From the book: Lubrication Excellence 2004 Conference Proceedings

(An excerpt from the "Best Practices in Bulk Lubricant Storage and Handling" paper.)

Above the tank’s oil level and beneath the roof of the same tank lies the headspace. Every tank produces different conditions within its headspace as the contents of oil mist, dirt and water vapor vary considerably. A high percentage of moisture and solid contaminants that enter lubricating oils and hydraulic fluids in storage vessels must pass through the headspace.

Breathers are necessary to exclude contamination. The breather needs to have a particle size and capture efficiency similar to what the transfer oil filter is expected to have. For example, if the oil filter that is used when discharging the lubricant out of the tank has a 10-micron filter and 90 percent capture efficiency (Beta 10 = 10), then the breather performance should be the same or better. If the lubricant is a hydraulic fluid, then the breather usually requires fine breather filtration – around 3 microns. Gear oils by comparison may need only 10- to 20-micron filters at 90 percent capture efficiency.

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These contaminants can often be found in oil prior to machine startup: preservatives, paint, moisture, rust, particles, dust, dirt, welding spatter.

Reservoirs should be sealed tightly from unwanted ingress of air, dust, falling debris, and liquid sprays (such as washdown sprays). Inspection hatches, filter covers, vertical-entry float switches, flanged return ports, breather/blower attachments, etc. should be designed to have elevated connections and sealed tightly. Inspection hatches, where used, should have an elevated lip around the opening, compression clamps and gasketing.

Consider system design in filter selection. If the application has cyclic flow rates, the increase in flow can dramatically alter the efficiency of the element during the surge. If this cylinder cycles at a fast rate, the filter is subjected to extremely high flow rates as the equipment cycles. This can cause leakage past the filter and over the bypass valve as the pressure drop increases, perhaps inducing element collapse.