An oil treatment unit is running in the workshop and a layer of fine droplets suddenly appears on the outside of the tank; they are wiped away and come back a little later. The first reaction on site is usually that it is leaking, but a fair share of these cases are not leaks at all: the tank wall temperature is below the dew point of the surrounding air, and the water vapour in the air condenses on the wall. The two situations are handled in exactly opposite ways, one calls for a shutdown and an inspection of the seals, the other for a check of the temperature control and the ventilation. Tell them apart before starting work.
First establish where the droplets are forming
Position is the first dividing line. Condensate appears on the outside of the tank, especially on the upward-facing surfaces that lose heat quickly, such as the tank top, the upper edge of the tank cover flange and the upper half of the tank wall; leakage usually appears at flange joints, threaded connections, weld seams and around pipe connections, and often takes the form of a continuous wet trace or a string of oily droplets rather than a uniform film over a whole area. Condensate runs down the tank wall and leaves a water trail from top to bottom; a leak point generally leaves no trace above it, and the wetness always creeps upwards from below.
Another way to judge is to look at what the liquid is. Condensate contains almost no oil and feels only like water; the liquid coming out of a leak carries oil, leaves an oily mark when wiped on white paper, and the pool nearest the leak point has the higher oil content.
Why condensation forms: the relation between tank wall temperature and ambient temperature
The physical condition for condensation is simple: the temperature of the tank wall surface is below the dew point of the surrounding air. The working temperature range of the machine is 45–80 °C, with the upper limit pulled down to 70 °C when the water content is high; in normal running the tank wall is hot and should not condense. There are, however, several situations that bring the tank wall temperature down: at a high ambient temperature the heat cannot be dissipated; while the tank has just been filled and the oil has not been heated up yet, the whole machine is still cold; and incoming oil at a very low temperature pulls the wall cold.
The ambient temperature range marked for the machine is -20–45 °C. In an environment close to 45 °C, with poor workshop ventilation, the difference between the tank wall and the environment is squeezed to a very small value, and then the wall only has to be slightly cooler than the air, only below the dew point temperature, for water vapour to be released. It is more likely to appear when the workshop floor has just been washed down, or shortly after rain, when the humidity is high.
Three checks that do not require opening the machine
The first check is to wipe the wall dry and watch how quickly the droplets come back. Wipe the whole tank wall dry with a dry cloth and look again half an hour later: if the droplets reappear over the same area, the wall as a whole is below the dew point and it is condensation; if only the line of the joint becomes wet again, the problem is on the sealing face.
The second check is the trend. Condensation usually appears when the ambient temperature rises, when the humidity increases, or when the machine has just been switched on and has not entered its working state; once the tank wall temperature has come up, it disappears on its own. A leak does not disappear as the temperature rises and often gets worse when the oil pressure rises.
The third check is the vacuum level. The working vacuum of the ZYD-100 is -0.08 ~ -0.0999 MPa and the ultimate vacuum is ≤5 Pa. If the tank seal is genuinely leaking, the vacuum level during the pumping stage will clearly fail to reach the working range and will not keep improving as the pumping time is extended; if it is condensation, the vacuum level is normal, because the sealing face is not leaking.
| What is seen on site | The more likely cause | How to confirm it |
|---|---|---|
| Droplets form evenly over the whole upper half of the tank wall and reappear after a while | Tank wall temperature below the ambient dew point, condensation | After wiping dry, check whether the position and the extent of the reappearance are the same |
| Liquid seeps out on its own at flanges, threaded joints and weld seams, with an oily trace | Leakage from a static sealing face | After shutdown, carry out a pressure holding test and observe whether the joint holds |
| Appears when the machine is just switched on and the oil is not yet hot, and disappears by itself after running for a while | Condensation while the machine is in a cold state | Record how the time of appearance relates to the oil temperature |
| During the vacuuming stage the vacuum level fails to reach -0.08 MPa and stops improving | Leakage at the sealing face, the negative pressure cannot be established | Check how the vacuum reading relates to the length of the vacuuming time |
| Appears when the workshop has just been washed down or the humidity is high after rain, and lessens after ventilation | Ambient humidity too high, causing condensation | Check the humidity of the day and the ventilation conditions |
Two principles for handling it
- Until a leak has been confirmed, do not tighten any flange or connection: dismantling and refitting while a slight leak is present easily scores the sealing face, and after the repair it leaks more easily than before.
- Once condensation has been confirmed, deal with the environment first rather than with the machine itself: improve the ventilation and keep cold air from blowing directly onto the tank or coming from an air conditioning outlet.
- When a machine that has been out of service for a long time is started again, condensation is most likely during the period in which the tank warms up from a cold state, and this part of the process has to be recorded.
- If droplets keep appearing at the same position but wipe off as oil rather than water, treat it as a leak and stop watching it as condensation.
The water on the tank wall: first establish whether it is forming over the whole wall or in one particular seam. The first is a matter of the environment, only the second is a matter of the machine.
