Chemglass AF-0350 Welch Vacuum Pump - Manual (Page 7)

2.2 Leak Detection
2.2a Large Leaks
The importance of eliminating all leaks in a vacuum system is obvious when it is realized that a leak into the
system, at atmospheric pressure, expands in volume by a factor of 750,000 to 10,000,000 or more. The pump
must remove this added volume to maintain the desired vacuum. Fortunately a number of effective techniques
for leak detection have been developed. Large leaks can be located by pressurizing the system and painting
the suspected area with a thick soap solution. Escaping air will produce soap bubbles.
2.2b Small Leaks
Small leaks may also be detected by spraying a suspected area with acetone or gases rich in hydrogen, and
observing a sudden change in pressure on an electrical gauge. The difference in calibration of these gauges,
for air and other gases, will produce a distinct change in the pressure reading. To use this method of detection,
the system must be under vacuum and the gauge sensing tube must be located between the pump and the area
to be probed. Use extreme caution, as these materials are highly flammable!
2.2c 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.
2.3 High Pressure Operation
DOUSEAL two-stage pumps are designed to be most efficient when operated at or near their ultimate
blankoff pressure. The pump is designed to remove air from a vacuum system at atmospheric pressure to a
deep vacuum. When the pump is operated at elevated pressures up to about 10 Torr for long periods of time,
the pump will run hotter. At elevated ambient temperatures under these conditions, the thermal protection
switch on single phase motors may cute out. Use of an exhaust filter with coalescing element is required.
CAUTION
Do not run this pump for prolonged periods of time at or above
10 Torr. The pump will overheat, excessive oil mist will be emitted
from the exhaust and the pump will eventually seize.
Leaky vacuum systems, too large a vacuum chamber for pump size or high gas loads to the vacuum systems
are causes for extended runs of the pump at elevated pressure. If you need additional information, please
contact Welch Vacuum at 847-676-8800, extension 1, for further information and precautions.
2.4 Shutdown Procedures
2.4a DuoSeal Shutdown
A few simple precautions are all that is necessary when shutdown is in order. If a gauge is connected to the
system, first isolate 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 is best to drain the oil refill with a fresh charge.
7