Operation - Welch CHEMSTAR 1374N Owner's Manual

Section 2: OPERATION
2.1 Starting Procedures
2.1a Starting a CHEMSTAR Pump
Before attaching the pump to a system it is well to familiarize yourself with the function and action of the pump
which you have now acquired. Remove the intake and exhaust port plugs and temporarily provide a stopper for
the intake and a dust cap for the exhaust. Review the power requirements as described in Paragraph 1.6.
2.1b Cleanliness
Take every precaution to prevent foreign particles from entering the pump. A fine mesh screen is provided for
this purpose in the intake passage of all CHEMSTAR pumps.
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 Shutdown Procdures
2.3a CHEMSTAR Pump 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 isolate the gauge, then turn off the power and open the system to atmosphere. If the pumps removed
from the system, over the intake port with a rubber stopper or suitable cover to protect the pump against contami-
nation 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 and refill with a fresh charge.
2.4 The Principle of Gas Ballast
2.4a The Effects of Unwanted Vapor
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, oil 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 recirculated to the chambers of the pump where
it is again volatilized causing increased pressure within the system.