I recently joined a Passenger RoRo Ferry as a temporary relief Second Engineer. I only had a few minutes handover with my Eastern European predecessor, during which time he informed me that none of the oil mist detectors on the 4 medium speed main engines were working, because the main air compressors were worn and oil was passing through into the compressed air system.
On investigating further I found that the Oil Mist Detectors were made by Schalter Automation. In this type 7 bar air goes through a sintered filter, a variable orifice to limit the air flow, a pressure reducing valve and then into an eductor/ejector/venturi and then vents through a drain to the bilge. ( The makers call this an air pump, which is perhaps not the best translation into English)
The venturi creates a vacuum at its narrowest point (in accordance with Bernoulli's Theorem & the "Steady Flow Energy Equation"), this is connected to the oulet of the oil mist sensor housing and draws air from it, creating a partial vacuum inside which in turn draws air from the crankcase into the sensor chamber.
It is therefore totally impossible for oil, water or any other contamination in the air supply to effect the readings. Also, in this case the hinged covers on all the detector housings had been left open, for reasons unknown!!!!
YOU SHOULD ALWAYS AVOID WORKING ON THESE WHEN THE ENGINE IS RUNNING, IN CASE YOU CAUSE THE ENGINE TO SHUTDOWN.
Having read the makers instrucction manual and found the tools and spares kit, I closed the doors of the detectors & attached the "U" tube manometers to them in turn. In every case the vacuum was so great that all the water was sucked out of the manometer and into the measuring head!!.
I thoroughly cleaned the lenses and ports inside the detector heads, renewed the circular, sintered air filters under the covers, and checked the cover gaskets were in good condition.
Note:- The rough surface of these filters should be on the outside, air is drawn from the engine room, through these filters and across the lenses of the light beam, to prevent them becoming obscured by oil. The filters are a "throw away item that can not be cleaned effectively. If no spares are available, as a temporary, emergency measure, they can be cleaned using Electrical solvent cleaner, but this is not very effective.
Always check the cover gasket, I have seen cases where a "home made" cover gasket has completely blocked these air intake filters, there should be a cut away in the gasket allowing the air to pass between the cover and the casing from the bottom of the casing.
Next I closed the variable flow orifice fully and refitted the "U" tube manometer (freshly filled with water), and slowly opened the orifice until I had the correct vacuum reading, according to the instruction manual. This orifice acts a safety device, preventing a big increase in the vacuum if the reducing valve diaphragm fails.
Then I adjusted the pressure reducing valve until I had the correct vacuum for that adjustment (slightly less than for the orifice) in accordance with the maker's manual.
All the Detectors appeared to be functioning perfectly in Port, however, when the ship was at full speed at sea, one detector went into alarm with the top red LED, No. 14 illuminating. Checking with the instruction manual, this LED indicates incorrect vacuum in the Detector.
This can happen because when the engines are all stopped, the engine room supply fans create an overpressure, above atmospheric pressure in the engine room, as the load on the engines increase, they consume more air from the engine room, and this overpressure reduces. The Crankcases are directly connected to atmosphere outside the engine room through the crankcase breathers or vent pipes so the pressure inside the engine stays constant at all times.
Because of this, after initial adjustment with the engines stopped, it is sometimes neccessary to make a very fine adjustment of the vacuum with the engines at full power.
Once the Oil Mist Detectors were all adjusted correctly and proved to be working correctly, all the variable orifices and reducing valves were locked in position with locking wire, to prevent any accidental adjustment.
If a continuous sequence of red LEDs are lit, from the bottom of the collum upwards, that is an indication of the level of oil mist, more light = higher mist level. If an individual LED lights up, refer to the makers manual as it is warning of a specific fault in the oil mist detector rather than the oil mist level.
Sometimes it is possible to get a spurious alarm and even shut down of the engine caused by water in the Lubricating Oil. This can usually be spotted by looking inside the oil mist detector, if there is a deposit of emulsified oil inside, the alarm was caused by water vapour. Then you have to check the crankcase, oil coolers and purifiers for water leaks.
Another cause of false alarms can be piston blow past.
I hope this helps someone out there.
It is always better to ask a stupid question than to do a stupid thing.