System Cleanliness Part 4
Hydraulic components should never fail when the system is clean. In 1979 the SAE adopted SAE J 1227 to stem the rise in catastrophic system failures. To that end the manufacturers of components have identified the ISO cleanliness levels that their components need to offer acceptable service life. SAE J 1227 Section 5.1 recommends that all components including piping (SAE J 1227 Section 2.4) be as clean as the most critical component within the system. Due to the technical challenges of producing ISO 14/12 pipe, the market's only choice in the past was to use pickled pipe. The problems outlined in Articles 1 & 2 have plagued the market for decades and the only hope lay in trying to clean up the system after fabrication.
These methods have not been consistently successful. Under optimum conditions the contaminants in piping would come out if it were just particulate contamination. But the problems confronting the installer are far greater than that. The rust and scale found in pickled pipe is electro-chemically bonded and anything short of the physical removal of base metal around and under the rust and scale is not going to be effective in eliminating these problems.
While all systems must be flushed, it is important to be aware of the limitations in order to avoid the pitfalls of expecting more than is possible.
SYSTEM FLUSHING USING THE SYSTEM PUMPS is one of the most common methods of flushing. ASTM D4174 Section 9.5.1. advises against this due to insufficient fluid velocity. Failure to achieve the desired Reynolds number will leave the system dirty.
CONTRACT FLUSHING is also used to flush systems. The limitations that need to be observed with this method are making certain that sufficient flow is available throughout the system. Return lines and headers will need to see 30 feet per second also in order to be properly cleaned. A typical 4" return line will need a flow rate of 1175 gpm to achieve 30 feet per second. Flushing units of this gpm using positive displacement pumps are not widely available in North America. Keep in mind the flushing capabilities in your region when sizing the pipes to be used so that an efficient flush can be realized.
ACID / CAUSTIC flushing is specifically prohibited in ASTM D4174 Section 22.214.171.124. This is due to damage that will occur to the base metal when corrosives are used.
Absolute filtration is essential for both flushing and system operation. Using high beta ratio filters to compensate for pipe that has rust and scale on the I.D. will frequently have disappointing results. The kind of system upsets that will dislodge the upper layer of rust and scale are shock waves and pressure spikes. These events tend to put large quantities of metallic debris into the hydraulic fluid stream. The filters are unlikely to be of much assistance in protecting components in these scenarios for two reasons: 1. If the particle count is high enough the filter will be overwhelmed and the discharge will have an ISO number higher than the components can handle. 2. The efficiency of the filter is likely to be less due to non- laminar flows and fluctuating pressure differentials, both of which are not seen in the laboratory testing used to rate the efficiency of filters.
Maintenance will always be a wild card in hydraulics. The dirtier the system the more often the filters need to be replaced. Should the filter go to bypass, then the potential for component failure increases dramatically. It is important to use pipe that complies with SAE J 1227 Section 2.4 so that there is no further debris added to system from the pipe after flushing. This will lower maintenance costs and minimize warranties.
Pneumatic projectiles do not remove particles efficiently. The published removal efficiency rate on particles of 50 micron size is 50% and declines to less than 14% for15 micron size. This reduction translates into less than one ISO range number. Additionally, REDOX advises against using them due to the wiping action that will remove the rust inhibiting qualities of the VSI oils used on REDOX pipe.
The following three steps will add extraordinary value to your systems and eliminate most warranty issues:
Determine the cleanliness needs of the most demanding component in the system per SAE J 1227 Section 5.1.
Specify that piping meet that cleanliness using recognized standards. The standards REDOX follows are in our TECH Sheet.
Use SAE J 1227 as guideline to specify flushing criteria in conjunction with an approved particle counting method (particle counter or Millipore test) to determine that system cleanliness has been met.
Integrating these standards into your system design specifications will dramatically lower the installer's cost while simultaneously increasing the customers up-time profit. The long term benefit associated with good customer satisfaction can be measured in hundreds of thousands of dollars in future revenue.
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