Duplex strainer · Switchable strainer
Two strainer chambers and a changeover unit in one housing: the strainer elements can be cleaned while production continues. Duplex strainer and switchable strainer are two names for the same design. Both always refer to the strainer that is switched over for a screen change instead of being shut down. Internationally it is also called a duplex filter or duplex basket strainer, and in plant engineering a changeover strainer.
Every duplex strainer is built to your process data. We design nominal size, pressure rating, material, changeover variant and mesh size together with you – supplemented by the equipment that suits the installation point.
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Operating principle
In operation, the medium flows through one of the two chambers. When the differential pressure rises, the flow is switched over to the second chamber. The first chamber is then free: the strainer element can be removed, cleaned and put back. The plant keeps running in the meantime.
Changeover unit
The changeover unit determines installation space, operating force and price. We build all three variants on the same housing – and all three can be automated.
The robust variant for larger nominal sizes. Two valves per line, operated by hand via levers, with angle support leg and fixing holes for bolting down.
Four individual ball valves, coupled by a linkage: tight shut-off, low maintenance and easily accessible. Preferred in stainless steel.
Two three-way bodies handle inflow and outflow, one lever switches both. This is the most compact design and the first choice where installation space is tight.
Automation
Whether butterfly valves or ball valves – the changeover can be carried out with pneumatic or electric actuators. Switching is then no longer done on site but from the control system.
Strainer element
As standard, the duplex strainer works with a support basket in 1.4301 and a fine mesh screen in 1.4401 – standard perforation Qg 10-12, one element per chamber. The mesh size ranges from 0.05 to 10 mm, standard 1 mm.
Fine mesh, filter bag or both. The housing stays the same – only the element is changed. A filter bag can be fitted in the same support basket instead of the fine mesh, turning the duplex strainer into a switchable bag filter. Both together are possible as well: the bag does the fine filtration, the screen keeps coarse particles away from it.
The bag filters finer than any wire mesh: polypropylene felt from 1 to 300 µm, polyester felt from 1 to 200 µm, nylon on request – in bag sizes 1 to 4. Where the fine mesh ends at 50 µm, the bag goes down to 1 µm. In return it is a replacement part, not a permanent component, and the pressure rating in this design is up to 10 bar.
The right filtration grade is not set by the medium but by the component downstream. How it is selected is explained under Selecting the mesh size for the duplex strainer →.
Designs
About half of all our strainers leave the works with additional equipment. On the duplex strainer, the two most common questions concern the same task: when to change, and how quickly the cover can be opened.
On a duplex strainer every move counts, because the element is changed more often than on a single strainer. Eye bolts with wing or ring nuts, hinged eye bolts or clamps replace the set of spanners – the basket can be reached without tools.
On a duplex strainer it is not an accessory but the change signal: only the differential pressure shows when a changeover is due. Options range from the gauge and the switch contact to a pressure transmitter connected to the control system.
Design parameters
Values for the standard design. The final design is based on your process data – further nominal sizes and pressure ratings are available on request.
Dimensions
Dimensions of the standard design with a changeover unit of four butterfly valves: H and H1 are the overall heights, L the face-to-face length, ∅D2 the housing pipe. The weight applies to the empty carbon steel design. A dimensioned drawing for your nominal size is included with the design proposal.
| Nominal size | H [mm] | H1 [mm] | L [mm] | ∅D2 [mm] | Screen area [cm²] | Volume [l] | Weight [kg] |
|---|---|---|---|---|---|---|---|
| DN50 | 765 | 700 | 650 | 114.3 | 2 × 415 | 12 | 95 |
| DN65 | 865 | 780 | 700 | 139.7 | 2 × 575 | 21 | 110 |
| DN80 | 915 | 830 | 800 | 168.3 | 2 × 695 | 32 | 155 |
| DN100 | 1,050 | 965 | 920 | 219.1 | 2 × 1,150 | 65 | 220 |
| DN125 | 1,210 | 1,120 | 1,055 | 273 | 2 × 1,750 | 115 | 340 |
| DN150 | 1,260 | 1,175 | 1,100 | 273 | 2 × 1,750 | 135 | 350 |
| DN200 | 1,540 | 1,430 | 1,350 | 355.6 | 2 × 2,770 | 270 | 670 |
| DN250 | 1,695 | 1,580 | 1,500 | 406.4 | 2 × 3,450 | 415 | 850 |
The screen area is given per chamber – the duplex strainer has two. The table shows the common sizes; we also build DN300, designed to your connection dimensions. Dimension sheets and STEP models are available on request.
Special sizes
The table above shows the standard. Anything beyond it is engineered as a custom design: larger nominal sizes, different face-to-face lengths, a geometry that follows the available installation space rather than the standard.
The photo shows a duplex strainer DN300 in carbon steel, painted RAL 5010 blue, with manually operated shut-off valves. This is the upper end of the data sheet range. At this size, weight and accessibility help shape the design. It then has to be clarified where the load goes and how the cover is opened. Removing the strainer basket by crane is also part of this.
Applications
Wherever an interruption costs more than the second housing. Likewise wherever a lot of dirt accumulates and frequent cleaning is therefore needed.
Manufacturing
Cutting, welding, surface finishing and testing are done in our own workshop. A different face-to-face length, a special material or an additional socket is therefore not a surcharge on a catalogue part but part of the design.
Enquiry
Tell us the medium, flow rate, operating pressure and operating temperature. From this we design nominal size, screen area and material. Only nominal size and pressure rating are mandatory – if you do not know them yet, select “please advise” or “not sure”. Add further strainers with “+ further item”.
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Yes. Both names refer to the same design: two strainer chambers in one housing, switched via a changeover unit. Duplex strainer is the name used in our catalogue; switchable strainer describes the function and is at least as common in operation – both mean the same unit.
In German the unit is called Doppelfilter or Umschaltfilter; another common English term is duplex filter. Not to be confused with the material duplex stainless steel, a special stainless steel that we also process on request.
As soon as downtime costs more than the second housing. With a simplex strainer, the line has to be depressurised for a screen change; with a duplex strainer, the flow is switched over and operation continues. With a high dirt load, cleaning is also more frequent – then every shutdown adds up.
Where installation space is tight, the 3-way ball valves, because one valve handles both inflow and outflow. For larger nominal sizes, the four butterfly valves. If switching is to be done from the control system, the pneumatic actuator with position indicator is added.
As standard, DN50 to DN300. Up to DN150, PN6 to PN40 is possible; above DN150, PN6 to PN16. Larger nominal sizes and higher pressure ratings are engineered on request.
By the differential pressure. It is the only reliable indicator of the degree of fouling – the service life cannot be planned by the calendar. The options, from the gauge and the switch contact to the pressure transmitter, are explained under Differential pressure on the strainer.
Yes, also for housings from other manufacturers. The quickest way is with the dimensions of the old part: outside diameter, height, handle or bottom design and the mesh size. A photo also helps.
All basic types compared: Industrial strainers
Replacement strainer baskets and fine mesh screens for duplex strainers are made to size or to sample – for our own ranges as well as for other manufacturers’ strainers. To the spare part →
Tell us the medium, flow rate, pressure and temperature as well as the component to be protected. You will receive a design proposal with nominal size, changeover variant, mesh size and material.
