Cost-Effective Alternatives for Flushing Turbine Oil Systems

Oil flushing is a catch-all term used to describe a variety of activities for removing lube system contamination and cleaning the internal components of your system. ASTM D6439 defines flushing as "circulation of liquid through the lubrication system or a component, when the turbine is not operating, to remove contaminant."

There are a multitude of ways to restore lube system cleanliness. In order to select the proper oil flushing method, you should consider the overall plant objectives, the types of contaminants in the system and the condition of the lube system components.

To Flush or Not to Flush

When required and done effectively, oil flushing is a high-value maintenance practice for your machines. Restoring system cleanliness can add years of life to your equipment. However, oil flushing is a disruptive practice to any lube system and carries significant risk if not performed appropriately. Determining if your system needs a flush is the first step.

There are many situations when oil flushing is required, such as when commissioning new machines or re-commissioning machines that have been idle for a period of time, after a system component fails and leaves broken pieces in the lube system, after a filter collapses and releases contaminants back into the system, when an incompatible fluid has been inadvertently added to the system, when changing to a new lubricant brand or formulation and the compatibility is not understood, or when oil degradation products such as sludge and varnish are in the system.

Unless there is a catastrophic failure, the most common time when turbine oil users contemplate a flush is between oil changes, but is a flush required each time you change your turbine oil? There is not an industry-accepted practice for determining this. Removing degraded oil and deposits from the internal components of your lube system certainly seems to make sense before charging your system with new oil. You wouldn’t want to take a bath in a dirty bathtub. However, oil flushing can be costly. Is it really necessary?

Flushing is not required between oil changes if the lubricant system is free of sludge, varnish and other deposits; the majority of old oil can be removed from the system; or the new turbine oil is of the same type and brand as the in-service oil or extensive compatibility tests have verified its compatibility.

On the other hand, flushing between oil changes should be performed when there are indications that sludge, varnish or other deposits may be in the system; the current in-service oil is in poor physical or chemical condition and it is not feasible to remove more than 98 percent of the oil from the system; or the new charge of oil will be a different formulation that may not be compatible with the current in-service lubricant.

Impact of Putting New Turbine Oil into a Contaminated System

Since the primary mode of turbine oil failure is oxidation, the life and performance of a turbine oil will be dependent upon the health of its antioxidant system. Recharging a dirty lube system with new oil can have a significant impact on the life and performance of this new oil. Oil degradation products are reactive species that will rapidly deplete the fresh antioxidants. Other properties such as acidity, water separability and foam inhibition can all be impacted by residual varnish and sludge in the system. The following examples illustrate why putting new oil into a contaminated system can have adverse effects.

A Ship’s Hydraulic Fluid Sump

Large ships often use rust and oxidation (R&O) inhibited oil as hydraulic fluid to control the steering in their controllable-pitch propeller systems. The ship described in the chart above conducted annual maintenance overhauls and changed its hydraulic fluid when the acid number rose higher than 0.20 milligrams of potassium hydroxide per gram (mg KOH/g). The ship did not perform flushing between these oil changes. The chart shows the result that this maintenance practice had on the life of the oil.

Notice that the useful life of the fluid decreased after each of the three oil changes, from five years after the first change to three years after the second change and one year after the third change. Significant losses in fluid value were also realized after each oil change cycle. Even though the same fluid was used and the system operations were nominally equivalent, the oil changes were less effective in extending the useful life.

When the ship reached 9 years of age and the third oil change was required after only a year of service, a system flush was performed. This removed all of the degradation products in the system. This oil flushing restored system cleanliness and resulted in the new oil lasting once again for five years.

A Large-frame Gas Turbine Reservoir

When the oil in a large-frame gas turbine was changed, an oil flush was not performed. The used oil was degraded, and the plant experienced valve performance problems due to varnish in the system. The impact of not adequately cleaning the system prior to recharging with new oil is shown in the tables below.

Table 1. Adding new oil to a varnished gas turbine significantly impacted the quality of the fluid.

