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Hydraulic systems rely on a variety of filters to keep process fluids clean and protect sensitive system components. Suction filters play an important role in this filtration process by capturing suspended particulates and pollutants as fluids pass through them and into the pump. Located within the pump’s hydraulic reservoir, they act as the last line of defense against debris entering the pump and potentially causing component damage or system failure.

In this article, we address some commonly asked questions regarding the use of suction filters in hydraulic systems.

What Is the Function of a Hydraulic Filter?

Suction filters are one of the many types of filters that help keep hydraulic systems in optimal condition. Most systems integrate multiple filters that are installed in various locations to keep fluids clean as they move through the hydraulic circuit. These filters work together to remove the millions of particles larger than 1 micron (.001 mm) that enter the hydraulic system every minute of its operation. As these particles can cause damage to the system’s components and hydraulic fluids are so easily contaminated, it is critical to design a hydraulic filtration process that is effective and efficient at protecting the system and, consequently, improving its lifespan.

Where Should a Suction Filter Be Located in a Hydraulic System?

Proper placement of filtering elements in a hydraulic system is key to successfully preventing the infiltration of solid foreign matter and the subsequent system damage. However, the location is subjective; where a filter should be installed depends on the type of filter.

For example, suction filters are designed to protect the pump from particulates in the reservoir. Therefore, they should be installed on the suction port or pipeline of the pump. On the other hand, inline filters are designed to filter fine particulates from the fluid stream before it flows back into the reservoir. Therefore, they should be installed within the fluid return line of a hydraulic system. When these filters are positioned correctly, the pressure of the return flow is strong enough to force fluid through the filter’s fine screen but not so strong that a more complicated filter design or housing is required.

Do Suction Filters Carry the Risk of Cavitation?

Hydraulic systems have complex pressure profiles that must be taken into consideration during the installation of filtration elements. In terms of filtration efficiency, the flow and pressure conditions surrounding the pump intake make it an optimal location for a suction filter. However, hydraulic pumps are extremely sensitive to restrictions in the intake line, and suction filters have the potential to cause cavitation and mechanical damage if not designed and maintained properly.

One of the main ways suction filters can trigger cavitation is by becoming clogged with debris, which causes a pressure increase in the suction line. Fortunately, this risk can be mitigated in the following ways:

• Following proper filter maintenance. Cleaning or replacing the suction filter on a regular basis is vital to ensuring the fluid’s pressure remains within a safe range.
• Installing a relief valve. Suction filters can be equipped with relief valves that activate in response to increasing pressure. These valves help protect the pump from damage, even when the filter becomes clogged.

<Learn more about out why suction filters are used in used hydraulic systems.>

How Often Should You Change Hydraulic Suction Filters?

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Clogged suction filters can cause pressure increases or decreases within a hydraulic system, which leads to reduced system efficiency and/or system damage or failure. Keeping the system running effectively and efficiently necessitates cleaning or replacing the filters on a regular basis. In most hydraulic systems, filters should be changed when their dirt-holding capacity reaches roughly 80%.

There are several approaches to gauging the condition of a suction filter:

• Manually monitoring pressure drops. Clogged filters can cause a noticeable drop in pressure since fluids can no longer pass through them freely.
• Integrating differential clogging indicators. Differential clogging indicators allow for continuous monitoring, alerting the operator when the pressure drop reaches a critical level.
• Employing advanced monitoring systems. More advanced visual or electronic monitoring systems can provide higher accuracy filtering element information, which makes it easier for system operators to keep filters properly maintained and optimally operating.

The suction filters at DOMS, Inc. feature a highly efficient design that facilitates easy inspection and cleaning, allowing them to be reused multiple times. When combined with a regular maintenance schedule and effective monitoring system, these well-designed filters play an important role in preventing pump damage and keeping the hydraulic system operating at optimal capacity.

What’s the Difference Between a Suction Filter and a Suction Strainer?

Suction filters and strainers are both designed to remove solid contaminants from process fluids, protecting downstream equipment from damage and improving system efficiency. However, despite sharing some functional similarities, filters and strainers are two distinct components with different operating principles. The main differences between them include:

• Size of particles being removed: Featuring a coarse mesh screen, strainers are designed and positioned to capture larger particles from process fluids as they flow into the hydraulic pump. On the other hand, filters have finer screens that are effective for trapping smaller particulates before they enter the pump.
• Flow resistance: Strainers provide low resistance to fluid flow, creating a relatively small pressure drop as fluids pass through them. In contrast, fluids passing through the finer screens of filters will typically experience a noticeable pressure drop.

