United Vacuum – Consider your options

When choosing a vacuum pump for specialty gas applications, it is important to consider many factors, including pump type, pump size and vacuum level required.

There are four major types of pumps that may be used:

• Single stage oil sealed rotary vane vacuum pumps
• Dual stage oil sealed rotary vane vacuum pumps
• Single stage oil sealed rotary piston vacuum pumps
• High vacuum dry pump

The size of the vacuum pump should correspond to the size of the manifold. The larger the manifold, the larger the pump needed.

Choosing the right type of vacuum pump for your application also depends partly on the vacuum level required. A single stage rotary vane vacuum pump is well suited when working in the range of 27” to 29.9” Hg. A two-stage rotary vane vacuum pump or single stage rotary piston vacuum pump works best when working in the range of 100 microns or less - where the cost of oil changes is an issue, however, a dry pump should be considered.

To determine the pumping speed, which determines the pump down time for your system, the following information is needed:

• The annular volume of the manifold and bottles on the manifold. Note: never base your volume on pressurized cylinders!
• The effective conductance, or ability of gases to flow, through the system. This is determined by the smallest orifice, which is almost always the cylinder valve.
• The ultimate vacuum that must be achieved.
• The desired pump down time from atmosphere to desired vacuum.

There are important considerations in the design of the entire system, as well. Sizing of the vacuum inlet lines has a big effect on vacuum pump performance - keep all connections to the inlet of the pump as large as possible. The larger you can keep the manifold and all of the connecting pipes, tubing and hoses up to your restricting orifice (which is usually the cylinder valve), the faster your pumping speeds (pump down time) will be. Also, always be sure to factor in the size of your smallest orifice as this will be your restricting factor.

The exhaust side of the vacuum pump is just as important. Try to make the piping on the exhaust side the same size as the exhaust port on the vacuum pump. If the pump exhaust line is too small, back-pressure will be created in the pump. This will cause the vacuum pump to overheat and possibly seize, and will also cause premature seal failure. Make sure that there are no shut-off valves in the exhaust line because if a valve is closed while running the pump it will cause severe damage to the pump. The integrity of the exhaust line is critical to prevent leaking of flammable or toxic gases into the working environment itself.

If the vacuum pumps are to be operated in a restricted flammable area, then the electrical component specifications must comply with the area designation. This applies to motors, electrical valves, instrumentation and also control electronics.

The gases that are being pumped from the gas cylinders can present hazards and should be handled carefully to ensure safe operation of the system.

It is highly recommended that you follow routine maintenance schedules for your vacuum pump. For oil sealed pumps it is critical that the oil be changed on a regular basis. The best gauge to determine if the oil needs to be changed is by examining its clarity; if the oil is discolored or milky, change it as soon as possible. On pumps equipped with exhaust filters, examine the filters quarterly for discoloration and saturation. When filters are saturated or discolored, change them as soon as possible.

By properly installing and maintaining your vacuum pump, it will remain in service for many years.