A few years ago, I wrote an article for the newsletter on this subject. Much of this information is repeated here, so if it sounds familiar please bear with me. There have been some recent trends and a lot of you were not subscribers at the time, so I felt that a review and update might be in order.

Conventional hydraulic machine design has typically included a pump suction strainer (most commonly a 40-micron steel mesh), surface pressure filter elements (typically of the pleat design) downstream of the pump and another surface media return filter. Sometimes another surface element will be used for offline filtration, sometimes called a "kidney loop" or "recirculating system". For decades, this has been considered sufficient to meet OEM cleanliness standards, so what's wrong with it? Well, one thing that is wrong with it is that the most common cause of failure - more than all other causes combined - is still contamination. Clearly there is room for improvement in filtration.

The suction strainer element is called a "full flow" element. There should be little if any pressure drop across it. If there is, pressure in the suction line of the pump can drop very low and cause the pump to cavitate. Often suction strainers are specified by the system designer, but this is mostly by convention than based on actual system need. Check the pump manufacturer's website. You may be surprised that most of them no longer recommend strainers or filters. They should not be needed if the other system filters are properly specified and reasonable fluid maintenance practices are in place. If getting maximum pump life is your primary concern—and it should be—then it’s far more important for the oil to freely and completely fill the pumping chambers. Even mild cavitation can cut a pump's useful life by over 50%. A 40-micron filter will certainly not protect the pump and if there is a danger of heavy sludge, bolts, tools and such entering the pump, someone is not being very careful with machine reliability and there probably is no filter that can help.

I'm often asked the difference between surface and depth filtration - and which should be used. Conventional surface filters are normally constructed from thin sheets of material folded into many sections and then turned into a multi-ported star shape to allow a relatively large surface are to be used in a small space. The filter fits inside a bowl. They may be in the form of a replaceable cartridge or permanently fitted inside a throw=away bowl. The thin sheet is full of pores which trap the solid particles as the fluid passes through them. Typical materials used for the media are:

Paper filter elements, though often quite efficient, are ruined by water which makes them soggy and the pores close, often causing the filter to collapse.

Another design uses a single filament of metal wound into a cylinder.

Depth filters are constructed with a thick layer of material with small passages through which the fluid must pass. The particles become tracked in the passages. Typical depth filter media materials are:

Surface filters allow a large amount of flow with a minimal pressure drop, but they really don't do a very good job until they have been in service a while. As contaminants collect on the media, smaller and smaller particles are trapped. Unfortunately, as the filter gets dirtier, the pressure drop across it increases. So, while the filter gains in effectiveness, it may not provide adequate protection when it is first installed. And as the pressure drop across the filter increases, the danger of collapse or rupture also increases.

Recently, a fluted media was developed which provides alternating flow paths and allow more filter media per unit volume. In the future, this type of media could replace the pleat technology found in conventional surface filters.

Depth filters however will remain more constant in their particle-trapping ability. System protection begins immediately and is less affected by trapped contaminants. The pressure drop does not increase appreciably until the media begins to become saturated. Metal fiber or fiberglass will give the longest filter life and are least likely to collapse under high pressure drops. They can also be impregnated with water absorbing material.

But before you replace all of your surface filters with depth media, there are limitations to be taken into account. One big one is cost. Depth media is considerably more expensive than surface media when compared on a basis of number of gallons of fluid passed through over the lifetime of the filter. And the flow rate the filters can withstand is considerably lower than most surface filters. Several depth filters would need to be connected in parallel in order to meet the flow requirements of most hydraulic machines.

A high quality surface filter is therefore more practical for pressure and return lines. A depth filter however will work much better for offline filtration and system flushing since these actions are usually performed at low pressure and considerably lower flow rates (it is recommended that the flow rate of most offline filtration is at least 10% that of the system flow). With a very high beta rating, a depth filter used in offline filtration can provide excellent protection to any hydraulic machine. Many of our customers use our flushing machine not only to reclaim their oil but also for offline filtration. For more information about our flushing machines and high beta rating depth filters, see the Hydraulic System Flushing section on our website.

• In-plant Troubleshooting
• Leakage Problems
• Pressure Settings
• Shock Problems
• Hydraulic Reliability Assessments
• Hydraulic Troubleshooting Manual Development
• Startup Consulting and Recommendations
• Heat Problems
• Repeated Component Failures
• Speed Problems

One pound chicken wings
1/2 cup butter
1/2 cup Texas Pete (or your favorite hot sauce)
Flour
Vegetable oil
This is the original recipe from the Anchor Bar and Grill in Buffalo, NY. Why tamper with perfection? Deep fry the wings in vegetable oil until golden. Be sure they're done by cutting to the bone the thickest part of the biggest wing. While the wings are frying, melt the butter and add the sauce. Increase the heat by adding cayenne pepper and/or habanero sauce if you're an iron man. If you're a wuss, get a milder sauce by changing the hot sauce/butter ratio. Add just enough flour to thicken the sauce sufficiently to stick to the wings. Put the wings and the sauce in a deep bowl, then shake the bowl until the wings are fully coated. Pour any remaining sauce over the wings if you like them wet. If you prefer dry wings, bake at 400^o until the sauce hardens.

This is determined by deliberately contaminating clean oil with particles of a specific size and taking samples both upstream and downstream of the test filter, measuring the difference between the two. This can also be performed using laser particle counters upstream and downstream of the filter. Contaminated oil from the Injection Reservoir is added to the clean oil in the Test Reservoir and directed through the test filter. Since the oil downstream will be cleaner than the oil upstream, more contaminants are injected to keep the inlet oil dirty. The test process continues until either the pressure drop across the filter reaches a predetermined amount or the flow through the filter, as measured by a downstream flow meter, drops to a specific rate.

The measured beta rating will change as the test continues. The initial beta rating will increase as the test continues because, as contaminants collect in the filter media, they actually become part of the media and help trap more contaminants. This is more pronounced in surface media than in depth media, so as a rule depth filters will have a more level beta curve. The beta curve shows the stability of the filter. As the beta value of the filter increases, the pressure drop across the filter will also increase. And, as the pressure drop increases, the flow rate through the filter will decrease. This continues until either the pressure or flow reaches the predetermined point and the amount of time this takes is the indication of overall filter life expectancy. The ideal filter would have the most gradual pressure drop increase, the most stable flow and the highest life expectancy which would indicate the greatest capacity for holding contaminants.

Though the depth filter tends to have the most level beta curve, it typically will have a greater pressure drop to flow rate comparison. They usually work best at relatively low flow rates, such as in an offline (recirculating) filtering system. Most of these will have a flow rate of only 10-15% that of the main system. Surface filters, while they allow greater flow rates and lower pressure drops, may not provide adequate protection when first installed.

So the beta rating alone doesn't really tell the whole story - to make a truly fair comparison of filters, the beta curve tells you much more. Be suspicious of filters that don't provide the curve - if it's good, they will probably be proud of it! Also, remember that these ratings are determined artificially under controlled laboratory conditions. The true test of a filter is how it performs in the field.