OTL TM PUMPS FOR HANDLING ACIDIC / COROSSIVE VAPOURS
Chemical processes which generate corrosive vapors and which require in-plant vacuum of 1mm.Hg. (Abs.) always pose a challenge to design engineers of vacuum pumps.
Single or double stage high vacuum pumps in conventional design will not give satisfactory performance even if all components are made in SS 316. These components may not corrode but are not suitable as far as the friction between working parts is concerned and the pump will have tendency to seize frequently.
In designing Once Through Lubricated (OTLTM) vacuum pumps, basically accepted three factors, which are responsible for the corrosion of pump components, are taken into consideration.
• Whenever any material is in vapor stage, its corrosive properties are drastically reduced.
• Whenever condensation of vapors inside the vacuum pump occurs, maximum corrosion of the Components takes place.
• Corrosive vapors contaminate lubricating oil very badly and this oil should never be Re-circulated if the corrosion is to be eliminated.
These three factors are given prominence while designing Once through Lubrication ( OTLTM) vacuum pumps.
In OTLTM vacuum pumps, the first stage (Connected to plant ) is located on the top of second stage (exhaust stage ), so as to drain off any condensate accumulated inside the pump by gravity whenever the pump is stationary. During the running, the condensate is continuously thrown out. Since no condensate accumulates inside the pump, no corrosion takes place.
The operating temperature of the pump is maintained between 50 to 80 degrees centigrade by the radiator cooling arrangement. At high temperatures, the materials sucked in by the pump in vapor form remains in the vapor form throughout its stay in the pump until it is exhausted, thus eliminating corrosion.
In OTLTM vacuum pumps fresh oil is continuously injected by an independent force-feed lubricating pump (at about 120 ml/hr. to 320 ml/hr. depending upon quantity and nature of vapors). This small quantity is not recirculated but it is thrown out from the exhaust immediately, thus totally eliminating the cause of corrosion.
In OTLTM series of pumps, the bearings on both, drive end and non drive end are separated from working chamber by two VITON oil seals. The SKF bearings are filled with high melting point grease. The vanes are of toughened phenolic fiber. The stator, rotor and end covers are of cast iron.
In addition, we insist upon the follow up of operating procedure as under :
• The pump should be run alone for about 15 minutes so as to attain the operating temperature of 50-65 degree C. before connecting to the plant.
• Before stopping the pump, it should be isolated from the plant and run alone with a bleeder valve open for ingesting fresh air for about 3 minutes. This will drive out all the traces of vapors inside the pump and later flush with oil that will cover all parts like rotor, stator, end coves, vanes and will not expose them to atmosphere.
With the design features and meticulously followed warming up and air bleeding operations, the OTLTM series of pumps, although they are made of cast iron components will not corrode with corrosive vapors and will give long and trouble free service.