Duplex Filters Manufacturer

Your Premier Duplex Strainer Choice for Filtration

Filson duplex strainer, as well as duplex type strainer or dual strainer, is a main basket strainer type manufactured by Filson. It is a perfect choice to provide continuous operation without any downtime during basket cleaning or change out.

Filson supplies various duplex strainers types, such as 2 inch duplex strainer, 6 inch duplex strainer, duplex oil strainer, duplex water strainer, stainless steel duplex strainer…

All these duplex strainers can be put into use for chemical industry, oil and gas industry, pulp and paper industry, pharmaceutical industry, metals and mining industry, water and waste management, power industry, petrochemical and refining.

Leave your name, email and requirements of connection size, flow rate, operation temperature, strainer housing and other duplex strainer specifications in the information sheet, our professional engineering sales will show you a suitable duplex strainer.

Filson Duplex Filters

2inch Duplex Strainer
Filson 2 inch duplex strainer is a type of basket strainer built to fit a fuel, oil or water piping system and it is used to remove large particles of dirt and debris.
3inch Duplex Strainer
Filson manufactures 3 inch duplex strainer to adjust a 3 inch size pipeline. It can be made of stainless steel, carbon steel, cast iron and PVC to meet customer specifications.
4inch Duplex Strainer
Filson 4 inch duplex strainer provides a continuous work with two seperate strainer baskets working by turns. Spare both time and labor and improve the filtration efficiency to the maximum at the meantime.
6inch Duplex Strainer
Filson Metal Filter Cartridge is an ideal choice for backwash filtration, high temperature and harsh environment applications. It can be cleaned and re-used for many times.
8inch Duplex Strainer
Filson manufactures duplex strainers with a wide range of connection size from 1inch to 24inch. And 8inch duplex strainer is one of them to block contaminants and unidentified matters from liquid, viscous and gaseous media.
10inch Duplex Strainer
Filson provides 10inch duplex stainer in a high design and manufacturing standard according to ANSI and ASME. And it can be customized to fit your specific needs.
12inch Duplex Strainer
Filson 12inch duplex strainer, with special material and robust construction, can be applied in many industrial usage including chemical, pharmaceutical, water and waste management.
Duplex Oil Strainer
Filson duplex oil strainer, also named duplex fuel oil strainer, is an ideal strainer for oil handling systems. And it is also applied for those applications which require a continuous supply of oil without any interruption.
Duplex Water Strainer
Filson duplex water strainer is the best choice for a water system. It captures the particles, solids and impurities of the passing water flow to filter the medium. With two independent strainers, it allows a high working efficiency.
Stainless Steel Duplex Strainer
Filson stainless steel duplex strainer is mainly made of stainless steel 304 or stainless steel 316. The special material makes it possess a good rigidity and high strength and with excellent temperature and corrosion resistance, stainless steel duplex strainer is always applied under tough conditions.

Why Filson Duplex Basket Filters

Filson designs and manufactures high-quality duplex basket filters and strainers to top brands in different industries. Like the Chemical industry, Power plants, air-conditioning and refrigeration technology, automotive and construction machinery industry, Shipping processes, Lub oil filtration, Ballast Water, and so on.

Filson duplex filter mainly consists of two welded steel housing with covers that are fastened via screws and nuts. You can download this duplex filter PDF catalog to learn more about Filson duplex filters.

If you also need single basket strainers or a portable filtration system, Filson can also help with this. We request your inquiry on any problem with our duplex filters.

Filson Duplex Strainer Manufacturer Video BackGround


  • 100% No Leakage
  • Welded Filter Housing
  • Change-over System
  • High Dirt Holding Capacity
  • Improve the filtration efficiency by two distinct strainer baskets working by turns
  • Remove the particles, solids and other contaminants of the passing fluids
  • Protect pumps, meters and other expensive components in the processing system
  • Provide remarkable quality duplex strainer with a competitive price
  • Design and build the most stable construction to avoid unnecessary malfunction
  • Multiple usage for industrial applications of liquid, viscous and gaseous media filtration
  • Wide range of connection size, working pressure&temperature, end connection and material to fit customer requirements
  • Less cleaning times with a low maintenance costs
  • Easy cleaning and installation
  • Available to be designed and manufactured in ANSI and ASME standards
  • Long life service and reusable
  • High precise of strainer basket to capture any size of impurities
  • Raw Material: stainless steel 304 or 316, carbon steel, cast iron, PVC
  • Type: 2inch duplex strainer, 3inch duplex strainer, 10inch duplex strainer, duplex oil strainer, duplex water strainer, stainless steel duplex strainer…
  • Connection Size: 1’’to 24’’
  • End connection: threaded, flanged
  • Operating Temperature: -190℃—600℃(upon material and request)
  • Flow Capacity: up to 15000L/min
  • Flitration Ratings: 1μm to 10000μm
  • Pressure Capacity: up to 100 bar
  • Applicable Medium: oil, stream, water, gas…
  • Lubrication system
  • Industrial or marine oil burners
  • Chemical plants
  • Cooling lines in power plants
  • Oil and gas industry
  • Pulp and paper industry
  • Pharmaceutical industry
  • Metals and mining industry
  • Water and waste management

When the system is in operation, the two separate duplex strainer basket works by turns to realize a non-stop working process. The work of a Filson duplex strainer is basically to remove impurities such as debris, huge particles and other dirt from fluids. It also guarantees a smooth flow of the fluid in the system.

The system also contains a valve handle installed between the two baskets to switch the flow to one strainer while the other is being cleaned or changed out. On some Filson duplex strainers, the valve will be designed and built to work automatically and the independent strainer will perform a self-cleaning operation.

Make sure that your order of new Filson duplex strainer or replacement meets all the specification as your present application. And then, it will ensure that the pressure of the system is always at a constant level.

Filson Duplex Filters: Your Premier Filters and Strainers Supplier

Filson knows duplex filters and strainers. If you are looking for a reliable duplex filters manufacturer, you are in the right place. Whether you need to purify your oil, water, or even gas, we can custom perfect duplex basket filters for your application.


