When a machine that has been out of service for more than a month is brought back into use, the problems tend to come out all at once on the first start: the vacuum is slow to draw, the pump starts with an abnormal noise, the oil temperature will not come up, and the alarm goes off as soon as the power is applied. Behind these symptoms are four kinds of change that take place during the standstill: ageing and hardening of the seals, moisture absorption or oxidation of the pump oil, deterioration of the oil standing in the tank, and moisture and dust on the pipework and the electrical components. There is a fixed order for dealing with these, and working through it in that order rules out most of the causes without having to dismantle the pump first.
The order to follow is: seals and interfaces → pump oil → the oil in the tank → blockage in the pipework → electrical and earthing → heating and temperature control → no-load verification. Do the checks that do not require dismantling the pump first, then the ones that do, and the energised test last.
Step one: the seals and the interfaces
Rubber parts that have stood for a long time or been soaked in oil harden and lose their elasticity; old seals may have left an impression on the flange face, and the sealing face is easily damaged by tools during dismantling. The check is to open up the tank cover, the sight glasses and the pump connections one by one and look: are there cracks in the seal rings, permanent flattening, a sticky or hardened surface; pinch one between finger and thumb and check whether the elasticity is still there. The criterion is that the seal rings are free of cracks and permanent deformation and that they give a clear rebound after being squeezed. The action is to replace the aged parts, clean the sealing faces before assembly and apply a grease compatible with the oil. Any dismantling has to be done with the machine stopped, the pressure released and the power disconnected.
Step two: the pump oil and the pump set of the vacuum pump
During the standstill the pump oil takes up moisture and emulsifies, it oxidises and turns black, and the pump seals and shaft seals may also leak slowly. The check is to look at the sight glass of the pump: is the oil level within the marked range and is the colour clear and bright; emulsified pump oil appears milky and translucent. Too little oil is the most dangerous condition, because on restart the pump runs in a partly dry friction state and the bearings are the first thing to fail. The action is to stop the machine, disconnect the power and change the pump oil, topping the level up, and at the same time to look at the drive parts of the pump for corrosion or an abnormal clearance.
Step three: whether the oil in the tank can still be used
Oil with water in it that stands for a long time forms a separated layer, and sometimes an emulsion layer, on the bottom of the tank, and oil with a high water content may already have produced acidic substances. The check is to take a sample from the bottom and a sample from the top through the sampling port, compare the colour, look for separation and for any smell, and measure the water content where necessary. The criterion is that the water content measured to GB/T 7600 should not exceed 3 ppm and the breakdown voltage measured to GB/T 507 should be not less than 65 kV; only when both are met is continued use worth considering. If it fails, drain the tank, clean it and change the oil; if it passes, it is still advisable to run at a reduced load the first time and to watch for any abnormality.
Step four: whether the pipework is blocked by deposits
If the oil left in the pipework has oxidised and thickened, deposits build up at the bends, in the valve elements and at the oil inlet strainer; the residue on a strainer that has stood for a long time cakes into a solid block. The check is to open up the oil inlet strainer and see whether it has caked, to turn the valve spindle by hand and see whether it is stiff, and to look for settled solids in the sight glasses. The criterion is that the spindle turns freely and that the strainer is free of caked blocks. The action is to stop the machine, release the pressure, disconnect the power and wash it out; the strainer and the seals are to be replaced where necessary, and a blocked strainer must not be fitted back and put into service.
Step five: electrical and earthing
During the standstill the distribution cabinet may have been in a damp or dusty environment, the terminals are easily oxidised and loosened, and the insulation parts may have absorbed moisture. The check is: open the control cabinet and look for oxidation, looseness and corrosion on the terminals; measure the insulation with an insulation tester; check the incoming phase sequence as described in the machine manual and carry out a phase-loss test; re-measure the earth resistance with an earth resistance tester and compare it with the record from before the standstill. The criterion is that the limit for the earth resistance follows the machine manual and the electrical rules of the site, that the re-measured value does not exceed the value on the earlier record, and that the phase-loss and phase-sequence protection operates reliably under simulated conditions. Retighten the terminals and apply moisture protection; where the insulation is low, establish the cause before replacing the components.
Step six: calibration of the heating and the temperature control
After a long period out of use, the condition of the surface of the heating elements and the position of the probes may have changed, and the reading of a temperature probe that has stood for a long time may have drifted. The check is to energise the machine and raise the temperature with oil in the tank and the level confirmed to cover the heating elements, and then to compare the measured oil temperature with the value set on the panel. The criterion is a temperature control range of 45–80 °C, with the upper limit brought down to 70 °C where the water content is high, and the deviation between the measured value and the set value within the range permitted by the machine manual. The action is to calibrate or replace the probe and to measure the insulation of the heating circuit.
How to verify the first start
Do not admit the oil and go to full load straight away. Work through four steps, each with a clear pass condition:
- Run only the vacuum pump and draw a vacuum at no load, with no oil and no heating. See whether the reading settles near the working vacuum of -0.08 MPa; the ultimate vacuum indicator is ≤5 Pa, and at no load it should be clearly better than under load.
- Energise the pump set separately and jog it, listening to the sound and watching the current. Stop at once if anything is abnormal, and do not run it continuously for long.
- Confirm that the level and foam detection are working and that no alarm is left unreset, then take up low-load processing, starting the feed rate at the lowest step.
- For the first run, choose oil with a low water content and few particles; once the system has settled down, process the difficult oil.
| Symptom | Possible cause | How to confirm | Action |
|---|---|---|---|
| The vacuum is clearly slower to draw after the machine is restarted | Ageing of the seals, a small leak at a new interface | Apply soapy liquid point by point to find the leak, check the elasticity of the seal rings | Stop the machine, release the pressure, disconnect the power, then replace the seals and retighten the interfaces |
| The pump starts with a dry friction noise and the pump oil sight glass turns white | The pump oil has absorbed moisture and emulsified, the level is too low | Look at the level and the colour, and smell the oil where necessary | Stop the machine, disconnect the power, change the pump oil and top the level up |
| The oil temperature will not come up after start-up | The temperature probe has drifted, the insulation of the heating circuit has fallen | Compare the measured oil temperature with the set value, measure the heating circuit | Calibrate or replace the probe and check the heating supply |
| The water content is clearly high after the first run | The residual oil in the tank has deteriorated and has mixed with the new oil | Take a sample and measure the water content (GB/T 7600) | Drain and clean the tank, change the oil and reprocess at a low load |
| The motor does not start or runs in the wrong direction after the power is applied | The incoming phase sequence is wrong, the terminals are loose | Check the phase sequence as described in the manual, tighten the terminals | Stop the machine, disconnect the power, correct the wiring and test the phase-loss and phase-sequence protection |
| The level display does not agree with the sight glass | The infrared level probe is affected by residual oil | Compare the mechanical level with the displayed value | Clean the probe and recalibrate it |
On the first start after a long standstill, draw a vacuum at no load first, then jog the pump set, confirm that the alarms can all be reset, and finally run the system at a low load until it settles down.