Varian 949-9350, Nstruction Manual

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TECHNICAL INFORMATION
GAS BALLAST VALVE
When the pump sucks in vapors, these condense
during compression and mix with the oil, forming
an emulsion (an aqueous vapor, for example) or a
solution (organic solvent vapors, for example).
A number of problems arise from this, such as the
impossibility of obtaining high vacuums, the altera-
tion of the properties of the oil, could cause scaling
on parts of the pump.
To avoid this, during compression at the second
stage, atmospheric air is let into the pump through
an adjustable valve, or “gas ballast valve”, located
at the top of the pump.
In this way, the discharge valve of the second
stage opens through the effect of the atmospheric
air before the partial pressure of the vapor reaches
saturation point, thus preventing condensation
from occurring. The vapors are expelled mixed
with air.
ANTI-SUCKBACK DEVICE
The pump is equipped with a special anti-
suckback device to avoid air pressure rises and/or
oil backflow towards the evacuated vessel when
the pump is switched off. This device has a shutter
which automatically closes the suction duct.
In this way the pump and vacuum system are
completely isolated from each other and air can
enter the pump without any risk for the vacuum
produced in the system.
The device includes some special features,
namely:
-
drive obtained avoiding any form of contami-
nation of the inlet duct by fluids (oil and/or air)
used to command the shutter. Thanks to this,
when the pump is started again, the pump-
down to vacuum conditions is extremely fast
as these contaminants are not present and no
degassing is therefore required;
- suction flange maintained closed even when
the pump is idle, so that pollutants cannot en-
ter from the environment and no oil can over-
flow from the pump.
10 87-900-943-01(E)