Welch Vacuum 1376N Chemstar - Manual (Page 28)

CAUTION
Hydrous acids are well-known for corroding the interior of the vacuum pump.
Particularly if acidifi ed oil is allowed to remain in the pump for extended periods
of time.
The fi rst step for reducing the rate of corrosion due to hydrous acids being pumped is to dilute the fl ow of them into the
vacuum pump. By adding dry nitrogen to the hydrous gas stream fl owing 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 fl ow 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.
WARNING
A cold trap concentrates the hydrous acid. Exercise caution
when cleaning trap to avoid acid burns to workers.
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 Section3 C.) since oxygen and water in the air will only accelerate the
corrosion rate.
The oil should be regularly checked for its pH during the initial start-up days of the experiment or process - and on at least
a weekly basis thereafter. The oil can be checked with pH paper. If the pH drops 2 units below the reading you obtain
using new oil, the oil should be replaced.
If you fi nd that the oil needs to be changed very frequently, you should consider using oil fi ltration system cat. No. 1418G
(See Section 2 C.) with the cellulose element which can absorb over 250 cc’s of hydrous acids.
If the fl ow of strong exceeds 10% of gas fl ow, you must switch to a perfl uoropolyether oil such as Fomblin YO6/6 (Welch
Cat. No. 1408C-06, 1 Kg) - Fomblin is a registered trademark of Ausimont. Perfl uoropolyethers are very resistant to
chemical attack by strong acids.
CAUTION
Failure to use Fomblin Y06/6 in applications where strong acids are pumped
at concentrations above 10% of gas fl ow will lead to degradation of the
DIRECTORR gold oil and to the CHEMSTAR pump failing.
B. Weak Acids: Acetic Acid (HOAC), Carbonic Acid (H
2
CO
3
)
Weak acids can cause a vacuum pump to fail just as easily as a strong hydrous acid - particularly if they are above
ambient temperature when they enter the vacuum pump. The same recommendations for strong acids are applicable to
the weak acids.
CAUTION
Welch does not recommend CHEMSTAR pumps for the gel drying application
unless you are trying to run off of one pump both a centrifugal concentrator
and a gel dryer. If you use a CHEMSTAR pump, you must use a liquid trap
and a cold trap to protect the vacuum pump. NOTE: The reason is Welch had
developed a gel dryer vacuum system employing a room temperature trap (Cat.
No. 1424-61 for 60 Hz operation and cat. No. 1424-51 for 50 Hz operation)
specifi cally for this application. The Welch gel dryer vacuum system does not
work with centrifugal concentrators. Please call our technical service for a
brochure and complete information.
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