Geothermal Steam Transmission - Darcy Assumptions Check

[rbel038]

Im doing a preliminary sizing (my first) of a geothermal fluid collection system from 4 wells to a turbine unit. The design pressure drop is something in the order of 1 - 1.5bar over 2km through the main transmission line. The steam is dry out of the well and im hoping to get some opinion on what the correct assumptions should be made in preliminary design using darcy weisbach equation for steam.

My methodology as follows:

1. Specified mass flow at the turbine side. Steam is around 5deg superheated and experienced colleagues say this is about correct (its all dry steam out of the well).
2. Darcy pressure drop using average density and viscosity (for Re) through each line.

Assumptions im not sure on:

1. Assuming saturated conditions at each node , this means the enthalpy appears to drop through out the pipeline (which would not bring about superheating)
2. Should i assume isothermal conditions instead? It is all insulated (thickness , conductivity no idea). This should allow superheating as expected by others here in the office.

Your views are appreciated , Thanks

 

[TD2K]

For the Darcy question, the rule of thumb for assuming a compressible fluid can be treated as incompressible fluid is that the change in density over a segment is less than 10% of the initial. If your segment is longer than that, I would break the segment into smaller sections until the density change over a segment is less than 10%. That's trivial with any sizing program and pretty easy even with a spreadsheet model.

For the temperature question, I'd look at the steam flow through the system and do some preliminary heat loss calculations. The 3E plus program (free off the Internet) is pretty good for this. That should give you a reasonable estimate over what fraction of the 2 km pipeline you have superheated steam and where the heat losses get you to saturated steam (then you start to have some condensation in the pipeline). 5C is not much superheat in any event, it's not going to affect your hydraulics.