Comment by Jan on 27 Sep 2009 at 16:26:57

responses: 4
Thank you for the reply.
Let say that Steel pipe is still tight, and gas due to the permeation through HDPE will accumulate and will create pressure outside of HDPE pipe.
After depressurization of HDPE pipe pressure of permeationed methane between HDPE pipe and steel pipe may be so high that HDPE pipe will collapse?
Did I understad this correctly?
If yes, I will propose to create openings on steel pipe, and these openings will be connected to open surface, so methane could excape, and pressure level between HDPE pipe and stell pipe will be atmosheric pressure. Is this solution ok?
Still I'm interested in the calculation:
What is the volume of diffused methane in one year, per meter of pipe, when
HDPE pipe has outside diameter 110mm, inside diameter 97,4mm, and methane has temperature 20Â°C, pressure inside of pipe is 600kPa ? Pressure outside HDPE pipe is atmospheric.
Thank you for the answer.
Comment by Composite Agency on 28 Sep 2009 at 16:46:50

responses: 3
Pressure inside is 6 Bar?
Collapse resistance pressure for unaged thin HDPE cylinders is approximated by:
Pcollapse = 0.365 * E * (t^2/r^2) with r >> t
With E is the Young Moduls of HDPE (08. Gpa, unaged), t is the thickness of the wall (6.3 mm), r is the radius (48.7mm) gives:
48 Bar.
Now since the maximum gas pressure in the interface between HDPE and Steel will never exceed the maximum system pressure (6 Bar), the interfacial pressure differential will always stays much smaller then the collapse pressure (48 Bar). After a certain time of loading the HDPE modulus will decrease, hence the Pcollapse will decrease. In fact this decrease will determine the service life of the suggested system.
If you still need to know the exact amount of Methane that diffuses per square meter, we need an estimation of the interfacial thickness between HDPE and Steel?
Kind Regards,
Composite Agency
Comment by Jan on 28 Sep 2009 at 17:23:09

responses: 2
Thank you for the explanation.
Still I'm interested in the calculation:
I just dont understand why is important to know space between HDPE and STEEL, when there is atmospheric pressure and space is ventilated, to prevent cumulation of methane.
For me is important volume of diffused methane.
Here are the conditions.
What is the volume of diffused methane in one year, per meter of pipe, when
HDPE pipe has outside diameter 110mm, inside diameter 97,4mm, and methane has temperature 20Â°C, pressure inside of pipe is 600kPa ? HDPE pipe is inside steel pipe which has outside diameter 133mm and inside diameter 124mm. Pressure outside HDPE pipe is atmospheric.
Thank you very much, Jan
Comment by Composite Agency on 28 Sep 2009 at 17:42:17

responses: 1
Jan, OK point taken.
You are interested in a worst case gas flux of Methane through HDPE (note that if the Methane accumulates in a non ventilated environment or a ventilated environment with a slower resulting removal rate than the diffusive flux, Methane pressure will buildup in the annulus, which will reduce the flux as a function of time  then the interfacial space and removal rate is of importance).
The bootstrap calculation for the worst case flux of Methane through HDPE at 6 bar is as follows:
Permeation of Methane in HDPE in ambient conditions is (CheFEM database figure):
9E13 [ m3stp / m3 bar ] [m2/s].
For a pipe with a thickness of 6.3 mm this gives:
9E13 [ m3stp / m3 bar ] [m2/s] / 6.3E3 [m] = 1E10 [m3stp / m3 bar ] [m/s]
STP density of Methane is: 7E2 [gram/m3]
At 6 bar it is: 4E3 [gram/m3]
The flux in gram per square meter of pipe now follows from:
1E10 [m3stp / m3 bar ] [m/s] x 4E3 [gram/m3] = 4E7 [gram/ m2 s]
Per year this flux equals:
13 [gram / m2 year].
Good luck,
Composite Agency