PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.A.2.5Self cleaning velocityAnother important factor for subwater pipelines is to prevent deposits inside the pipe and to preventaccumulation of air/gas.To check the pipelines capacity for self-cleaning, we introduce the flow’s shear stress (τ) :Dτ = ρg⋅ ⋅ I4A.2.12)ρ = density of water (kg/m 3 )g = acceleration of gravity (= 9.81 m/s 2 )D = internal diameter (m)I = incline of friction drop line∆ hLTo be self-cleaning the shear stress shall be ≥ 4 N/m 2Example 8Check if the pipeline Ø 400 mm PE SDR11 in example 6 is self-cleaning?Solution:11,03We must find the incline of the friction drop line: I = = 0. 004425000.32722Hence using A.2.12): τ = 1000 ⋅ 9.81⋅⋅ 0.0044 N/m = 3.5 N/m4As we see the shear stress is < 4.0. We therefore must expect some deposits in the pipeline.In such a case it can be useful to install equipment for flushing and use of cleaning pig.A.2.6Air transportAir and gas accumulations are the ”worst enemies” for subwater pipelines.To handle the problems there are 2 possible solutions:a) Prevent air from entering the pipelineb) Provide a sufficient velocity in the pipe to transport air/gas through the pipelineAir/gas accumulations in a pipeline will/can bring :− reduce the hydraulic capacity− entail flotation or vertical displacementIf possible, we recommend method a) to be the safest solution.For an outfall pipeline the outlet chamber must be constructed in a way that air can not enter thepipeline. It means that you have to take into account :−−−Lowest low water level in recipient / source (LLW)VortexFluctuations in water level due to sudden change in flowIn most cases this means that top of the outfall pipeline in the point where it leaves the chamber shallbe in the range 0.5-1.5 m below LLW.For inlet pipelines the maximum under-pressure shall be less than 4 mwc to avoid air release fromthe water. Siphon constructions are normally not recommended.For both outfall pipelines and intake pipelines, we recommend avoiding high points in the trace.Side 25 av 84