Level detection looks like the simplest part of the whole arrangement: measure a height and judge whether it is high or low. But its errors trigger two kinds of consequence directly. A reading that is too high means that oil spills and that water enters the vacuum chamber; a reading that is too low means that the pump runs dry and that the heating section burns dry. So the criterion for choosing a level arrangement is not the accuracy, it is whether the failure mode can be caught.
Float: simple, but it hates dirt
The float type works with a float that follows the liquid surface, and the change in the buoyant force on the float or in its position is converted into a switch signal. The structure is straightforward and the maintenance is straightforward, and it is the most common arrangement.
Its failure modes are just as straightforward: the impurities and the sludge in the oil stick to the float, it seizes up and no longer follows the liquid surface; with an oil of a high viscosity the float lags, the liquid level has already passed the set point and it has not moved yet; if the float breaks or the seal fails, the ball can drop into the system and become a new source of contamination. The more viscous the oil, the more obvious these problems become.
Siphon type: no contact with the medium, but it depends on the air line
The siphon type uses a thin tube led out of the vessel and reflects the height of the liquid level through a change in the air pressure. The advantage is that the measuring parts do not touch the oil, which suits a corrosive medium or a case where no opening can be made, and it also makes it easy to transmit the signal to the control room over a distance.
Its failures concentrate on the air line: if the pressure tube leaks or leaks slightly, the reading drifts; if the line is blocked by oil mist and impurities, the reading sticks and does not move; a pressure tube that is too long or has too many bends introduces a lag, and the signal cannot keep up when the level changes quickly. As long as the air line is intact it is very stable, but a fault is not easy to find at the first moment.
Infrared type: no contact, but sensitive to the interface
The infrared level detection decides the level from the reflection or the transmission of the infrared light at the oil surface, it does not touch the medium at all, and it is the most suitable for a sealed, corrosive case where no opening can be made.
Its failures come from the interface itself: a strong reflection on the oil surface or bubbles in the oil can cause a false trigger; the oil mist on the lens makes the signal drift gradually; the depth of the colour of the oil and the foam covering the surface both affect the decision. The difference between a clean, light oil and a dark heavy oil is especially noticeable.
The practical effect on the protection of the equipment
The consequence of a false high level is the overflow and the ingress of water: oil on the floor is a safety risk, and water entering the vacuum chamber means damage to the equipment. The consequence of a false low level is the pump running dry and the heating section burning dry; a vacuum pump running without oil is damaged quickly, and a heating section without oil simply overheats.
The level signal therefore has to be set in both directions when it enters the protection logic: a high level alarm, a high high level trip, a low level alarm and a low low level trip, with one step of operating time left on each threshold, so that the same alarm value is not made to carry the protection of both directions at once. Using the infrared level detection together with the foam detection and the automatic defoaming as a matching set is precisely what makes this two-direction protection real.
| Arrangement | Principle | Typical false alarm or failure | Suitable medium and duty | Role in the protection |
|---|---|---|---|---|
| Float | Conversion of the buoyant force and the position | Seized by the impurities, lag with a high viscosity, the ball falls out | Clean oil of not a high viscosity | Low cost, needs regular cleaning |
| Siphon type | Change of the air pressure in the pressure tube | Drift with a leak, sticks when blocked, lag | Where no opening can be made, where remote transmission is needed | Depends on the integrity of the air line |
| Infrared type | Reflection and transmission | False trigger on the reflection, oil mist on the lens, foam covering the surface | Sealed, corrosive, no contact permitted | No risk of contact, the drift has to be guarded against |
- The protection logic has to be set in both directions, with different thresholds and different actions for the high and the low level.
- Keep the detection point clear of the foam zone and of the splash-back zone of the return oil.
- Clean the float regularly, check the siphon line for leaks regularly, wipe the infrared lens regularly.
- Leave a time gradient between the alarm and the trip, do not go from one to the other in a single step.
In my view the value of the level detection is not in how accurately it reports, it is in the fact that when it reports wrongly it does not drive the equipment in a worse direction.