Filson: Eminent Duplex Strainer Manufacturer and Supplier for Worldwide

Filson is a veteran duplex strainer supplier with over 20 years of manufacturing experience. Under the support of advanced equipment and professional engineers, Filson designs and builds excellent duplex strainers to customer specifications. And our orders can reach 100 units per month.

Filson duplex strainer is generally made of stainless steel 304, stainless steel 316, carbon steel, cast iron, and PVC(polyvinyl chloride) materials with high temperature and corrosion resistance to be reused in the production process.

Among those materials, cast iron is used most commonly because it costs the least for simple manufacturing methods. Cast iron duplex strainer also has good corrosion resistance in the water and many other services.

Filson carbon steel duplex strainer is used under higher temperature conditions or where the thermal and mechanical shock is considered as an issue. This makes duplex strainers made of carbon steel popular in the oil and petrochemical industries.

Filson PVC duplex strainer is a cost-effective option for a filtration system. Its special material makes it extremely helpful for some harsh chemicals. While Filson stainless steel duplex strainer is needed in a tough environment with high temperature and corrosion and is widely used in chemical, food, and pharmaceutical industries.

By the way, Filson provides a custom service. Except for the four standard materials mentioned above, you can customize other materials duplex strainer from Filson. Just tell us your specific materials, pressure classes, and designs, our engineers will try their best to fit your specifications.

Compare to the common duplex strainer, the Filson duplex strainer has a larger flow capacity of up to 15000L/min and can work with pressure up to 100 bar. And with compact and steady construction, the Filson duplex strainer has a longer service time than the others.

In addition to the duplex strainer, Filson also manufactures the simplex strainer. The biggest difference between them is the quantity of the strainer chamber. Simplex basket strainer has one single strainer chamber while duplex strainer has two of them.

Double strainer chambers for Filson duplex strainer mean higher working efficiency for the whole production process. The primary benefit of it is to provide uninterrupted downtime for maintenance and cleaning.

As for the insert baskets of a Filson duplex strainer, there are many options, no matter for single or multiple baskets, it’s up to your request.

Filson duplex strainer provides an outstanding effect in removing particles and solids among water, gas, oil, and steam to protect the valves, pumps, meters, and other important components downstream.

If you have the need for a duplex strainer, Filson will always be here to supply you with a suitable one.

More details to be known in the following ways:

Phone: +86-157 3695 8886


Duplex Filter Manufacturer

  • DN 15 to DN 1000

We can custom duplex filters from DN 15 to DN 1000 with pressure up to 32Mpa. Numerous connection and flange styles upon request.

  • Welded Version, Steel/Stainless Steel

Filter Housing can be made of carbon steel, 304 SS, 316 SS… Elements are available in micron rating from 3μm to 1000μm

  • Large Ball Change-Over Available

Compact and easy-to-operate, ball-switch duplex filters have reliable sealing and require minimal storage space for installation

  • Works for Any Liquids

Tailor-made duplex filters for any liquid like mineral oils, lube oils, water etc.

Filson duplex filters are also called duplex strainers, twin basket filters which are used to remove large particles of dirt and debris from fuel, oil or water piping system.

parker duplex filter replacement

Actually the duplex filters are consists of two simplex filters, the duplex filter structure mainly include the basket filter housing, valves and basket filters.

every part of a duplex filter

If you are looking for duplex filters that have excellent filtration capability, then Filson duplex filter can be your better choice.

Filson duplex filters can be designed based on your detailed requirements and we provide full support on the duplex filters training, duplex filters installation, duplex filters cleaning, duplex filters troubleshooting, duplex filter element replacement and more. Not even a duplex filter manufacturer, you need a real partner which will fully support your business.

Filson low pressure duplex filter

Filson duplex filters are with automatic change over system which is very easy to operate even for a newbie. The duplex filter working principle is very easy with the ball valve system.

Filson duplex filters include duplex basket strainer, low pressure duplex filter, medium pressure duplex filter, high pressure duplex filter and fabricated duplex filter.

Our duplex filters can be made of carbon steel, 304 stainless steel or 316 stainless steel based on your detailed request. Every Filson duplex filters are with the 18 months warranty on quality and you will be happy on our duplex filter quality.

duplex filter drawing

If you do not know what is a duplex filter, the Filson is your best teacher on duplex filter, actually, we have over 20 years manufacturing experience on duplex filter, any question you have, freely to ask us.

duplex filter details

Even you bought other brands duplex filter, like Eaton duplex filter, Hydac duplex filter, or any other brand duplex filter, Filson can always help you out.

Large Flow Duplex Filter

So, send your detailed request about duplex filter now and let me know your problem. You need to tell us below details to get a detailed quotation:

  • Duplex filter Material. Carbon or SS304,316,316L…?
  • The flow rate and Filtration accuracy
  • Simplex or duplex filter
  • Duplex filter Quantity you need

Why full welded Duplex Filter

duplex strainer manufacturer

  1. The welded duplex filters are flexible and lightweight compared with Cast Duplex Filters
  2. We can custom the duplex filter inlet and outlet direction, installation dimension and connection type
  3. There is no MOQ limit on the duplex filter, you can only import one strainer at one time

Duplex Filter Element

Basket Filter Element

Filter element is the core of the filter.

The bypass valve is pressure relief to protect the element, the sender is alarming to protect the element, the drain valve is draining the polluted oil to protect filter element.

Duplex Filter Bypass Valve

1.The role of the bypass valve: after a period of time using of the filter elements, pressure differential increased to the expected limit, the bypass valve that installed in oil inlet direction, feel the pressure differential and automatically open.

2.The purpose is to prevent the filter pressure differential continue to rise that deforming filter element and continues supplying oil to the system

3. Classification(according to different  function)

Duplex Filter Indicator

The duplex filter indicator will tell you when you need to change and clean the filter element

More Detailed images of Filson Duplex Filters

The surface of the Filson Duplex filter

Surface of a duplex filter

The connection of Filson Duplex Filter inlet and outlet

connection of duplex filter


New Change Over designs of Filson Duplex Filter

New designs of Filson Duplex Filter


Stainless Steel Filson Duplex Filter

stainless steel duplex filter


Filson basket duplex strainer

Filson Duplex Filter also called duplex strainer is composed of two single filters and a switching valve.

