PE Pipe Technical Catalogue (PDF) - Pipelife Norge AS

Pipelife Norge ASPE CATALOGUE-SUBMARINE APPLICATIONS, PIPELIFE NORGE AS, December 2002.In practice, ∆v in A.3-23) may be positive or negative :positive,negative,as caused by shutting a valve at the end of a transmission line,or by starting a pumpas caused by a pump failure or by a sudden change of hydraulicconditions that reduce the flow-rate and speed.Example 10Find the size of the water hammer for a PE100 pipe SDR 17.6 if the change in water velocity= 0.15 m/s (reduction).Solution :From table A.3.3.1 we getHence using formula A.3-23) :c = 282 m/s− 0.15⋅282∆p == − 4,3 mwh =9.81- 0.44 barThe pressure is an under-pressure.This result must be added to other external loads to check the risk for buckling.Assuming the required space of time to shut a valve to be from one to two minutes, when operatedproperly, the maximum surge pressure should be in the range :∆p max = 10 – 15 % times the pipes pressure rating PN (bar)If water hammer repeats regularly over a pipe’s service life, it may cause fatigue failure.As a rule of thumb, a PE-pipe can sustain 10 7 oscillations of amplitude + 0.5 x nominal pressurewithout diminishing its service lifetime.Under-pressure will never lead to fatigue, only ovalisation.A.3.4Temperature stressesIf a pipe is exposed to a change in temperature, it will try to adjust its length if it can move freely.The change in length ∆L can be expressed :∆ L = α ⋅ Lo ⋅ ∆Tα = thermal expansion coefficient (≈ 0.2⋅10 -3 ºC –1 )L o = initial length at installation∆T = change in temperatureA.3-26)As we see the change in length is independent of the diameter and the wall thickness.Example 11How much shorter will a PE-pipe be if it is installed in sea water at 4ºC when it had a length of3000 m at 20ºC in the production factory ?Solution :We apply formula A.3-26) and get :∆L = 0.2 ⋅ 10 -3 ⋅ 3000 ⋅ (4-20) m = -9.6 mThere have been some real examples where subwater pipelines have been too short due to changein temperature.Side 38 av 84