Duplex strainer is a key part when it comes to oil filtration.

Keep “Zero Leakage” of your equipment is always a target during production.

A high quality duplex basket strainers will help you solve these problems completely!

Medium pressure duplex basket strainer

Also , a perfect duplex strainer will reduce possible downtime, with robust and compact structure.

In this article, I am going to guide you to dig the duplex strainers ,you will learn:

  • Definition of duplex filters
  • Parts and components of duplex strainers
  • Duplex filter working principle
  • Types & classification of duplex strainers
  • Duplex strainer installation process
  • Troubleshooting a duplex filter
  • Filtration standards of duplex filters
  • Applications of duplex filters in industries

Whether you’re a newbie or a professional in the hydraulic systems industry, this information will be handy to ensure continuous operation.

Now, let’s go straight to the subject of this article:

What is Duplex Filters?

Duplex straineralso called duplex filter, is made of two chambers.

It is in these chambers that all contaminants (dirt or debris) are removed.

The degree of filtration will depends on the size of pores in the sieving system.

In most cases, sieve sizes range from 3,5,10,20,25,50 to 100μm.

Furthermore, they come in different designs that can handle a range of flow rates and pressures.


Low pressure Duplex Filter

                                   A low pressure duplex filter by FilSon Filters

Like the name suggests, a duplex filter assembly has two separate container/chamber housings.

Their two chambers have identical filters.

This unique design makes these hydraulic filtration systems suitable for continuous operation.

That is, at any instance, whenever the first chamber is on-line, the second chamber remains on standby.

Therefore, just in case the first filter needs maintenance or replacement, you’ll simply switch to the second filter.

Depending on the duplex filter design, the switching can either be automatic or manual.

Key Components/Parts of a Duplex Strainer

I want you to review the two images below and then compare them to that of the low pressure duplex filter above.

Low pressure Duplex Filter drawing

                                           Side view of the low pressure duplex filter


Top view of low pressure duplex filter

                                            Top view of low pressure duplex filter

As you can see from this duplex filter design, this accessory is an assembly of different parts.

Normally, the number of parts will depend on the type of duplex strainer (you’ll learn more about this classification later in this article).

Some of the most common parts of duplex filters include:

· Duplex filter housing

It is this chamber that hosts the filter elements.

This section determines the capacity of any filtration system and dirt holding capacity.

Normally, it is designed such that the basket strainer can guarantee continuous operation.

They are made of either cast iron, carbon steel or stainless steel.


Super dual-phases stainless steel: UNS S32760 duplex filter

The choice of a particular material depends on the hydraulic system or water piping system.

Quite a number of filter housings feature quick and easy to the open covers.

This makes it easy to change filter elements, thereby reducing any possible downtime.

· Drain System

The drain system is located on the duplex filter housing.

That is, one is at the bottom, while the other is just above the outlet port.

The one at the bottom is a clean side drain that helps to remove sediments from the hydraulic filtration system.

On the other hand, the drain system just above the outlet is called a dirt side drain.

Normally, the dirt side drain removes particles and dirt thus you can maintain the cleanliness of the entire system of the filter.

Again, you’ll use the drain plug to empty the system when replacing filter components.

It helps to keep the working area clean.

· Sealing Systems

The duplex strainer housing is fitted with an O-ring sealing.

This eliminates any possible leakages thus it’s easy to maintain the system pressure.

· Vent Plug

Vent plug is located on the upper section of the filter housing.

The vent plug purges excess air, thus helping to maintain system pressure at equilibrium.

· Lever Handle

You can easily switch from one chamber to another by turning the lever handle.

It is an easy to operate accessory.

· Clogging Indicator

It indicates when you should change the filter elements.

Therefore, you don’t have to open the duplex filter housing to inspect the hydraulic filter elements.

· Inlet/outlet

The inlet port is located on the cylindrical section, which can either be on same or opposite sides.

Usually, the inlet port is on the upper segment while the outlet is on the lower segment.

· Bolt Connections

They hold various parts of the oil filtration system in place.

· Hydraulic Valve Systems

A duplex oil filters have a many types of hydraulic valves.

Depending on the unique design of the system, it may have check valves, bleed valve and counter balance valves, just to mention a few.

Each of these valves performs different functions such as:

Balance valve; it is mainly for pressure relief and ensures smooth changeover whenever you’re replacing filter elements.

