Microbiological or microbial contamination is basically the growth and colonization of bacteria in the oil. And as is the case with all organisms, these bacteria require certain conditions to survive and grow:

• Water – Emulsified in the oil, 500 ppm (0.5 ml per liter of oil) is sufficient.
• Food – Carbon, nitrogen, and phosphorous (readily supplied by the oil).
• Oxygen – Air is typically present in hydraulic oil at 7% to 10% by volume.
• Optimum temperatures – 24C to 49C (75F to 120F).
• Dark, stagnant, or low flow areas – The reservoir in hydraulic systems
• Suspended particles – Assist with initial transportation and colonization.

While all of the above ingredients are required to sustain bacterial growth, water is the key ingredient. So in terms of prevention, keeping the oil dry is a biggy. Another operating factor that can be influenced without too much difficulty is operating temperature. Just as with the storage of food, we all know to prevent the growth of bugs, food should be stored at a low temperature in the refrigerator or kept hot at high temperatures.

A hydraulic system operating temperature above 50C will similarly prevent the survival and growth of bacteria. And if you’ve been reading my stuff for a while, this advice may appear to go against the grain. After all, I’m a big advocate of decreasing hydraulic system operating temperatures, NOT increasing them. That said, it’s all about the degree — literally. Matched with the correct viscosity oil, 50C to 65C is not an operating temperature of a magnitude that is going to cause other reliability issues.

A third preventative factor, and something which is hydraulic equipment reliability “101,” is the maintenance of low particle counts in the oil. Bacteria need water to grow, but they also need a host on which to coalesce and start a colony.

Put simply, the key to preventing a whole host of hydraulic problems, including this one, is to keep the oil DRY, CLEAN, and COOL. The “cool” part is a relative term. Compared to 85C, 50C is cool. But as you now know, allowing the hydraulic oil to run wet and warm is a mistake, because it’s the perfect environment for bugs to grow.

Yours for better hydraulics knowledge,

Brendan Casey