The demands placed on hydraulic systems constantly change as industry requires greater efficiency and speed at higher operating temperatures and pressures. Selecting the best hydraulic fluid requires a basic understanding of each particular fluid's characteristics in comparison with an ideal fluid. An ideal fluid would have these characteristics:
Although no single fluid has all of these ideal characteristics, it is possible to select one that is the best compromise for a particular hydraulic system. This selection requires knowledge of the system in which a hydraulic fluid will be used. The designer should know such basic characteristics of the system as:
Each of the following factors influences hydraulic fluid performance:
Viscosity - Maximum and minimum operating temperatures, along with the system's load, determine the fluid's viscosity requirements. The fluid must maintain a minimum viscosity at the highest operating temperature. However, the hydraulic fluid must not be so viscous at low temperature that it cannot be pumped.
Wear - Of all hydraulic system problems, wear is most frequently misunderstood because wear and friction usually are considered together. Friction should be considered apart from wear.
Wear is the unavoidable result of metal-to-metal contact. The designer's goal is to minimize metal breakdown through an additive that protects the metal. By comparison, friction is reduced by preventing metal-to-metal contact through the use of fluids that create a thin protective oil or additive film between moving metal parts.
Anti-wear - The compound most frequently added to hydraulic fluid to reduce wear is zinc dithiophosphate (ZDP), but today, ashless anti-wear hydraulic fluids have become popular with some companies and in certain states to reduce loads on waste treatment plants. No ZDP or other type heavy metals have been used in the formulation of ashless anti-wear fluids.Note that excessive wear may not be the fault of the fluid. It may be caused by poor system design, such as excessive pressure or inadequate cooling.
The pump is the critical dynamic element in any hydraulic system, and each pump type (vane, gear, piston) has different requirements for wear protection. Vane and gear pumps need anti-wear protection. With piston pumps, rust and oxidation (R & O) protection is more important. This is because gear and vane pumps operate with inherent metal-to-metal contact, while pistons ride on an oil film.
When two or more types of pumps are used in the same system, it is impractical to have a separate fluid for each, even though their operating requirements differ. The common fluid selected, therefore, must bridge the operating requirements of all pump types.
Foaming - When foam is carried by a fluid, it degrades system performance and therefore should be eliminated. Foam usually can be prevented by eliminating air leaks within the system. However, two general types of foam still occur frequently:
Surface foam is the easiest to eliminate, with defoaming additives or by proper sump design so that foam enters the sump and has time to dissipate.
Entrained air can cause more serious problems because this foam is drawn into the system. In worst cases, it causes cavitation, a hammering action that can destroy parts. Entrained air is usually prevented by properly selecting the additive and base oils. Caution: certain anti-foam agents, when used at a high concentration to reduce surface foam, will increase entrained air.
R & O - Most fluids need rust and oxidation inhibitors. These additives both protect the metal and contain anti-oxidation chemicals that help prolong fluid life.Also linked to the foam problem, is fluid viscosity, which determines how easily air bubbles can migrate through the fluid and escape.
Corrosion - Two potential corrosion problems must be considered: system rusting and acidic chemical corrosion. System rusting occurs when water carried by the fluid attacks ferrous metal parts. Most hydraulic fluids contain rust inhibitors to protect against system rusting. The tests used to measure this capability are ASTM D 665 A and B. To protect against chemical corrosion, other additives must be considered. The additives must also exhibit good stability in the presence of water (hydrolytic stability) to prevent break down and acidic attack on system metals.
Oxidation and thermal stability - Over time, fluids oxidize and form acids, sludge, and varnish. Acids can attack system parts, particularly soft metals. Extended high-temperature operation and thermal cycling also encourage the formation of fluid decomposition products. The system should be designed to minimize these thermal problems, and the fluid should have additives that exhibit good thermal stability, inhibit oxidation, and neutralize acids as they form.
Although not always practical or easy to attain, constant moderate temperature and steady-state operation are best for system and fluid life.
Water retention - Large quantities of water in a hydraulic fluid system can be removed by draining the sump periodically. However, small amounts of water can become entrained, particularly if the sump is small. Usually, demulsifiers are added to the fluid to speed the separation of water. Filters can then physically remove any remaining water from the hydraulic fluid. The water should leave the fluid without taking fluid or additives with it.
Temperature - System operating temperature varies with job requirements. Here are a few general rules: the maximum recommended operating temperature usually is 150° F. Operating temperatures of 180° to 200° F are practical, but the fluid will have to be changed two to three times as often. Systems can operate at temperatures as high as 250° F, but the penalty is fairly rapid decomposition of the fluid and especially rapid decomposition of the additives - sometimes within 24 hours!
