Engine Detector - for Engine Crank Case, chain case and Gear Box monitoring More Information below

Oil Mist Detection in Diesel Engines

You may be aware, MCA and IMO, together with other Marine Societies, are very concerned about the number of fires that start in machinery room spaces. Places most at risk are engine and purifier rooms. However, other areas have their own problems and these include bowthruster rooms, steering gear and hydraulic pumps. Figures produced suggest that up to 65% of machine room fires are the result of oil mist.  

What is Oil Mist

There are two ways oil mist can be formed. One source is minute leaks in pressurized in oil lines and fittings which produced a atomized spray of oil into the atmosphere. The second source is when oil hits a very hot surface and boils.

Danger occurs when high pressure type of leaks of oil mist are formed with a particle size of 3 to 10 microns that builds up to a hazardous concentration of mist in the atmosphere. At levels of saturation conditions are truly hazardous, and if no action is taken a fire can start. The ignition temperature for this type of oil mist can be extremely low depending on the fuel that is being atomized. 

Other ways an oil mist can be generated is when drops of oil hit a hot spot or surface and boils. When oil mist is produced by boiling the particle size is then 3 to 10 microns. This mist is visible and is known as "blue smoke". The larger and hotter the hot areas is the quicker the oil mist is produced. At this stage a temperature as low as "150°C can cause ignition.

How to prevent Crank Case Explosions

When a crankcase explosion occurs within the confines of a diesel engine crank case, the crankcase door is vulnerable to damage. Where a set of correctly designed pressure relief valves are fitted, the pressure is relieved and it is unlikely that external structural damage will be experienced. In both instances, however, injuries to operators can occur if they are close by when the event occurs. There is also the added risk of a secondary explosion caused by the ingress of fresh air into the confines of the crankcase.

An oil mist detection system can stop damage to the engine and most importantly injury to engine room personnel by allowing early detection of engine wear and bearing damage which could lead to an explosive environment within the engine.

Oil mist equipment when used in the correct way, can therefore be used as a health monitoring facility to assist in the determination of the condition of the machinery, so that the necessary servicing can be highlighted at an early stage of a condition change taking place.

As already stated there is an inherent oil mist level within the engine that acts as a base line. If the oil mist readings are manually or electronically logged the increased levels of oil mist will become apparent as wear takes place. Remember wear starts from small beginnings and with awareness of this, preventative action can be taken

The QMI Oil Mist Detection System

QMI have for the past 18 years been developing the Multiplex^TM system to detect oil mist in diesel engine crank cases and in sensitive operating environments. Our efforts were directed to overcome a number of problems that were well known to users.

What is most needed in an Oil Mist Detector?

1. A fast and accurate response - This can only be achieved with a detector mounted on each crank case to detect the level of oil mist at the source of the problem. This eliminates unnecessary pipe-work where oil mist can condense and take time to travel to the detector.

2. No false alarms - To make this possible light absorption must be replaced with light scatter so there can be no mistake between oil mist and dirty lens.

3. Direct reading of a known value such as mg/l and having a linear output.

4. Must have a known maximum oil mist alarm level, not one that can be changed at will.

5. Must work on direct readings taken in each compartment of the crankcase.

6. Must not use compressed air for calibration, as this tends to contaminate the lens so that it is not possible to distinguish between a false alarm or an engine problem. Also contaminated sire tends to gum up the valves within this type of oil mist detector.

7. It must be possible to directly read the oil mist levels in each crankcase compartment simultaneously without scanning.

8. It should be able to log oil mist levels either manually or electronically.

9. No siphons or valve that can block and stop the oil mist from reaching the detector.

10. Last and most important, an oil mist monitoring system that has the monitor in the control room and not on the engine.

QMI uniquely uses the light scatter method to measure oil mist concentration. This is also known as Nephelometry.

 

The measurement of oil mist using the Nephelometry light scatter method greatly differs from conventional Obscuration light absorption used by other suppliers.

Light scatter has the advantage of being linear in output and has a true zero. This means it is possible to quantify the oil mist as a measurement in mg/l

The practical answer is to install and Oil Mist Detection System that will detect oil mist as it is being diffused into the atmosphere which will alarm long before it saturates the atmosphere to a danger level.  It should be noted that steam and smoke have approximately the same particle size, so an oil mist detector should be able to detect these parameters, if the right system is used - which is a bonus.

Crankcase Oil Mist Detectors

These are mounted at each crankcase compartment, gear or chain case housing and thrust bearing housing, if it has its own space. The detector incorporates LED optos and a PCB. A microprocessor that is built into the unit, sends 5 sets of information to the Monitor each half second via a cable connection to the Monitor where is proceeds to give all the relevant readings to the operatives. There is no interaction between the detectors as they are independent of each other.