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Rotary vane vacuum pump maintenance and oil change

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Oil consumption, oil contamination and oil changes in Rotary vacuum pumps

What is vacuum pump oil used for?

The oil serves to:

lubricate moving parts
seal moving parts against atmospheric pressure
seal the valve
fill the dead space below the valve
to seal the various operational spaces one from another

In all pumps it is possible to check the oil charge during operation using the built-in oil level sight glass. During continuous operation in particular, it is necessary to ensure that the oil level never falls below minimum . During a pumping process, oil-sealed rotary vane pumps will emit oil vapors from the outlet port due to the high operating temperature. This leads to oil loss to an extent which will depend on the quantity of gas or vapor which is drawn into the pump. Larger oil droplets can be retained by installing acoarse oil separator in the discharge line. This will reduce oil loss considerably. The fine oil mist filter installed in some pumps will also retain the finest oil droplets so that no oil at all will leave the outlet of the pump. Oil loss is reduced practically to zero as the oil which is separated out is returned to the pump. For pumps without an integral fine separator this device is offered as an optional extra.

Removing and fitting of the internal demister of the Leybold TRIVAC D 40-65 B

Take a look at this video for a demonstration on how to fit a de-mister on a Leybold TRIVAC D 40-65 B

How often should the vacuum pump oil be changed?

If an oil-sealed rotary pump is operated without an oil separation and return device, then it will be necessary to expect a certain amount of oil consumption, the extent of which will depend on the size of the pump and the nature of the process. In the worst case this can amount to about 2 cm3 for every cubic meter of air pumped. Greater oil loss must be expected when operating the pump with gas ballast.

If the pump oil has become unusable due to exposure to process vapors or contaminants , the oil will have to be replaced. It is impossible to formulate any hard-and-fast rules as to when an oil change will be required, since the operating conditions will determine how long the oil will remain good. Under clean conditions (e.g. backing pumps for diffusion pumps in electro-nuclear accelerators) rotary vane vacuum pumps can run for years without an oil change. Under extremely “dirty” conditions (e.g. during impregnation) it may be necessary to change the oil daily. The oil will have to be replaced when its original light brown color, has turned dark brown or black due to ageing, or has become cloudy because liquid (such as water) has entered the pump. An oil change is also necessary when flakes form in corrosion protection oil, indicating that the corrosion protection agent is exhausted.

Steps to follow when changing the oil

The oil change should always be carried out with the pump switched off but at operating temperature. The oil drain opening provided for each pump is to be used for this purpose. If a pump is more seriously contaminated, then it should be cleaned. The applicable operating instructions are to be observed in this case.

Selection of the pump oil when handling aggressive vapors

If corrosive vapors (e.g. the vapors formed by acids) are to be pumped, then a PROTELEN® corrosion protection oil should be used in place of the normal pump oil. These types of vapors will then react with the basic (alkaline) corrosion protection agent in the oil. The continuous neutralization reactions will exhaust the corrosion protection agent at a rate depending on the quantity and acidity of the vapors. The oil will have to be changed more frequently in accordance with these factors. Corrosion protection oils are either very hygroscopic (adsorbing water) or they easily form emulsions with water. Consequently, a pump which is filled with corrosion protection oil will absorb moisture from the air if it is out of service for an extended period of time. A pump filled with corrosion protection oil must not be used to pump water vapor since the lubricating and corrosion protection properties of the oil would be adversely affected. Once the oil has absorbed water it will no longer be possible for such pumps to achieve their ultimate pressure. Oil-sealed pumps should, under normal operating conditions, not be filled with corrosion protection oil. LVO 100 oil is preferred when pumping air, water vapor and non-corrosive organic vapors in so far as there is positive protection against the vapors condensing inside the pump.

Measures when pumping various chemical substances

This discussion cannot provide complete coverage of the many and varied application fields for oil-sealed vacuum pumps in the chemical industry. Our many years of experience with the most difficult of chemicals applications can be used to solve your particular problems. Three aspects should, however, be mentioned briefly: pumping explosive gas mixes, condensable vapors, and corrosive vapors and gases.

Explosion protection

Applicable safety and environmental protection regulations must be observed when planning and engineering vacuum systems. The operator must be familiar with the substances which the system will be pumping and take into account not only normal operating conditions but also abnormal situations, operating outside normal parameters. The most important aids to avoiding explosive mixtures are – in addition to inertization by adding protective gases – maintaining the explosion limit values with the aid of condensers, adsorption traps and gas scrubbers.

Protection against condensation

Leybold pumps offer three options for keeping vapors from condensing in the pumps:

The gas ballast principle (See Fig. 2.14). This increases considerably the amount of vapor which the pump can tolerate.
Increased pump temperature. The rugged design of our pumps makes it possible to run them at temperatures of up to 120°C. This raises the tolerance for pure water vapor, for example, by a factor of five compared with normal gas ballast operation.
Using vacuum condensers. These act as selective pumps and should be sized so that the downstream gas ballast pump will not receive more vapor than the amount corresponding to the appropriate vapor tolerance.

