Oil Mist Detector

INTRODUCTION

High temperatures, in excess of 200°C that occur on bearing surfaces under initial failure conditions, can lead to a rapid generation of oil vapour. When the hot vapour contacts the relatively cooler atmosphere of the crankcase it condenses into a fine mist, with typical particle sizes of around 0.5 to 5 microns in diameter. When the density of these particles reaches between 30 to 50 mg/l (Milligrams per litre), depending upon the type of oil, an explosive condition exists.

A fire or explosion needs three constituents: fuel, oxygen and an ignition source. Remove one of these and no explosion will occur. Similarly, within the crankcase, the three constituents which could cause an explosion are air, oil mist and an ignition source, the "hot spot". Using optical measuring techniques, oil mist density can be measured at levels as low as 0.05 mg/l and give early warning of a rise in oil mist density.

OIL MIST DETECTION SYSTEM (OMDS) PARTS

Oil Mist Detection (OMD) techniques have been used to monitor diesel engine crankcases for potential explosive conditions and early detection of bearing failures. The systems available rely mainly on analysing the optical density of oil mist samples drawn from the crankcase compartments, through pipes to the detector.

The system comprises three main components:

Detectors Engine

Junction Boxes

Control Unit

System can comprise up to 64 detectors directly mounted on the crankcases of up to 8 engines, allowing both main propulsion and auxiliary generators to be monitored at the same time. Each detector communicates electronically over a serial data link via the engine mounted junction box with the control/display unit which is designed to be mounted within the Engine Control Room. This eliminates the need to enter the machinery space in alarm conditions.

Each detector continually monitors the oil mist density in the crankspace to which it is connected. In addition, it self checks for any internal faults. The control unit interrogates each detector in turn, notes its address and the oil mist density value. For each engine the average oil mist density is calculated and stored. Each detector signal is then compared in turn with the stored average. A positive difference (the deviation) is then compared with a pre-set, but adjustable reference (the deviation alarm level) for that engine (or detector). If it is greater than the reference a deviation alarm is given.

CONSTRUCTION AND PRINCIPLE OF WORKING

• It basically comprises two parallel tubes of similar size, each having a photo-electric cell (these are light sensitive and generate an electric current which is proportional to the intensity of the light falling on their surface) fitted at one end. The lenses seal the tubes but allow light to pass.
• Two identical beams of light from the centrally positioned same lamp, are reflected by mirrors to pass along the tubes on to the cells.
• One of the tubes contains clean air and is called the reference tube. The other tube has connections through which the vapours of the crankcase are drawn by means of an electrically-driven extractor fan.
• Sampling points are normally fitted to each cylinder crankcase and their connections are led to a rotating-selector-valve, which repeatedly connects each sampling point to the measuring tube, in proper sequence.
• If the concentration of the oil-mist is present in a certain sample, light shall be obscured before reaching the cell of the measuring tube, the electrical balance will be disturbed and an alarm will be operated.
• At the detection point, the rotator stops to indicate, which sampling point is to be investigated. After testing, the samples are led away to the atmosphere.