volume of gas initial in dipping tube

Hello Mohamed,

This is simply the volume of gas in the tank at the start of the simulation. Please see the section "Treat as Junction" in the following wiki: Modeling Reference - Hydropnuematic Tanks. This is the volume of gas that corresponds to the hydraulic grade during the initial conditions (unless using a bladder). It is important to remember that the tank is only treated as a junction in the initial conditions. During the transient simulation it is still treated as a hydropneumatic tank. 

Regards,

Scott

Thanks Scott.

But it is an input i have to enter. i want to know how to calculate this number?

will i use P1V1 = P2V2 ?

P1 is atmospheric pressure.

v1 is compression chamber volume.

P2 is (Hydraulic grade initial - tank elevation + atmospheric pressure)

V2 is volume of gas initial which i need to calculate

Mohamed,

Yes, you are on the right track.

When the "Treat as junction?" option is set to "true", it means that you want the tank to "float" on the system during the initial conditions (pressure in the tank settles to be equal to the normal pipeline pressure). The "volume of gas (initial)" is the volume that the trapped gas compresses to when the pressure inside the tank is equal to the initial conditions pressure.

It is ultimately up to your engineering judgment, but the method you mentioned is a possible approach to determine the initial gas volume to use. The wiki article that Scott mentioned has some similar examples where two iterations of the gas law PV=nrt (simplified as PV=K) are used. Since you know the compression only starts to occur when the water level rises to the bottom of the dipping tube, you can use the full compression chamber volume for the initial gas law calculation, with an initial pressure of zero (zero gage, 1.0 absolute), calculate the "K", then use the gas law equation again with the initial conditions pressure P (expressed in absolute pressure) , the K you first calculated, and solve for V for the initial gas volume that the chamber compresses to at that pressure.

I have now added these details to the dipping tube section of the article.