The other main fire-resistant fluids are synthetic types. They are made from mineral oil, but have been processed and contain additives to obtain a much higher flash point. It takes more heat to start them burning but there is not enough volatile materials in them to sustain burning. These fluids may catch fire from a pot of hot metal but quickly self-extinguish after leaving the heat source.

Synthetic fluids retain most of the qualities of the mineral oil from which they are derived, so most hydraulic components specify no operating restrictions. However, most of these fluids are not compatible with common seal materials so seal specification changes are usually necessary. Special consideration must be given to handling of synthetics because they can cause skin irritation and other health hazards. Also most synthetic fluids require protective epoxy paint for all components in contact with them.

Of all the fluids discussed, synthetics are the most expensive. They can cost up to five times more than mineral oil.

No matter which fluid is chosen, design the circuit to work in a reasonable temperature range; install good filters and maintain them; and check the fluids regularly to see if they are within specification limits.

A good operating temperature range is between 70° and 130° F with the optimum being around 110° F. A rule of thumb would be: warm enough to feel hot to the touch but cool enough to hold tightly for an extended period. Overheating hydraulic fluids is second only to contamination when it comes to reasons for fluid failure.

Continuous filtration of any hydraulic system is necessary for long component life. Fluids seldom wear out but they can become so contaminated that the parts they drive can fail. (The filter section of this book offers some good recommendations on keeping fluid clean.)

Even with the best of care, any hydraulic fluid should be checked at least twice a year. Systems located in dirty atmospheres may need to be checked more often to see if a pattern exists that requires special consideration. Pay close attention to the sampling process and packaging procedures recommended by the test facility that will process the sample. Expect a report on the level of contamination plus an analysis of the additive contents, water content, ferrous and non-ferrous material amounts, and any other problem areas the test facility finds. Use this information to know when to change fluids and to check for abnormal part wear problems.

New oil or other fluids from the supplier are not necessarily clean. The fluids are shipped in drums or by bulk, and there is no way of knowing how clean these containers are. Some suppliers offer filtered oil with a guaranteed contamination level at added cost. Otherwise, about the lowest level of contamination from most manufacturers is 25 microns.

Anytime a system needs new fluid, it is best to use a transfer unit, Figure 2-1, with a 10-micron or finer filter in the loop. Another way of filtering new or refill fluid is with a filter permanently attached to the reservoir, Figure 2-2. In this arrangement, the breather or other possible fill points should be made inaccessible.

The filter cart shown in Figure 2-1 can also be used to filter any hydraulic unit in the plant. Instead of this filter unit sitting idle except when filling systems, set it up at a machines power unit for a timed run. Place the suction hose in one end of the reservoir and the return hose in the opposite end. This adds a continual filtration loop to any machine even when the machines main pump is shut off. Run the cart until the fluid is clean and then move is to another power unit. Repeating this process on a regular schedule can assist the hydraulic units filters and add extra life to the fluid and the hydraulic components. This process may also show a pattern on machines that have a contamination problem.

Hydraulic fluids should be stored in a clean dry atmosphere. Keep all containers closed tightly and reinstall covers on any partially used drums.

Never mix fluids in any hydraulic system. Make sure all containers are clearly marked and segregated so fluids will not be mixed with one another. Mixing fluids can result in damage to components and some combinations are very difficult to clean up. Be especially careful when mineral oils and synthetic or water-glycol fluids are used in different parts of the same plant.

Fluids are the lifeblood of any hydraulic system and should be given the utmost care.