PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.Example 9Let us return to example 7 and 8.We choose to dig down the pipe in a trench with 1 m soil cover. Decide the safety factor againstσbuckling in this case. Assume short time E-modulus for pipe = 1000 Mpa, ,D = 0.05 γ = 20 kN/m 3and mod. Proctor for soil = 80%.Solution :First we decide the pipe’s ring stiffness from A.3-21) :1000 ⋅1000SR =kPa =312 ⋅ (33 −1)2.54 kPaCorrection factor, σ, due to ovaling is taken from A.3-20) : α = 1 - 3⋅ 0.05 = 0.85E s 1 is found from fig. A.3.2.2.1 : E s 1 = 600 kPa ⇒ E t 1 = 2⋅ 600 kPa = 1200 kPaUsing formula A.3-19) assuming F= 1.0 we get the buckling pressure :5.63q = ⋅12.54 ⋅1200⋅ 0.85 kPa =264 kPa = 0.264 MPa≈ 27 mwcWe realize that the pipe now can withstand an external pressure corresponding to approximately27 mwc.Compared to example 7 and 8 there is an external soil pressure caused by the cover, ref. formulaA.3-22)q t = (20-10) ⋅ 1 kN/m 2 = 10 kN/m 2 = 0.01 Mpa ≈ 1 mwcTotal external pressure is :q t = under-pressure + soil pressure = (2 mwc + 1 mwc) = 3 mwcq 27Safety factor against buckling : F = = ≈ 9q 3tBy installing the pipe in a trench with soil cover the safety factor has increased from 0.85 to ≈ 12.This indicates that we ought to install pipes in trenches if they are exposed to significant externalforces and the SDR class is high.Regarding subwater pipelines it may be economically favourable to reduce the SDR-class comparedto installing the pipe in a trench.A.3.3Water hammerWater hammer (pressure surge) occurs in a pipeline when there is a sudden change in the flow.The result is a pressure wave going backwards and forwards in the system.The most common reason for pressure surge is sudden start and stop of pumps or closing/openingof valves. Even if there is installed frequency converter on pumps the electric power supply can fail.Exact calculations of water hammers are complicated and must be carried out by computerprograms.However there is a simplified method that gives an indication of the maximum and minimumamplitude of the pressure wave. This method will be presented below.For intake and outfall pipelines water hammer is normally not a problem if the pipes are not directlyconnected to the pump, but suddenly closing of gates must be avoided.Change in flow will be damped in the intake and outfall chambers.The area of the chamber should be designed for the expected variations in flow.Side 36 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.In such cases the amplitude of the fluctuations will be in the range of ± 1 m above maximum andbelow minimum operating level respectively.For transit pipelines and intake and outfall pipelines connected directly to pumps, the water hammercan imply damage to the pipe if the pressure class is too low.The most critical is normally the under-pressure which can reach values >10 mwc if there aresignificant high points in the trace.To reduce the pressure surge, we can install flywheel mass on the pumps or connect pressurevessels.Such solutions are most often economic favourable compared to reduction of the SDR ratio for thepipe, but depend on the length of the pipeline and the diameter.It shall also be mentioned that pressure surge can occur during sinking of PE-pipes [12].The size of the water hammer is derived from the general relationship of surge∆v⋅ c∆ p =A.3-23)gThe surge pressure/water hammer is said to linearly depend on the pressure wave speed, c, in waterinside a pipe. ∆v is the change of water flow speed (acceleration/retardation) andg = 9.81 m/s 2 . The pressure wave speed, c, is given by :1/ 2E o⎡ s ⎤c = ⋅2 ⎢ ⎥ A.3-24)(1 − ν ) ⋅ ρ ⎣Dm⎦E o = short time modulus of elasticity, ref. table A.1.2.ν = 0.4-0.5 = Poisson ratioρ = density of waters = wall thicknessD m = D o -eWith rewriting, we get an equation of c as a function of the pipe SDR class :E o 1c = ⋅A.3-25)21/ 2(1 − ν ) ⋅ ρ (SDR −1)Surge is a short time condition (few seconds) under which a PE pipe, applied to a constant long termstress, returns to its initial E-modulus at time zero.In table A.3.3.1, we have calculated the pressure wave speed for PE100 and PE80 materials asa function of SDR class.Polyethylene PipePressure wave speed in water inside a PE-pipe c m/secSDR33 (PN322) SDR26 (PN4) SDR17.6 (PN6) SDR11 (PN10)PE100E o = 1050 N/mm 2203PE80E o = 800 N/mm 2 180230200282250263320ν = 0.45Table A.3.3.1 Pressure wave speed for PESide 37 av 84
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