Rotary vane pumps are considered wet, positive displacement pumps, with the term “wet” denoting that the gases being pumped are exposed to oil. The significant characteristic of oil sealed rotary vane (OSRV) pumps is the use of oil as a sealant, which is not found in ‘dry’ pumps.
This oil and its propensity to exit with the pressurised gases render OSRV unsuitable for many uses, unless high-tech oil traps and accessories are employed. However, such devices are never 100% efficient.
As a result, although oil-sealed rotary vane pumps are very effective they are not suitable for every situation.
The oil employed in oil-sealed rotary vane pumps performs four essential functions:
- provides the seal between the rotor and the vanes;
- forms the “Duo” seal between the vanes and the stator chamber;
- cools the rotor by transferring heat to the outside of the housing;
- and provides corrosion protection to metal parts from the gases being pumped.
How do rotary vane vacuum pumps work?
In an oil-sealed rotary vane pump (or dry vane mechanism) an offset rotor fitted with vanes that slide in and out of their housing within a stator ‘chamber’. The vanes rotate and ‘trap’ a quantity of gas which enters through the pump’s inlet port and the volume between the rotor and stator decreases. The resulting compressed gas exits the outlet port to the atmosphere.
Oil is a fundamental part of rotary vane pumps and to ensure their most efficient long-term performance, as well as effective and trouble-free operation, follow these eight top-tips:
1. Oil type
The oil employed for oil-seal rotary vane pumps is commonly a distilled mineral oil which has been molecularly doctored to provide a stable fluid with a low vapour pressure. In addition, if the pump is exposed to reactive or corrosive gases, the oil will be further processed to remove any impurities. Where a high concentration of oxygen or other chemically reactive gases are present, highly inert, man-made lubricants are recommended which must not be exposed to temperatures above 280°C. If the incorrect oil is used in a chemically aggressive environment, it can break down and leave a tar-like residue, which will block the internal passageways, leading to overheating and failure due to insufficient lubrication. (N.B. not just any engine oil will do!)
2. Venting condensates to avoid corrosion
Starting up the vane pump from cold (especially if the ambient temperature is low and air damp) will cause condensates (e.g. water vapour) to collect in the oil. To reduce this happening, run the pump on zero load with the gas ballast valve open to purge the oil of any condensate entrapped in the oil, until the pump is warm. When pumping is over and before switching off, run the pump (again on zero load with the gas ballast valve open, to purge the oil of condensates) before shutting down. Following this process will significantly reduce the possibility of corrosion.
3. Exhaust mist filters
Oil-sealed rotary vane pumps by their very nature are “oil-wet” and, when operating, expel (with the gas being transferred) some oil as mist. An oil mist filter will capture this expelled oil. After leaving the pump, the gas passes through the mist filter which, via an elemental-filter, reduces the mist into oil droplets and “sumps” it. This captured oil can either be drained manually, or through other accessories, for a return to the pump. The filter element is a consumable product and must be periodically replaced.
4. Preventing condensation
If the oil is contaminated with water droplets “gas ballasting” will clean it. When the pump’s gas ballast valve is opened, the additional flow of air (or other non-condensing gas) absorbs water vapour and allows it to be expelled from the pump. (N.B. The additional flow of air through the pump will create more oil mist at the exhaust, so it is important to check oil levels.) Depending on contamination levels, gas ballasting may take several hours to achieve.
5. Avoid/prevent loss of oil
An effective exhaust valve is essential in avoiding/reducing oil losses. A common valve design uses an elastomer with a metal backing plate (which limits the movement of the rubber part of the valve). All-metal valves are susceptible to “suck-back” if the pump stops under vacuum with oil leaking past the valve and being “sucked” back through the pump and into the vacuum chamber. Therefore, in addition to oil-mist collection and recycling, an appropriate ‘exit’, exhaust, valve is essential to preventing/reducing oil losses.
6. Oil colour
Most oil reservoirs for oil-sealed rotary vane pumps are transparent units which (assuming that the glass or plastic has not been prematurely and permanently discoloured) will enable the colour of the oil to be viewed and checked against a colour chart. Such colour charts, which are readily available from reputable pump and oil suppliers, are specific to each different type of oil. Any change of colour (particularly a darkening) will indicate an unacceptably high level of degradation, contamination and/or condensate. Such regular checking of the oil colour, particularly if the oil mist is being filtered, collected and recycled should be an active part of the pump’s maintenance schedule.
7. Oil change
The method of draining off used oil and refilling the pump with new oil is not dissimilar to that employed for a car engine. That is too say: make sure that the pump is warm, drain the oil using an extension port (if available), fill with clean oil (allowing the first part of fresh new oil to flush out the final part of the old oil), run the pump for a short time, again drain off the oil, refill with clean oil to the correct level, and then ensure that the drain plug is closed properly.
8. Pump servicing
In addition to setting up a pro-active servicing schedule (based upon hours of pump duty) it is important to inspect the pump at regular intervals between servicing. The following are indicators that all is not well: visible/audible leaks, excessive vibration, unusually high oil consumption, discoloured oil, elevated running temperature, excessive time and/or inability to reach pressure/vacuum levels. Any of these indications is evidence that seals and/or vane tips need to be replaced.
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