Power at Work: When to change hydraulic system filters

Wednesday, May 5, 2010

Spending $100 on replacing a filter at the proper time is a much better investment than forking out $10,000 for a new hydrostatic motor assembly

by RALPH WINFIELD

As our hydraulic systems become more sophisticated and the tolerances/clearances between metal components are decreased into the micron range, fluids must be kept cleaner to prolong the life of system components. Changing system filters with the correct filters – on time – can eliminate both the cost of repair parts and down time.
Some hydrostatic motor assemblies can cost as much as $10,000. Some high clearance sprayers will have four of these motor assemblies – one at each wheel. It does not require great math to determine that replacing a $100 filter on time can be a very good investment.

There are effectively three things that can cause problems with the hydraulic fluid – particles that can be metallic or dust; water; or oxidized fluid caused by overheating the fluid. The last two must be taken care of without expecting the filter(s) to do the job.

Many sophisticated hydraulic systems will have temperature gauges and/or fluid cooling systems in place. You should insist that the operator adheres to the maximum temperature limits specified by the equipment manufacturer. If the fluid is oxidized from overheating, it must be replaced.

Water must be kept out. Most mobile hydraulic systems, such as the ones on tractors, combines and sprayers, are very vulnerable to contamination by water.

Because these hydraulic systems go through wide temperature changes, the volume of hydraulic fluid changes significantly between cold start-up and hot run times. Thus, the system must have a reservoir breather that lets air out when the fluid expands, and lets it in when the fluid cools – every time the system is shut down.

As you know, air carries moisture. The changed air brings moisture into the reservoir through the breather.

If and when dew point temperature is reached on the inner surface of the reservoir, condensation of water will occur there. Breather caps have been developed that can strip water from the air on the way in and add it back to the discharged air.

The other significant source of water is at cylinder rod seals. You will note that the only mechanism to keep water (and dirt) out of the working cylinders is a wiper seal pointed outward. If it is damaged, it should be replaced immediately, because the high-pressure fluid seal behind it is pointed inward. It locks tightly around the cylinder shaft only when hydraulic fluid pressure is brought to bear on the seal lip.

Have you noticed that virtually all vertical or nearly vertical hydraulic cylinders are now positioned with the exposed rod pointing down? That orientation minimizes the amount of water challenging the wiper seal for entry into the cylinder and the hydraulic fluid.
Filters must be selected and changed. Filters are used primarily to remove small, hard particles. Therefore, it is important that we understand filtration efficiency as established by the International Organization for Standardization (ISO), the National Fluid Power Association (NFPA), and the American National Standards Institute (ANSI).

The most common indicator listed on virtually every reputable hydraulic fluid filter is at least one Beta ratio (ß). The Beta ratio is determined by counting the number of particles over a given size upstream of the filter and dividing that number by the number of particles over the same given size downstream from the filter, from the same volume of fluid.

For example, if the selected particle size is 10 microns and the upstream count is 1,000 and the downstream count is 50, the ß10 = 1000/50 = 20. As you can see, the higher the Beta ratio, the more efficient the filter is at removing particles of the selected size and larger.

As you would expect, when the filter media collects particles, its resistance to flow increases, therefore the pressure drop across the filter increases. This provides an excellent method for us to determine the time to change filters.

Every filter head assembly should have an optional pressure sensitive bypass valve in it. For filters in the fluid return to reservoir line, the most common bypass valves will open at 25 pounds per square inch pressure difference (25 psid). Some will be 44 psid.
Upon cold weather start-up, when the fluid is more viscous, that valve will open to let fluid bypass the filter until the fluid warms up. So it is very critical that the machine not be put to work until the fluid has warmed up and the bypass valve has closed.

How do you know? Some machines with electronic dash panels will have a light, usually red, that remains on when the bypass valve is open. Most other filter heads will have a 1/8 NPT plug located on the upstream side. (Please, if you are installing a new filter head assembly, make sure that the fluid flow direction follows the arrows shown on the head. If it is plumbed backwards, the bypass valve will not work on cold start-up and the filter media will be much less effective.)

If you have filter heads with the 1/8 inch NPT plugs, buy a small gauge and install it on the fluid entry side. They are available from many hydraulic supply companies and are cheap – under $10.

By observing the differential pressure gauge, you can determine the extent to which the filter flow is being restricted by particles that have been collected in the filter media. Remember, the fluid is returning to an unpressurized reservoir in most instances. (In some applications, the reservoir can be pressurized to about 10 psi by regulated air pressure from the engine turbocharger. This is done to help keep water out at those cylinder seals I noted earlier. If in place, this boost pressure must be added to the gauge reading to get the correct differential pressure.)

Filter media. For most of our more sophisticated hydraulic systems, we should be buying fluid filters that have synthetic media versus cellulose media. The cellulose media will provide Beta ratios in the order of 20, while synthetic media will provide much higher ratios – in the order of 75.

Therefore, it is critical that you buy hydraulic fluid filters that are recommended by or meet the equipment manufacturer or supplier specifications if you want to maintain the manufacturer's warranty.

When to change hydraulic system filters. While the hours between changes as shown in the operator or service manual are a good guide, the use of a differential pressure gauge or indicator offers a better way to determine the extent of plugging.

For example, in most installations where a 25 psid bypass valve exists, a gauge or indicator that shows 15 psid indicates the filter element is at 90-95 per cent of capacity and should be changed.

Other higher-pressure systems where 44 psid bypass valves are standard can have indicators or pressure gauges that trigger recommended filter changes at 28 psid.
One further point to remember: If pressure indicators are suggesting filter changes at more frequent intervals than the manufacturer recommends, the hydraulic fluid should be tested to determine the source of the higher than expected contamination levels. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.