The oil in the vacuum degassing chamber is atomised into fine oil droplets, and part of those droplets is carried along with the extracted gas into the pumping path. The gas eventually goes into the vacuum pump and the oil mist goes with it. In the cold section of the pump these droplets condense and return to the pump oil, and once the pump oil has been diluted the ultimate vacuum cannot be reached. The working vacuum of the machine is -0.08 ~ -0.0999 MPa and the ultimate vacuum is ≤5 Pa; when these two figures cannot be reached, in many cases the pump has not failed, but its working condition is being held back by what comes before it.
How the oil mist enters the pumping path
The purpose of atomisation is to spread the oil over a large enough surface area so that the gas and the water dissolved in it can escape more easily. Some of the spread droplets are small enough to be carried off by the gas flow. That is exactly why atomisation improves the degassing efficiency, and it is also where the cost arises: the droplets carried off by the flow meet the cooler inner wall of the pipework, part of them condense again into larger droplets that fall back into the chamber, and the rest are carried on by the flow.
Every section of the pumping path is at a different temperature. The section close to the chamber is hot and the oil mist stays liquid; towards the vacuum pump the pipe wall temperature falls step by step, and in the parts below the dew point droplets start to separate out. If this path contains elbows, valves or bag-shaped sections, the droplets collect in these places and oil builds up.
Where the loss falls: three observable results
The first result is a change in the condition of the pump oil. Once the pump oil has been diluted by the condensate from the oil mist, its colour becomes lighter and its viscosity drops, and a layer can be seen on the oil surface. The second result is a worse ultimate vacuum: pumping down an empty chamber can approach ≤5 Pa, but with oil in the machine, or after it has run for a while, that value clearly cannot be reached. The third result is an unsatisfying end to the gas content: the gas content of the oil is ≤0.01% under GB/T 423, and if the system still falls short on the last part of that figure, it is often related to the actual efficiency of the pumping stage.
The standard vacuum configuration of the ZYD-100 is an X-100 vacuum pump and a JRP-500 roots blower. The division of work between the two stages is this: the roots blower provides the compression, pushing the gas pressure up stage by stage, and the X-100 vacuum pump finally draws the gas away and exhausts it. The roots blower does not discharge the gas to atmosphere itself, it only changes the pressure. If oil mist accumulates in the pipework ahead of it, the compression efficiency is affected as well, because the amount of gas entering this section has increased while the pumping capacity of the blower has not.
| What is seen | The possible cause | How to confirm it |
|---|---|---|
| The pump oil becomes lighter in colour and a layer appears on the oil surface | Oil mist condensate mixes into the pump oil and dilutes it | Take a pump oil sample and compare its colour and condition with new oil |
| Pumping down an empty chamber does not reach ≤5 Pa, and it is worse with oil in the machine | The efficiency of the pumping path has fallen | Record the ultimate vacuum separately with the chamber empty and with oil in it |
| Oil builds up at the elbows and valves on the pumping path | Condensate separates out in the cold section and stays there | After shutdown, open up and inspect the low points and the bag-shaped sections of the pipework |
| The longer the machine runs continuously, the harder it is to bring the gas content down to ≤0.01% | The exhaust efficiency of the pump falls as the condition of the pump oil degrades | Compare the gas content records taken before and after a continuous run |
| The temperature or the noise at the outlet of the compression stage changes | More gas is entering the compression stage | Record the running noise of the pump and the condition of the exhaust |
How to keep control of this section of pipework
- Record the data for the vacuuming stage and the degassing stage separately: what level is reached with the chamber empty, and what level is reached with the oil in it. Keep the two figures apart, because the difference between them is itself the size of the loss in this section.
- Inspect and change the vacuum pump oil at the intervals recommended by the manufacturer, and do not wait until the ultimate vacuum drops before dealing with it; by then the loss has been running for some time.
- When inspecting the machine after shutdown, look closely at the low points of the pipework, the elbows and the valve bodies; these are the places where condensate is most likely to remain.
- Record the date on which the pump oil was changed and the reason for the change, so that there is a time reference when the condition of the pump oil is assessed next time.
- For long continuous runs, or for operating conditions with a high water content in the oil, put the cleaning interval for the low points of the pipework into the plan rather than waiting for the vacuum alarm.
What the vacuum pump draws away is gas, and what it carries away with it is oil mist. In the pipework the two are one and the same gas flow, but the loss is charged to the pump's account.
