Formula: a von EModul Em Gleichung) If the pulling in of the pipeline takes place in temperatures other than 20 ° C, the pit length can be corrected as follows: Equation 15: (Formula: Ermittlung des maximalen Krümmungsradius … |
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