Except Regular Changing of Filters, Monitoring Oil Level, Leakage checking, System cleaning & Flushing there are many other points to enhance reliability of hydraulic system. Below are some check points for the same –
1. Make Regular Check Drive Motor Current Drawn – Many Hydraulic Pump Tests can be carried out to check the health of Hydraulic Pump like Pump Case temperature, Case drain Flow etc. A similar test is Current drawn by Electrical Motor. The amount of Electrical Power required to drive Pump depends on Hydraulic system Pressure and Flow. As the Pump wears, its internal clearance increases consequently internal leakage
increases. This result decreasing Pump Output. As pump output decrease, Power required to drive Pump also decrease proportionally. Therefore the drive motor current drawn will also decrease. Periodically checking of these current drawn is an excellent way to track Pump Wear. A record should be made to maintain for base line reference when system operating normally.
2. Make Regular Check of Temperature – All Hydraulic system generates certain amount of heat (Approximately 25% of Electrical Power, depends on system type) as no system is 100% Efficient. When a system component wear, internal clearance increases, this lead to increase bypassing Oil, resulting excessive heat generated. By using some heat detecting device or infrared camera, this excessive bypassing can be detected. Have in mind that heat will generate whenever pressure drop occurs. Localize heat always occurs in any device that meters flow like Flow control, Proportional & Servo Valves controls as flow restricting by same. A hot relief valve or a severe temperature increase between a suction line and case drain line (Externally Drain Pump) usually indicate a problem. Most Piston pump bypass 1-3% volume (vane pump up to 5% volume), this bypass oil be heated as having no useful work. When pump wears, bypass oil increases causing temperature increase in bypass line. The amount of case drain flow vary from one manufacturer to another, the key is to make initial temperature record when pump is relatively new. In general hydraulic reservoir oil Temperature Should be between 38°C – 50°C. It is estimated that below 60°C temperature oxidation rate is slow but above this temperature, oxidation rate approximately double for every 10°C temperature increase so oil life reduce by 50%. Modern formulation of Hydraulic oil contain vital additive package to help the fluid to satisfy essential operating functions, unfortunately high temperature can deplete all such additives particularly rust inhibitors, foam depressants, antioxidants & Antiwear ingredients.
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3. Regular Checking Of Valves – On any Hydraulic system one or more valve could be closed while the system runs. These include Relief Valve used with Pressure compensating pump, Air Bleed Valve, Accumulator Dump Valve etc. The Tank Line of all Valves should be check regularly with a temperature gun to verify that they are closed and there is no oil being flow to tank back. Any type of wear out in these valve develop leakage path that allow high pressure hydraulic fluid to leak to a low pressure port by creating heat and raising oil temperature. Internal leakages are hidden culprits. There can
be various causes of internal leakage like component surface wear, incorrect tolerance during part manufacturing, etc. The most common is basic Wear & Tear of components. Relief Valve used in conjunction with Pressure compensated variable displacement pump if not set properly (set at lower pressure than Pump compensator setting) will also generate heat. In many cases it is recommended that all types of Hydraulic valves should be tested frequently (no longer than 12 months) which depends on desire level of performance, Valve condition, Service condition etc.
4. Liston for Abnormal Sound – Different types of noise related to different issues can provide valuable clues which can help to diagnose the problem and get it correct before it becomes a major problem. Regular sound checking should be made especially with hydraulic Pump. It generally understood that pump size & pressure have about equal effect on hydraulic noise that pump will generate. In addition pump speed having about 3 times greater effect on noise rather than pump pressure & Size (That’s why some Pump manufacturer recommend for lower speed electric motor). If it is high pitch whining sound, most likely because of Cavitation. Cavitation is formation & collapse air cavities. Hydraulic oil contain approximately 9% dissolved air, if Pump not getting required amount of liquid, this dissolve air pulled out of oil on suction side & collapse on pressure side. If same not corrected, Pump will deliver a reduce flow until it eventually Catastrophic failure. Running a Pump for many hours in Cavitated condition to warm-up oil can greatly shorten Pump life. The most common cause of Cavitations is Faulty Suction Strainer, Excessive Oil Viscosity or higher Electric Motor RPM. If Pump has a knocking sound like marbles rattling around, may be because of Aeration problem. Aeration is pseudo cavitation, It occurs whenever outside air enters into suction port of Pump. Cause of aeration includes air leak in suction line may be because of loose connection, Oil level too low (below suction point), Line Crack or any improper seal fitting or a bad shaft seal of fix displacement Pump. NFPA recommended for the highest point of Suction strainer must be at least 3” below of minimum oil level. Air may be enter through the Cylinder Rod seal, this can happen on cylinder mounted with rod up and having no proper counterbalance. Gravity load may cause a partial vacuum on Cylinder Rod end. Cylinder seals are generally not design to seal air out, so even a good seal can leak air. Sometimes because of cracked mounting bracket lead to pump shaft out of alignment, wear the seal. A misaligned coupling can also cause a shaft seal to wear permanently as well. Any foaming of fluid in oil reservoir is another sign of aeration as air existing will cause aeration. A whistling or erratic hissing sound is related to pressure relief valves, which design to open if system pressure reached at certain level. Pressure Relief Valve is not design to remain open and dump pressure continuously. A continuous hissing sound indicate either relief valve set too low or stuck open and continuously releasing pressure. An erratic whistling sound indicates relief valve set incorrectly or damaged.
5. Take Regular Oil Samples and have analyze the same – From medical prospective, it is important for all of us to have regular drawn our blood sample and analyze it especially as we old. By doing so our medical professional can catch issues before they become severe problem. Likewise we have to analyze our hydraulic systems lifeblood (Hydraulic oil). A regular oil testing must be done for every 500 – 1000 Hr. working (depending on equipments duty cycle, Operating Environment and how critical is Machine operation) and also test after any maintenance activity that expose the system with external environment. Oil contamination is usually gradual so its regular testing will be enough to identify problem before it become serious. Overheating and excessive water are two most critical conditions that can upset a fluids chemical unbalance relatively quickly. Many techniques available to analyze the oil like, Particle counting, Spectroscopy, Ferrography etc. An oil analysis should be done for at least below parameters –
(a) Particle Counting & Size Distribution – By particle counting and Size distribution, many problems can be easily determined. New technologies not only count particles but provide more details, from where the particles coming. Gradual wear & tear of equipment is caused by silt particles (4-6 µ) and sudden breakdowns cause by larger particles (> 14 µ).
(b) Water contamination – Hydraulic oil become cloudy when having water contamination above its saturation level (typically 300 ppm at 20°C although depends on fluid base type, additive package, temperature & pressure). It can destroy additives and reduce lubricating film strength allowing wear to occur with high frictional heat. Water reduces lubricant film thickness and load carrying ability.
(c) Viscosity – This is the fluid resistance to flow under gravity. Oil viscosity may vary within a range of 25% up or down the initial viscosity for acceptable limit. Oil viscosity is expected to rise over time and use, loss of viscosity is more serious than increase.
(d) Total Acidity No. (TAN) – TAN measures acidic by products formed during oxidation. Every oil has a maximum TAN value (Typically 1.0 mg/KOH gr). New fresh oil having 0.2 mg/KOH gr. TAN value use as a guide to follow the oxidative degeneration of oil in service. An abrupt rise in TAN would be indicative of abnormal operating conditions like overheating. Similar type some more check points are also there to enhance the reliability of Hydraulic systems. It is important to develop a Preventive maintenance schedule for hydraulic system and adhere with it.
