Hydraulic fluids are subjected to various kinds of contamination. Contaminants can affect the physical and chemical properties of fluids. If the contaminants are not monitored or controlled, the fluid is likely to be subjected to various kinds of failures. These failures include the deterioration of the fluid properties and consequent fluid breakdown. At the system level, the contamination can affect the performance and service life of the components and cause their erratic operation, increased heat generation, frequent fluid replacement, catastrophic system failure, and higher costs.
Contamination in Hydraulic Fluids
Fluids are exposed to various types of contamination (particles, water, air, compounds, and biological matters). Dust, dirt, and sand are contaminants can enter the system from outside. The system itself can generate metal particles from component wear or other byproducts from chemical reactions.
Contaminants are the natural enemy of hydraulic components and systems. 70 to 80% of the failures are due to the adverse effects of contaminants. Even minute particles can damage them due to the existence of minuscule clearances in today’s system components. Excessive water contamination is liable to accelerate the aging process of the system fluid.
Solid Particles
It includes hard particles (dust, dirt, silica, wear metals) and soft particles (elastomers, fibres). Silts are particles less than 5 μm; chips are particles greater than 5 μm. Silt particles of a size corresponding to the typical tolerance are most dangerous to the system than larger chip particles. The system itself can generate metal particles from component wear. Abrasive particles scrap metal from the surfaces of components. The freely-circulating particles can cause premature wear of parts.
Chemical Contaminants
They are formed by the breakdown of additives, due to chemical reactions. The reaction products can generate other contaminants in the form of acids or oxidants, in the presence of water/heat. They can cause physical and chemical changes in the additive elements. These changes can lead to the deterioration of additives and subsequent fluid breakdown.
Water
Water is introduced into the fluid by the condensation of humid air. Unprotected reservoir opening, leaking seals, and ineffective heat exchangers are other means of entry of water. Fluid can dissolve water up to its saturation point. Above the saturation point, water remains in the free or emulsified state. A mineral fluid can permit water content up to100 ppm (0.01%). The moisture is capable of providing the oxygen for the chemical reactions.
Air
Air can exist either in the ‘free state’ or in the entrained state. An air pocket trapped in a part of the system is an example of free air. Air bubbles typically less than one mm in diameter dispersed in the fluid medium is the entrained air. Air can enter into the fluid medium through system leaks, pump aeration, or reservoir fluid turbulence. Air entrained can cause cavitation and foaming, as it cycles through the system. The entrained air tends to make the system operation spongy and system response weak
Contamination Control
Contamination control involves the removal of particles, water, air, sludge, acid, and chemicals. Particles can be removed by installing correctly-sized filters at appropriate locations. Removal of acids, sludge, gums, varnishes and other oxidation products requires the use of an adsorbent filter with active elements. Magnets installed in the tank can remove the ferrous particles and rust matters.
Air contamination can be eliminated by providing air bleeds and diffusers. Water can be removed by installing a water-removal filter or a vacuum dehydrator. Heat can be removed by installing a heat exchanger.
Fluid Cleanness Standards
Cleanliness of fluids needs to be monitored for maintaining components of hydraulic systems at a satisfactory level. Many national and international organizations have developed standards for specifying ‘particle size classification’ and ‘contamination concentration levels’. The particle size classification standard is the ISO 11171:2010. The standard for specifying the contamination concentration levels is the most widely used ISO 4406:1999.
Particle Size Classification Standard
The standard ISO 11171 specifies the three-dimensional size of particles (i.e., 4, 6, and 14 microns), for representing the concentration levels of fine as well as coarse particles. Earlier standard ISO 4402 specified the two-dimensional size of particles (i.e., 5 and 15 microns).
Fluid Cleanliness Level Standard
ISO 4406:1999 specifies the cleanness level of a given sample of fluid by a three-number range code representation, such as 18/16/14, based on the number of particles of size greater than 4, 6, and 14 microns respectively, present in one ml of the sample fluid.
|
Range code |
Number of particles per ml | |
|
> |
<= |
|
|
21 |
10 000 | 20 000 |
| 20 | 5 000 |
10 000 |
|
19 |
2 500 | 5 000 |
|
18 |
1 300 | 2 500 |
| 17 | 640 |
1 300 |
|
16 |
320 |
640 |
| 15 | 160 |
320 |
|
14 |
80 |
160 |
|
13 |
40 | 80 |
| 12 | 20 |
40 |
| 11 | 10 |
20 |
Interpretation
|
Particle sizes |
>4μ | >6μ | >14μ |
|
No. of particles (say) |
1510 | 406 | 133 |
|
Range codes and representation |
18 | 16 |
14 |
|
18/16/14 |
|||
Typical Cleanliness Level Targets
Hydraulic equipment manufacturers, fluid suppliers, and fluid power associations have established target fluid cleanliness levels applicable for the general types of hydraulic components.
| Components |
Range code |
| Flow control valves, cylinders |
17/18/15 |
| New, unused fluid |
17/18/15 |
| Gear pumps/motors |
20/17/14 |
| Vane pumps/motors |
19/16/13 |
| Piston pumps/motors |
19/16/13 |
| Directional & pressure control valves |
19/16/13 |
| Proportional valves |
18/15/12 |
| Servo valves |
17/14/11 |
Hydraulic Fluid Analysis
Fluid analysis can be carried out to ascertain the health of a fluid medium. Fluid analysis essentially counts the number of contaminant particles, detects its level of oxidation, identifies the component wear, determines the condition of the additives, establishes the overall level of contamination, and verifies the composition of the fluid. If the analysis meets the necessary cleanness target, then we only need to continue to maintain the filters and retest the fluid periodically. If not, appropriate corrective actions must be taken to rectify the problems.
A good fluid analysis first establishes target cleanness level, sampling location, and testing frequency. Ensure that the sample taken from the system is a representative sample of the system fluid. Depending upon the sensitivity, analysis can be conducted on a fluid sample by (1) Patch test, (2) Laboratory analysis, and (3) On-line fluid monitoring.
Patch test
It is a simple visual analysis of the fluid sample extracted from a hydraulic system, using a fluid analysis kit. It consists of 100x magnification field microscope, filter test patches, a vacuum pump to extract fluid samples, sample bottles, and visual correlation charts or photographs. 100 ml of fluid is passed through the filter media of the test patch. The patch is then dried and analyzed under the microscope for both color and content and then compared to the reference photographs of known particle concentration levels to determine the approximate ISO cleanness code and the type of particles captured on the patch.
Laboratory Analysis
The laboratory analysis is a complete scrutiny of the fluid sample. Most of the laboratories offer the following important fluid parameters:
- Viscosity
- Neutralization number (Total Acid Number, TAN)
- Water content
- Particle counts
- Spectrometric analysis for finding the wear metals, trending graphs
Online Fluid Monitoring
With the advancement of computer technology and the introduction of sophisticated online fluid monitoring instruments, analysis of a fluid can be done on-site in a consistent manner while the system is in operation. Today’s on-line contamination monitoring instruments can detect changes in the quality, contamination level, and chemical composition, of a given volume of fluid. It can also measure the amount of wear metals present in the fluid.
Authored by JOJI Parambath, Founder/Director, Fluidsys Training Centre, Bangalore
Reference:
JOJI PARAMBATH, Industrial Hydraulic Systems – Theory and Practice, Universal Publishers, Boca Raton, USA, 2016.
http://www.universal-publishers.com/book.php?method=ISBN&book=1627340580
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