Contact the Suction Filter Experts at DOMS Incorporated Today

Suction filters play an essential role in preventing particles from entering hydraulic pumps and causing component damage or system failure. However, they must be well-designed and properly maintained to work as intended. Partnering with an experienced filtration solutions provider can ensure you receive the right filtering element for your system and application.

At DOMS Incorporated, we manufacture high-quality suction filters, strainers, and flow diffusers for a wide range of OEM industries. Our suction filters are carefully designed to protect hydraulic pumps, ensuring consistent system performance and enhanced system longevity. The superior performance of these filters comes from their unique design, which includes the following features:

• Rounded root and crest pleat design. Traditional suction filters feature an accordion-style peaked pleat design, which is inherently inefficient and challenging—and occasionally impossible—to clean. Our suction filters utilize an easy-to-clean rounded root and crest pleat design that allows for 100% filter area utilization to overcome these limitations. In addition to enhancing efficiency, the specialized design allows the filter to be reused over and over.

• End caps with a unique mechanical lock design. Our filters have end caps that are flared for a tapered channel construction. The cap, center core, and element are assembled with a high-strength, two-part epoxy resin adhesive, forming a robust filtration unit that can withstand extreme vibration, shock, and other harsh hydraulic system operating conditions.
• Optional relief valves. For enhanced system protection, our suction filters can be equipped with relief valves upon request. These valves automatically activate in response to pressure buildup, which helps prevent pump damage when filters become clogged or blocked.

With both standard and custom filtration options available, we can work with you to find a personalized solution that optimizes your hydraulic system’s efficiency and protects the pump from damage. To learn more about our specific products and services, check out our suction filters page or contact us today. To discuss your suction filter requirements with one of our experts, request a quote.

Suction Line Filtration in Modern Hydraulic Systems

Many times in my career I've seen the topic of suction line filtration become the subject of doubt and debate. Believe it or not, I still hear viewpoints of experienced technicians and engineers vary from "never" to "always". And when hydraulic equipment owners and operators ask about my angle on this subject, their reactions to my reasoning range from "was blind, but now I see" to "I should like to challenge you to a duel!"

One thing is certain, though - suction filters have been around for decades. I consider this fact alone to be the main reason why there are so many people who view the use of suction filters as something highly beneficial if not mandatory. In this post I will explain why I am convinced that suction filtration is vastly overrated, and why the use of suction filters in modern hydraulic systems (with very few exceptions) can only be explained by blind tradition following and nothing else.

Tradition is beliefs handed down from generation to generation, and since a lot of currently active technicians have acquired most of their hydraulic knowledge through hands-on practice and colleague advice, it's perfectly natural for them to follow their company convention without question. So, if their "mother shop" has always installed suction strainers, there is no reason for them not to follow and defend this practice.

Even one of the best sources of current technical information - the manufacturer catalogs - is no stranger to bias, and if you want to use them to educate yourself in modern filtration technologies, you will have to filter out "slogans from the past" and concentrate your attention on bare facts. Although many brands did carefully change or even completely exclude the description of suction filtration advantages from their catalogs, you can still find phrases like:

• "...Suction filters are used as safety filters to protect pumps from gross contamination which can cause them to grip."
• "...Pumps incorporated in hydraulic systems must be protected from coarse containments which when not removed by any other filtering devices may gain access to tank."
• "...Suction strainers protect pumps from damage "
• "...suction filters, on the suction line, protecting the pump from possible coarse contamination"
• "...suction filters are used for functional protection of the downstream pump in the circuit (although only for the avoidance of coarse contamination)"
• "...suction strainers provide coarse filtration to protect hydraulic pumps from large size contaminants which could cause a catastrophic pump failure."
• "...thanks to their function of protection from coarse contamination (due to lack of initial cleaning or to an accidental ingression) they take care of the pump, ensuring a safe and efficient operation."
• "...suction filters protect hydraulic pumps and control systems from solid contaminants"

All of the above was copied from leading manufacturer websites and their downloadable catalogs, and if you like, you can easily google for the source.

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Clearly, such statements coming from respectable sources make it very easy for someone with little industrial experience to recognize suction filters as some kind of universally recommended "pump protectors". Plus our common sense tells us that if we want to "stop contamination", we must do it at the very beginning of the "pumping process", therefore the suction line is the most logical place for a filter. Well, logical - perhaps yes, but practical and justified? - A careful consideration must take place in order to answer this question. So then why don't we do just that? - Let us consider the conditions, in which most suction filters operate, and let us carefully examine their advantages and disadvantages.

