Dry Nitrogen Purging Of Oil Case; Section 4. Recommendations For Pumping Corrosive Gases; Sulfuric Acid (H 2 So 4 ), Nitric Acid - Welch CHEMSTAR 1374N Owner's Manual

C. Dry Nitrogen Purging of Oil Case
When pumping on corrosive gases, these gases can have a significant 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 fitting 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 fluid can result in as much as a 10°C
lowering of operating temperature and reduced corrosion rate.
2.) Continuous degassing of the pump fluid and purging of the oil reservoir typically extends pump oil life.

Section 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 SO ), Nitric Acid (HNO
2 4
Hydrous acids are well-known for corroding the interior of the vacuum pump.
Particularly if acidified oil is allowed to remain in the pump for extended
The first step for reducing the rate of corrosion due to hydrous acids being pumped is to dilute the flow of them into the
vacuum pump. By adding dry nitrogen to the hydrous gas stream flowing into the vacuum pump, you can reduce the
concentration of the acid passing through the pump. Ideally you want the hydrous acid to occupy less than 1% of the
gas flow into the pump. If you go above 5%, we recommend an acid neutralization trap, cat. Nos. 1420H-21 or 1420H-20
(See Section 2 C.).
A cold trap is also a possibility for preventing the ingestion of hydrous acid into the vacuum pump. For most common
hydrous acids, a cold trap cooled to -198°C using liquid nitrogen is needed.
A cold trap concentrates the hydrous acid. Exercise caution
Use of the dry nitrogen purge will facilitate the removal of acid from the oil reservoir. Avoid using air as the diluting
gas(See Section 3 B.) or through the purge (See Section 3 C.) since oxygen and water in the air will only accelerate the
corrosion rate.
).
3
CAUTION
WARNING
when cleaning trap to avoid acid burns to workers.
periods of time.