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primarily by the effect of the environment on the fluid rather than by the type of pumping action. Humidity only affects requirements for the pump casing. When operating temperature extremes are specified for a hydraulic system, the operating temperature of the fluid, not the ambient temperature, is usually the critical factor. Minimum operating temperature is normally set by the increase in fluid viscosity as operating temperature is decreased. When the fluid viscosity is increased to the point where inlet conditions can no longer keep the pump completely full, cavitation, with possible pump damage, occurs. Fire resistant fluids have a higher specific gravity than petroleum oils and higher viscosity at lower temperatures. They may also contain water which can vaporize at lower pressures or higher temperatures. Thus, pump inlet conditions are more sensitive when these fluids are used. High altitudes can produce similar effects when the fluid reservoir is not pressurized. Because pumps are designed for specific fluids, failure rates of seals can increase if alternate fluids are used. Above allowable operating temperatures, many oils will be too thin to maintain proper lubrication at high-load points, and may progressively deteriorate a graphite seal as a result of oxidation. Under elevated temperatures, some seals may harden. See Chapter 3. 10.2.4 Interference For rotary displacement pumps, the interference problem must be seriously addressed since very small distortions of rotors will decrease the clearance causing rubbing or direct impact between the moving parts of the pump. Thermal expansion can also pose a threat if there is no care taken in the proper selection of materials. Improper installation can also lead to interference problems. With centrifugal pumps, cavitation significantly increases the interference problem because cavitation causes vibration and imbalance. Interference can be avoided by designing the parts with appropriate elastic and thermal properties so that excessive load or temperature won't significantly deflect internal parts. Manufacturing tolerances must be carefully maintained. 10.2.5 Corrosion and Erosion Consideration must be made for other possible failure modes such as erosion corrosion. Erosion corrosion is an acceleration of the rate of corrosion attack in metal components due to the relative motion of a corrosive fluid and a metal surface. Erosion is dependent on the rate of liquid flow through the pump and also the angle of attack at which the fluid impinges on the material. A combination of erosion and corrosion can lead to extremely high pitting rates. The analysis of pump reliability must therefore determine if there are abrasive solids in the fluid. Pumps 10-10 Revision C

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