Hydraulic fluids Pour point

Pour point is the lowest temperature at which a fluid will flow. If the hydraulic system will be exposed to extremely low temperatures, it is a very important specification. For... 

Hydraulic fluid a fluid supplied for use in hydraulic systems. Low viscosity and low pour point are desirable characteristics. Hydraulic fluids may be of petroleum or nonpetroleum origin. 

Paraffinic and naphthenic (cycloparaffinic) stocks may be used for the formulation of lubricating oils, each with favorable characteristics for particular uses. Paraffinic stocks are generally preferred for their superior lubricating power and oxidation resistance. Naphthenic stocks, on the other hand, have naturally lower pour points, i.e, they maintain flow characteristics at lower pour-points than paraffinics (Table 18.8) and are better solvents, features which are more important for applications such as heat transfer, metal working, and fire-resistant hydraulic fluids [33]. Any residual aromatics in the lubricating base stock will have been removed before formulation by solvent extraction, using N-methylpyrrolidone, furfural, or less frequently today, phenol (Eq. 18.39). 

The lower cost dialkyl benzenes are used in a wide variety of industrial and metalworking products. In particular, their sulphur-free chemistry has led to extensive use as rolling and drawing oils for copper. The synthesised alkyl benzenes, even when their chemistry is optimised, generally exhibit poorer properties than PAOs. However, their excellent solvency and low pour point make them suitable for lubricants designed for extremely low-temperature operations in arctic greases, gear oils, hydraulic and power transmission fluids. 

Phosphate esters are also used as hydraulic fluids in civil aircraft where thermal stability is less important than their low-temperature viscosities and cold flow properties. Trialkyl and alkyl-aryl phosphates are used which, when formulated with a VI improver, give fluids with pour points of -55 to -65°C and a VI of 170-300. They are also chosen for other low-temperature applications for conditions such as those found on North Sea, and similar weather condition, oil rigs. 

Mineral-based hydraulic fluids are controlled in the USA by military specification MIL-PRF-5606 and in the UK by Defence Standard 91-48 although the resulting grades of product are effectively the same and both are covered by the same NATO designation H-515 [21]. The most widely used fluid has excellent low temperature properties with a maximum pour point of -60°C due largely to the naphthenic nature of the base oil. The fluid also contains ... 

Phosphate esters are produced from phosphorus oxychloride with various alcohols or phenols, or combinations of these hydroxyl compounds. These fluids generally have good thermal and oxidative stabilities and fire-resistancy. However, because of their high polarity, poor Vl-pour point balance, facile hydrolysis and inferior elastomer and paint compatibility, their use in general lubrication is limited. The major use for phosphate esters is in fire-resistant hydraulic oils. 

---