Suction Filters in Modern Hydraulic Systems

    And once again I am talking about suction filters...

    Suction filters and strainers have been around for decades, and this, probably, is the main reason why there are still people, who think that use of suction filters is beneficial. Use of suction filters in modern hydraulic installations can only be explained by blind tradition following and nothing else.

    There are many articles on the web about this topic, and what I am writing here is no news, but every now and then I stumble upon a new project, well designed by apparently experienced engineers, and yet there you have it, a practically unserviceable suction filter inside an oil tank, just like a bird poop on a freshly washed new car smack in the middle of the windshield.

   The most common answer I get when I ask what was the main reason to justify the use of suction filtration, is something like "the equipment will work in dust (sand, sawdust, flour, dry paint, dirt, etc...) contaminated environment, so some extra filtration can't hurt, right? To prevent the dust from entering the pump, right?" Well, wrong!

   First of all, suction filters, as a rule, will have high micron ratings and low beta ratio (filtering efficiency), which by definition make them a piss poor filtering solution. And secondly, even a filter with the largest screen can condition suction line, and actively reduce life span of the pump.

   If you want to know exactly how a pump feels when a suction filter is used, try to run a ten miles marathon wearing this. Those of you who had done military service will know what I am talking about. So if you are still defending the use of them suction filters, you should consider wearing this at all times, as it also will protect you from particle contamination, (and, as an additional benefit, will give you a mysterious S.T.A.L.K.E.R. Shadow of Chernobyl touch).

   The only place where you might consider (consider!) installing an oversized suction filter, would be a closed circuit charge pump inlet. Mainly because in this situation we have low flow rates and sensitive equipment conditions. But I, personally, prefer pressure side filtration for charge pumps, which is a way better option as it allows the use of lower micron rates and more efficient filters.

   Instead of worrying about not allowing the dirt to enter the pump, you should worry about not allowing the dirt to enter the OIL. If you are projecting a hydraulic system that will allow enough oil contamination to need a suction filter, than that is a deficient design.

   I am aware that there ARE no-other-exit situations that, in fact, mandate the use of a suction strainer. I have seen myself in such situations, and, against my principles, have applied suction filters for large open circuit pumps, BUT those were the situations when the oil tank construction was made against all common sense by people, who had absolutely no idea on how a hydraulic oil tank is made, with NO possibility to rectify the situation and the urgent and undisputable need to put the equipment in service. The obvious disadvantages of suction filtration were overpowered by the benefit of the pump not exploding five minutes after the startup due to welding balls inside the oil tank. Even in such situation you should oversize the filter. In fact, you should always oversize suction filters!

   Of course, this is the technical side of the problem. There also is the commercial side. From the commercial point of view a suction filter is another serviceable element a client will have to pay for in the future. As long as the equipment passes the warranty period without major failures, the use of suction filters is also a "polite" way of reducing sevice life of a pump, which a client will most probably buy at the same shop he bought the equipment.

   So, suction filters are good for business and bad for pumps (with very-very rare exceptions), consider this next time you design or order a piece of hydraulic equipment.