Use and Maintenance of Vacuum Pumps

1.0 DESCRIPTION

    Vacuum pumps remove gases or vapour from closed systems. Purists who specialize in low pressure systems dislike the word 'vacuum' because, in the physical world, there is no such thing as a true vacuum or absence of gas. In this article we will continue to refer to any device designed to provide low relative pressure as a "Vacuum Pump".

    In practice, low pressures are necessary in order to remove vapours and some suspended particles in laboratory applications. Pumps can be as simple as one used to displace fluids (think of a syringe) or as complex as those required to simulate deep space.

2.0 CHARACTERISTICS

2.1 Aspirators and Low Pressure systems

    In a laboratory, the means to move the vapour can be a simple venturi system driven by water flowing from a tap. Such pumps are used in aspirating liquid from surfaces where an intermediate container traps the liquid but prevents the liquid from entering the pump mechanism. Other systems can move quite large volumes of gases like a household vacuum cleaner which moves large amounts of air (2,000 litres per minute) but at relatively low differential pressure.

2.2 High Vacuum

    High vacuum pumps are used in pulling high differential pressures (ie. "deep vacuum") such as those used for freeze-drying or evacuating moisture from refrigeration circuits. Generally these pumps displace very small amounts of gases - less than 30 litres per minute of free air. The low pressures that can be achieved are about 1 micron of mercury column - like pressure in deep space.

3.0 HIGH VACUUM DUAL STAGE PUMPS (for laboratory applications)

    These high differential pressures are achieved by generally using two stage rotary vane pumps. The pumping action is done in two cylinder-like chambers with rotating shafts on which several vanes "scoop" the vapour and force it through a system of one-way reed valves.

    The tolerances between all moving parts are so close that by-passing vapour is minimized. Oil is used in these pumps to cool, reduce friction, lubricate and to further reduce vapour by-pass. Different manufacturers recommend specific types of oil with characteristics that optimize the performance of their pump designs.

    All high vacuum pumps generally cost over one thousand and sometimes several thousand dollars. Therefore selection of the appropriate pump for the application is critical.

4.0 OPERATION

    Generally, one should always read the instructions for proper use of vacuum pumps. While today's designs are fairly rugged, these devices will not stand up to abuse.

4.1 What Goes Wrong

    Oil in the pump is most critical. Running the pump without oil or with low oil levels is a killer. Not only will the pump not pull the vacuum but the pump will eventually require expensive repair. Leaking oil seals should be replaced as soon as possible. This is a relatively inexpensive repair.

    Regular oil changes are necessary for all pumps. Oil changes depend on the amount of use. The more hours of duty, the more frequent it is necessary. Also, a harsh environment may contribute increased stress. In time, the oil accumulates debris from:

(1.) Natural wear of the moving components - friction,

(2.) Particles suspended in the vapour that is being moved - dust, microscopic particles, etc.

(3.) Foreign contaminants that evaporate in rarified vapour - organic acids, chemicals.

This debris forms a heavy sludge that rapidly degrades the function of the pump. Contaminants can break down the seals, cause worn bearings and shafts and corrode reed valves. (see image)

4.2 Regular Maintenance:

    Like an automotive engine, the oil must be kept clean by changing regularly in order to provide optimum performance and reduce expensive repairs. A regular scheduled maintenance program should be introduced that enforces this discipline. Some manufacturers suggest oil changes at least every year.

    On-site preventive maintenance consists of flushing the pump of old oil and debris, filling with new oil, changing worn motor belt, and testing the pump vacuum performance

    If your facility does not have a technical service person or group, the best way to ensure this maintenance is achieved is to engage an outside service provider. Pumps may be set up on a scheduled preventive maintenance program where service is performed on site at least once per year or may be delivered to our service shop.

4.3 Repairs

    When the vacuum pump fails to operate to the manufacturer's specification, repair service may be required. Caltech can repair, test and warrant the operation of your vacuum pump. Pumps are serviced in accordance with manufacturer's specifications and tested to design tolerances.

    Refurbishment must be performed in our shop and includes cleaning of interior and exterior parts, minor interior repairs and parts replacement, exterior refinishing, and pickup and delivery. Any pump requiring major repair should proceed only with an estimate.

5.0 SUMMARY

    High vacuum pumps costing thousands of dollars require disciplined maintenance. The cost for such maintenance is an investment in the longevity of the pump. Caltech can provide this service or can suggest a regime to implement regular maintenance.

    Caltech can also refurbish your pump for a fraction of the cost of a new replacement. 

Copyright 2002 R.L. Andersen, Caltech Tech Services