3 min read

Varnish in Turbine Oil Systems

Sun, Nov. 18, 2012

Despite thousands of dollars and hours spent trying to mitigate the effects of varnish in turbine oil, many still suffer from this ongoing yet solvable issue. When combustion and steam turbines fall victim to unit trip or fail-to-start conditions, varnish is the usual suspect.

Sludge and Varnish in Combustion Turbines varnish bearing

Combustion turbines (CTs) use lubricating and hydraulic control fluids for a wide range of applications. All CTs are prone to varnish with GE reporting that all units are expected to have varnish issues over their lifetime. Peaking units are expected to experience varnish problems first because of the thermal cycling of the lubricant and the settling time lubricants have in some units.

Surfaces Prone to Varnish Deposits  

Varnish issues frequently occur on bearing surfaces, the inlet guide vane (IGV) servo and on gas or liquid fuel control valves. Any varnish deposit can cause failure. Once the varnish starts to deposit, the servo valves can stick, slow or even fail to operate. Fail-to-start conditions and valve positioning errors, which force manual start requirements on plants, are frequently the result of varnish deposits. With the growing number of peaking gas turbines, the varnish issue will remain front and center unless an effective varnish mitigation program is in place.  

Modern Gas turbine lubricants use Group II base stocks which have better resistance to oxidation but have significantly lower solubility characteristics. While these lubricants are normally highly effective, their capacity to hold oil breakdown products in solution is limited. If these lubricants are operated beyond their saturation point, they will form varnish deposits. If oil breakdown products are not allowed to accumulate, the saturation point is never reached and these lubricants cannot form varnish.

Common Locations of Sludge and Varnish in Turbine Lube Oil 

The following are examples of where sludge and varnish might occur in turbine lube oil:

  • black crusty deposits on mechanical seals
  • gold adherent films on valves
  • charcoal-like deposits on babbitt sleeve bearings
  • gooey-brown accumulations on oil filters
  • black scabby deposits on mechanical seal surface thrust-bearing pads
  • carbonaceous residue on mechanical surfaces

Now that you know where sludge and varnish are likely to deposit themselves, read on to learn strategies for managing these deposits and residues. 

Mitigation Strategies for Varnish Prevention and Removal 

Effective mitigation strategies prevent the accumulation of oil breakdown products, allowing high oil solubility to be maintained at all times, which eliminates the potential for varnish to form. Reactive mitigation strategies use particulate removal devices, which must wait for oil breakdown products to accumulate in the lubricant (sometimes for years) for saturation to occur and then for varnish to form before they can start to work. By this time, the risk event to the turbine is already present so these systems have missed their window of opportunity.

Particulate removal devices by definition can only capture the spillover that falls out of solution, and do not address the root cause of varnish formation which is the accumulation of soluble breakdown products.

How SVR Systems Vanquish Varnish 

SVR systems remove breakdown products as they are created so that accumulation never occurs, varnish products never form and existing deposits are dissolved back into the highly soluble oil. By removing the dissolved breakdown products, the lubricant additives will have improved performance and longer life because the oil breakdown molecules are no longer providing an offensive line, and the additives will be more likely to reach their intended target which is oil oxidation.

 

Download one of our Soluble Varnish Removal Case Studies!

Written by Aaron Hoeg

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