How much mass is released to decrease the pressure

[wasmaro]

Homework Statement

I have a pressure vessel with saturated water and vapor at 1 MPa. I'd like to determine how much vapor mass is released if I lower the pressure to .9 MPa.

Any ideas how I can do that?

Relevant Equations

First law of Thermodynamics

My best estimate was to find the water height and assume it remains constant when venting the air. I know the initial weights of vapor and liquid ( I know the total mass of substance inside the vessel), so I find the new density of water and vapor and calculate the "new" mass and take the difference from the initial mass. However, that's obviously wrong since the water height should not be constant.
Any suggestions?

 

[BvU]

See Anorlunda's reply for this same question.

Post an enthalpy balance and consider temperature

 

[Chestermiller]

Problem Statement: I have a pressure vessel with saturated water and vapor at 1 MPa. I'd like to determine how much vapor mass is released if I lower the pressure to .9 MPa.

Any ideas how I can do that?
Relevant Equations: First law of Thermodynamics

My best estimate was to find the water height and assume it remains constant when venting the air. I know the initial weights of vapor and liquid ( I know the total mass of substance inside the vessel), so I find the new density of water and vapor and calculate the "new" mass and take the difference from the initial mass. However, that's obviously wrong since the water height should not be constant.
Any suggestions?

Are you saying that there is air in the head space? Is the temperature remaining constant, or is the system insulated? or something else?