Seal compatibility - In most systems, seals are selected so that when they encounter the fluid they will not change size or they will expand only slightly, thus ensuring tight fits. The fluid selected should be checked to be sure that the fluid and seal materials are compatible, so the fluid will not interfere with proper seal operation.Most fluids are evaluated based on their ratings for rust and oxidation (R & O), thermal stability, and wear protection, plus other characteristics that must be considered for efficient operation:
Fluid life, disposability - There are two other important considerations that do not directly relate to fluid performance in the hydraulic system, but have a great influence on total cost. They are fluid life and disposability.
Fluids that have long operating lives bring added savings through reduced maintenance and replacement-fluid costs. The cost of changing a fluid can be substantial in a large system. Part life should also be longer with the higher-quality, longer-lived fluid.
Longer fluid life also reduces disposal problems. With greater demands to keep the environment clean, and ever-changing definitions of what is toxic, the problem of fluid disposability increases. Fluids and local anti-pollution laws should both be evaluated to determine any potential problems.
Synthesized hydrocarbon (synthetic) hydraulic fluids contain no waxes that congeal at low temperatures nor compounds that readily oxidize at high temperatures which are inevitable in natural mineral oils. Synthetic hydraulic fluids are being used for applications with very low, very high, or a very wide range of temperatures.
The overwhelming majority of hydraulic components and systems are designed to use oil-based hydraulic fluids. No wonder; these fluids rarely present significant operating, safety, or maintenance problems. Unfortunately, there are circumstances where using oil-based fluid should be avoided. One common fluid power application is in an environment with potential ignition sources - an open flame, sparks, or hot metal. In these environments, a leak spraying from a high-pressure hydraulic system could cause a serious fire and result in major property damage, personnel injury, or even death.
Even though most oil-based hydraulic fluids have relatively high flash/fire points (>300° F), small leaks in a high-pressure system can produce a finely atomized spray that can travel significant distances. If an ignition source is encountered, complete ignition of the spray envelope can occur. The alternative is to use a hydraulic fluid that eliminates or significantly reduces this hazard: any of several fire-resistant hydraulic fluids (FRHFs).
Apart from isolated segments of basic research, little progress was made in developing suitable FRHFs until the end of World War II. During the war, tragic incidents related to hydraulic fluid fires and major property losses at steel mills and foundries graphically illustrated the urgent need for something to be done. Similar incidents in captive environments such as coal mines during the rapid post-war industrial expansion helped motivate a major joint research effort between government and industry. This work was directed at developing fluids that could replace oil-based hydraulic fluids at a reasonable cost and with no significant reduction in hydraulic system performance. Two basic approaches were undertaken. One involved the introduction of water into the fluid to act as a "snuffer" if the fluid ignited. The other involved synthetic, non-aqueous products whose chemistry resisted burning or generated products of combustion that helped extinguish any flame.
Commercial products in both categories evolved during the 1950s and 1960s and are still in use today. In the early 1970s, an additional synthetic type of fluid was introduced to address many of the drawbacks inherent in the earlier types. Since the introduction of each type, many improvements have been made in fire resistance, anti-wear properties, and overall quality.
Water glycol and invert emulsion constitute the major fluid types of water-containing products. Water glycol is a true solution of a glycol (such as ethylene glycol) in water, along with a variety of additives to impart viscosity, corrosion protection, and anti-wear properties. A shear-stable thickener, which has improved over the years, represents the novel technology aspect of the fluid. Water glycol contains approximately 40% water. Despite a number of drawbacks, water glycol is the dominant FRHF on the market today and is used in a wide variety of applications.
An invert emulsion also contains approximately 40% water but is a stable emulsion of water dispersed in oil. The outer phase, oil, represents the wetting surface; the inner phase, water, provides the fire retardant-element. Oil-soluble additives provide anti-wear properties, corrosion protection, and emulsion stability. Inverts, at one time, were commonly used but are losing favor in industry today.
Synthetic fluids initially were represented by a class of chemical compounds known as phosphate esters, which are reaction products between phosphoric acid and aromatic ring-structure alcohols. These fluids are extremely fire resistant and have widespread industrial use, as well as military and aircraft service. However, their popularity has declined because of environmental, cost, and compatibility factors.
The other type of synthetic fluids in use are synthetic hydrocarbons, more specifically, polyol esters. These fluids are the reaction products between long-chain fatty acids (derived from animal and vegetable fats) and synthesized organic alcohols. These products contain additives to impart anti-wear properties, corrosion protection, and viscosity modification. Fire resistance results from a combination of high thermal properties and physical characteristics. This is the most recent category of FRHFs and has gained widespread and growing use.
The term "fire resistant" often is misunderstood or interpreted to be overly inclusive; it seems appropriate to standardize the terminology and review the accepted test methods for judging the fire resistance of a given fluid. First, there is no single property or test of a fluid, such as flash/fire point, auto ignition temperature (AIT), etc. that will quantitatively rate its relative fire resistance. This has led to a "simulated incident" approach in which tests are designed to replicate a worst-case scenario in typical applications where fluid power is used near a potential fire hazard. Fluids generally pass or fail these tests, and those that pass are incorporated into an Approval Guide or List of Qualified Fluids.