Table 2. Estimated financial impact of not cleaning the lube system between oil changes

As can be seen in Table 1, the antioxidants have depleted 26 percent after one week of use. This is due to the antioxidants reacting to the residual contamination left in the system. The varnish potential has also escalated to beyond the normal classification for this application. In this case, not cleaning the system before a new fill of oil resulted in approximately 30 percent less life. The oil’s performance will also be diminished, possibly impacting plant reliability and availability.

Understanding the Reactivity of Varnish Components

A simple experiment was set up to understand the reactive nature of varnish and the impact it has on new oil. A filter that contained significant varnish deposits was first rinsed with petroleum ether to remove the oil, leaving the varnish on the media. The varnish-loaded media was then placed in a beaker of new oil and allowed to mix for one day. Fourier transform infrared (FTIR) analysis was then performed on the oil after 48 hours. The oil changed color significantly and dissolved some of the varnish components in the filter. The addition of degradation products to the oil could also be seen on the FTIR spectrum.

The analysis revealed that a significant concentration of amine antioxidants was lost from the new fluid. This experiment demonstrated the negative impact degradation products can have on new oil additives.

Options for Flushing Turbine Oil Systems

While a number of different oil flushing techniques can be employed, they generally can be categorized into three groups: chemical flushing, mechanical flushing and solubility-enhanced system cleaning.

Chemical Flushing

With this method, surface-active chemistry is used to remove varnish and sludge from the system. The surface-active cleaning agent is typically an oil-soluble solution composed of a naphthenic base with detergents, dispersants and corrosion inhibitors. The surface-active agent is often added to the in-service turbine oil 48 hours before the outage at a rate of 10 percent. Supplemental filtration is usually required to handle the high amount of contamination that is released during the flush. 

The in-service oil and cleaner are then drained from the system, and a sacrificial flushing oil is used to rinse the remaining cleaner from the system. Frequently, multiple partial charges of flushing oil are needed to adequately dilute the cleaner in the system. Sometimes water-based fluids with cleaners are employed as the flushing fluid.

Table 3. Overview of turbine oil flushing technologies

Mechanical Flushing

When performing a mechanical flush, heated oil is pumped at a high velocity throughout the system using external pumps. The flow rates are typically three to four times the normal flow rate and require a minimum Reynolds number of 4,000 to achieve turbulent flow. Bearing jumpers are installed to increase bearing supply and return flow rates as well as to protect bearing surfaces from abrasive contaminants. Other components such as system headers and valve blocks are also isolated from the flush. Supplemental side-stream filters (typically bag filters) are installed to collect contaminants pushed out of the system from the turbulent oil flow. Manual cleaning of components often accompanies a mechanical flush.

Filters before (left) and after an oil flush (right)

Solubility-enhanced System Cleaning

For this oil flushing method, a compatible solubility-enhancing agent are blended into the in-service turbine oil three months prior to the planned outage. This chemistry greatly improves the oil’s deposit control characteristics. It works by increasing the solubility of in-service fluids so that the oil has the ability to readsorb system deposits like sludge and varnish.

During the outage, the in-service oil and solubility-enhancing agent are drained from the system, and the cleaned reservoir is recharged with new oil. No sacrificial rinsing oil is required since the solubility enhancer is compatible with the in-service oil.

The ability of an oil to redissolve deposits depends upon it having negative free energy from a thermodynamic perspective. Le Chatelier’s law (the equilibrium law) governs the balance of this energy and does so by dissolving more deposits into the oil. Adding the solubility-enhancing agent to the lubricant increases its solubility, providing the required kinetic forces to redissolve deposits back into the fluid.

In many cases, a slip-stream chemical filtration system is connected to the turbine oil three months prior to an outage. The filtration system must have the capability of removing dissolved degradation products from the oil. This allows the contaminants to continually be removed from the oil and restores the ability of the fluid to dissolve more contaminants.

Oil flushing services often come with a combination of technologies and flushing techniques bundled together. However, in all cases, it is critical to match the various technologies with your oil flushing objectives and resources.