Size expansion vessel to reduce pressure loss

Hi all,

Given:
I have a pressure vessel of 1000M^3 filled with water with 8 bar pressure.
On this pressure vessel I have installed a valve and a expansion vessel on top of the pressure vessel.
The expansion vessel is prepressurized to 8 bar as well.

Question:
when I open the valve i lose 20 liters of water.
The decrease of water will result in a pressureloss in the vessel.

Question 1: what will the new pressure be (without an expansion vessel)
Question 2: What will the new pressure be with an expansion vessel

To make it easier: the expansion vessel will be filled with water also.

 

[JBA]

If both vessels are liquid full then the pressure measured at the level of the outlet will be the combination of the 8 bar imposed pressure plus pressure head of the contained liquid. lf all vapor has been vented from the expansion vessel and the liquid is essentially incompressible then the 8 bar of imposed pressure will quickly drop to zero (the actual time will depend upon the vessel material's modulus of elasticity and pressure stress on the vessel).

If there is a vacuum breaker attached to the top of the expansion vessel, the pressure measured at the outlet level will then be a function of the pressure head of the draining liquid until the vessel is emptied to the outlet level.

If there is no vacuum breaker on the expansion chamber, once the initial 8 bar pressure reaches zero, the draining liquid will begin to develop a vacuum in the vessels' volume above the liquid level that is proportional to the increase in that volume. This vacuum will continue to increase until: 1) the vacuum is equal to the backpressure at the vessel's discharge point (assuming no air is drawn into the vessel through the draining discharge connection) or: 2) the vacuum reaches the vaporization pressure the liquid in the vessel; or, 3) at some point, the vacuum exceeds the buckling strength of the vessel and the vessel collapses.

 

[CWatters]

How rigid is the tank? 20L is quite a small percentage of 1,000,000L.