In the United States, two test protocols have evolved and are generally regarded as benchmarks in the industry. One was developed by Factory Mutual Research Corporation (FMRC). Their original intent was to use the test results in the risk-assessment programs of those insurance companies under the Factory Mutual System umbrella. The test has since become the chief qualification for commercial companies using FRHFs; all fluid suppliers submit products seeking "FMRC Approval." The FMRC Approval Guide lists over 300 FRHFs from approximately 50 suppliers. Factory Mutual's program is now global in scope.
FMRC addresses the definition of FRHF in the following excerpt in their introduction to the hydraulic fluids sections of their Approval Guide: Less flammable hydraulic fluids approved and listed here have been tested to evaluate fire hazard only. All presently available fluids will burn under certain conditions. In each case the fire hazard has been reduced to an acceptable degree, meeting the Approval Standards of FMRC; other fluid properties are not investigated.
This paragraph accurately puts the intent of FRHFs into the proper perspective. They are not fireproof but, rather, they significantly reduce the potential hazard associated with oil-based products. In the FMRC tests, the fluid is conditioned to 140°F, pressurized to 1,000 psi in a steel cylinder, and discharged through an oil burner-type nozzle. The spray generated is intended to simulate a high-pressure hydraulic system leak. A gas flame is passed through (not retained in) the spray envelope at two distances downstream of the nozzle. There may be local burning at the point of flame entry, and the pass criteria dictate that any flame must self-extinguish when the ignition source is removed; no flame may propagate back to the nozzle. This process is repeated 20 times, and the burn duration timed. Any burn duration over 5 sec is considered a fail.
A second test uses the same spray directed at an inclined metal channel heated to 1,300°F. In this test, the spray is continuous for 60 sec. The criteria are:
1. The spray in contact with the channel may not burn, or
2. If spray ignition takes place, fluid rolling off the channel cannot continue to burn, and the flame cannot follow the spray if directed away from the channel.
If these conditions are satisfied, the fluid is approved. Statistics are not available, but many products in all of the fluid categories described do not pass this test.
The Mine Safety & Health Administration (MSHA) has had in place for many years an evaluation program for qualifying fluids that are used underground, primarily in coal mines. MSHA testing is similar to FMRC's in the sense that a spray mist of the candidate fluid is generated. However, the ignition mechanism is somewhat different in the MSHA test. Under this procedure, a spray mist is directed continuously at a variety of ignition sources that include an open gas flame, a welding arc, and burning rags. The pass criteria are that localized burning in the spray mist extinguish within 5 sec, and there can be no sustained propagation along the spray axis. They also have an AIT criterion and a wick test to assess the rate of evaporation of water from a candidate product. MSHA tests also have a relatively high rate of product rejections.
Since both of these tests involve fluids submitted by the supplier to the testing agency, both FMRC and MSHA have comprehensive manufacturer auditing programs in which quality-assurance programs are carefully evaluated and monitored by periodic, on-site inspections. This may include retests of approved fluids.
In addition to these "third party" ratings of FRHFs, many companies have developed their own fire-resistance tests that must be considered in addition to a product having FMRC approval. Again, these tests generally follow the simulated incident philosophy and are specific to the type of industry involved. Examples of these include exposing the candidate fluid - in spray or non-spray form - to a hot manifold, molten metal, heated blocks of a representative metal, burning rags, hot sand, etc. The evaluation criteria may be no burning, limited burning, no smoke, non-propagation, etc. Minimum AIT and flash/fire point temperatures also are used either independently or in combination with a test described above.
In all of these tests, a product is either approved or rejected; there is no ranking or rating of approved products. This aspect, the occasional lack of reproducibility, and the absence of service history of a fluid has led FMRC to develop a new test that will quantify the relative fire resistance of various fluids. The test procedure involves measuring the heat release of a fluid under a fixed-burn condition and combining this value with a separately determined measurement of the energy required to initiate burning. These values are used to establish a Spray Flammability Parameter for each product evaluated. This test and a new approval standard currently are under review by FMRC and have not been formally adopted.
The major problem facing a designer converting a hydraulic system from an oil-based fluid to FRHF is selecting the particular type that will minimize the cost of conversion and maximize the operating and safety benefits. The choice becomes a trade-off of characteristics associated with each type. Each product group offers advantages and disadvantages for any given application. It is beyond the scope of this article to attempt to make recommendations for certain end-users, but the major attributes and shortfalls of the various fluid types can be addressed.
Significant improvements continue to be made with both water glycol and polyol ester fluids. The impact of more-stringent environmental regulations will be more strongly felt in the next few years and may even restrict the choice.
The motivation for converting from an oil-based fluid will also strengthen as waste control regulations expand for any product containing oil. In some areas, "hydraulic oil" already is considered a hazardous material. As their prices decrease, fluids having the capability of being non-toxic and readily biodegradable will further expand the motivation to replace oil-based hydraulic fluids.
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