When oil traces appear under the machine, most people's first reaction is to wipe them up and carry on. But leaks come in two kinds and the handling differs considerably: static seal leakage and dynamic seal leakage. More troublesome is a third form that is easily overlooked, a vacuum leak. A vacuum leak neither leaks oil nor makes a mess, it cannot be seen on site at all, and it shows only as a slowly falling vacuum level.
Static seals: the leak is at a fixed position
A static seal is a seal between two fixed parts, including flanges, pipe fittings, gaskets, O-rings, and the sight glass and its seal ring. Its characteristics are very clear: the leak point does not move, the leakage rate is proportional to the differential pressure at that point, and the higher the pressure the faster it leaks. The leaked oil usually runs down the outside wall and collects at the lowest point, so the position of the oil trace is not necessarily the leak point; the trace has to be traced back up along its path.
How to judge it: stop the machine, release the pressure, then place a piece of paper or packing material at the suspected position and tighten it again. If the leakage stops, that is the point. A soapy liquid leak test works just as well: under negative pressure the bubbles keep coming. Note that this procedure has to be carried out with the machine stopped, the pressure released and the machine cooled.
Dynamic seals: the oil trace is thrown off by the rotation
A dynamic seal is a mating face with relative movement, typically the pump shaft with its seal or a mechanical seal. Its behaviour is completely different from that of a static seal: the oil is thrown off by the rotating shaft and leaves curved, streak-shaped traces on the machine casing or on the floor, sometimes appearing only at one angular position; the shape of the trace follows the speed, and at high speed the oil is thrown further. The leakage rate is usually small but continuous, so a floor that has just been wiped shows fresh oil in the same place a little later.
There is one situation that is easily misjudged: the oil dripping at the seal may in fact come from a static joint higher up and simply run down along the shaft. The way to tell them apart is to follow the trace upwards, find where it starts, and only if it starts at the stuffing box of the shaft seal is it a dynamic seal.
Oil leakage and vacuum leakage: two different hazards
- Oil leakage: it contaminates the floor and the surrounding equipment, it hides other leak points, it saturates the filter medium prematurely, and it carries a risk of scalding and fire. It is a visible hazard.
- Vacuum leakage: it does not change the appearance of the machine and no oil escapes, it shows as a slow fall in the vacuum level, a longer pumping time and a higher energy consumption. On site the pump oil or the pump itself is often suspected first, and the real leak point is found last.
- Both together: the most common form, where the same joint both leaks oil and draws in air. Here the oil leak is in fact the clue that leads to the vacuum leak.
How to locate the leak point
First, a soapy liquid leak test. With the machine stopped and the pressure released, let the system build up negative pressure slowly and apply the soapy liquid point by point to the joints, the flanges, the seal rings and the sight glasses; a continuous string of bubbles marks the leak point.
Second, the section isolation method. Split the system into several sections, close and isolate them one at a time, and watch the rate at which the vacuum level decays. The section whose isolation produces a marked drop in the decay rate is the section that contains the leak. This step reduces the search from the whole machine down to a short length of pipework.
Third, listen. With the system under negative pressure, listen close to the pipework for a faint, continuous flow noise; a vacuum leak is not hard to pick out when the surroundings are quiet.
Fourth, trace the oil back. The oil on the floor follows a path: start at the lowest end of the trace, work up along the outside wall, and the first place where an oil film appears is the actual source of the leakage.
| Symptom | Assessment | How to confirm | Action |
|---|---|---|---|
| The oil trace is in a fixed position and varies with the pressure | Static seal leakage | Stop the machine, release the pressure and verify by retightening with packing material | Replace the sealing parts or reprocess them, and check the threads and the flange face |
| Curved, streak-shaped oil traces thrown off with the speed | Dynamic seal leakage | Trace the oil back and confirm that it starts at the shaft seal | Stop the machine, replace the shaft seal or the mechanical seal, and check the alignment |
| No oil trace, the vacuum level falls slowly | Vacuum leakage | Section isolation method + soapy liquid leak test | Replace the sealing parts once the point is located, then re-check the whole system |
| The same joint both leaks oil and draws in air | A damaged sealing part | Open it up and check whether the sealing face is scored | Replace the sealing part; light damage to the sealing face can be repaired |
Safety precondition
The precondition for every leak-finding action is that the machine is stopped, the pressure released and the power disconnected. Removing a flange under pressure sprays oil, and disconnecting terminals with the power on carries a risk of electric shock. The soapy liquid used for the test has to be wiped off completely: residual moisture entering the vacuum system contaminates the pump oil directly and affects the vacuum level.
Oil leakage is visible, a vacuum leak is not. When the vacuum level falls for no apparent reason and no leak point can be found, suspect the seals first rather than changing the pump oil first.