Simple in structure and easy-to-use, it is specially designed to offer continuous operation, even during duplex filter element is changed or cleaning.

duplex strainer manufacturer

Filson Duplex Filter is available in a wide range of versions, including duplex oil filters, dual basket strainers, duplex bag filters, automatic duplex basket strainers, duplex lube oil filters, stainless steel duplex strainers, high flow duplex filters and other duplex type strainers with Filson replacement elements.

Basket Filter Element

Filson Duplex Filters can handle flow range from 25 L/min to 15000 L/min with working pressure up to 32 Mpa.

With wide range of filtration precision 3 ~ 1000μm, Filson duplex type strainers provide a simple and cost effective way to separate particle impurities from hydraulic & lube oil systems.

Gear oil

Filson Duplex Filters adopt phosphating for surface treatment and painted by polyurethane topcoating for strong corrosion resistance.

Filson duplex filter housing made from SS 316 & SS304 are mirror polished to resist adhesion of dirt and rust.

They are generally installed in pressure or return pipelines to protect fluid system safety.

stainless steel duplex filter

Compared to other duplex filter manufacturers, Filson duplex filters use high quality hydraulic fittings and filtration material that will allow for quick set up and minimize oil spills.

All FilSon duplex filter housing and duplex filter elements get through strict air-tight and leak testing according to ISO 4406 quality standard.

Filson Duplex Type Filters are ideally used to replace many brands of dual filters in function and dimensions, such as Pall duplex filters, Hydac duplex filters, Hayward duplex strainers, Indufil duplex filters, Eaton duplex strainer model 50, Mueller duplex basket strainers, Keckley or Titan duplex oil strainers and Hilco duplex oil filters…

With the Filson’s rich experience in duplex filtration industry, we can help you remain competitive by being your OEM partner for the Filson Duplex Filters.

Whether for standard, custom or different structure, Filson Duplex Filter offers a perfect solution for all filtration systems.


  • Numerous Connection Variants
  • Custom Modifications Available
  • Nominal size: DN 15 up to DN 1000
  • Fineness: 3 – 1000 µm (other on request)
  • Filter Housing: Carbon Steel/Stainless Steel
  • Wide variety of Filter Elements Material optional

Filson Duplex Filter Working Principle

Duplex oil filter allows continues operation even during element is changed for which is clogged by contaminants.

After opening the balance valve and then turn the directional valve, the other filter begins to work; the clogged element should be changed in time as a standby.

Duplex filters are ideal for applications where continuous flow must be maintained, with no flow interruption for strainer basket cleaning.


  • Heavy-duty mining
  • Metallurgical machines
  • Electricity
  • Chemical&petrochemical
  • Wastewater treatment
  • Paper&pulp
  • Sea water filtration

Filson Duplex Filter Design

  • Ball valve, butterfly valve, cam-operated valve can be chosen to switch the working vessel from the dirty side to clean side without interrupting operation.
  • Balance valve between the two filter housings is used to reduce the pressure shock during the switching operation.
  • Clean/dirt side drain remove particles and keep duplex oil filter inside clean.
  • Vent plug for air purging and can provide dual strainer smooth changeover.
  • Fast-to-operate lever handle is flexible, lightweight, and quiet operation.
  • Pressure gauge  shows when to replace duplex filter elements.
  • Duplex filter housing is available in SS 304/316L, duplex stainless steel 2507, etc.

Filson Duplex Filter Changeover Procedure/Duplex Filter Cleaning Procedure

  1. When the indicator is alarming, begin to replace the filter elements.
  2. Open balance valve of the duplex oil filter and keep around 5min.
  3. Turn the middle of the switch handle, so that the arrow point to the working filter housing.
  4. Turn off the balance valve.
  5. Loosen vent plug to bleed air and loosen hex head screws on duplex filter cover, then remove the cover.
  6. Open the drain valve on the filter housing, discharge contaminated media; flush the interior of duplex filter housing with clean, processed, filtered oil.
  7. Pull filter element from the filter head and verify correct element replacement. Wire mesh duplex filter elements can be cleaned.
  8. Install element in duplex filter housing. Make sure element o-rings seat properly into the head, making sure that the notch on the element lines up with the notch in the head.


  • Switching valve must be turned in place as required
  • After the indicator alarm, must replace the filter elements immediately.
  • when replace the filter elements, you should check whether there is damage to the seal, and do not omit seals when installed.

Duplex Strainer: The Complete Guide for Importers

I know you’d wish to remove debris, dirt or large particles from oil, water or fuel without shutting down the entire system.

Yes, you need a duplex strainer to ensure continuous operation of the piping system.

You know, shutting down some systems is both impractical and uneconomical.

That’s why I want to help you choose a suitable duplex strainer.

The best part?

I will walk you through every aspect, from basic definition, working principle, installation to technical data of duplex basket strainers.

By the end of this guide, you will be able to import the right duplex basket strainers.

Chapter 1: What is Duplex Strainer?

Duplex strainer is a closed system, with two separate basket strainer chambers.

The two chambers work independently.

In fact, each strainer has a dual basket filter element configuration.

Look at this:

Now, this is what happens:

As one chamber is working, the other remains on standby.

When the chamber that was working gets clogged, simply switch to the one that was on standby.

Therefore, you can use a duplex strainer continuously without stopping the system.

Basically, there will be no downtime even when you’re cleaning or doing routine maintenance.


I know you’re probably asking yourself this question:

How do you make the change-over between the on-line and standby chambers?

Every duplex strainer has an integral ball-valve or plug system.

By a single operation or turning the lever, you will initiate the changeover mechanism.

Also, the duplex strainer may come with a diverter plug.

It allows medium to flow either right or left.