Leybold - Vacuum Fundamentals graphics

Fig. 2.14 Diagram of pumping process in a rotary vane pump without and with gas ballast when pumping condensable substances.

a) Without gas ballast

Pump is connected to the vessel, which is already almost empty of air (70 mbar) – it must thus transport mostly vapor particles
Pump chamber is separated from the vessel – compression begins
Content of pump chamber is already so far compressed that the vapor condenses to form droplets – valve pressure is not yet reached
Residual air only now produces the required overpressure and opens the discharge valve, but the vapor has already condensed to liquid droplets in the pump.

b) With gas ballast

Pump is connected to the vessel, which is already almost empty of air (70 mbar) – it must thus transport mostly vapor particles
Pump chamber is separated from the vessel – now the gas ballast valve, through which the pump chamber is filled with additional air from outside, opens – this additional air is called gas ballast
Discharge valve is pressed open and particles of vapor and gas are pushed out. The overpressure required for this to happen is reached very early because of the additional gas ballast air. Condensation cannot take place.
The pump discharges further air and vapor

Corrosion protection

Oil-sealed pumps are already quite satisfactorily protected against corrosion due to the oil film which will be present on all the component surfaces.

a) Acids

Our pumps are fundamentally suited to pumping acids. In special situations, problems with the oil and with auxiliary equipment attached at the inlet and/or outlet may occur. Our engineers in Cologne are available to assist in solving such problems.

b) Anhydrides

CO (carbon monoxide) is a strong reducing agent. When CO is being pumped it is therefore important that gas ballast is not using atmospheric air but rather that inert gases be used at best (e.g. Ar or N2). Inert gas ballast should also be used when pumping SO2, SO3, and H2S. A corrosion inhibiting oil is also to be used when handling these three anhydrides. Carbon dioxide (CO2) can be pumped without making any special arrangements.

c) Alkaline solutions

Normal LVO 100 pump oil is to be used to pump alkaline solutions. Sodium hydroxide and caustic potash solutions should not be pumped in their concentrated form. Ammonia can be pumped well with the gas ballast valve closed. Alkaline organic media such as methylamine and dimethylamine can also be pumped satisfactorily, but with the gas ballast valve open.

d) Elementary gases

Pumping nitrogen and inert gases requires no special measures.
When handling hydrogen it is necessary to make note of the hazard of creating an explosive mixture.
The gas ballast valve may in no case be opened when dealing with hydrogen. The motors driving the pumps must be of explosion-proof design. ATEX regulations apply.
Oxygen: Particular caution is required when pumping pure oxygen!
Specially formulated pump oils must be used for this purpose. We can supply these, accompanied by an approval certificate issued by the German Federal Materials Testing Authority (BAM), following consultation.

e) Alkanes

The low molecular weight alkanes such as methane and butane can be pumped with the gas ballast valve closed or using inert gas as the gas ballast and/or at increased temperature of the pump. But important – Increased explosion hazard!

f) Alcohols

Once operating temperature has been reached, methanol and ethanol can be extracted without using gas ballast (LVO 100 pump oil). To pump higher molecular weight alcohols (e.g. butanol) the gas ballast valve will have to be opened or other protective measures will have to be implemented to prevent condensation.

g) Solvents

Acetone: Can be extracted without difficulty; wait until normal operating temperature is reached.
Benzene: Caution – vapors are highly flammable. Ultimate pressure is degraded by dilution of the pump oil. Mixtures of air and benzene are explosive and flammable. Work without gas ballast. Inert gases may possibly be used as ballast gas. ATEX regulations apply.
Carbon tetrachloride and trichlorethylene: Trouble-free pumping; nonflammable and non-explosive but will be dissolved in oil and therfore increase the ultimate pressure; wait until normal operating temperature is achieved. Keep the gas ballast open when pumping carbon tet and other non-flammable solvents. Use LVO 100 pump oil.
Toluene: Pump through the low-temperature baffle and without gas ballast. Use inert gas, not air, as the gas ballast.

Operating defects while pumping with gas ballast – Potential sources of error where the required ultimate pressure is not achieved

a) The pump oil is contaminated (particularly with dissolved vapors). Check the color and properties. Remedy: Allow the pump to run for an extended period of time with the vacuum vessel isolated and the gas ballast valve open. In case of heavy contamination an oil change is advisable. The pump should never be left standing for a longer period of time when it contains contaminated oil.

b) The sliding parts in the pump are worn or damaged. Under clean conditions our pumps can run for many years without any particular maintenance effort. Where the pump has been allowed to run for a longer period of time with dirty oil, however, the bearings and the gate valves may exhibit mechanical damage. This must always be assumed when the pump no longer achieves the ultimate pressure specified in the catalog, even though the oil has been changed. In this case the pump should be sent in for repair or our customer service department should be contacted.

c) The measurement instrument is contaminated (see page on Maintaining Gauges).

Potential sources of error when the pump no longer turns

Check the pump electrical supply.
The pump has been left standing for a long time containing contaminated or resinous oil.
The pump is colder than 10°C and the oil is stiff. Heat the pump.
There is a mechanical error. Please contact our customer service department.

Oil leaks at the shaft

If oil is discharged at the shaft, then the shaft seal in the drive bearing will have to be inspected and possibly replaced. The design of the pumps makes it possible to replace this ring easily, following the operating instructions provided with the unit.

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Vacuum symbols

A glossary of symbols commonly used in vacuum technology diagrams as a visual representation of pump types and parts in pumping systems

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