Steam / condensate drain pipe process?

[Vitalis]

Is the process in a drain system isenthalpic or isentropic? We have two situations in our sootblower steam system which normally operates at 200 psig and 750F.
1. During initial warm-up, a steam trap discharges liquid condensate. When the saturated condensate at 200 psig discharges to the very low pressure drain line (eventually to a zero pressure condenser), does it flash to steam at constant enthalpy within the pipe, or does it follow a (nearly) isentropic process with a change in enthalpy?
2. During further warm-up, a thermal drain valve discharges superheated steam at 200 psig and 750F. What is the process in the low pressure drain pipe in this case? (nearly isenthalpic, isentropic, other?) Thanks!

 

[TD2K]

In both cases you would have a constant enthalphy process.

When you drop from 200 psig to essentially atmospheric across a valve or a steam trap, you are going to have lots of turbulence. The process will occur at anything but under constant entrophy.

The drain piping is more interesting (after you've gone through the trap or whatever). If the velocities within the drain system are low enough that you have limited turbulence, that process could be taken as a constant entrophy expansion (meaning the flow down the pipe and loss of pressure is reversible). Condensate collection headers however typically run at fairly high velocities because of the volume of the flash steam produced and thus I would expect it to more be a constant enthalphy process rather than constant entropy.

It will be interesting to see the other comments.

 

[sterl]

Treat the Drain Lines as Constant Enthalpy because in 2-phase, near saturated flow: As pressure reduces even due to friction, the flow accelerates because the reduced saturation pressure causes a small proportion phase change; and at near-saturated conditions the normal analysis predicated on Boyle's Law for Expansivity PV relationship is inconsistent with the accompanying mass exchange between liquid and vapor.