Do not worry, I will learn more about the ball and plug system in chapter 3.

But, that’s not all.

Other duplex strainers, have the diverter cartridge instead of diverter plug.

This special cartridge functions the same way as the diverter plug.

Of course, the medium flows similarly as in the other design.

The best part?

Diverter cartridge ensures an excellent seal shutoff between the twin chambers.

This gives operators ample time to open, clean and replace the duplex filtering cartridge.

Furthermore, in the process, there is no flow interruption.

Below is an image of duplex strainer courtesy of Hayward:

Duplex strainer

Duplex strainer

The truth is, there are many systems that require an uninterrupted flow system:

  • Chemical plants
  • Compressor
  • Cooling lines for power plants
  • Condensers
  • Industrial or marine oil burner
  • Ink supply lines of printing presses
  • Lubrication systems
  • Water lines for cooling towers and pumping stations in water collection points like lake and rivers.

In all these systems, in case the duplex strainer is clogged or there is a breakdown, simply switch to the other chamber.

It’s a simple process – just push the lever.

FilSon duplex strainer

FilSon duplex strainer

Advantages of Duplex Strainers

Depending on the design of the duplex strainer, they guarantee the following key advantages:

  • The ball-valve system provides seal together with technicians’ protection during routine maintenance.
  • It does not allow for any downtime during basket screen cleaning or maintenance as stand-by chamber will come on. They are suitable for continuous operations.
  • It offers a greater filtration area than simplex strainer thus exhibits an immense filtration capacity.
  • Its single lever eases the operation, simplicity and safety. Also, it allows for low pressure drop across the strainer.
  • Duplex strainers come in different sizes, to meet the flow requirements in most industries.

Simply put:

A duplex strainer is a simple unit that houses opposing outlet and inlet flange.

The unit has identical twin chambers tightly held with bolts to the central ball-valve assembly.

Also, the double strainer is available with either flanged or threaded end connections.

Duplex Straining Element

This is an important component of the duplex strainer.

The standard screens are usually for filtering water, oil, gas acronym WOG, etc.

Other liquid filter media are also competitively priced or customized.

Basket straining element

Basket straining element

The basket straining element is made of straight or staggering perforated plate.

At times it is covered with mesh widths and wire-cloth material.

As the fluid flows through the basket, the element traps all solid contaminants.

Its design is such that, fluid flows from inside to outside.

Depending on the specific needs of your application, you can use a different straining element.

In most cases, the twin strainers are made from any of these materials:

  • Bronze
  • Carbon steel
  • Iron
  • Stainless steel

Please, I want you to note the following:

         i. Iron is commonly used because it is affordable compared to the other materials. It is also resistant to corrosion.

        ii. Carbon steel is suitable for applications with higher temperature, mechanical or thermal shock. It’s a perfect choice for petrochemical and oil industries.

       iii. Bronze has better corrosion resistance properties. Also, it can withstand mechanical and thermal shock more effectively as compared to iron. On the contrary, it has a limited temperature range.

        iv. Stainless steel the most suitable housing material for duplex strainers where corrosion poses the greatest challenge. That is why most chemical, beverage and food, and pharmaceutical industries widely use stainless steel strainers.

Green duplex strainer

Green duplex strainer

Anyway, that was just a by the way – but I guess the type of basket strainer material is critical, you can’t just ignore it.

Moreover, a duplex strainer may have other accessories such as:

  • Air eliminator
  • Equalizing lines
  • Mounting feet
  • Specialty basket options
  • Welded heating jackets (a component for both the carbon and stainless steel housing assembly)

And of course, whenever you’re buying duplex strainers, they should come with differential pressure gauges and electrical switch units.

A differential pressure unit will help you in checking the basket chamber operation conditions.

You will know when the system is clogged and needs cleaning.

More advanced duplex strainer gauges feature an alarm system.

You can easily know when the chambers need cleaning.

In a nutshell:

A duplex strainer has two independent chambers.

You can switch from chamber A to B or vice versa, to ensure continuous flow in pipe systems.

Now, let me take you a step further and explore how a duplex strainer works.

Chapter 2: Working Principle of Duplex Strainer

In this a chapter, you’re going to learn how a duplex strainer works.

It is an essential information for both duplex strainer importers and buyers.

This way, you can make an informed decision when evaluating various specifications of a duplex strainer.

Anyway, let’s get straight to the main subject of this chapter.

The integral ball-valve system design provides an instantaneous shutoff and bubble tight sealing at the point of diversion.

This helps to eliminate potentially dangerous and costly leakage into the unused chamber during cleaning and routine maintenance.

The image shows various parts of duplex strainer courtesy of Mueller Steam Specialty:

Parts of a suplex strainer

Parts of a duplex strainer

Furthermore, it also reduces suction operation risks like air ingress.

At the same time, it gives access to other duplex chamber parts without necessarily taking the strainer off the system.

Now, with that, let’s go ahead and learn how a duplex strainer works.

But first, I want you to watch this video:

The working principle of duplex strainer is very simple.


They have no moving parts.

Of course, you can see that from the video above.

And by the way:

It is important to order a duplex strainer with appropriate pressure and flow rating that match your pipe systems.

Besides, the inlet and outlet openings should match the piping flow.

Logically, the proper orientation of flow will begin at the “inlet” and flow peripherally out to the “outlet”.

Both the inlet and outlet are labelled on the duplex strainer.

The two chambers of the duplex strainers come with a pleated or smooth filter elements through which the fluid flows from the inner to the outer piping.

The filter chamber is kept under pressure while working to allow the fluid to flow via the basket filter element in the designed directional flow.

In fact, the fluid flow shifts to the other chamber without filtration interruption once the set threshold is attained.

At the beginning, there will be a low pressure drop across the basket strainer.

Of course, at this point, the basket strainer will have virtually no or very little contaminants.

As the fluid flows through the duplex strainer, contaminants or parts begin to build up within the basket strainer.

This causes clogging and more pressure drop.

So, what does this imply?

