PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

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Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.For transit pipelines it must be possible to remove air in the manholes at the shoreline when startingup the water transport during general operation and in case of repair work.For sewage transport the retention period must not exceed the time limit for H 2 S emission.As an indicator, 4 hours retention period should not be exceeded (depends however on operatingtemperature).In solution b) the critical speed, U c , must be obtained by the flow to remove air bubbles presentin the pipe.The critical speed of water, U c , is given by : U c = f (D i sin α) A.2.13)D i = pipe internal diameter (m)α = pipe gradientA simplified expression gives U c as a function ofgD iU= k ⋅A.2.14)c gD ig = acceleration of gravity (9.81 m/s 2 )The factor k is displayed in Fig. A.2.6.1 as a function of sin αThe curve of k in Fig. A.2.6.1 is applicable for α= 0º →90º.1,2√ g ⋅ D iU c1,0K =0,5α ≤ 5 oα > 5 o0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0√ sin α1 o 2 o 5 o 10 o 20 o 30 o 40 o 50 o 60 o 90 o(α)Fig. A.2.6.1 Critical velocity for transport of air in a pipeline.Example 9Calculate the critical velocity for transport of air in a pipeline with slope α = 10º and internaldiameter D i = 500 mm.Solution :From fig. A.2.6.1 gives : k = 0.75If we insert this value in A.1-13 we got : U c = 0.75 ⋅ 9.81⋅ 0. 5 m/s = 1.66 m/sAs we see, the system requires quite a high velocity to transport air.Side 26 av 84
Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.If the velocity in the pipe is higher than 1.66 m/s, air bubbles are carried away with the water.If the speed is less than 1.66 m/s, air bubbles will move backward to be released onshore providedthere are no high points in the trace.This is a theoretical consideration. In the real case there is a diffuse transition for U c .Formula A.2-13 gives however an qualified indication.Side 27 av 84
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