A three way ball valve; Together with handle, are mainly used for switching direction from the dirty filter side to the clean side without interrupting operation. This reduces the chances of shutting down the entire system during part changeover.

· Filter Element

A filter element is enclosed within the filter housing.

It is the main component that removes any possible contaminants in the fluid.

Different types and sizes of filter elements

Different types and sizes of filter elements

The size of the filter pores depend on the degree of cleanliness level that conforms to the ISO standards.

I know this subject can be overwhelming, especially if you’re new to the oil filtration systems.

Again, as I mentioned earlier, these duplex oil filters come in many designs and shapes.

Therefore, the best way to know your system better is to request for the duplex filter manual/brochure.

Duplex Filter Working Principle

Before we even start discussing the working principle of duplex filters, it is important to know how you can specify this accessory.

For a fact, any duplex filter assembly that does not meet the recommended criteria will obviously fail.

First, you need to know that fluid contamination may double or even triple the operation cost of hydraulic system.

This because it causes:

  • Particle buildup in hydraulic systems that may lead to component failure.
  • Mixing undesirable components lead to system breakdown
  • Internal leakages that lowers efficiency
  • Sludge build up results in high cost of maintenance

With these in mind, now let’s review some of the critical aspects to consider whenever you’re specifying a duplex strainer.

Duplex Strainer Specifications

Whenever you’re planning to buy a new duplex strainer for any hydraulic systems, it is important that you specify the following:

  • Hydraulic fluid flow rate
  • Size of filter pores (measured in μm)
  • By-pass pressure setting
  • Operating pressure of the system
  • Viscosity of the fluid
  • Operating temperature range
  • Physical dimensions of the filter system

Of course, to come up with a comprehensive list of specifications, you need to be succinct on your system parameters.

With these, you can either go for the standard designs or custom made duplex filters.

Technical specification of duplex basket strainer series

Technical specification of duplex basket strainer series

Now, let’s move to another important segment of these hydraulic filtration systems:

Duplex Strainer Installation Process

First, you need to install the duplex filters properly and to correct specifications. Here is what you should do:

A duplex filter fully connected in the hydraulic circuit

A duplex filter fully connected in the hydraulic circuit

  • At the factory, a duplex filter assembly is carefully packed to prevent possible damage during shipping.
  • Verify the operating pressure of the system. All hydraulic filters come with a nameplate detailing every specification.
  • Before you begin the installation process, you need adequate ventilation, place the unit close to the installation area, provide a shielded electrical unit and a reasonable clearance around the duplex filter.
  • Prepare the mounting area and provide support to prevent any possible vibrations.
  • Install the duplex strainer and secure it appropriately with locking bolts
  • With the duplex filter in place, now connect the input and output connections
  • Connect the electrical and bleed line connections. You need to follow standard wiring procedure as indicated in the product manual.
  • Ensure the lever in the right position – either fully right or left. This will depend on the configurations of the duplex filter. NOTE: every procedure in the user manual is mandatory and must not be ignored.
  • Also, remember to purge the air.

Well, installing and operating a duplex filter is a simple process.

However, if you don’t follow the recommended instructions as stipulated in the manual, it may cost you a lot.

With that simple process, let me now take you through the working principle of these oil filtration accessories.

When must the filter be replaced

As I had mentioned earlier, duplex filters provide continuous and consistent filtration.

This is due to the two separate filter elements.

Cross section of duplex strainers

                                                            Cross section of duplex strainers

With the duplex strainer already installed, it receives hydraulic fluid or liquid through the inlet pipe.

Depending on the position of the flow control lever, it will direct the fluid to one of the filtration assemblies (either left or right).

The fluid passes through the filtration system where all contaminants are removed.

Meanwhile, during the fluid filtration process the system compares the input pressure and output pressures.

If the back pressure exceeds a specific threshold, the clogging indicator will turn on.

As a result, you can then switch to the second duplex strainer assembly.

Normally, you’ll use the selection lever to choose which of the two filters should be in use depending on the change in the back pressure.

The selection lever design is such that it can swing through 180°.

The lever handle (in blue) can turn to an angle of 180 degrees

                                      The lever handle (in blue) can turn to an angle of 180 degrees

By turning the lever to any of the directions, one duplex filter assembly will be in use while the other filter remains on standby position.

During this process, a check valve will prevent any back flow to the hydraulic filter assembly that is not in use.

So, at this point, you can service that filter assembly.

This is to ensure that it’s ready for use once the other filter assembly gets clogged.

