Varnish removal technologies can be classified into two main categories: 1) Particulate removal including electrostatics, balanced charge agglomeration and depth media filters, and 2) Dissolved breakdown product removal including ion exchange and ion charge bonding technologies.
As discussed above, varnish is the consequence of dissolved breakdown products accumulating past the point of saturation. Since the key to prevent varnish is to prevent saturation, only varnish removal technologies that remove the dissolved oxidation products and return the oil to an unsaturated state can prevent varnish.
While particulate removal technologies are excellent tools, and are capable of removing insoluble contaminants once saturation is achieved, they are not able to prevent saturation and therefore not able to truly solve the varnish issue. For this reason ion exchange and ion charge bonding technologies are preferred.
Important considerations when selecting a varnish removal system
1. Does the system work during normal turbine operation when the varnish is dissolved in the oil?
While particulate removal technologies offer very important benefits, such as reducing wear, they are ill equipped to remove varnish because it can take up to 72 hours to form after oil is cooled to ambient temperature. This is why the ASTM D7843 varnish removal test requires a 68-72 hour sample "aging" period. In an effort to accelerate this transition, some particulate removal systems use a cooler on the inlet to try and force dissolved varnish precursors out of solution before they go through the filter. Using an ion exchange or ion charge bonding system that can remove varnish precursors when they are dissolved is much more effective and will offer 24/7 protection regardless of turbine operating condition.
2. What is the flow rate of the system?
Optimal lubricant varnish prevention using ion exchange requires a system that cycles the entire reservoir volume >1x/day through the filtration system. Because the flow rate is constrained based on the volume of ion exchange resin used, attention needs to be paid to the filter size. For lubricants with high varnish levels requiring restoration, higher reservoir exchange rates may be in order.
3. What is the cost of ownership over the lifespan of the system?
The cost of ownership of a varnish removal system includes the initial cost of the system, maintenance and consumables over its lifetime. The latter can be calculated by the resin bed volume of the filter (e.g. per in3 or cm3). Special care is required when evaluating this key point. As stated in #2, the volume of resin used also determines the flow rate of the system, so attention must be paid to the filter size.
4. What is the support for the system?
Quality vendors should be available for on-site start up and training as well as ongoing monitoring to document the success of the system. In addition, users need to evaluate the oil analysis used to determine initial oil condition and show success as standard testing often misses key indicators.