A vacuum level that will not come up makes the vacuum pump the first suspect on site. But there is another common reason when the pump itself is sound and the pipe joints are tight: condensate in the low points of the vacuum line. It runs downhill into the pump cavity, emulsifies the pump oil and pulls the pumping speed down, and what finally shows up is a degassing result out of specification, while the real cause sits in a slope nobody was watching.
The condensate is produced by the degassing section itself
Degassing does not pump the oil dry, it draws the gas out of the oil. The oil is atomised in the tank, dissolved gas and water leave with the droplets, and this mixture of oil vapour and gas is pulled into the pipework. The pipework is colder than the tank, so part of the mixture condenses on the inside wall and finally gathers at the lowest point of the pipe run. That liquid is the condensate.
How much of it there is depends on three conditions: the higher the water content of the incoming oil, the more is produced; the higher the oil temperature, the more oil vapour enters the pipework; and the longer the pipework with more bends, the stronger the cooling and the denser the condensation points. The working vacuum of the ZYD-100 is -0.08 ~ -0.0999 MPa and the limit vacuum is ≤5 Pa, and at that vacuum level the temperature and the pressure inside the pipework are low enough for water to condense, so condensate is not something that only appears in an abnormal condition.
What happens when it runs back into the pump
If the vacuum pipework is installed with a fall along the flow direction, the condensate runs towards the lowest point. If that lowest point happens to sit on the pump inlet side, the liquid is drawn into the vacuum pump. The consequences come in two steps: first the droplets strike inside the pump cavity at high speed, the pump oil is emulsified and thinned and turns cloudy; then the oil film loses its load carrying capacity, bearings and rotor wear, and the pumping speed of the pump falls away.
Once the pump output drops, the first thing to show is the degassing efficiency. The oil rate has not changed, yet the working vacuum no longer reaches -0.0999 MPa, and the gas content stays outside ≤0.01% (GB/T 423). Pulling the pump apart at that point usually finds a pump with degraded oil, while the real problem is in the pipework.
What a collection point looks like from outside
| What is seen on site | Possible state of the low point | Where to look |
|---|---|---|
| Pump oil turns cloudy and emulsified, the vacuum falls back slowly after start-up | Condensate from the low point is drawn into the pump cavity | Walk the pipe run on the pump inlet side and find the low points |
| The pump sounds rough and the running current is above the usual value | Ingested liquid raises the resistance inside the pump | Stop and open the pump, check whether the oil is emulsified |
| Pump oil is consumed noticeably faster | The diluted oil can no longer carry the film load | Check the pump oil grade against the replacement record |
| Oil traces or drips appear on the outside of the pipe run | The low point is full and the surplus is seeping past the joint | Open the flange at the low point and check how much has collected |
How to confirm whether a low point holds liquid
- Stop the machine and let it cool down first, because condensate gathers more tightly at the low points once the pipework has cooled.
- Walk the pipe run section by section along the flow direction, and look closely at bends, tees and reducers, because these are the places that form a low point.
- Put a container in place before opening the flange at the low point, because the amount of condensate is itself the evidence and should not simply be drained away.
- After the inspection, retighten and fit new sealing parts, because a joint that is not put back properly becomes a new leak point.
- Write down the position of the low point and the volume collected, and check it again at the next shutdown to see how fast it builds up.
How the pipework fall and the supports should be arranged
Prevention is cheaper than cleaning. The vacuum pipework has to be installed with a fall along the flow direction so that the condensate always runs away instead of collecting in one place; the pipe supports have to follow that fall as well, and a rigid support must not push a reverse slope into the pipe. The fall is set out in the pipework drawing and is not adjusted by feel on site, because a fall that is too small and a fall that is too large produce two different kinds of trouble.
If the pipework on site really cannot be installed with a fall, a drain point can be added at the low point and the condensate released at regular intervals, which costs far less than waiting for the pump oil to emulsify. The ZYD-100 uses an X-100 vacuum pump and a JRP-500 Roots blower to form a two stage vacuum, with the Roots blower behind the vacuum pump acting as the first stage booster, so the pipework has to be arranged along the real connection order of the two stage system rather than treating both pumps as one stage.
The condensate in a vacuum line does not disappear on its own, it only follows the fall into a pump that was never meant to take in liquid.
