It has been said that there is a time and a place for everything. Unfortunately, in the case of particle counting and ferrous density testing, there seems to be a bit of confusion on the proper time and application (place) to perform these tests.
Direct Reading Ferrography
Direct reading (DR) ferrography measures the amount of ferromagnetic wear debris in an oil sample, commonly referred to as the ferrous density reading. The results of DR ferrography are generally given in terms of DL for particles greater than five microns in size and DS for particles less than five microns.
DR ferrography works by running the sample through a precipator tube over a high-powered magnet. Larger ferromagnetic particles attract to the magnet allowing them to gather at one end, while the smaller particles gather over the exit end. Light is then transmitted through the sample. Photo detectors measure the amount of light passing through the sample, resulting in the DL and DS values.
An advantage of DR ferrography is the information that can be derived from the results. While simply an index value, some equations can be applied to find total wear particle concentration (WPC) and the percent of large particles (PLP).
The particle quantifier (PQ) is another ferrous density device using the Hall Effect to determine the ferromagnetic particle concentration of an oil sample. The Hall Effect is a measurable induced voltage across a sample under an applied magnetic field and current (Figure 1). In general, the higher the concentration of ferromagetic wear debris present, the higher the observed Hall voltage.
Figure 1. Hall Effect
Similar to DR ferrography, the particle quantifier gives the measured ferrous concentration as an index value. However, this is where the similarities between the two end.
PQ results are given as a single index value versus the two values provided in DR ferrography. There is no separation of size.
The PQ is not sensitive to particle size. When utilized in conjunction with atomic emission spectroscopy (AES), several evaluations can be made. If both the PQ and AES values increase, it is likely that many small particles are being generated. However, if PQ increases and there is no change or a decrease in AES ferrous debris, this suggests large particles are being generated which indicates an abnormal level of wear.
Particle counting in industrial gearboxes will tell the same story as particle counting in a hydraulic system or pump application - cleanliness. When establishing an oil analysis program that is proactive in controlling contamination, particle counting is a vital component to the routine test slate.
Two types of particle counting are generally performed: optical and pore blockage. Optical particle counters typically utilize a sensor that measures the amount of laser light lost when crossed by a particle, which then relates to a specific particle size. Pore blockage particle counters work through a transducer measuring either the amount of pressure rise or flow decay that occurs as the oil sample passes through a sensor containing a set number of specifically sized pores. In this method, larger particles that cannot pass through the sensor are trapped while smaller particles get trapped in the open gaps between the larger particles and the screen (Figure 2).
Figure 2. Pore Blockage
Particle count results are used to obtain the ISO cleanliness level. ISO 4406:99 reporting structure gives the cleanliness as a three-digit value (for example, 18/16/13). Optionally, it is acceptable to report the cleanliness as a two-digit value (16/13). These digits correspond to the number of particles detected at the >4, >6 and >14 micron levels for the three-digit code and the >6 and >14 micron levels for the two-digit codes.
Using particle count data in conjunction with other routine tests can help differentiate between a dirty gearbox and a gearbox filled with excessive contamination. Table 1 presents actual test results from three similar conveyor gearboxes. These gearboxes are located in different areas moving various types of product.
When viewing the data, it appears fairly obvious which gearbox has an excessive amount of ferrous wear. With the size limitations of atomic emission spectroscopy being five microns or less, it can be concluded that GB 1 likely has a good amount of large ferrous wear particles as a result of continuous contamination.
Keeping equipment clean is key to prolonging equipment life. As an additional note, according to Figure 3, the owner of the above referenced GB 3 will enjoy an equipment lifespan of three times longer than the owner of GB 1, simply by employing proactive levels of contamination control and removal. Additionally, if the cleanliness level were to achieve the recommended target level of 16/13, a lifespan increase by a factor of four would be realized, provided all parameters remained optimal.
Figure 3. Life Extension (Noria Oil Analysis coursebook)
As with most oil analysis tests, it is necessary to understand how to choose which test to perform. If the equipment being tested is prone to ferrous wear, some type of ferrous density may be beneficial. Conversely, if sampling from a sleeve bearing housing where nonferrous debris is the major wear component, ferrous density testing would not be required.
Performing particle counting depends on the level of cleanliness that needs to be achieved. When monitoring particle contamination, it is important to understand that issues such as filtering capabilities, contamination control measures and overall criticality become vital in setting appropriate target values and alarm limits.