When Oil Flushing Is Necessary

Flushing takes several forms, depending on the system and issue being addressed.

By Mark Barnes, PhD CMRP, Des-Case Corp.

If an oil has been left in service for too long or a system has become heavily contaminated, it is always advisable to flush the system when changing oil. Failure to do so can lead to contaminants being left behind since it’s not always feasible to drain every drop of oil from a system. Failure to remove byproducts of oil degradation can also shorten the life of the new oil since deposits left behind can act as a catalyst and degrade the new fluid.

While the singular word flushing might suggest a simple, single process, flushing can take several forms, depending on the nature of the system and the issue that is being addressed. The correct and most effective will depend on oil type, contaminant type, and overall system complexity. Following are some of the more common flushing methods and when they should be used.

Draw-down oil flush: A draw-down oil flush involves nothing more than draining the old oil, re-filling with new oil of the same type, operating the system for a short period of time, then draining and re-filling a second time. While this can be effective if the purpose of flushing is to remove minor water contamination or if the system has been topped off with the wrong viscosity of oil, draw-down flushing is rarely the most effective strategy. 

Mechanical cleaning: If baked-on deposits are present in the oil sump or reservoir, it may be necessary to physically remove deposits with a scraper or wire brush. Mechanical cleaning is often required if the oil drain has been over extended or if there is a large difference between the oil temperature in the reservoir and the rest of the system. Significant temperature differences tend to accelerate the deposit baking process.

Solvent or chemical cleaning: When an oil drain has been over-extended, there will often be sludge or varnish build up in the system. To remove these deposits, it may be necessary to use a solvent or other chemical to dissolve and suspend this material back into the oil, so that it can be removed by draining the system. Old-school mechanics will tell you that using kerosene or diesel fuel is a good strategy in this instance. This is typically not recommended.

Most oil suppliers provide a flushing fluid that is specifically designed to be used with certain oil types. These system cleaners are typically added to the old oil in low concentrations (5% to 10%). The system is then operated under low load for a period of time (12 to 24 hr.), the oil drained, and the system re-filled with new oil. For hard-to-remove deposits, there are stronger/more aggressive solvents and chemicals such as acids, caustics, and solvents such methyl-ethyl-ketone (MEK). These chemicals, while effective, can pose a health and safety risk due to their acute toxicity.

Flushing through offline filtration<bold>: In some cases, removing oil-degradation deposits can be achieved without ever changing the oil. This is done by connecting an offline filtration unit to the system using filters specifically designed to remove degradation byproducts. Several types of media can effectively remove deposits, though their efficacy will depend on the nature of the deposits and how they were formed. The two most common media are depth media cellulose elements, which are most effective at removing oxidation byproducts in anti-wear hydraulic fluids, and polymer resin filters, which are effective at removing thermal deposits in turbine oils.

Any flushing through offline filtration will, by its very nature, be slow. The process involves stripping soluble and insoluble byproducts from the fluid, thereby giving the oil greater solvency to further dissolve and suspend material throughout the system. For heavily contaminated, complex systems, it can take several months to effectively remove deposits.

High-velocity flush: Significant deposits may form in systems that use extensive piping or hoses. Removing these deposits may require a high-velocity fluid flush. Under slow (low) flow rates, fluid in pipes and hoses tends to travel in a calm, laminar fashion, like a river in mid-summer when water levels are low. By increasing the flow rates, it’s possible to cause the fluid to flow in a turbulent fashion. Just like a river in flood stage erodes its banks, this turbulent flow will scour the walls of the piping helping to remove deposits. To ensure turbulent flow, flow rates need to be increased to create a Reynolds number greater than 4,000. While this is relatively simple for low-viscosity fluids, generating turbulent flow can be tricky with higher viscosity oils.

Reverse-flow flush: In some cases, reversing fluid flow during flushing can help dislodge stubborn contaminants.  This is often done with high-velocity flushing. Jumpers may be required to by-pass contamination-sensitive components such as valves.

Intermediate flushing fluid: When changing from one oil type to another, it is sometimes appropriate to use an intermediate flushing fluid. This might be necessary when switching to an oil that is known to be incompatible with the old fluid, or when switching oils from different API base-stock categories. Where additive incompatibility is expected, a simple strategy is to perform a draw-down flush, using an intermediate fluid of a similar viscosity grade but with no- or low-additive content. An R&O oil is often a good choice for this procedure. The system should be drained, filled with the flushing fluid, then re-drained before finally refilling with the new oil type.

Extreme care and extensive flushing are required when one of the oils involved in the switch is from the API Group V category. While not always the case, several Group V oils are incompatible with hydrocarbon mineral oils from API Groups I-III, as well as synthetic hydrocarbon-based oils from API Group IV. In this instance, the intermediate flushing fluid must be compatible with the existing and new oil types.

The most common mistake here is accidentally mixing polyalkylene glycol (PAG) fluids with mineral or synthetic hydrocarbon fluids. PAG and hydrocarbons react and form a gel that can quickly plug system filters and oil clearances. Whenever switching from a hydrocarbon to a PAG or vice-versa in applications such as plant air compressors or gearboxes, an intermediate fluid, most commonly a polyol ester (POE) fluid should be used. Figure 1 provides some guidance as to when a flush may be necessary based on base-oil type. Whenever switching oils of different types or manufacturers, it is always advisable to ask about base oil and additive compatibility and, if necessary, flush the system during the oil change.

With the cost of oil ever increasing, along with an increased focus on reducing our carbon footprint, flushing systems during oil changes is becoming a standard procedure that helps ensure new fluid lasts as long as possible. From removing baked-on deposits to removing water or particle contamination, flushing is a critical step in ensuring that oil, the lifeblood of our machines, will protect our assets and deliver optimum reliability and asset availability. EP

Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management, oil analysis, and contamination control.