It is time to switch to the next duplex strainer (the one which was on standby).

You need to monitor the basket strainer for high pressure drop so that you can clean it in time.

This will prevent possible back pressure or the basket strainer bursting (due to high inlet pressure).

The best part?

Modern basket strainers come with a clogging indicator.

From the clogging indicator, you will know when to switch to the next chamber and clean the clogged one.

Normally, the clogging indicator can be in the form of a differential pressure gauge.

It monitors changes in pressure within the duplex strainer chambers.

In fact, there are duplex strainers with alarm systems.

Once they get clogged the alarm goes off, hence a time to switch and clean the basket strainer.

But don’t confuse this with an increased pressure drop due to sudden drop in fluid flow rate.

The problem could be in the piping system.

So basically, fluid will flow to one chamber of the duplex strainer through the inlet.

Once this chamber gets clogged, an alarm may go off or you’ll see a high pressure drop.

Switch to the next chamber which was on standby.

At the same time, clean the clogged chamber.

Let the fluid flow until chamber two gets clogged then direct flow again to the chamber that was in standby.

You can see the working principle of duplex strainer is simple and straight forward.

Chapter 3: Duplex Strainer Structure

One of the main reasons for installing a duplex strainer is to protect other equipment and accessories downstream.

These may include compressors, pumps, valves and other sensitive components.

For that reason, you need a duplex strainer with a better design structure.

That’s exactly what I am going to discuss in this section.

Technical drawing of a duplex strainer

Technical drawing of a duplex strainer

Over years, there has been a tremendous development in the structure of duplex strainers to meet system or medium requirements.

This allows you to give the best in-pipeline protection in various industries such as chemical plants, petrochemical plants, process plant and refineries.

The following units form the main structure of a duplex strainer:

  • Body
  • Filter and strainers
  • Duplex Strainer Covers

FilSon Duplex strainer structure

FilSon duplex strainer structure

Now, let’s explore some of these essential structures:

· Duplex Strainer Body

The body is an important component of duplex strainer structure.

Since you are now more enlightened about what duplex strainers are, it would be wiser to understand its structure.

Most duplex strainers consist of dual filter chambers.

The chambers are fitted with permanent strainers and the duplex unit mounted either horizontal/vertically.

The new technology has made most manufacturers to rely on the novel-modular structural design concept that allows for use of fabricated body.

The design philosophy comes with a number of advantages:

  • Makes it possible to have a variety of body materials.
  • High pressure specifications easily achieved.
  • Such body materials require minimal maintenance routine.
  • Compact design makes the strainer take up minimal space during installation.
  • Other customized designs, the screening media can be accessed without necessarily taking off the cover.
  • Most designs guarantee a reliable performance on-line.
  • Provide great ease during installation in existing systems.

The table below is courtesy of Hydac indicates a summary of standard filter housing technical specifications:

No. of Screen Baskets/ Housing

Filter Area/ housing


Filter Element TypeFilter Materials and Filtration Ratings [µm]Permiss. Diff. pressure across element [bar]
Slotted TubePerforated Plate




















Each strainer body may consist of one basket element or a string of small basket elements installed carefully.

· Filter & Strainers

Before we proceed further, how about you get a glimpse of filter and strainer structural difference.

In essence, filter and strainer are semantically related words each standing for different meaning depending on the duplex filter design and specification.

Alternatively, a strainer could be referred to as a coarse filter.

The duplex filter design depends on:

  • Flow rate
  • Materials/ resistance
  • Operating temperature
  • Operating pressure
  • Particulate density in the medium
  • Required filtration rating
  • Type of contamination
  • Type of medium
  • Viscosity

The following graphs of pressure drop and strainer sizing curves are used to design strainers:

Basket Filter Design Curves courtesy of Hydac

Basket Filter Design Curves courtesy of Hydac

Strainer sizing chart

Courtesy of Hayward Filtration

But this alone is not enough.

You need to know the relationship between the solid particle dimension and required screen specification.

Below is a strainer’s mesh sizes table courtesy of Project Materials:



The standard mesh screen sizes ranging from 20 to 200 mesh.

And please have this in mind.

The mesh size does not necessarily indicate the particle retention ratings of the cleanable screen.

The number and diameter of inch/wire is what determines the particle size retention.

However, for critical applications, the cleanable screen should be chosen through a special particle retention ability.

And the table below shows a summary of filter elements technical specifications courtesy of Hydac as per the industry recommendations:


TypeConnection Size Din DNMaterialsPressure range*







Stainless SteelCarbon Steel
Without int. corrosion protectionWith int. corrosion protection









PN 16






-10 to 90



































For finer medium straining, the common practice is to reinforce the basket perforations with a wire-mesh liner.

This is because the screen alone would not suffice.

The real trapping and collection of these coarser impurities in any pressure system is done by the screen filter element.

They are conveniently placed inside the strainer bowl.

According to strainer industry, the screen openings is known as mesh, while the size of the opening through the basket filter media is called perforations.

The best and most common type of material is stainless steel.

Now, let’s take a look at the duplex strainer general structure.

Basically, a duplex strainer is designed and manufactured with dual-parallel basket strainers.

They ease and protect of the in-line of the filtration mechanism.

During cleaning and routine maintenance, the inline piping is bypassed.

· Duplex Strainer Covers

The duplex strainer cover forms an integral structure of this mechanical device.

It should be easy to open, allowing for cleaning and maintenance.

Types of Strainer Covers

Some of the main types of covers include:

  • Bolted Covers

In most strainers, the cover is usually bolted.

The bolts are not permanent as they are loosened to take over the cover to easily access the basket.

Duplex strainers with bolted covers

Duplex strainers with bolted covers

According to researches, bolt held covers are the strongest and highly recommended for high pressure systems.

  • Clamping Yoke Cover

Tee-shaped and threaded handles are used to lock together cover to the strainer’s body.

Generally, the cover gets attached to the vessel with a hinge-mechanism, which gives the ease during removal.

Duplex filter with a clamping york cover

Duplex strainer with a clamping yoke cover – Photo courtesy: Philips Pump LLC.

