"Would you recommend an offline vacuum pump/filtration unit for a hygroscopic oil if water content is on the high side?"
To better comprehend the characteristics of polyglycol oils, you need to understand the makeup of the raw materials used to formulate them. Polyglycols are made from a mixture of ethylene and propylene. When these two compounds react with oxygen, ethylene oxide (EO) and propylene oxide (PO) are formed. Polyalkylene glycols are then produced by polymerization.
Depending on the specific blend of EO and PO, the oil’s behavior will greatly increase or decrease with water. Typically, polyglycols are mixed with an EO/PO ratio of 50:50 or 60:40. This blend will determine the water solubility and hygroscopicity.
By nature, polyglycol oils tend to absorb water from the air. A 1:1 ratio can absorb up to 10 percent moisture at ambient temperature and a relative humidity of 80 percent. For this reason, careful consideration should be made when selecting a polyglycol for the application. Most polyglycol oils can perform well even with the presence of water, but there will still be uncertainty as to the water’s effects.
A simple solution for filtering out water from a polyglycol oil is to use a filter constructed of cellulose media. When selecting a filter cart, be sure to take into account the high viscosity of oil used. Consideration will also need to be made for the amount of moisture. The greater the moisture problem, the harder it will be to remove it.
Vacuum dehydration is another alternative for water removal. Applications with large lubricant volumes or critical systems could be considered for this type of method. Vacuum dehydration units can be rented or purchased, but their high cost of use often leads to many organizations choosing to perform a simple oil change.
Keep in mind that removing the water is only a Band-Aid. You must find out how the moisture is making its way into the reservoir. This will prevent you from having to spend money on labor and parts for a problem that will occur again in the future.
For reservoirs with open vents, install a desiccant breather. Any moisture in the air will be trapped by the desiccant before entering the reservoir, reducing the amount of moisture that the oil will absorb. For reservoirs with faulty seals, oil coolers and washdown applications, more in-depth analysis will be required to come up with a proper solution.