- Buyer's Guide
This is Part Three of a four-part series on flushing. Part One (May-June issue of Machinery Lubrication) addressed the topic of when to perform a flush and the possible consequences of delaying the procedure. Part Two (July-August ML) covered the maze of flushing tactics and discussed the similarities and differences between flushing and oil reconditioning.
Part Three establishes a rationale to select the best tactics for the specific machine and conditions requiring flushing. The final part of the series will assemble them into a program of steps needed to achieve a complete and successful flush. If you haven’t already read the first two parts, I suggest you review them before proceeding with the strategies outlined below. Articles from past issues of Machinery Lubrication magazine can be found in their entirety at www.machinerylubrication.com.
It stands to reason that selecting the wrong flushing tactic and strategy can be not only costly and time-consuming but potentially ineffective. Carelessly opening a machine and introducing foreign fluids may also present risk to the machine’s future reliability - more harm can be done than good. Because of such concerns, there are many cases where the flushing job may well exceed the practical limits of the on-site maintenance staff. This could be due to the complexity of the flushing procedure or the need for specialized auxiliary equipment. If your job is large and complex, it may be good advice to consult with professional flushing service providers before proceeding with a do-it-yourself plan. In such case, you may want to confirm that their approach to flushing will be tailored to your specific needs. Some flushing contractors perform the exact same type of flush for all clients and applications.
Figure 1 graphically depicts the flushing strategies discussed below. Across the top are the four potential steps that comprise a flush sequence. Note that each flush strategy does not always require the use of each of these steps. A black dot indicates that the step in the sequence is deployed for any particular strategy. No dot means that it is skipped or unneeded. The four steps are described below:
Original Oil - this is the old oil that was in use at the time the decision to perform the flush was made. In some cases, this oil may have already been drained from the machine.
Flush Fluid - this is a liquid that has been introduced to the machine after the drain of the original oil. This could be an oil, a solvent, a detergent, a caustic, blends etc. as described in the corresponding flushing tactics.
Rinse Fluid - as the name implies, the rinse fluid is used to rinse out the flush fluid, including suspended sludge and deposits that were successfully dislodged from the internal surfaces of the machine. In certain cases, more than one rinse may be required depending on the flush fluid and the rinse procedure. The rinse fluid in most cases is a lubricating oil of the exact or similar type (perhaps only lower in viscosity) as the new oil that will be reintroduced into the machine following the flush.
New Oil - after all proper inspections, the machine is ready to receive a charge of new replacement oil. Once done, oil analysis should be employed to confirm that no remnants of the flush or rinse fluids remain.
The flushing strategies below are arranged in order of their inherent complexity, time to perform, risk to the machine and overall cost. It is sensible that the strategies further down this list should be avoided if possible. Note that the descriptions of these strategies are brief and generic. Information on more detailed procedures should be obtained from original equipment manufacturers, lubricant suppliers, etc. before proceeding.
Double Oil Change. When a machine’s internal surfaces are not yet exhibiting signs of distress despite the offensive presence of sludge or insolubles, the best strategy might simply be a double oil change. The first drain carries out a large portion of the contaminants and degraded oil. New oil is then introduced and circulated through a fine filter until operating temperature is reached and the oil is turned over a minimum of four times. It is then drained as well. A patch test, blotter, total insolubles or other suitable oil analysis of the second oil just prior to the drain will define the success of the procedure and perhaps the need to perform yet another sequence of the drain and fill.
Table 1 presents a variation of the double oil change because it is sometimes difficult to perform a complete drain of the full-system fluid volume. Table 1 shows the effect of partial drains (60 to 99 percent, versus 100 percent) of the original fluid and the effect of a partial refill (25 percent) in removing the undesirable contaminants.
Simple Power Flush. The power flush uses a portable filter cart or other high-velocity flushing rig. In some cases, a wand tool can be used to manually direct the flushing fluid in an effort to lift sediment and sludge from the sump bottom or break off deposits that have formed on internal machine walls. This can be performed without an oil change or prior to a single or double oil change. In large circulating oil systems, power flushing is usually performed in steps corresponding to specific zones within the system that must be cleaned. Blocking valves and hoses are often used to partition the targeted zones during the flush steps.
Advanced Power Flush. The advanced power flush is the same as the simple power flush with the addition of more aggressive deposit-removing tactics as listed in Table 1. The selection of one or more of these tactics is usually triggered by an inspection of the machine’s internal surfaces and/or a knowledge of the root cause that led to the need for flushing. Experience with flushing tactics corresponding to the flushing conditions may be the primary basis for the decision. In some cases, these tactics may be an attempt to achieve a successful flush without reverting to the more risky introduction of aggressive chemicals into the machine discussed below.
Chemical Power Flush. Using foreign chemicals such as solvents, detergents, caustics or acids should be considered flushing strategies of last resort. How these chemicals might disturb the machine’s reliable operation is always uncertain. For instance, such chemicals may dissolve internal coatings or surface treatments. They might attack elastomers used as seals or bladders. They might soften adhesives and binders used in the construction of filters. Remnants of these chemicals may clash with the oil (base oil and additives) that is returned to the system after the flush. And finally, they may adsorb into the grain boundaries of machine surfaces and later retard the performance of surface active additives such as rust inhibitors and antiwear agents.
Still, there are times when the chemical power flush is the only viable solution - the lesser of the potential evils. In such cases, laboratory testing in advance and seeking sage advice from professionals in the field are highly recommended.
Mechanical Cleaning. One could say that mechanical cleaning is not really a flush. This is because in most cases the machine must be fully or partially disassembled to gain access to surfaces to be cleaned. As such, there is no application of a flush fluid to the machine during the procedure. Often the deposits that require mechanical cleaning have formed in localized regions of the machine where they present operational risk. An example might be enamel-like deposits on the spool and bore of a servo valve that is restricting actuation. On a larger scale, diesel engines, compressors and gearboxes are often torn down for mechanical cleaning, typically involving the use of scrapers, brushes and solvents.
Besides experience, an important part of defining the correct strategy and tactics therein comes from the inspection of the machine, as illustrated in the sequence shown in Figure 1. This inspection should be repeated before a machine is returned to service to verify that a successful flush has been achieved. Likewise, a final oil sample should be taken and analyzed to confirm that residual flushing fluids or loosened deposits that could potentially compromise lubrication and system operation don’t remain in the new oil.