This type of duplex strainer is expensive as compared to the standard ones.

  • Automatic Type

Duplex strainers designed with automatic type need no manual cleaning as the name suggests.

Here, the basket is made with a circular screen and made to rotate, and water inlet is routed to the basket.

This type of duplex strainer is typically for larger water supply systems.

For instance, lakes and rivers water inlets used for process and cooling.

Now, we are going to look into various duplex strainer structures.

You can remember in chapter 1, we mentioned ball-valve and plug system.

There are somewhat four types of duplex strainer structures differentiated by the medium flow diverter mechanism and size.

Let’s begin by the medium flow changeover mechanism type:

Ball Type Duplex Strainer

A ball type duplex strainer effectively removes debris and dirt from medium flow as well as provide protection on other sensitive components.

It completely shuts off fluid for cleaning of the filter element.

The table provides a size selection table for ball type duplex strainer structure courtesy of Hydac:

SizeBody & Cartridge MaterialConnectionSeat/SealDiverter BallsRating Standard (Metric)
¾”, 1”, 1-1/4”, 1-1/2”, 2”, 2-1/2” 




TFE/ Buna N*



Stainless Steel

125# ANSI 200 PSIG @ 150F (PN16 13.8 bar @66C)
1”, 1-1/2”, 2”, 2-1/2”, 3”, 4”Flanged
¾”, 1”, 1-1/4”, 1-1/2”, 2”, 2-1/2”


Carbon Steel

Stainless Steel


150# ANSI

200 PSIG @ 150F (PN16 13.8 bar @66C)

1”, 1-1/2”, 2”, 2-1/2”, 3”, 4”Flanged

It is usually fitted with a unique ball-valve cartridge flow diverter that helps isolate the two basket chambers as well as to prevent medium overflow.

The system is activated using a handle to divert the system flow direction into the other free basket chamber.

Since the handle needs low operating torque, only human effort can easily turn the lever over.

Take a glimpse at a sketch of ball type duplex strainer technical details courtesy of Hayward Filtration:

Duplex Strainer Sample Technical Schematic

Duplex Strainer Sample Technical Schematic

Therefore, whenever the strainer basket requires to be cleaned, you should turn the lever-handle thereby setting the used chamber free.

The position of the lever-handle indicates the chamber in service.

Plug Type Duplex Strainer

This type of duplex strainer is economical, simple and trouble-free design fitted with pressure-rated plug valve to the straining chambers.

The image shows the interior section of a plug type duplex strainer structure courtesy of Hayward:Duplex Strainer Sections

Duplex Strainer Sections

Just like ball type duplex strainer, lever-handle is used to accomplish the medium change-over by turning it through a 900 arc manually by hand(s).

The table offers the duplex strainer selection chart courtesy of Hayward Filtration:

SizeBody MaterialPlug MaterialEnd ConnectionsSealsRating
5”, 6”, 8”IronIron or BronzeFlanged 125# 

Buna N

200psi@ 100F. 5” and 6” sizes.

150 psi @ 100F. 8” size.

BronzeFlanged 150#
6”, 8”Carbon SteelBronze or Stainless SteelFlanged 150#
6”, 8”Stainless SteelStainlessFlanged 150#Viton

This duplex strainer structural design permits no operation interference because a special plug port is infused in the diverter mechanism plug.

The entire medium flow diversion takes about 30 seconds.

The following is a sample sketch of technical details in this strainer courtesy of Hayward Filtration:

Duplex Strainer Technical Drawing Sample

Duplex Strainer Technical Drawing Sample

The integral stop automatically places the plug each time at the exact right place.

The duplex strainers come with an in-built lifting jack that is used to lift up the diverter plug off its port when operating the lever-handle.

After the diverting the medium flow, the same lifting jack is used to place back the plug.

The lifting jack works even better under extreme pressures.

How about we continue with duplex strainers according to sizes:

Large Duplex Strainers

As the duplex strainer size increases, the use of the ball or plug type diverter design becomes cumbersome and impractical altogether.

In such instances, sliding-gate valve arrangements are preferable to change the medium flow direction.

The table shows the various sizes of large duplex strainer, where you can choose from courtesy of Hayward Filtration:

SizeMaterialEnd ConnectionSealsPressure Rating
10” to 12”IronFlanged 125# 

Buna N

200 psi @ 100F
BronzeFlanged 150#
14” to 18”IronFlanged 125#150 psi @ 100F
BronzeFlanged 150#

The valves-discs are accurately and precisely actuated by a set of hand-wheels, and synchronized to work inversely.

That is, as one basket housing opens to the medium flow, the other basket chamber is completely sealed off.

Here is a technical sketch on a large duplex strainer:

Large Duplex Strainer Technical Drawing

 Large Duplex Strainer Technical Drawing

You may come across other huge duplex strainers that use synchronized butterfly-valves to change medium flow course.

Do you have any question on duplex strainer structure and design?

You can leave a quick comment below.

Meanwhile, let’s explore another important of duplex strainers.

Chapter 4: Duplex Filter Element Working Principle

In chapter 2, we learned about duplex strainer working principle, and here you are going to expound on duplex filter element working principle.

You will hardly notice the difference between the two; duplex strainer and duplex filtering cartridge. Although, they both serve the same purpose.

Filter element

Filter element

The duplex filter assembly is designed to ensure a clean filtered medium supply in an oil filtration system under pressure.

The duplex filter element should give a reliable oil purification mechanism for long periods before being changed.

An ideal duplex filter working principle should allow for overhaul of filter media while running, and the duo filter element in the assembly provides redundancy.

Also, the duplex filter element surface is critical in determining the differential pressure, hence, it is fitted with an electrical switch, which closes contacts to alert a lodged filter.

The duplex filter element consists of innumerable housing containing with a couple of 3 µm filters, check valve and a differential pressure switch-system.

For offshore applications, extremely compact design is recommended to promote smooth operation as well as a standby option in a single duplex unit.

