TECHNICAL CONCEPTS OF HIGH-SPEED OIL FLUSHING. DIFFERENCES WITH FILTRATION
When one talks about the term Contamination Control in Industry, it is frequent to hear the word Flushing as one of the necessary measures to reduce the particle content in lubrication or hydraulic system.
In this paper, we intend to correctly define this term and to clarify the main differences with the term Filtration.
It is vital that all the parts involved use the same terminology to work properly and professionally in the field of lubrication engineering and its practical uses in the industry.
Keywords: Flushing, Reynold, Filtration.
There are various ways to define a Flushing procedure; here we’ll quote the one that appears in ASTM D-6439-05 Standard . It reads circulation of liquid through the lubrication system or a component, when the turbine is not operating, to remove the contaminant.
It is important to highlight that the procedure is pointed out as the right one to remove contaminants from the mechanical equipment, its lubrication system or components. It is not focused on cleaning or filtering the oil, rather it is focused on cleaning the system, the mechanical components and pipes.
Several aspects have to be considered to achieve a successful Flushing procedure, that is, a procedure that removes the greatest amount of dirt from the system and in the shortest time possible. The most important ones are:
1- It has to be performed before the first start-up of the equipment (Commissioning), or in scheduled shutdowns (major outages).
2- Achieve a turbulent flow regime. Regularly, the Reynolds number should be greater than 4,000 to ensure that the different fluid layers inside the pipe have better action over the internal walls of the pipe.
3- Use of the high-efficiency filter media. Filter media with a βx factor higher than 200 must be used to ensure that the particles removed are extracted from the system.
4- Use of in-situ oil analyzers. It’s very important to have analysis equipment of ISO 4406 code (Particle Content) to perform a follow-up of the procedure and determine when the cleaning has ended.
The above mentioned will give Reliability and Availability to the System, allowing to obtain a start-up without shocks and the optimization during the times of start-up and scheduled shutdowns, concepts which are more and more demanded to increase the Annual Production.
To achieve a turbulent regime, it is necessary to know the diameters of the tube which will be intervened, and to make the following equation:
Re = 21.200.Q / (V.d) Q [lts/min]
V [Cts], at the operating temperature
In practice, to increase the Re, one can choose to use external high-flow pumping equipment, a fluid of lower viscosity for the work, or simply to hit it up to reduce its viscosity. Generally, one chooses a combination of all the previous options.
Thus, for example, to perform a Flushing in an 8” pipe (203.2 mm), using ISO VG 32 oil, and taking into account an operating temperature of 40°C, the minimal necessary flow to fulfill the requirements of the Standard would be 1,230 lts/min.
In relation to high-efficiency filters, it is essential to use a set of external filter media with a βx > 200 factor, remembering that this βx factor indicates how efficient the filter media is in the removal of particles. In this case βx > 200 means that the efficiency of removal will be higher than 99.5% in particles greater than x microns.
These filters should be mounted in the complete flow of circulation. This is why, frequently, large surfaces are required since we are working with high flows.
Going back to the main point of our paper, we will indicate now the differences between the procedure we have just defined and the one which tends to be wrongfully used in the industry under the name of Flushing.
When the word Flushing is mentioned, one is many times simply referring to an external kidney loop filtration of the oil tank or the system reservoir. Even if we use high-efficiency filter media, the reader will easily understand that the reach of this procedure is much more limited than that of the Flushing defined by the Standard. In this case, we simply intend to filter the oil, take it into the right ISO Code and not perform a cleaning in the system, which is the main goal of the concept Flushing previously defined.
This filtration procedure may be carried out with the system in operation using much lower flows, since the use of higher flows here, would generate turbulences inside the reservoir which may affect the normal operation of the lubrication or control system.
It is normally advised to perform Flushing at the moment of the first start-up of the equipment and at every scheduled shutdown to maintain the system clean during the life span of the machine, thus, preventing the lube oil contamination. During the operation of the machine, and according to the results of the daily oil analysis, it may be necessary to connect external filtration equipment to reduce the number of particles present in the system.
Combining, and not confusing, the Flushing and Filtration procedures during the life span of the equipment, one can achieve a longer duration of the mechanical components, and, therefore, greater reliability of the equipment. Through the bearing life span calculation, or life extension tables for bearings, hydraulic systems, engines, gears and other mechanical elements, it is proven that the lower the contamination level, the longer the life span of these elements. 
The following table summarizes the main aspects of both terms and their differences:
Goal / Target
Start-up or Scheduled Shutdown
In the following graphic, we can observe how the use of high-efficiency filter media, instead of the filter media of the equipment, ensure a shorter duration in the flushing process.
On the other hand, the table below shows the main differences between a conventional flushing and one carried out under the most demanding premises of ASTM D-6439 Standard. It summarizes the main points where advantages in reliability and service performance time can be achieved.
To achieve commissioning or a start-up after a scheduled shutdown, it is essential to carry out a flushing procedure according to the standards, thus ensuring the removal of all the dirt from the system and in the shortest time possible to improve the production time of the intervened system.
Likewise, during the operation of the equipment, special attention must be paid to the presence of contaminants in the system with the aim of removing them, and, in this way, guarantee the longest useful life of the mechanical elements. In this case, the external filtration resource must always be taken into account.
It is extremely important to be acquainted with both procedures and apply them in the appropriate times and ways to obtain the best results from a technical and economical point of view.
 ASTM D-6439-05 “Standard Guide for Cleaning, Flushing, and Purification of Steam, Gas and Hydroelectric Turbine Lubrication System”
 Lubricación por Niebla de Aceite y su impacto positivo sobre la vida de los rodamientos [Oil mist lubrication and its positive impact on the lifespan of the bearings]. Work presented in the I Taller Argentino de Tribología [I Argentinean Workshop in Tribology], May 2012, Bahía Blanca.