About 15 years ago, I added a new file to my library of technical literature - a folder into which I put a single magazine article on flushing procedures. Today the file is bulging with papers, articles, procedures and industrial standards on this single subject. When you consider the large number of machine types in use, each with special needs and the unique problems that flushing must restore to a clean and healthy state, it is no wonder that so many different approaches and technologies have been deployed. Both service and equipment suppliers continue to offer new answers to the age-old problem of how best to purge contaminants and lubricant degradation products from the bowels of the machine.
My column in the May-June issue was entitled “When to Perform a Flush.” It addressed the many machine conditions which threaten operational reliability and fluid health… conditions serious enough to justify the often-expensive activity of removing adherent surface deposits and low-lying sediment. In that column, I discussed the many root causes that can lead to the need for a flush and the potential consequences of delaying or not performing the flush at all.
Because flushing activities present a maze of options, I’m covering the subject in four consecutive issues of Machinery Lubrication. This column is Part 2 of the series and will describe 11 flushing tactics. In review, flushing can be broken down into the following elements:
Flushing Tactics. These are single discrete activities directed at removing unwanted deposits, sediment and risk-prone fluid suspensions.
Flushing Strategies. Strategies are a program of one or more tactics and related steps needed to achieve a complete and successful flush.
Let’s begin this tactical discussion by distinguishing between two closely related cleanup activities: oil reclamation and machine flushing. Unlike flushing, oil reclamation (also known as reconditioning) does not have to involve the machine and its surfaces. It is simply a process of removing health-threatening contaminants from the bulk oil. In certain cases, this may include acid scavenging. For large systems, it may be followed by bleed-and-feed or other top treatments to restore depleted additives and dilute soluble impurities.
As mentioned in the following list of tactics, the removal of harmful contaminants (both soluble and insoluble) from bulk oil can beneficially impact the removal of pre-existing sludge and varnish. It can also substantially mitigate the future formation of internal machine deposits. This has led to a blurred line between the definitions of oil reclamation and flushing. The confusion basically comes from these two cross-linked statements: (1) It can be safely stated that a machine’s internal environment free of deposits, sediment and sludge will by default result in extended oil life expectancy; (2) In a similar fashion, a bulk lubricant scrubbed free of soluble and insoluble impurities by oil reclamation will have a measurable impact on machine service life, cooling and friction. In certain instances, the reconditioned oil can even be an effective agent in removing of varnish-like deposits. This explains why many of the flushing tactics mentioned below are also classical technologies used for oil reclamation.
Choosing the wrong tactic can be not only wasteful, but also risky from the standpoint of potential system upsets and negative side-effects. Anytime you introduce unusual fluid chemistry, temperatures, pressures, flows and turbulence there can be adverse consequential effects to the machine, its seals and the lubricant. Now on to the tactics … the list below describes in limited detail, the practices and technologies used by at least 95 percent of flushing activities in industry.
Drawdown Filtration/Separation. This is the mildest of the flushing strategies. Because many machines have no onboard filtration, the use of periodic filter carts and oil reclamation equipment not only can clean the oil (drawing down the contaminant level) but can also remove loosely deposited sludge and sediment.
High Turbulence, High Fluid Velocity, Low Oil Viscosity. Flushing is improved by enhanced fluid dynamics near machine surface boundaries. The approach involves increasing fluid velocity (sometimes two to four times the normal flow rates) and/or reducing oil viscosity during the flush. Typically, a Reynolds number in the range of 4,000 to 6,000 is generally targeted. Use the search engine at www.noria.com to get information on Reynolds number.
High Flush-Oil Temperature. This strategy also reduces viscosity and increases turbulence, and in addition, it increases oil solvency to aid in the scrubbing of tenacious deposits. Target temperatures range from 175ºF to 195ºF.
Cycling Flush-Oil Temperature. Some practitioners have discovered that shocking the machine with large temperature shifts helps break loose crusty deposits during the flush. They use coolers and heaters to cycle the oil’s temperature repeatedly over a range greater than 100ºF.
Pulsating Oil Flow. Rapidly changing oil flow rates caused by pulsation have been found to help dislodge pesky contaminants from nooks and crannies.
Reverse Oil Flow. By changing fluid flow direction, some contaminants and surface deposits are exposed to bending fatigue reversals and can be dislodged and freed into the oil.
Wand Flush Tool. This tactic is used for wet sumps, gear boxes and reservoirs with convenient access to hatches and clean-out ports. A wand on the end of a flushing hose is used to create high-velocity oil flow to blast away deposits. Alternatively, the wand used in suction mode can be effective at picking up bottom sediment on the sump floor.
Charged Particle (Electrostatic) Separators. Some suppliers of these proprietary reclamation technologies have successfully removed varnish from machine surfaces as well as submicron soft contaminants in the oil, known to be a precursor to varnish and sludge.
Solvent/Detergent Flush. Various solvents and detergents have been used to concoct flush fluids with different degrees of success. These include mineral spirits (petroleum distillates), diesel fuel, motor oils and detergent/dispersant packages. They are typically added to the flush fluid at concentrations of 5 percent to 15 percent, followed by a rinse. Compatibility problems (with the oil, seals and machine surfaces) are the primary concern. Always consult machine and lubricant suppliers before these chemicals are introduced.
Chemical Cleaning. These are chemically active compounds, typically caustics and acids, that aid in the removal of the most adherent organic and inorganic surface deposits. The oil must first be removed completely from the system. Following the flush, these chemicals should be thoroughly rinsed from the system, often followed by pacification. Always consult machine and lubricant suppliers before employing chemical flushes.
Mechanical Cleaning. This generally involves the use of scrapers, brushes, abrasives and sometimes an ultrasonic bath. Often, chemicals are also used as the machine components are washed one at a time using a parts-cleaning station.
Table 1 summarizes the application and probable effectiveness of these technologies in removing contaminants, sediment and deposits from machinery. So there you have it, 11 tactics for flushing.* In the next issue, I will discuss how best to use these tactics in a strategic flushing program to maximize their effectiveness and reduce costs and risks.
*Many of these tactics have been discussed in articles published in Machinery Lubrication or Practicing Oil Analysis magazines.