Q & A

"I suffer cold start-up problems in the morning with respect to the filters. How can I overcome this?"

Typically, the problems associated with cold start-ups include the filter differential pressure exceeding the limit owing the higher viscosity to the cooler oil. This results in the pressure differentialindicator tripping and the filter going into bypass mode allowing unfiltered oil to pass through.

If the system has a very short warm-up period, and there are no critical components, it may be worth considering allowing the filter to go into bypass mode for a short period, but remember to install an indicator that either has a thermal lockout or has an automatic reset function. As the filter blocks-up in the normal course of events, the period of bypass will increase, and this can be monitored by the length of time the auto-reset takes to activate. However, regular particle counting is essential to highlight a problem with the bypass valve sticking in the open position.

However, on critical systems, possibly at an outdoor hydraulic plant with a longer warm-up period, ensure the filter is selected according to the worst-case viscosity conditions as a result of the lowest ambient temperature likely to be encountered. This will result in a larger than required element at operating conditions, it should result in a longer element life requiring less frequent changes.

Today's Tip

• Avoid the use of suction line filters because they can cause problems such as cavitation in the pump. However, ensure that the suction line at least has a protection strainer of 200 µm rating to help keep small hard objects from damaging or jamming the pump. Always check with the pump manufacturer before fitting any devices immediately upstream of the pump as these can possibly affect the performance of the pump and cause problems downstream in work-end components.

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Book Bits

From the book "Wear Debris Analysis":

Trend Analysis is the process by which a change in machine condition is determined from an examination of changes in specific sensors or output.

Different systems exhibit different trends. For instance, the wear debris generated from a system involving gears and bearings may well be quite high initially, particularly if no attempt has been made to 'run-in' the system. Thus there is a gradually reducing level of debris until the system settles in. Then comes the acceptable very low wear rate associated with well lubricated surfaces, which perhaps increases just slightly. Finally, the machine begins to show signs of fatigue or fracture and particle generation increases at an ever increasing rate. These three stages of wear are shown in the classical 'Bath-tub' curve.

Another changing feature of wear with time, is that the wear rate begins to fluctuate. After the 'running-in' period, the wear rate is expected to be reasonably uniform, although it may rise slightly. However, as conditions begin to become less favorable wear may be less or more in terms of particle size generation as well as with total quantity. Scatter is thus seen as a trend indicator.

The third sort of trend indication is that associated with the shape of particle - its morphology. This is less easy to measure than the gravimetric level or 'size' of particulate; but, by using the analysis mentioned later in this chapter for defining particle shape, two or more possible specific indicators of shape may be trended to detect the point at which a serious change in debris particle shape occurs.

For more information about the book "Wear Debris Analysis" go to:

http://www.noria.com/secure/product_detail.asp?catalogid=38

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