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TM 10-4310-391 -13&P
II-2c. Fine Leaks
Locating very fine leaks requires a helium-sensitive, mass-spectrometer leak detector. This Instrument will
locate leaks which cannot be detected by any other method Numerous fine leaks can have the total effect of a
large leak.
II-3. SHUTDOWN PROCEDURES
II-3a. DuoSeal Shutdown
A few simple precautions are all that Is necessary when a shutdown Is in order If a gauge Is connected to the
system, first Isolated the gauge, then turn off the power and open the system to atmosphere If the pump Is
removed from the system, cover the Intake port with a rubber stopper or suitable cover to protect the pump
against contamination and loose particles If the pump has been contaminated In service and Is going to be
shelved for a prolonged period it s best to drain the ail and refill with a fresh charge.
II-4. THE PRINCIPLE OF VENTED EXHAUST
II-4a. The Effects of Unwanted Vapors
Systems which contain undesirable vapors cause difficulty both from the standpoint of attaining desirable
ultimate pressures as well as contamination of the lubricating medium A vapor Is defined as the gaseous form
of any substance which Is usually a liquid or a solid Water, ail and mercury vapors are three of the more
common vapors encountered In typical vacuum systems When such vapors exist In a system, the vapors or
mixtures of gas and vapor are subject to condensation within the pump; the precipitated liquid may thus
ultimately dissolve or become emulsified with the lubricating medium This emulsion is re-circulated to the
chambers of the pump where it Is again volatilized causing Increased pressure within the system.
II-4b. The Presence and Removal of Condensate
Condensation takes place particularly In the compression stroke of the backing or second stage of a two-stage
pump The compression stroke Is that portion of the cycle during which the gas drawn from the Intake port Is
compressed to the pressure necessary to expel it past the exhaust valve Condensation takes place when the
ratio between the Initial pressure and the end pressure of the compression Is high that Is, when the mixture of
vapor and gas drawn from the Intake port Is compressed from a low pressure to a high pressure By adding air
through the vented exhaust valve to the mixture of vapor and gas being compressed, the pressure required for
delivery past the exhaust valve Is reached with a considerably smaller reduction of the volume of the mixture,
thus, depending upon the amount of air added, condensation of the vapor Is either entirely avoided or
substantially reduced.
II-4c. Pump Function Without Vented Exhaust
In a pump functioning on a contaminated system and operating without the vented exhaust feature,
compression within the stage takes place In the normal manner until the saturation pressure of the
contaminating vapor contained with the mixture of gas and vapor Is reached The saturation pressure of water
vapor Is that pressure and corresponding temperature at which the dew point of the vapor Is reached and
condensation occurs The saturation pressure of water vapor at an ambient temperature of 20°C Is 17 5 tort,
while at 600C, the approximate operating temperature of a pump, the saturation pressure Is 149 torr The
external side of the exhaust valve Is subjected to atmospheric pressure Consequently a compressive force
somewhat greater than atmospheric pressure Is required to open the valve and permit expulsion of the gas
Sometime during increased compression of the mixture of gas and vapors, the saturation pressure of 149 torr
for the water vapor Is reached and the vapor condenses The condensate Is then allowed to emulsify with the ail
which Is re-circulated within the pump stages thus providing continued contamination of the system.
II-4d. Pump Function With Vented Exhaust
On the other hand, when ballast air at atmospheric pressure Is supplied to the compression stroke by means of
the vented-exhaust valve, the partial pressure of the unwanted vapor becomes a very small part of the total
pressure of the mixture of gas, vapor and newly supplied air The vapor Is thus prevented from reaching bits
saturation pressure corresponding to the temperature of the pump and Is finally expelled from the pump as a
vapor.
II-4e. Controlled Ballast Flow
Some degree of vacation In ballast flow may be obtained by the amount of opening applied to the vented-
exhaust valve. Two or more turns of the valve are sufficient to open it wide With the valve open,
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