Oil-injected rotary screw air compressors are one of the toughest applications there is for any type of lubricant or coolant. But when you try to perform both critical functions with a single fluid, and continuously pump large volumes of reactive air and contaminants directly into it, it is no wonder that oil-related reliability problems are so common with these compressors.
Depending on the nature of the contaminants that are pumped into the oil and the specific type of oil being used, these oil-related problems often call for a deep cleaning of the compressor or a flush, and converting to a different type of oil that is better suited for the compressor’s environment. In either case, a thorough flush of the compressor is often the best recommended course of action.
The following information is provided to help compressor users understand when flushing their air compressors may be warranted and the proper procedures for an effective flush that will achieve the optimal outcomes.
Reasons for Performing a Flush
There are several reasons to perform a flush on a rotary screw air compressor, including:
1. Converting Oil Types
Compressor OEMs often standardize on one specific type of oil when filling their compressors. However, their “factory fill” may not be the best choice for your compressors’ specific operating conditions, which over time can eventually lead to oil-related problems in the field.
The different compressor base oils most used are mineral oils, synthetic hydrocarbons (SHCs), Polyalphaolefins (PAOs), Diesters, Polyol esters (POEs), and Polyalkylene glycols (PAGs). These different oils and their additives may be incompatible with each other and as a rule should never be mixed.
Mixing incompatible oils can lead to problems including foaming and reduced service life.
2. Removing Contaminants
Dirty, degraded oil can leave harmful contaminants (solids & acids) trapped in dead zones within the compressor after a routine oil change. Sometimes this can be as much as 25% of the compressor’s total fill volume.
Residual contaminants left behind can immediately begin to degrade the new oil, deplete additives, and significantly impact its performance and service life.
3. Cleaning Varnish and Deposits
Oils like PAOs, SHCs, and mineral oils are known to form varnish and deposits that plate out on compressor internals when run to failure. Varnish is a compressor killer, negatively impacting viscosity, oil flow, cooling, and lubrication, which if ignored can lock up a compressor’s air end and result in expensive downtime and repairs.
Diester oils, which are commonly blended with other synthetic oils to improve solvency, will form acids and sludge in the presence of water, which is always present in air compressors. Like varnish, these solids will also negatively impact lubrication and cooling.
Flush Fluids
Diester-Based fluids are most used for flushing due to their relative low cost, compatibility with most compressor oils. They have excellent natural solvency which aids in cleaning varnish, sludge, and solid deposits. Diester based flush fluids fall into one of two categories:
- “Strong” Diester Fluids: These fluids typically contain higher concentrations of diester and lower doses of additives which gives them maximum solvency and are best for short-term use to clean compressors with a history of varnish or sludge. These fluids can be aggressive to the compressor’s elastomers and should only be used for short periods of time (up to 8 hours).
- “Weak” Diester Fluids: These fluids have lower diester concentration and lower solvency which makes them better suited for compressors with little to no varnish and conversions to different oil types. These weaker diester fluids also have higher doses of additives, making them suitable for extended use (up to 500 hours) and more compatible with compressor elastomers.
Flush Procedure:
Regardless of the reason, the basic process of flushing a rotary screw air compressor remains consistent. The only variables to consider would be the type of flush fluid to use, the duration of the flush, and the number of flushes potentially required for the specific issue at hand.
- Warm-Up: Run the compressor with the oil to be replaced until it reaches normal operating temperature (approx. 180°F / 82°C).
- Shutdown: Shut down the compressor, lockout/tagout power supply, close the isolation valve, and vent the compressor to atmosphere. (follow manufacturer’s recommendations)
- Drain: While the oil is still warm and free flowing, thoroughly drain all the fluid as possible from the main sump tank, oil cooler, air-end, and fluid lines.
- Filter Change: Replace the compressor’s oil filter.
- Fill: Fill the compressor with the proper flush fluid.
- Run: Restart the compressor and run with flush fluid:
Stronger diester concentrations: up to 8 hours
Weaker diester concentrations: up to 500 hours
- Monitor: Monitor compressor discharge temperature and the differential pressures of the oil filters and separators. (Flush fluid may mobilize contaminants that can foul components and result in rapid rise in temperatures and differential pressures.)
- Drain: Thoroughly drain the flush fluid from the compressor, observing its appearance and contamination level to determine if additional flushes may be needed. If anything unusual is detected, perform another flush and repeat until the fluid looks normal.
- Fill & Replace: After the final flush refill with new oil, replace separators and oil filters, and run the compressor at full load and normal operating conditions. Record the relevant data (time/date, discharge temperature and pressure, and oil filter and separator differential pressures).
- Oil Sampling: Sample the new oil within the first 24-48 hours to establish new baseline properties and evaluate the effectiveness of the flush. Assuming the new oil’s baseline properties are well within their normal limits, continue sampling the oil as follows:
Conversion Flush: If the oil had no history of varnish, sludge, or solids, resample the oil again at 1,000 hours, and then regularly at 2,000-hour intervals.
Cleaning Flush: If the oil had a history of varnish, sludge, or solids, resample every 500 hours up to 4,000 hours, while continuing to monitor temperatures and differential pressures. If after 4,000 hours in service the oil properties and compressor temperatures and pressures remain within their normal limits, increase the oil sampling interval to every 2,000 hours.
(Note: Converting from a silicon-based compressor fluid requires a special procedure not covered here. Consult Fluid Metrics for more information.)
Summary: In summary, flushing of rotary screw air compressors is recommended to address a variety of oil-related compressor problems. Whether performed to avoid compatibility problems when converting to a different type of compressor oil, or to remove residual contaminants or varnish that have accumulated from years of continuous service and oil degradation, flushing is a valuable tool air compressor users can use to avoid more expensive problems down the road.