Wet Chemistry

Oil-Filled Rotary-Vane Vacuum Pumps

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Maintenance

Vapor loads from wet chemistry applications require more attention to maintenance. The pump oil will become contaminated more rapidly than with a clean and dry application. The oil will need to be changed more frequently. Discolored or brown oil is contaminated. If you run your application with gas-ballast open, the oil level should be checked more often.

Proper use and good maintenance will help your vacuum pumps and your applications run better. Misused pumps in wet applications will need repairs / replacement more frequently. Simple vacuum pump maintenance is discussed in a separate document.

Oil Change Schedule

Contaminated rotary pump oil is by far the major performance and failure problem. You need to create your own maintenance schedule, based on how and what applications you run in your lab. For a clean and dry application, the pump can run about 3000 hours between oil changes. For a pump that runs 24x7, that's 125 days or approximately 4 months. For other applications, the oil may need to be changed much more often. In some cases, the pump oil may need to be changed every week or after every run. When vapors are pulled from samples in wet chemistry applications, the oil becomes contaminated very rapidly. In the sight glass, the oil will look darker only if it is already contaminated. When determining your oil change schedule, you may want to know the true condition of the oil. Drain the oil into a clean plastic or glass container. If the oil looks dark or contains particles, then it's time to change the oil. If the oil looks like new oil, or has a little haze, then pour it back into the pump at the oil fill port.

Vapors

In a wet chemistry application, vapor from liquids will be pulled into the pump. Any vapor can condense in the pump and contaminate the oil. Don't forget water vapor -- it is especially bad, since it promotes oxidation (e.g. rust) inside the pump.

When a pump is used to pull vapors that contaminate the oil, severe pump damage can occur when the pump is idle for extended periods, unless vapor control procedures are followed. See below.

To handle vapor, you need to minimize vapor condensation in the pump. You do this by running the pump hotter and giving it a chance to expel vapor. The pumps are generally designed with a gas-ballast to handle vapor loads in this manner.

Vapor Control with Gas-Ballast

The gas-ballast control is a key feature built into pumps to handle vapor loads. When pumping gases that contain vapors, it is important to run with the gas ballast open to avoid condensation of the vapors and resulting oil contamination.

The gas-ballast control is usually a knob on the side or top of the pump. Turn it counterclockwise to open. The amount of opening depends on your pump type and how heavy the vapor load. When the gas-ballast begins to open, you will hear air rattling in the pump and may see small air bubbles in the oil; this is normal.

Running with gas ballast open will limit the ultimate vacuum and will cause the pump to run hotter, but the system will work more efficiently with vapors. Gas ballast will also produce more oil mist (may look like light smoke) from the pump exhaust port. Monitor the oil level in your pump. Depending on the situation, the oil level in your pump could become dangerously low in a few days. To control the oil mist in your laboratory, you may want to install a good-quality oil mist filter and/or vent the pump exhaust into a fume hood. It is also important to route the exhaust vent initially downward so that the vapor condensate does not drip back into the pump.

Gas-Ballast Operating Procedure

Use gas ballast before, during, and after your application run. WARNING -- For some pump types, leaving your application connected to an idle pump with gas-ballast open may cause pump oil to be sucked back into your application.

Vapor Traps

When pumping vapors, it is advisable to use a suitable vapor trap before the inlet to the pump. A vapor trap can be important for the performance and service life of your pump. Cold traps work best. Dependable and effective cold traps use refrigeration systems or liquid nitrogen. Alternatively, canisters filled with activated alumina, charcoal, or other desiccants can be placed between an application and the vacuum pump. The effect of desiccants is limited and requires frequent baking to expel vapor.

Vapor Explosion Hazard

The explosive power of a flammable substance is realized when it is vaporized and ignited with a spark. Inside an oil-filled rotary-vane vacuum pump there is always an oil mist, which is a form of vapor. If the oil is contaminated with a flammable substance, then the oil mist can be ignited by a spark. When pumping gases that contain quantities of flammable substances, use a cold trap before the inlet to the pump. Change the pump oil frequently to remove flammable contaminants.

Estimating Vapor

You may want to know how much vapor your pump is pulling. Applications vary widely, and operation can be a factor. You can weigh your sample before and after an application cycle. Weight lost will be as vapor during the application cycle. Figure the vapor rate - how much vapor was given off over the time period. Vapor rate is often expressed in grams per hour (g/h). Compare with the manufacturer's specification for maximum vapor pumping rate. Rotary-vane pumps with a pumping speed of 4 to 10 cubic feet per minute (cfm) typically can handle upwards of 60 g/h of vapor. See your pump manual for details.

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Disclaimer

The use of scientific laboratory equipment involves risk of personal liability and property damage. Gaver Services provides suggestions to improve the efficient use of high vacuum pumps and assumes no responsibility for how an individual may interpret or apply this information.

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