Understanding the Impact of a Valve on Fluid Properties Upstream and Downstream

[AAMAIK]

I want to know the relation between the properties upstream of the valve and those downstream.
Assume that the throttling device is small that not enough surface area is available for Q to occur. Changes in kinetic and potential energy are negligible.
If I define my system to be the fluid upstream of the valve and through the valve. what would happen to the properties that describe the condition of my system as it goes through the valve?
If my fluid is liquid then the pressure exerted on the valve will be ρgh where the height of the fluid column to the valve. However, if my system is composed of gas then pressure by the collision of the gaseous molecules with the surface of pipes in which the gas is flowing through. If on the other side of the valve the pipe is evacuated of air nothing is present then the pressure is lower this causes the fluid upstream to move, but what happens to the pressure will it stay the same, decrease?
I am not sure whether this analysis is appropriate for this situation.
Another scenario what if downstream the valve another fluid is present what happens to the pressure of the fluid upstream if I consider it as my system

 

[Ross Hanna]

I'm not sure exactly what properties your considering and what your trying to achieve as such. However depending on complexity, maybe the Bernoulli equation could help? Though you said changes in potential and kinetic energy are negligible so i don't know if this is what you want.

 

[AAMAIK]

I want to determine how the pressure and enthalpy change as the system flows through the valve.

 

[russ_watters]

I want to determine how the pressure and enthalpy change as the system flows through the valve.

Could you give us a specific scenario to analyze please. What you provided is too vague.

 

[AAMAIK]

I define my system to be the gas upstream of a valve, the valve is closed. I want to examine how the properties that describe the current state of my system change as I open the valve namely pressure and enthalpy. To simplify matters, I assume that the surface area of the valve is small enough such that no heat transfer takes place. I want to consider the changes in pressure and enthalpies in two scenarios, one if downstream the valve is empty (no matter is present) and the other if gas occupies the space downstream. Is the analogy to consider the gas downstream as a piston and the gas upstream contained inside a cylinder pushing against that cylinder. In addition, the external pressure acting against the gas upstream is increasing as it moves along because more gas molecules downstream are accumulating.