Romianne:
But isnt there higher water content at higher pressure? Shouldnt I get more kg of water per Sm3 at 5 MPa than at 4 MPa and therefore a higher dew point temperature? Can you please explain it to me in the simple way, I am really lost
From your additional post I understand that the value 50 kg of water per milion Sm3 is the humidity at saturation at 4 MPa and temperature of -30 C.
Consider the equation given in the document you provided:
P*Y_i=P°
Where P: Absolute pressure, Y_i: Mol fraction of water at gas phase (water content) , and P°: vapor pressure. You must fix two of these quantities to obtain the third value, so your question "Shouldnt I get more kg of water per Sm3 at 5 MPa than at 4 MPa and therefore a higher dew point temperature?" doesn't quite make sense, the response is "it depends", the kg of water depends on what temperature you set, the value of dew point temperature depends on what initial water content you set.
Look at what shan says:
The higher natural gas dew point temperature corresponds to the higher pressure. If you keep the stream temperature constant at -30C, water condensate will appear when you increase the pressure from 4 MPa to 5 MPa in the condition your natural stream dew point is -30C at 4MPa. Therefore, the water content in the gas phase is less than 50 kg/MMSm3 due to formation of water liquid phase.
In this case, keeping the temperature at -30C and increasing the pressure from 4MPa to 5 MPa, translates in a smaller water content, you must specify two values (P and T) to get water content.
I have dry natural gas at 5 MPa and 15 C. There is a measurement which says the water dew point of this gas is -30 C, but it is dew point recalculated to 4 MPa; it is not water dew point at real pressure of my system. I found out that at -30 C and 4 MPa, gas is saturated with 50 kg of water per milion Sm3.
I understand that if I increase the pressure, my dewpoint is gonna increase too, but probably at like -28 C. But my system is at 15 C, so I dont have to worry about any condensation and my gas still contents 50 kg of water per milion Sm3. Am I right? My system is closed.
From you first paragraph, we have:
P water content dew point
5MPa x -30C
4MPa 50 -30C
x must be smaller than 50 kg of water per milion Sm3.
In the second paragraph you have a different comparison:
P water content dew point
5MPa 50 y
4MPa 50 -30C
yes you are right, y will be greater than -30C, you must do the calculation to confirm that "y" will have a safe margin as compared to the operating temperature of 15C.
Best regards.