Step by Step guide on how to replace element from Duplex strainers

Element replacement

 Element replacement step

So this is what you need to do:

  • Open the vent valve and blow down the valve. This will ensure all the dirty fluids drain off the filter.
  • Now open the upper lid and remove the filter
  • Examine the filter element. If it is in good shape, you can clean and then reuse it again. However, if it’s faulty, then you have to replace it.
  • Return the filter element to its original position and install the upper cap.
  • Close both the vent and blowdown valves.


Finally, use the air bleed valve to purge air from the serviced duplex filter.

By doing all these, your filter assembly will be ready to be on standby.

You can repeat the same process when the other filter gets clogged.

With all these in mind, I am sure now you can install, understand working principle and clean clogged duplex strainers.

In case you have any question on these three technical aspects, don’t hesitate to contact our technical team.

Next, I want to take you through another critical aspect.

It is about the various types of duplex filters that available in the market.

Types & Classification of Duplex Strainers

The demand for specialized duplex strainers has increased in the recent past.

This is due to the dynamic needs of various production and manufacturing industries.

It is for this reason that there are different ways of classifying duplex filters.

Let’s take a look at some of the most critical variables that play an integral in this classification process.

Types of Duplex Filters based on Pressure

Broadly, we have three main types of duplex strainers and they include:

· High Pressure Duplex Filters

This hydraulic fluid filtration accessory has a unique construction and design for high pressure applications.

It can handle a maximum pressure of 32Mpa, with a pressure loss of ≤0.5Mpa.


High Pressure Duplex Filter

FilSon Filters high pressure duplex filter

Most of these high pressure duplex filters are made from either cast iron carbon steel or stainless steel.

Typically, the choice of a particular material will depend on the nature of the application.

Of course, there are standard designs and custom options depending on the specific requirements.

Most of these hydraulic filters have a flow rate of 30 to 660 liters per minute.

· Medium Pressure Duplex Filters

These hydraulic filtration systems play a pivotal role in many industries.

From their structural designs, you can clearly see the difference between medium and high pressure duplex filters.

industrial strainers manufacturer

 FilSon Filters medium pressure duplex filter

They have a working pressure of 21Mpa, with a pressure loss of ≤0.1 to 0.35Mpa.

Depending on their structural design, the flow rates may vary from 160 to 400 liters per minute.

· Low Pressure Duplex Filters

In case you need a duplex strainer for low pressure applications, then this is a perfect alternative.

The operating pressure is 1.6Mpa, while the pressure loss is ≤0.35.

Low pressure Duplex Filter

FilSon Filters low pressure filters

Depending on the specific requirements of your unique applications, you can choose a duplex filter with a flow rate of 25 to 1,300 liters per minute.

Don’t get me wrong, I am not saying that these are the only low, medium and high pressure duplex filters available.

No, there are so many designs available.

Apart from these, there are other classification criteria such as:

  • Flow rate: You can opt for high to low flow rate duplex filters. A good example is the high flow rate filters that can handle 1,300 to 15,000 liters per minute.
  • Nature of the fluid: Again here, you can choose from high viscosity to low viscosity duplex filters.

Also, I want you to note that dimensions and technology may also form a pivotal role when it comes to classifying the different types of filters available in the market.

Remember, it is the nature of the application that will determine the types of hydraulic filter you should buy.

Applications of Duplex Filters in Industries

The design of hydraulic filters ensures fluid flow and filtration in two different filters.

This implies that, you can easily switch from one filter assembly to another.

With these filters, downtime will definitely be a thing of the past.

This makes hydraulic duplex filters a perfect choice for all applications that require continuous flow such as in:

  • Foodstuff and chemical processing industries
  • Oil drilling and processing industries
  • Mining industries
  • Power industry
  • Pharmaceutical and drug industries
  • Water and waste management industries
  • Firefighting industries, etc.

In all these applications, duplex filters remove all contaminants to ensure safe and optimal operation at all times.

Therefore, you don’t have to worry about partial or permanent blockage that may be too costly.


In short, duplex strainers play a pivotal role in hydraulic systems.

They guarantee continuous hydraulic fluid flow, by allowing users to alternate between various filter assemblies.

This is a critical aspect, especially for companies that intend to reduce downtime and maximize productivity.

Here at FilSon Filters, we offer a wide range of duplex filters for virtually all hydraulic system applications.

For inquiries about our products, feel free to contact our technical team.

Further Reading