Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/756816
46 | November - December 2016 | www.machinerylubrication.com is needed. If the viscosity never thickens to the point where the oil ceases to flow, then a heater is not required. Pour Point Identifying the temperature at which an oil will stop flowing doesn't have to be an engineering experiment or incredibly diffi- cult. Look for one property in particular — the pour point. During this test, an oil sample is gradually cooled and its flow measured. Once no movement is seen after five seconds, the pour point is recorded as the previous temperature when flow was observed. For machines that operate at low temperatures, the pour point is very important. If possible, select a lubricant with a pour point at least nine degrees lower than the lowest expected ambient temperature. This ensures the lubricant remains fluid enough at these lower temperatures so as not to hinder its move- ment or splashing characteristics. Waxes Wax is another lubricant constituent that affects the pour point. Waxes are commonly found in Group I paraffinic mineral-based oils. While they help contribute to a higher viscosity index, waxes can congeal and cause the lubricant to gel at colder temperatures. In the refining process, great lengths are taken to remove as much wax as possible. One method known as chill dewaxing involves the oil being mixed with solvents to help absorb some of the unsaturated hydrocar- bons, then chilled to congeal the wax and filtered to remove as much of the wax as possible. This is commonly found in Group I and II base oils. Some Group II and most Group III oils undergo a process known as hydroisomer- ization. In this process, the normally straight paraffin chains are stressed and reformed into branched chains, which helps minimize the free wax and lower the pour point. If the oil in the machine is less refined or has a lower quality base oil, the wax content may be high enough that the oil gels much easier, leading to a much higher pour point and making the case for using an oil heater. Pour Point Depressants To combat the problem of waxes congealing, most lubricants are formu- lated with an additive known as a pour point depressant. This additive is commonly made from compounds of alkylated wax naphthalene, polymethacry- lates and alkylated wax phenols. As mentioned previously, when oil cools, the wax crystals in the fluid begin to congeal, leading to the gelation of the oil and a higher pour point. Pour point depressants inhibit the congelation of those crystals, keeping the oil more fluid and lowering the pour point. However, these additives only work down to certain temperatures based on the formulation and the wax content in the base oil. Once the temperature drops below a certain point, the pour point depressants are no longer able to keep the waxes from congealing. Contaminants Contaminants not only influence an oil's viscosity and overall health but also impact its pour point. Soot is a common contami- nant found in diesel engines that can increase an oil's viscosity. As the soot load builds, the viscosity rises as well. For example, after an engine is parked over- night during winter, the soot load contributes to decreased oil movement in the morning, which can result in delayed lubrication in the engine. Glycol is another contaminant often found in engines. Similar to soot, glycol can also raise an oil's viscosity and is one of the leading culprits of oil gelation in motor oils. Routine oil analysis can help to identify both of these root causes of impending lubricant and machine failure. Due to the propensity of engines to produce contami- nants, you frequently will see diesel engines in cold environments with oil-pan heaters installed to keep the oil fluid. Types of Oil Heaters There are two main types of oil heaters used in industry. The first and probably most common for industrial applications involving a reservoir is a submergible heater (also known as an immersion heater) with a probe that extends into the oil. The length of the probe and the heating capacity are based on the expected temperature and quantity of oil in the reservoir. If these types of heaters are used, they should be thermo - statically controlled, since the elements can heat up and cook the oil surrounding the IN THE TRENCHES 66% of lubrication professionals use oil heaters in their plant, according to a recent survey at MachineryLubrication.com DO'S DON'TS Properly size heaters based on target temperature and reservoir size. Oversize the heater for ease of use. Use thermostats to control heat and watt density. Simply use a switch or plug in to keep the heater on full time. Inspect heaters periodically to ensure function and the proper setting. Forget about the heaters and assume they are working. Install level switches for heaters that must remain fully submerged in oil. Allow the oil level to drop below the heater, causing premature damage. Only use heaters when the lubricant can't perform and no other lubricant options exist. Use heaters for lubricants prone to oxidative failure or machines heavily contaminated with fuels. Do's and Don'ts of Oil Heaters