HyQuest Solutions HS40C, User Manual, Page: 27 / 37

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Appendices
2. The ability of a gas to “pass through” liquids.
The first characteristic has already been demonstrated. The second characteristic can be appreciated by blowing
through a hollow tube (straw) that is immersed in water. No bubble will appear until the pressure in the tube is
sufficient to expel all the water from the tube.
Figure 3
I. If a tube is immersed in a liquid, it can be seen that the pressure at the bottom of the tube is that exerted by the
head of liquid above it.
II. If we connect a gas supply to the tube, the gas will act on the surface of the liquid in the tube and tend to force it
down. Assuming there is no change in level, the pressure at the end of the tube must be still the same.
III. If we increase the pressure so that the gas can push out all the liquid against pressure level A, then it follows that
the pressure at A will be the back pressure in the system. Also the pressure of each bubble of gas as it emerges
from the end of the tube will a that point have an internal pressure equal to the head, and this internal pressure
reduces until the bubble emerges at the surface at atmospheric pressure.
Therefore the gas acts like a plunger to keep the water out of the tube and in effect, precisely balances the head of
water that would naturally occur if the tube was opened to atmospheric pressure.
In other words, the system pressure (back-pressure) is supporting a column of water, equal to the pressure head.
Hence, any pressure measuring device, if connected to the pressure system will record the pressure head over the tube
outlet (orifice).
Figure 4 – depicts a water manometer connected to the system and it is recommended that – such an arrangement be set up in a
workshop for staff training.
Gas Purge Operational Characteristics
In the foregoing, the discussion has been limited to static head conditions, whereas in practice, the levels in stream are
constantly fluctuating.
“Rising” Situations