At any point, a duplex strainer will use one filter chamber while the other is on standby.

The lever-handle is instrumental in the choice of filter and chamber to use.

The position of the lever-handle indicates the filter under operation.

Duplex filter technical data

Duplex filter technical data

Before you set duplex filter element in use, it should be filled and vented.

And please, note the flow direction of the as indicated on the filter element.

This will make sure the medium enters the duplex filter element from the top.

But, remember:

Cylindrical elements are highly recommended for filtering coarse solid particulates.

As the oil purification systems filter gallons, the filter media in operation traps and removes impurities from the medium stream.

It eventually clogs or get contaminated, the pressure differential shoots up between the duplex strainer input and output flow.

As the pressure differential increases through the filter system, a preset limit is reached and pressure switches turn on because the contact closes.

The dirty filtering cartridge can be removed, cleaned or changed.

It then remains on standby for use once the other gets clogged.

The installation of two duplex filter element chambers allow for uninterrupted operation even during maintenance of duplex filter assembly.

In order to achieve low pressure loss, the medium is taken through a rather straight flow path that experiences minimal diversion in the directional flow.

A high surface-area pleated-screen provides a more effective filtration area.

You can place it in the medium flow path.

Chapter 5: Steps for Cleaning Duplex Strainer

In this chapter, I am going to walk you through how to clean a duplex strainer.

It is an important maintenance process.

Normally, when the duplex strainer is in use, it will eventually get clogged with solids and particles.

And as I had mentioned earlier, this leads to an increase in pressure difference.

Besides, it will also reduce the efficiency of the duplex strainer.

Remember, during this time, dirt will also accumulate in the filtering zone.

The good news is – with duplex strainers, you don’t have to shut down the entire system when doing the cleaning.

So is what you should do:

        i. You should wear all the necessary safety gear such as overall, gloves, goggles, etc. They will protect you from possible injuries.

       ii. Check both the pressure and temperature of the duplex strainer system. Furthermore, you should confirm that medium in the duplex strainer is neither hazardous nor cause and form of risk.

      iii. First, identify the chamber to be cleaned. Of course, you can identify it using the position of the lever handle. Therefore, you have to turn the plug handle in the position of the other standby chamber.

Change over lever handle

Changeover lever handle

         iv. Make sure the handle completely turns into place, this will protect the operator from leakage if a tight shut-off is not immediately incorporated.

         v. Do not rush in but carefully undo the drain plug-in that chamber to release pressure before fully accessing the chamber.

Position of the drain plug

 Position of the drain plugs

    v. Take out the cover nuts to lift out the cleanable screen to purge out the sediment or dirt lodged on the perforation of the screen.

Nuts holding duplex strainer housing

 Nuts holding the duplex strainer housing

           vi. Use pressure water to thoroughly clean and flush the chamber, but remember to protect the basket chamber screen from intentional contamination during the cleaning. The bottom section is clean or filtered part of the duplex strainer.

         vii. Continue to replace the washer and drain plug. Also, check whether the O-ring is still in condition otherwise, replace it.

Cleaning strainer

Cleaning strainer

        viii. Now, take back the screen into the strainer’s chamber and make sure that its handle protrudes above the top of the strainer body.

          ix. Check whether the cover seats on the handle correctly before touching the strainer housing units. Place the cover and cover nuts back to tighten it properly.

           x. Restore the plug into its original place to avoid damage to the O-ring. You may now move the lever-handle slightly to allow for partial medium flow into the cleaned bowl to vent-off air and precisely re-seat the bleed screw.

         xi. Now you can take a complete turn of the handle to resume straining on the cleaned chamber.

        xii. Alternatively, if you use an equalizing valve, it is possible to run it after refitting the top cover solely for the purpose of venting that chamber. However, the valve should be closed once the operation done and the cleaned chamber is ready for operation.

       xiii. The process should be carried out for the other chamber when it is clogged.

A recommended safe differential pressure to start cleaning of the screen is 20 psi since the there is a rapid rate of increase in pressure drop when the screen gets heavily clogged.

Caution: Ensure you don’t surpass the highest differential pressure to prevent the screen from collapsing.

Chapter 6: Duplex Basket Filter Element Changeover Procedure

In the earlier chapters, you learned that duplex strainer can run continuously with no shutdown.

However, it does not mean that those basket filter elements are perfect as at times they may breakdown or need an overhaul.

Step 1: Monitor the Clogging Indicator for Possible Changeover

For a fact, the basket filter elements also have a service life, after which they get wear-out or need changeover.

Pressure gauge is usually fitted around the filter basket to check the optimal contamination capacity of the strainer.

Once the clogging indicator alarms, it is recommended to immediately carry out basket filter element changeover exercise.

Note that duplex strainer basket filter element without contaminant indicator are advisedly changed twice annually.

On the other hand, the ones with contamination indicator are done once a year.

Clogging indicator

Clogging indicator

If the strainer contaminant indicator has never shown need for basket filter element replacement, then the by-pass valve could be no longer functional.

As an operator, you must be vigilant to prevent use of worn out filter elements.

Step 2: Check Pressure and Change Flow

First, before trying to remove the basket filter element, you must release pressure within the strainer, and make sure that specific unit is cooled off.

This is a very important safety measure, you must adhere to.

The isolation valves on the strainer make it easier to cut-off pressure supply to strainer from the source.

It also prevents possible bodily injury and flood when taking out the strainer plug.

Balance Valve

Balance valve

In the process, the medium is admitted slowly into the other standby basket housing through equalizing valve.

However, before rushing to open up the strainer, make sure you check whether there is any leakage, but if none continue to the next step.

Unfortunately, if there is leakage, shut-off the equalizing valve, vent and drain the basket filter element chamber.

Remove the basket chamber cover, address the leakage before re-assembling the unit.

Step 3: Remove the Old Basket from the Housing then Replace with a New One

When the fluid drains to a level below the top of the basket filter element, close the drain valve, and go ahead and remove the basket.

Take some time holding the basket filter element right above the chamber to drip.

