VAC recommends the use of a filter or filter regulator with the installation of any positioner regardless if it is a pneumatic, electro-pneumatic, or digital positioner. VAC offers both filters and filter regulators that offer both coalescing and particulate filtration. When selecting a filter for positioners it is important to select a unit that is both coalescing and particulate filtering.
Coalescing filters are designed to remove moisture from air systems. Coalescing filters have the ability to remove oil vapor which could be from compressor lube oils, condensed moisture, pipe joint compound residue, etc. The coalescing filters remove both water and oil from the system by collecting minute amounts of moisture until a droplet forms and allows the moisture to drop out of suspension from the air stream. Coalescing filters also have the ability to filter out particulate but the main focus is to remove moisture. The life of a coalescing filter is usually determined by the amount of solid particles that are caught by the element as they are permanently trapped in the element and will cause the element to “blind” causing a high pressure drop and decreased flow through the filter.
Particulate filters are designed to remove debris made of solid particles from the air stream. The size of the particle that can be removed from the air is dependent on the grade of the filter element. Where a coalescing filter will filter out particulate at a rated grade the same cannot be said of a coalescing filter removing moisture. Often times when a particulate filter is used in a system with a great deal of moisture present the filter element becomes “blinded” or unable to pass the air through it.
Selecting and installing a coalescing filter or filter regulator is a simple step to ensure that your valve positioner has clean/dry air to operate with.