But first, let's take a minute to define the vocabulary. Suction filters come in all shapes and sizes, but in general, any filter line will offer two "flavors" - namely internally and externally mounted. Internally mounted filters are, basically, wire mesh strainers that are screwed on a suction pipe inside the oil tank, while externally mounted filters are their more sophisticated younger brothers, whose distinguishing feature is the fact that they can be serviced from outside of the tank. In order to simplify things, hereinafter the former will be referred to as suction strainers, and the latter as suction filters. (I understand that this, obviously, is not correct, as all strainers are filters, but not all filters are strainers, but I wanted clear and short terms to separate these two types in the scope of this article).

All possible problems with suction filtration stem from the fact that these filters and strainers are mounted, well, in suction lines of pumps. And a hydraulic pump suction line is a very special place, where the maximum flow-driving positive pressure is a fixed value, defined by the sum of the oil head above the pump inlet and the internal tank pressure. This pressure is usually very low, especially in atmospheric and low-profile tanks. The fact that the suction line is "fed" by a fixed pressure source is what gives these lines their peculiar "reaction" to any type of flow restriction.

While restriction in a return line, a pressure line, or any other line fed by a positive displacement mechanism, will cause the oil pressure to rise before the restriction point, putting a restriction in the suction line will cause the pressure to drop after the restriction point. Pause and consider that for one second - any restriction in the suction line will cause the already low flow-driving pressure to drop even lower downstream the restriction point!

Any kind of barrier filter is a restriction point by definition. If it is not much of a choke when new, it will become one sooner or later due to particle loading, varnish deposit, an increase of oil viscosity, or something else, and what do we have downstream of our suction filter? That's right - the inlet of our hydraulic pump! And we all know that the lower the pressure in a pump inlet the greater is the chance of cavitation and the bigger is the mechanical stress on the rotary group components (especially in axial piston and vane units). In short - as the inlet pressure goes down so does the life span of the pump! Allow me to rephrase - any type of suction filter or strainer reduces the possible life span of a pump it's connected to.

To be completely fair, let us make a list of the strong and weak points of suction filtration:

Advantages of Suction Strainers

• if your so-called hydraulic system is subject to large floating and partially floating foreign objects inside the tank (cigarette butts, wine corks, plastic bags, tissues, oh yes - let's not forget flora and fauna, like leaves, insects, rats, and other small animals) and the tank is constructed in such a way that all this paraphernalia not only can fall inside but can also find its way into the suction line, then a suction strainer indeed is required to protect the pump.
• if you are a manufacturer of hydraulic equipment, a suction strainer is a "polite" way to reduce the pump life span, and also is another component your client will have to order sooner or later (from your company) especially if you told him that suction strainers should never be washed. Suction strainers are excellent for business!
• suction strainers can be used as conversation starters.
• suction strainers can be used as "slacker tell-tales" (see the explanation below).

Disadvantages of Suction Strainers

• they offer zero filtration value due to the high micron rating and low filtering efficiency of the wire mesh. All particles generated during normal operation will pass through the strainer like flour through a sifter.
• they reduce the possible life span of the pump they are supposed to protect.
• if left unserviced, they can cause cavitation and catastrophic pump failure.
• they are mounted inside the oil tank and therefore are hard to service.
• they are invisible from the outside, which often leads to situations where equipment owners either don't know or eventually forget that they exist.

Advantages of Suction Filters

• easily serviceable
• can replace butterfly valves usually used for servicing hydraulic pumps, sometimes offering a more compact installation
• can have extras - like an air bleed port for priming, drain plugs, etc...
• allow for relatively easy condition monitoring
• can be equipped with magnetic cores that capture ferrous particles
• can come in very large sizes (for some reason)
• can also be used as conversation starters
• can also be used as "slacker tell-tales" (explained below)

Disadvantages of Suction Filters

• most of them still have zero filtration value because they use elements made of the same wire mesh as the strainers listed above
• they reduce the possible life span of the pump they are supposed to protect
• if left unserviced they will cause cavitation just like the strainers
• models that use high-efficiency elements of adequate micron rating are only good for relatively low flows, however since they are subject to normal particle loading, and yet their "replace" delta P threshold is much lower than that of their "return" or "pressure" brethren, they are, basically, a waste of a good filtering media
• can be expensive, in fact as expensive as much more effective pressure filters of the same size

So, when you use suction filtration what you get is - expense, shortened pump life, a real risk of catastrophic pump failure if maintenance is skipped, questionable filtration, and not much of anything else. A classic situation when the risks outweigh the benefits!

Strainers are just not needed, period. Using a suction strainer is like opening an umbrella indoors to protect you from the rain outside - if your roof leaks, the umbrella does keep you dry, however, shouldn't you be fixing the roof instead of relying on an umbrella? And if the roof is OK, then what are you doing with an open umbrella indoors?

