Welch Vacuum 1376N Chemstar - Manual (Page 27)

27
CAUTION
Be sure the pump is operating at a pressure of 10 Torr or lower for continuous
operation. If not, discuss application with Welch Technical Service before
proceeding to prevent oil from overheating and breaking down resulting in the
pump failing.
CAUTION
Be sure to use argon or nitrogen which is dry rather than wet. Water vapor
mixed with corrosive gases can accelerate corrosive in the pump and lead to
shorten service life.
C. Dry Nitrogen Purging of Oil Case
When pumping on corrosive gases, these gases can have a signifi cant residence time in the oil case. The gases become
dispersed throughout the oil. The longer the residence time the more the corrosion observed.
Welch provides a ¼” compression fi tting positioned at the top of the oil case which is connected to a bubbler tube in the oil
reservoir. When pumping on corrosive gases, Welch recommends passing 2 to 10 cc/minute of dry nitrogen gas through
the oil bubbler at a maximum of 2 PSI.
Reasons for using dry nitrogen gas purging through the oil case are:
1.) Uniform injection of dry nitrogen into the oil reservoir and lubricating fl uid can result in as much as a 10°C lowering of
operating temperature and reduced corrosion rate.
2.) Continuous degassing of the pump fl uid and purging of the oil reservoir typically extends pump oil life.
Part 4 Recommendations for Pumping Corrosive Gases
Experiments and processes that employ corrosive gases are well known for shortening the life of vacuum pumps. Much
reason exists for this to happen:
1.) Particulates may be formed in the experiment or process. These particulates are then ingested by the vacuum pump.
The particulates can grind the moving parts of the forepump out of tolerance.
2.) Chemical reactions occurring in the process/experiment and on various surfaces between the chamber and the pump
may create a new group of compounds. The effect of these new compounds on the pump will be determined by trail and
error.
3.) The Corrosive gases may decompose the vacuum pump oil leading to the pump becoming oil starved. An oil starved
vacuum pump will eventually seize.
Welch has reviewed some of the common corrosive gases encountered in the laboratory and experimental test reactors.
Below are our recommendations on what step you can take to extend the service life of a CHEMSTAR pump in the
presence of corrosive gases.
A. Strong Hydrous Acids: Hydrochloric acid (HCI), Hydrogen bromide (HBO), Sulfuric Acid (H
2
SO
4
), Nitric Acid (HNO
3
).