The easiest misconception to fall into while watching the vacuum gauge on the degassing tank is that a needle sitting still at -0.08 MPa or a deeper reading means the gas has been drawn out of the oil. The vacuum level is a pressure reading; the gas content is a volume fraction measured to GB/T 423. They describe two different things on the same machine. A pressure that has reached its target shows only that the pumping speed keeps up with the leakage of the system. Whether the gas is stripped out and carried away depends on how well the oil is actually atomised and how long it stays in the tank.
The vacuum level reads pressure, the gas content counts gas
The working vacuum of the ZYD-100 is -0.08 ~ -0.0999 MPa and the ultimate vacuum is ≤5 Pa. These two figures describe the pressure level of the vacuum system in a particular state: the first is the working range held during normal degassing, the second is the depth the system can reach under limiting conditions. The target for the gas content in the oil is ≤0.01%, based on GB/T 423, which measures how much dissolved gas is still left per unit volume of oil.
The pressure gauge can be read at any time; the gas content can only be established by taking a sample and sending it for testing. There is no one-to-one conversion between the two: a machine can show a very good vacuum reading while the gas content measured from a sample is still above the target. The reverse also holds. When the vacuum level cannot be reached for the moment, the gas content need not be poor, because the oil may not yet have been fed into the vacuum zone in any quantity.
Whether the oil is really broken up into a spray
Degassing presupposes that the oil is atomised. Below the set temperature the viscosity is high, the atomised droplets are coarse, and the dissolved gas inside them does not escape quickly, so the degassing efficiency falls. The working vacuum range of the ZYD series and the temperature control range of 45–80 °C of the TYA series are used together, and the upper temperature limit is pulled down to 70 °C for oil with a high water content. A higher temperature is not always better: margin has to be kept below the flash point, and a higher temperature also makes foam more likely inside the tank.
To judge on site whether atomisation is really taking place, watch two things: whether the oil inside the tank has formed a steady rolling movement and surface disturbance, and whether there is a continuous flow of air at the exhaust end. If the vacuum level is steady but the oil in the tank hardly moves, what is being drawn off may be only the air left in the tank itself, and not the gas carried in by the oil that is fed in continuously.
| What is seen on site | Which stage it more likely sits in | The first check to make |
|---|---|---|
| The vacuum gauge settles quickly at the -0.08 MPa mark | The system pressure is at its target, the oil may not have entered in any quantity | Check the oil level in the tank and whether the oil inlet valve is open |
| The vacuum level is steady but the gas content is still high | Atomisation is incomplete, or the residence time is insufficient | Check whether the oil temperature has reached the set value |
| The vacuum level becomes shallower as the oil feed is increased | The gas load has exceeded the margin of the pumping speed | Reduce the oil feed and watch whether the reading recovers |
| Oil mist is carried out at the exhaust end | The oil-gas separation stage is heavily loaded | Check whether the separation stage and the automatic defoaming device are working normally |
| The vacuum level is steady before the oil temperature has reached the set value | What is drawn off is mostly the air left in the tank | Bring the oil temperature to the set value first, then observe again |
Residence time: drawn off is not the same as drawn clean
The gas content is the accumulated result. For a batch of oil running from the tank inlet to the tank outlet, even if every segment is degassing, if the total time is not enough the average for the whole batch cannot come down. The flow rate of the ZYD-100 is 6000 L/h, which is a rated value; how much oil the tank actually holds on site and how many times the oil circulates in the tank decide how much degassing time this batch really gets. The flow rate of the ZYD-200 is 12000 L/h and of the ZYD-300 is 18000 L/h. A higher flow rate means more oil passes per unit of time, so the degassing time depends more heavily on the tank and on the path.
One more point is easily overlooked: sampling itself changes the result. Once the oil sample leaves the machine, dissolved gas continues to come out of the oil or to return from the air, so the value measured on the sample is not the same as the value inside the machine at that moment. If a re-test on site gives a high reading, first establish how large that deviation is, and only then decide whether the machine itself has to be looked at.
- Confirm the temperature first, then look at the vacuum. Until the temperature control has settled at the set value, the reading on the vacuum gauge has no reference value.
- Use a change in the oil feed to test the pumping speed: reduce the feed and the vacuum level should recover, and the amount it recovers by is the margin of the pumping speed.
- Take the sample at the oil outlet, not with a dipper straight from the bottom of the tank, where the sediment and the foam would be carried into the sample.
- Record the oil feed, the oil temperature and the running time of every run on the same sheet, so that changes in the gas content have something to be compared against.
- Resetting the vacuum level as soon as it drops usually takes the investigation away; record first, then act.
How to identify the stage at fault on site
Work through the symptoms in order and most cases come down to one specific stage. First look at the temperature: if the oil has not reached the set temperature, atomisation is incomplete and any later vacuum reading, however good, means little. Second, look at the oil feed: with no oil entering, the vacuum level reflects the leakage of the tank rather than the degassing capability. Third, look at the time: if the temperature and the feed are both normal, check whether this batch had enough residence time in the tank. Fourth, and only then, look at the system itself, such as the state of the pumps, the free flow of the pipework and the correct position of the valves.
If the first four steps all check out and the gas content is still out of specification, then consider the initial state of the oil itself: oil with a high initial gas content loads the degassing process heavily, so the actual degassing performance of the same machine cannot be compared directly with that on oil of low initial gas content.
The number on the vacuum gauge answers whether the machine can pull the gas off; the gas content answers whether it has been pulled clean. Between the two questions sit the temperature, the atomisation and the time.
