- Buyer's Guide
Countless times, I've devoted this column to stressing the virtues of contamination control. With each passing year, we seem to receive more and more user feedback detailing and validating the business case for clean, dry and cool oil. Of course it's one thing to preach its value, but it's quite something else to actually achieve and sustain near-contaminant-free conditions. Therefore, it is this subject that I'll tackle with the current and a few future columns, beginning with exclusion and removal of water.
Keeping Your Machines Water-Tight
You don't have to remove what you don't allow in. Indeed, it's hard to question the logic of controlling water ingression. But because moisture is everywhere, achieving bone-dry through exclusion alone may not be practical or even necessary. Lubricating oils have different degrees of hygroscopicity (water-loving tendencies), making the control of all dissolved water an almost futile exercise. However, for many applications, it's the free and emulsified water that is the most destructive and, hence, the central target for control.
Exclusion relates to the process of preventing (excluding) the ingress of water from environmental, machine and process sources. Common points of water ingression include:
Water from make-up oil (some supply tanks can collect inches of water below the oil level)
Turbine gland steam seals (e.g., improper pressure regulation)
Defective vapor extraction system (too high can suck in steam while too low can fail to keep up with ingress)
Process water in-leakage from pulp and paper production, water treatment plants, sewage treatment plants, etc.
Oil cooler leaks
In-leakage of water past seals from washdown sprays, rain and flooding conditions
Reservoir and sump headspace vent/breather ingress
The top-line priority is to squelch ingression points through tight and well-managed ingression control. Deferring maintenance of worn seals, defective breathers and coolant leaks creates more expensive maintenance events in the future, including the possible cost of premature oil changes, flushing, oil dehydration and replacement of water-damaged parts. Monitoring and promptly closing-off ingression sites are by far the wiser use of maintenance resources.
Oil analysis can be an important strategy for providing red-code alerts to periodic water ingression problems. This can be as simple as daily visual inspection of BS&W (bottom, sediment and water) sight glasses, level gauges and live-zone oil samples. In addition to visible inspection, water also can be detected using a number of field-level instruments and pass/fail methods such as crackle tests and calcium hydride detectors. Once a new source of water is encountered, then corrective measures can be quickly deployed. When gross water contamination is observed, there is little need to quantify the exact amount in the oil.
Let the Machine Dry Itself
For normally hot-running machines, low to moderate amounts of water can move out of the oil by evaporation through the headspace and vent. The hotter the oil and the lower the relative humidity in the headspace, the faster the water extraction process will occur from evaporation. Of course, we can't ignore the many negative consequences heat imparts on the oil and machine. Therefore, this practice should be used sparingly.
Some companies use fans and instrument-air purges to keep their headspace relatively dry. Purging a headspace of moist air not only aids in the removal of water from oil but also can substantially control the ingress and condensation of atmospheric moisture. Lubricants with good water-shedding properties (demulsibility) can release free water quickly to the low zones of the reservoir. This water can then be periodically drained using manual or automatic methods. In sum, whatever can be done to help the machine and its oil to exclude and discharge occasional wetness by itself is always a good thing. However, the real world is often less friendly and, hence, there is a need to force the water out by human or mechanical intervention to achieve needed dryness.
The number of dehydration systems on the market seems to grow day by day. Some use tried-and-true methods from decades past, while others are more innovative and boast a range of associated performance benefits. Of course, only the best of the new-technology systems will likely survive to put pressure on the older technology systems to change or become obsolete. This, of course, is the well-known product life cycle.
Once oil accidently becomes wet, the first line of defense is to try to let the oil release as much of the water as possible by gravity separation in settling tanks. Heating the oil to around 160 degrees Fahrenheit can help facilitate the separation by lowering the oil's viscosity and increasing the differential specific gravity between the oil and the water. What remains behind in the oil as dissolved and emulsified water can then be scrubbed out using dehydrators. The following are generic descriptions of such methods:
Centrifugal separators: While settling tanks employ the forces of gravity (1 G), centrifugal separators can rev-up the process by generating as much as 10,000 Gs. Low-viscosity oils with good demulsibility work best.
Coalescing separators: These also work by the forces of gravity; however, settling is made more efficient by improving demulsibility. These separators use special filtration media to help pool together water droplets so they settle faster. Large water drops settle faster than small droplets (Stokes Law). Low-viscosity oils with good demulsibility work best.
Sparging and air-stripping separators: By forcing bubbles of warm, dry air through the oil, there will usually be fairly rapid dehydration as the air is allowed to purge. The moisture is drawn out of the oil by the dry (desiccating) air; it is simple but effective.
Dialysis dehydration: This technology deploys the use of a special porous membrane. On one side of the membrane is wet oil and on the other side is dry air. Just like the sparging method, the dry air preferentially soaks the water out of the oil and is subsequently purged.
Vacuum dehydration: These separators boil the water out of the oil to form vapor which can be released to the atmosphere or condensed as condensate. To enable the water to boil out of the oil at low temperature, the oil is exposed to a vacuum (typically 25 to 28 inches of mercury). Under vacuum, the water can quickly vaporize at temperatures as low as 150 F. The low temperature reduces the risk of thermal-oxidative damage to the oil and its additives.
Super-absorbent filtration: These filters employ the use of special polymers that form gels when exposed to water. The polymers can hold upward of 500 times their weight in water. Eventually, the filters become bloated with water, causing a pressure rise which signals the need for a changeout.
Each of these dehydration technologies comes in unique proprietary configurations from various vendors. And, as usual, they all have strengths and weaknesses and, as such, should be selected based on application and need. Articles on most of these technologies can be found at Noria Web sites, including www.machinerylubrication.com, by using the search engine.