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When performing an oil flush on your equipment, it is important to have a plan and procedures that take into account the criteria and materials used for deeming the system clean. This includes particles you cannot see with a naked eye. The width of an average human hair is approximately 40 microns, but particles as small as 2 microns can damage your system. How can you judge your system clean if you cannot see the material? There are several options.
The benefits of using a laboratory are controlled, certified results. The disadvantages are time and control. For example, a flush may have become the controlling factor of your outage, and you are ready to move forward with final testing to verify the flush. If this happens to be at 5 p.m. on a Friday, will your lab be open over the weekend? Where is the closest lab? Do you have to wait for shipping, processing and posting of results? In some rural areas, the wait time for expedited results can be anywhere from two to four days.
Patch kits have come a long way from the old microscope and counting of particles, but they still allow for human error. These kits are more of a quick, "down and dirty" method and should always be supported by sending samples to a laboratory.
Using an onsite particle counter is the fastest, most reliable method for verifying microscopic particles. Multiple types of particle counters are available, so make your selection based on the application. While light-refracting units are excellent for turbine and hydraulic fluids, they can give false readings with darker oils and oils with moisture contamination. Pore-blockage units will work with darker oils and moisture but require more care with handling and calibration. The best units will provide particle counts for both the International Organization for Standardization (ISO) and the National Aerospace Standard (NAS) cleanliness codes based on the manufacturer's requirements. Always check the unit's calibration before staking the success of your oil flush on the readings.
Since a particle count will not reveal all the existing material, it is vital to include a visual inspection as part of your cleanliness criteria. The most common inspection media is a 100-mesh screen. These screens come in different forms as required by equipment manufacturers.
The Wye strainer uses a cylindrical or "barrel" screen for inspection. While these screens are quick to change and fairly common, they tend to get crushed from repeated removal and installation. Contamination can also drop out into the screen cover and give a false positive.
Basket strainers are utilized in many oil flushes today. They accept full flow and have a sealed bottom to catch all materials during the inspection. Consider purchasing a unit with quick-release handles on top and a drain valve on bottom. This allows for ease of inspection. The disadvantages of basket strainers are their availability and cost. There is no standard basket or housing. You should also keep spares on hand in case a basket is damaged during the oil flush. This can prevent unnecessary delays.
The witch's hat strainer is not typically employed by oil flushing experts. However, some manufacturers require this type of strainer to be placed in the return header to keep contamination from getting into the reservoir. The witch's hat should only be used for gross contamination and inspected often. The screen's surface area causes the unit to have a tendency to tear or blow out. Another factor to consider is accessibility. The witch's hat requires a pipe spool piece for removal and installation.
Bag filters are frequently used in many turbine and compressor flushes today. The advantages of bag filters are their cost and availability. Bag filters are produced and distributed globally, and can be purchased in many micron sizes to meet your needs. A 36-inch, 1-micron bag filter can cost as little as $3. The problems with bag filters are that the housings can be costly and bulky, and the filters are nominal, which is to say they do not capture all the contaminants. A bag filter can also be difficult to inspect, often requiring you to cut open the filter and count particles.
Flange or "slip" screens are the preferred method of oil flushing experts. These screens can be easily dropped between two flanges in place of the existing gasket material. This facilitates ease of placement without unnecessarily disturbing the lube oil piping. If constructed properly, the screens can be cleaned and used numerous times throughout a flush.
It is best practice to utilize a layered approach when employing slip screens. For example, you might use gasket material, a 40-mesh screen for a rigid back, a 100-mesh screen on top of the 40-mesh screen and then another layer of gasket material, all glued together with a silicone adhesive. A handle can be added for easier installation and removal.
Slip screens enable the full-flow capture of particulate during the inspection phase. The downside of a slip screen comes into play if it is not built properly or becomes coated with debris. This could cause either a backup of fluid upstream or the screen to tear.
With all inspection media, it is recommended to insert valves on both sides of the inspection location. This allows for segregating the media from the system, preventing oil spills as the inspection is performed.
Finally, regardless of the media you choose, always employ a two-pronged inspection approach. Particle count for the material you cannot see and use inspection media for the material you can.