Plant Services - August 2008 - (Page 39) An Inside You must keep your hydraulic system clean of varnish or pay the consequences By Rob Profilet Story pecially troublesome. e effect of hydraulic-system valves sticking and operating erratically leads to impaired responsiveness and reduced oil flow, which reduces efficiency and increases maintenance costs. he competitive nature of the hydraulics market has created a trend of increasing power outputs along with smaller fluid reservoirs. is produces consequences for manufacturing plants. e operating temperatures of hydraulic fluids in these systems are significantly higher, leading to greater risk of oxidation and thermal degradation of the additives in the fluid. e result of that degradation is varnish, and it can cause unplanned downtime, higher maintenance costs and reduced profitability. Over time, varnish buildup increases the friction inside the valves, especially those with fine tolerances such as servo and proportional types, where this phenomenon can be es- T The lowdown on varnish Challenges for hydraulic oils Original equipment manufacturers report that the most frequent problems with high-performance hydraulic systems are: • Systems are getting smaller while the flow rates in the reservoir have increased. • The reservoir size and shape are not optimum for fluid life. • Oil flow rates are high compared to oil volumes. • Hydraulic systems are designed to have higher power densities. • Oil temperatures are higher. • Oil pressures have increased in general. The consequences of those challenges include: • Foaming and cavitation because oil spends insufficient time in the reservoir to release air and allow foam to collapse. • Shorter fluid life because of increased oxidation. • Poor hydraulic valve response because of sludge and varnish buildup. • Greater need to replace blocked filters. • Increased valve and pump wear. Oxidation, thermal decomposition and the natural process of additive consumption is what causes fluid to degrade as it ages. Additives – the performance-enhancing chemicals – are consumed during the fluid lifespan. e concentration of degradation byproducts increases as oil ages, eventually forming varnish. Because varnish is a polar material, it’s attracted to metal surfaces such as servo valves. Varnish starts as a sticky, soft residue. e sticky nature of this material allows it to attract wear debris, resulting in formation of a sandpaper-like surface. Eventually, the soft, sticky material changes to a hard lacquer that can be difficult to remove (Figure 1). Be assured that varnish harms equipment. Oxidized oil generally doesn’t lubricate very well. e result can be reduced oil flow, plugged fi lters, plugged valves, greater friction, poor heat transfer and elevated operating temperature. Because varnish acts as an insulator, it compromises the system’s cooling capacity. For example, in high-performance vane pumps, varnish adhering to the vanes can cause a vane to jam in the rotor slot. e consequences are increased noise, decreased volumetric and mechanical efficiency, increased energy consumption, side plate scuffing, rotary seal damage and possible bearing damage. 39 A .PLANTSERVICES. http://www.PLANTSERVICES.com
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