Sample Points Vital For Accurate Testing
Professional Skipper Magazine Issue 43 Jan/Feb 2005 (unedited)
Further to my last article, sample points. I cannot highlight enough how quick and easy, not to mention safe taking oil samples from a dedicated sample point is.I was working on a brand new vessel the other day. It had travelled only 20 hours before an alarm went off. The source was found to be a suspected blocked filter in the hydraulic system. The cause of this suspected blockage was thought to be grease from a gearbox unit that the hydraulic system operated.
I was asked to provide analysis of the oil from the two circuits as well as open the filter up for inspection. What stunned me was that this new multi-million dollar vessel had no sample points anywhere on the system that would enable us to take a clean sample.
To provide the best possible results oil should be drawn from the pressure side of the hydraulic system before the filter. Where possible the samples should also be drawn while the system is up to operating temperature and before it is shutdown. However, the only safe way to do this is via specific sample points.
These sample points are not only useful for drawing samples at any stage, but can be used for taking manual pressure readings as well. Filters should not be loosened to allow oil to flow or plugs removed from the side of filter housing to take samples.
These methods will not only lead to the possibility of impurities entering the oil, but also pose dangers if the system is still under pressure or started up by mistake. You may also be taking the sample from the wrong side of the filter element. Unfortunately taking samples from the main reservoir is also not accurate enough for this type of testing.
Stauff Corporation (NZ) Ltd manufacture and provide "Test Points" that are designed for up to 600 bar. They have a protective cap on them to keep out the dirt and dust and enable a sample to be drawn very quickly and safely while the system is in full operation. Again, as I have stressed before taking a sample while the system is still operating or just after it has shut down i.e. from an engine or gearbox will provide you with the best possible results.
Luckily the analysis of samples came back pretty much spotless. The filter also showed no signs of being blocked. The labs assessment of the filter was as follows:
"The filter element was cut open and the element material examined. The only indications of the filter being used are that it is wet with oil and there are small amounts of particulate trapped on the outer surface of the filter element. The particulate is a mix of typical dust/sand and metal. The largest being in the order of 0.5x1mm. Putting the element material on a vacuum Mobil DTE 24 (ISO 32) passed through at about 9psi. The assessment is that the filter is not blocked and the suggestion would be that the alarm is possibly faulty."I questioned the lab on whether there were any signs of grease and was told that there were no obvious indications that any had passed through or had been trapped in the filter.
On passing my report on to the vessel's engineer it was accepted that the oil and filters were in good condition and that any debris found in the filter was just the result of the system settling down after being installed.
The suspicion now fell on the level at which the sensors on the alarms were set at. It was felt that the alarms themselves were ok as they had turned off when the filter was changed, but that the setting on the sensor was possibly set to low.
With sea trials and hand-over coming up it was concluded that further analysis should be done at a later date to assess any outstanding issues as well as assisting to confirm this latest set of results.
Filter analysis or what is sometimes called Filtergram Analysis can be very useful. With this job we were only looking at what may have caused the filter to block. Filtergram Analysis on the other hand is a diagnostic technique that is similar to Used Oil Analysis.
Normally a used oil filter is removed and cut open. Sections of the filter paper can then be cut out and removed for testing. I find that this type of testing is very good when a piece of equipment is unable to be started or run-up to operating temperature for whatever reason. Unfortunately though, as there is a myriad of filters out there you need to supply a new unused filter as well. This is because the paper contains minerals that will show up in the analysis.
For the results to be accurate the wear-metal/mineral counts from the new filter element need to be subtracted from the used one, thus leaving results that can be relied upon to indicate wear from the equipment. This wear can also be looked at under the microscope to see how it was formed i.e. sliding, cutting or fatigue wear. Now that's another story.
Have a safe and merry Christmas. I look forward to catching up with you all in the New Year.
Written By Kelvin Hieatt
Managing Director Marine Diagnostics Ltd