Remove old basket from the housing

Remove old basket from the housing

Now, you can carefully unpack the new basket filter element, remove the packaging papers and reinstall the basket at the same position.

With basket filter element made of stainless metal, must be completely replaced because it is used.

Also, check the strainer cover seal and if it is damaged replace it.

Step 4: Reassemble the Duplex Strainer Unit

Reassemble the unit and secure the basket chamber cover.

To confirm the strainer’s chamber does not leak, open the equalizing valve to let medium flow from the vent-valve.

Take your time and check if there is any leakage otherwise close the vent-valve.

Parts of Duplex Strainer

Parts of a duplex strainer

Note that leakage around the strainer housing is due to poorly tightened bolts or cover knobs, damaged or dirt cover seal, drain plug and vent plug.

Please make sure that the threads of the drain and vent plugs are clean and smooth during reinstallation.

In case, you choose to use thread sealer, it should be compatible with line medium.

The newly fitted basket filter element and cleaned housing are ready for medium changeover.

Chapter 7: Applications of a Duplex Strainers in Piping Systems

You can use duplex strainers in many industries.

Just as I had mentioned, it protects the components downstream.

These components may include relief valves, pressure-reducing valves and many others that must be clean.

It is one of the best ways to increase their service lifespan.

Furthermore, using duplex strainers help improve occupational health and safety at work as well as protect the environment.

They are useful in oil purification systems.

Some of the main applications of duplex strainers include:

1. Duplex Strainers in Power Generation

For the fossil power generation engines to work at optimum performance specification, their fuel-gas combustion chambers require clean and dry supply of gases.

Insufficiency of pre-filtration implies that aerosols and solids can ingress the combustion chambers unhindered.

This introduces abrasion and wear in the block, and costly repair and maintenance.

Hyro powergeneration

Hydro power generation plant

Therefore, fossil power generation must use duplex strainers not only to clean the fuel but also to protect the machines and clean water for cooling.

It is equally required to strain transformer oil to curb clogging of the system.

2. Petrochemical Industry

Turbo machines are some of complex invention often used in the petrochemical industry.

They are fitted with dry-gas seals that are extremely sensitive to fluid particulates such as aerosols, condensates and solid particles.

Section of trubo machinery

Section of turbo machinery

The shaft movement creates a tiny opening of approximately 3 µm on the seal.

This gap allows the seal gas to flow, and to protect the seal for longer service life, the seal gases must be strained appropriately.

3. Petroleum Industry

Duplex strainers are useful in petroleum pipelines as in power generation.

They help remove and clean unwanted foreign impurities in gasoline and crude oil.

More specifically, fuel oil sometimes may contain gums, tars or other dirt which muffle the nozzles of fuel burner.

Hence, duplex strainers are installed in every oil burner to eliminate such impurities.

Section of a pipeline

Section of a pipeline

Also, duplex strainers are similarly used in the same way in refineries as well as in oil pipelines to protect meters and pumps from undesirable materials.

4. Duplex Strainer in Marine and Offshore Drilling

In order for ship engines together with its subsystems to work properly, you need clean and dry gases and fuel.

Therefore, it is very important to use duplex strainers to clean water drawn from the ocean or sea to avoid abrasive wear on the system components.

Marine applications

Marine applications

In offshore oil and gas drilling, duplex strainers are recommended to increase throughput of the mining process.

5. Chemicals Manufacturing Industry

By far, the chemical industry is largest duplex strainer user for their pipeline.

In fact, chemical pipelines cannot run optimally without duplex strainers.

Chemical manufacturing plant

Chemical manufacturing plant

There are different areas where pipeline duplex strainers mean equipment protection, cleaner products and simpler separation of unwanted solid materials from liquids.

6. Water Treatment Plants

Just like in chemical industry, water pipelines are also other biggest beneficiaries of duplex strainers.

They are essential in removal of gravel, sand, debris from wells, lakes and stream.

In addition, they trap birds’ feathers, insects, leaves and many more to prevent ingress of cooling towers, which are open to the environment.

They strain out lint and hair from large swimming pools before filtering the water.

Duplex strainers also form an integral part of desalinization equipment where they eliminate unwanted solid matters or dirt from the inlet before the salt removal process.

Water treatment plant

Water treatment plant

Industrial parks sewer pipelines incorporate strainers to fish out solid materials that can build-up and lead to blockage of the waterway.

7. Pharmaceuticals and Cosmetics Industries

Most products of cosmetics and pharmaceuticals that contain undissolved matter or undispersed clumps flow into pipeline strainers.

This helps remove such undesirable substances that ruin their products.

8. Paper and Pulp

In order to present the market with smooth paper finishes, there is need to discuss the problem of unseen and undispersed clumps contained in coatings.

Industrial Paper Mill on a River
A large paper mill located on the Amelia River in Fernandina Beach, Florida.

Paper pulp industry

Hence, paper and pulp pipeline strainers trap and hold the unseen lumps as well as clean traces of paper or pulp from waste water effluent during discharge.

9. Process Plants

Most process systems usually cost a colossal amount of money thus, demand protection while running.

This helps avoid damages from by-products, dirt and scale and constantly averting costly downtime by installing pipeline duplex strainers preceding the line.

Duplex strainers are conveniently positioned at the intake of condensers, heat exchangers and pumps to avoid clogging of nozzles and flow meter.

10. Food Industry

Here, pipeline duplex strainers help remove skins, pulp or foreign matters from blended fruit juices. On the other hand, they are used to take out lumps from honey and chocolate syrup.

I can’t list all applications here.

However, whichever the case, you will use duplex strainers in pipelines where continuous operation is paramount.


As you can see from this guide, duplex strainers play an integral role in most industries.

And, I am sure, the information in this guide will help you to buy efficient and reliable duplex strainer.

You need to consider so many factors, from type of material, design and ability to filter water, oil or any other fluid.

Feel free to contact us in case you have any questions on duplex strainers.

We are here to serve you, that’s why we offer free consultations on duplex strainers and other filtration systems.

Further Sources

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