But what about those suction filters? It kind of seems like they can actually make technical sense, because they are accessible, serviceable, capture metal particulate with magnets, and even do more than that, right?

Well... Firstly, there isn't much economical sense to them, since what can be called a "decent" suction filter with a high-efficiency element can be as expensive as a pressure filter rated for the same flow, and secondly - just think about this whole suction filtration "fashion" for a second - it is not that you can not find a clever way to circumvent all of the disadvantages stated above - technically it is doable. Hell, you can choose a micro-fiberglass multilayer suction filter the size of a truck, connect it to an oil tank with a 10-meter oil head above the inlet, and monitor the crap out of it with digital vacuum meters, and it will be safe and it will secure ideal oil conditions! However - all of these clever solutions are rendered unnecessary by other common types of filtration, like pressure, return and off-line coupled to a decent breather, as long as the tank starts up clean!

If you want to experience how a pump feels when you slap a strainer (or a filter) on its inlet, run a marathon while wearing this. I guarantee that it will keep you protected from contamination during the race, and as an added benefit, will give you that S.T.A.L.K.E.R Shadow of Chernobyl touch. You can tell me how you feel about suction filtration at the finish line.

The only place in a hydraulic system where I would consider (consider!) installing a properly oversized suction filter would be a suction line of a relatively small closed-loop charge pump, although I still prefer pressure side filtration for the charge circuits any day. The analogy here is different - when I think of a suction strainer connected to a large open loop pump, I imagine an adult person running with a gas mask on, however putting a suction filter to protect a charge pump of a closed-loop transmission is like putting a delicate baby in a sterile incubator. Note, however, that this still makes sense only for small charge pumps. Large closed loops should stick to pressure filtration of the charge flow.

A separate word must be said about the so-called magnetic filters. I have no experience with them, but I believe this technology has a future, especially with efficient magnet arrangements like, for example, in Magnom Filters. They do have a line of products that can be mounted on suction lines - and this is something I would consider using, even if it were just to test the efficiency of this type of filtration (which I just might do in the future).

Suction Filters and Strainers as "Slacker Tell-Tales"

And now, as promised, an explanation of the biggest (in my opinion) advantage of suction strainers and filters, namely, their ability to serve as perfect "slacker tell-tales".

Everything I have said so far about suction filtration is best applied to new projects. But what about already existing builds? If we follow the technical logic described above, we should remove a suction strainer or a filter whenever we come across one, shouldn't we? And to this, my friends, I respond the following:

Any hydraulic system is or will be inserted into a certain environment, and it is this environment that defines its operating conditions. Any environment consists of two parts - physical and human, out of which the latter will always override the former!

If a hydraulic power-pack is mounted in a cool and sterile room, but the maintenance is poor or inexistent, the operators use the filler cap as an ash-tray, and the tank is refilled from a dirty jerry-can only when the low-level alarm goes off, you can say that this system operates in extremely harsh conditions. Likewise, if you have a hydraulic system running next to a rock-crusher inside a mine, but the maintenance crew are properly professional and know what work ethic is, this system, however unlikely this may sound, is operating in much better conditions than the first one.

When you are building a new hydraulic system, you can resolve the physical environment at the design stage, but you will never determine the human environment before the system has operated for at least half a year! And that is when the above mentioned "slacker tell-tale" ability of suction strainers and filters comes into play:

When you happen to service an existing hydraulic system, originally equipped with a suction strainer or filter - inspecting it will give you a very precise clue about the human environment. When you inspect the old filtering element - if it is visually clean - you can safely remove it from the system. However, if you find a lot of "foreign material" blocking the mesh, it can only mean one thing - the human environment of this particular piece of hydraulic equipment consists primarily of slackers (hence "slacker tell-tale"), and even if you clean the tank, reinstall the missing filler cap, reinstall the missing breather, reinstall a hundred of other things that are missing, and do whatever you can to make things better for the poor unit - it won't matter! The slackers will make sure that the oil in the tank returns to its previous condition in a matter of days!

In this case (positive "slacker" test) I would advise (against all technical reason) to keep the suction strainer or filter. Better even - replace it and charge for it. Don't worry about the reduced life span of the pump and don't mention it! Such equipment owners have absolutely deserved their right to suction filtration!

Conclusion

For new projects - forget about suction filters and strainers, they are unnecessary. For existing builds - if your hydraulic system has them for the sake of having - remove them, if your hydraulic system needs them - it is not a good system. Consider this next time you design, order, or service a piece of hydraulic equipment.

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