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In terms of PAG there are distinct advantages in using these types of synthetic base oils. They provide a high viscosity index and excellent friction coefficient, they have a high temperature stability and a high shear stability, use of these lubricants provides less evaporation losses, reduced gas solubility and a maximum load carrying ability.


However, there are a number of myths that have arisen which might prevent users from considering these lubricants. That they are not compatible with common seal materials, that they might react with water like Esters, that the water content of PAG cause problems with bearings and corrosion, they cannot be used with white metals or mixed with mineral oils and are much more expensive than other synthetics.


The reality is that the conversion from a traditional hydrocarbon oil to a PAG does not have to be difficult if the application demands it. Conversion can be achieved and if it will be observed during the design process then the use of PAGs is possible.


It is true to say that PAG‘s are not compatible with some form of seal material such as natural rubber, leather or cork sealing materials. There are however many seal materials available to use for which PAG‘s are compatible in most applications and the use of testing can provide assurance in this regard.


In terms of water solubility, then water only reduces the viscosity of the lubricant depending on the type of PAG and whether it is in itself water soluble. Tests have also shown that there is no significant influence on performance from the water content of PAG causing problems with bearings and corrosion. If the system does not allow for the ingress of water, then there is also no reason why PAGs cannot be used with white metals.


The myth that you should never use a PAG when equipment is painted is a curious one. Certainly, PAGs are not compatible with Phenolic paints, however there are two-component lacquers available which are compatible with PAG.


Although mixing PAGs with mineral oil is to be avoided, with a good flushing procedure changing from mineral to PAG is possible and allowing for small residues in most cases is acceptable.


Finally, the myth busting question of cost. It is true to say that synthetics are more expensive to procure initially than mineral oils, depending on application the total cost of ownership over the life of the fluid could be lower and the costs for base oils are lower than PAO‘s.


It is not recommended to use PAGs under certain circumstances. For example, if the application does not demand it then there is no point converting over. Polyglycols should also not be used in very wet environments with a high risk of coming into contact with water contamination and white metal components. Similarly, if the equipment being used is known to contain any incompatible paint or seals. PAGs are also not recommended for aluminium/ steel tribological contacts involving chemical wear, and finally do not convert a system to PAG from hydrocarbon mineral oils without proper flushing and cleaning.


Compressor lubricants have different requirements depending on the type of compressor used. For screw compressors providing low to moderate high pressure and a moderate volume then typically the type of gases used in these systems include air, natural gas, refrigerants and chemical gases then typical Original Equipment Manufacturers (OEM) requirements are for low viscosity products lubricating both bearings and valve, along with cooling and sealing the system. For Vane compressors providing low pressure and low volume using air and natural gas, then lubrication requirements are for low viscosity products again providing lubrication to bearings and valves along with cooling and sealing the system.


Reciprocating (piston) compressors which tend to work under high pressure and moderate high volume using air, natural gases, refrigerants and chemical gases will have a need for high viscosity products lubricating bearing, pistons, cylinders and valves. Turbo compressors typified by low and high pressure accompanied by high volume and using natural and chemical gases require lubricating bearings and valves and again cooling and sealing the system.


If we consider PAG, PAO and Ester-based base oils for compressor applications then we can match the attributes of the lubricant with the requirements of the system. PAG-based compressor oil is compatible with chemical gases (ammonia, vinyl chloride and Continued on page 22


LUBE MAGAZINE NO.148 DECEMBER 2018 21


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