Start by defining what a "efficiency drop" is. The working vacuum of the ZYD-100 is -0.08 ~ -0.0999 MPa. A machine that holds steady near -0.08 MPa during the first ten minutes or so after the oil is admitted and the vacuum has been established, and then falls back slowly to around -0.05 MPa after several hours of continuous running, with the same batch of oil taking noticeably longer to process than last time, is what an efficiency drop means. A vacuum that will not reach the level in the first place is a different problem and has to be investigated in the order set out for the vacuum level topic.
A second pattern has to be separated out: a drop out of -0.08 MPa within a few minutes followed by a continuing slide, which is called a cliff-edge pressure loss and is usually related to the seals, the pump set or an abnormal oil level. It must not be handled as normal fluctuation. The investigation runs through five variables in turn: oil feed rate → water content of the incoming oil → viscosity and temperature of the oil → condition of the pump oil → differential pressure across the element.
Variable one: the oil feed rate
The symptom is that the same drum of oil takes longer to process than last time, and that the vacuum level falls back gradually as the oil in the tank builds up. The mechanism is that the feed rate determines how much water vapour and dissolved gas enters the vacuum chamber per unit of time, and it also determines the residence time of every drop of oil in the tank. To confirm it, reduce the feed rate by one step with everything else unchanged, hold the condition for ten minutes or so and see whether the vacuum level recovers; a recovery means the machine was overloaded. The action is to take the stable value after the reduction as the usable feed rate for the current condition. If that usable rate cannot meet the production schedule over the long term, re-check the throughput against a larger model such as the ZYD-200 at 12000 L/h or the ZYD-300 at 18000 L/h.
Variable two: the water content of the incoming oil
The symptom is that the incoming oil has a higher water content than last time, that the vacuum falls faster and that the amount of condensate drained off increases noticeably. The mechanism is that a large amount of water vaporises and enters the vacuum chamber, the gas load rises, the pumping speed of the vacuum pump falls, and the partial pressure of the water vapour in the tank raises the equilibrium pressure in its own right. To confirm it, take a sample at the oil inlet and measure the water content, and look at the figure measured to GB/T 7600 together with the result of not more than 3 ppm after treatment, then watch the amount of condensate drained off. The action is to remove the free water first in staged pre-treatment; where the water content is high, bring the upper temperature limit down to 70 °C and accept a slower rate in order to protect the vacuum system.
Variable three: the viscosity and temperature of the oil
The symptom is that the vacuum will not reach the level, or falls quickly, when the oil temperature is low, and that it improves once the oil has been heated. The mechanism is that the viscosity determines how well the oil disperses after it has been sprayed and atomised: at a high viscosity the droplets are large and the surface area is small, so water vapour and gas escape slowly from the oil, which amounts to a loss of degassing efficiency, and the viscosity in turn rises as the temperature falls. To confirm it, compare the measured oil temperature with the value set on the panel rather than looking only at the heater indicator lamp, then raise the temperature one step and watch the reading. The action is a temperature control range of 45–80 °C, with a suitable value chosen for the oil and the water content; if the temperature will not come up at all, go back to the heating system.
Variable four: the pump oil of the vacuum pump
The symptom is that the vacuum level slides down slowly while the pump body heats up and the pump note becomes coarse, with the pump oil sight glass turning white or showing bubbles. The mechanism is that the pump oil takes up water and emulsifies, its viscosity falls, its lubricating and sealing ability both deteriorate, and the pumping speed drops accordingly. To confirm it, stop the machine and look at the colour and the level of the oil in the sight glass; emulsified pump oil appears milky and translucent. The action is to stop the machine, disconnect the power and change the pump oil, topping the level up at the same time, and to establish where the water is coming from: condensate running back and excessive water in the tank both give the same result.
Variable five: the differential pressure across the element
The symptom is that the differential pressure rises steadily while the vacuum falls, the two changing together. The mechanism is that as the cake on the element surface thickens the flow resistance rises and the actual pumping speed of the pump falls. To confirm it, compare the inlet and outlet differential pressure readings with the total volume processed by the element and see whether the rate of rise of the differential pressure is proportional to the volume processed. The action is to run at a reduced rate to gain time and to arrange an element change; when the differential pressure alarm operates, follow the setting of the machine.
When the machine must be stopped, and when the conditions can be adjusted and the run continued
The following signals mean the machine has to be stopped and dealt with: the vacuum drops out of -0.08 MPa within a few minutes and keeps falling, with no recovery after the rate is reduced; the vacuum pump develops a clear abnormal noise or the pump body temperature stays high; the pump oil has emulsified and is bubbling; the oil has reached the temperature limit and still cannot be drawn; foam overflows from the tank, or the infrared foam detection operates and the automatic defoaming cannot hold it back; the oil circuit goes overpressure and the audible and visual alarm stops the machine; a burnt smell or a sour smell appears. The following signals mean the conditions can be adjusted and the run continued: the vacuum is only sliding down slowly and recovers to around -0.08 MPa after the feed rate is reduced by one step; the water content has risen but the condensate is draining normally; the oil temperature is below the set value and improves after it has been raised; the differential pressure is rising but has not yet reached the alarm value.
| Symptom | Possible cause | How to confirm | Action |
|---|---|---|---|
| The vacuum slides down slowly and recovers after the rate is reduced | The feed rate exceeds the degassing capability of the current condition | Reduce the feed rate by one step, hold for ten minutes or so and watch the reading | Run at the value after the reduction, or change to a model with a larger flow rate |
| It drops out of -0.08 MPa within a few minutes and does not recover | A seal leak or a fault in the pump set | Stop the machine and check for leaks with soapy liquid, listen to the pump and check the pump temperature | Stop the machine, release the pressure, disconnect the power, then check for leaks and check the pump |
| The amount of condensate drained off increases and the vacuum falls faster | The water content of the incoming oil has risen | Measure the water content of the incoming oil (GB/T 7600) | Pre-treat in stages and bring the upper temperature limit down to 70 °C |
| The vacuum improves after the temperature is raised | The oil temperature is too low and the viscosity too high, so the atomisation is poor | Compare the measured oil temperature with the set value | Adjust the temperature to a suitable value within the 45–80 °C range |
| The pump note becomes coarse and the pump oil sight glass turns white | The pump oil has taken up water and emulsified | Stop the machine and look at the colour and the level of the pump oil | Stop the machine, disconnect the power, change the pump oil and investigate the source of the water |
| The rise in differential pressure and the fall in vacuum happen together | The cake on the surface of the element is thickening | Compare the differential pressure readings with the total volume processed | Run at a reduced rate and arrange an element change |
- Record the feed rate, the water content of the incoming oil, the oil temperature and the vacuum level of each run on one line; the trend is more useful than a single set of values.
- Change only one variable in each reduction test, otherwise it is not possible to tell which one had the effect.
- Whether to stop is decided on the combination of signals, not on a single reading.
- An efficiency drop that keeps recurring has to be traced back to the feed stage, rather than being run at a reduced rate for ever.
For an efficiency drop, reduce the feed rate by one step as a comparison: if the reading recovers it is a question of the operating condition and the run can continue; if it does not, look at the pump oil and the seals.