5 replies to this topic

[Daniel89]

Hi,  I am working on a problem where a subsea gas pipeline valve shall be leak tested, by closing valve and lowering pressure by 70 Bar @ valve depth on one side of the valve and observe for pressure increase. After the leak test is concluded, Nitrogen is pumped into the pipeline to equalize pressure and open the pipeline valve again.

 

The gas in the pipeline originally is almost pure CH4. I have used SRK to calculate densities for varying pressure and temperature along the pipeline. 

 

I need to calculate what pressure I need topside, to equalize the pressure across the valve subsea with a mix of N2 and CH4, since the valve cannot be opened with any significant dP.

 

I have calculated the whole riser for pure N2 and CH4 and found what mass of gas for both cases, and the corresponding standard volume. I need to find some relationship number to calculate the correct Z factor for the mixed gas to find the mixed density. Is it correct to calculate Z factor based on mass relationship or sm3 volume of the two gases?

 

The pressure of CH4 before N2 pumping is x, and the goal pressure is y of the mix of CH4 and N2... 

 

Anyone that can help me to get on the right path? 

 

Best regards,

Daniel

 

 

[breizh]

Hi,

Check for Kay's rules applied to mixture of gas !

% are % mole. 

 

Good luck

Breizh 

[horatorres]

Check the following link

 

https://www.linkedin...o-torres-molina

 

I recommend to use  Hall Yarborought correlations and Kay´s mixing rules and check with the Katz Hight pressure equation and choose the larger Value

 

Horacio

[Daniel89]

Thx for the help so far, but if I understand you correct: If I assume no interaction between N2 and CH4 for simplicitys sake.

 

At pressure 100 Barg and temp 10 °C, Z factor for CH4 is 0,832 and for N2 it is 1,0091 .

 

Lets say I have a mixture of 70 mol% CH4 and 30 mol% N2. Is it a fair assumption that the mixtures Z-factor is: mol% CH4 * Z for CH4 + mol% N2 * Z for N2 = 0,7*0,832+0,3*1,0091 = 0,8851

 

Assuming no interaction between the gases? ( Binary interaction parameter between C1 and N2 is considered quite low) 

[breizh]

hi, 

No! 

You need to create a pseudo  component (mixture of N2and CH4)  with all the associated parameters to calculate Z using EOS . 

-Collect all the necessary data for each component (Pc, Tc , Mw , acentric factor, ... ) , i. e N2and CH4 .

-then calculate the pseudo component parameters .

 As an example , Pc of the pseudo component = Pc N2 *%mol N2+Pc CH4 *%mol CH4 and so on for the other parameters .

 

From your data and thermodynamic model SRK  Z=0.895  and density = 94.1050 kg/m3 .Does it make sense?

 

Good luck 

Breizh

[Pilesar]

This seems like a difficult problem to solve exactly. Or am I just overthinking it? The fluid is above critical pressure. Nitrogen pumped in top side will initially heat up substantially before being cooled in the pipeline. The nitrogen and methane will not mix very much initially and only diffuse slowly at the interface between the two fluids. And then there is the temperature gradient. I don't think you will want to wait for steady state before opening the valve. I think the assumption of no interaction between fluids is better than assuming complete mixing. There are so many assumptions you will need to make in your calcs that coming up with a range of topside pressures seems the best you can do.