A look at vacuum pumps 14-9

The idea is simple enough. The pump will be designed to pump liquid as well as gas or vapors. If you need to create a vacuum you can connect your piping to the suction side of the pump, but if you need a positive pressure you can connect the piping to the discharge side of the same pump.

We can use the vacuum we create to prime a centrifugal pump, evacuate a condenser, lower the pressure in a chemical vessel or do any thing else we can think of where a vacuum might be needed. The pressure side could be used to create a head or pressure in a nozzle application or a hundred other applications you might consider. The capacity needed, along with how much vacuum, or how much pressure, will dictate the type of pump you will need.

Vacuum can be measured in millimeters of mercury (Hg) or inches of mercury. Atmospheric pressure at seal level is 760 mm. or 29.9 inches. Any less than this amount is considered to be a vacuum. Hard vacuum starts at one millimeter of mercury. The industry calls this amount of vacuum "one Torr." At one Torr and harder vacuums any elastomer (rubber part) in the system will out-gas, shrink in volume and then allow air to leak into the system you are trying to evacuate.

As we learned from our discussions about conventional centrifugal pumps, these centrifugal do not work very well pumping gas or vapors, so we will be looking at a positive displacement type to do this job and there are a variety of designs available to us.

It is important to remember that packing does not do a very good job of sealing vacuum so you are going to have to install a good mechanical seal on the rotating pump shaft or rotor to prevent the intrusion of atmosphere into the system. A hydraulically balanced, O-ring design would be desirable.

Liquid ring pumps

In these very popular designs that are sometimes called "wet vacuum pumps", you start with a round multi-finned rotor spinning in an elliptical casing. The casing is full of water or some compatible liquid. The rotor throws liquid outward to the stationary casing where the solid ring of liquid rotates at the same speed as the rotor. The elliptical shaped casing causes the rotating liquid to recede from and enter buckets in the rotor. As the liquid is thrown away from the rotor it draws in gas and vapor through the inlet ports and discharges the gases through the outlet ports as the liquid is forced back to the rotor because of the elliptical shape of the casing.

Problems with liquid ring pumps:

Rotary lobe pumps

Dry screw vacuum pumps

Single stage oil recirculating, sliding vane pumps

Once through oiling sliding vane pumps.

Dry running rotary sliding vane pumps

Two stage, oil once through sliding vane pump

All of these very popular sliding vane designs have similar maintenance problems. The following troubleshooting hints should help you solve some of these problems:

Make sure the oil rate consumption is not changing suddenly.

Listen for an increase in noise level

High internal pump temperature or condensate in the lubricating oil can cause the sliding vanes to "skip" over the walls of liquid lubrication causing a "washboard" affect on the discharge side of the cylinder walls and little to no contact of the vanes on the suction side of the walls.

Troubleshooting

Upper rotor is stuck, but turns freely after the pump has cooled. Over heating is the problem. That is the reason the rotor turned freely after it cooled. Look for:

Upper rotor is stuck and cannot be freed.

Lower rotor is stuck

The pump ran alright last time it was used, but now it is locked up

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