Duplex Filter (Duplex Strainer): Complete Technical Guide – Working Principle, Design, Sizing & Selection
A duplex filter (also called a duplex strainer or changeover strainer) is a dual-chamber inline filtration device that enables 100% continuous, uninterrupted flow during basket cleaning or replacement. Unlike simplex (single-basket) strainers that require pipeline shutdown for maintenance, duplex filters use a diverter valve to switch flow between two independent filter chambers — eliminating downtime entirely.
This guide provides the complete engineering reference: working principle, design standards, bill of materials, screen mesh specifications, dimension tables, pressure-temperature ratings, differential pressure calculation, and application selection criteria — based on actual manufacturer catalogue data and ASME/API standards.
- Working Principle — Step-by-Step
- Design Standards & Codes
- Bill of Materials (BOM)
- Screen Mesh & Micron Ratings
- Dimension Tables by Pressure Class
- Pressure Testing per API 598
- Pressure-Temperature Ratings (ASME B16.34)
- Differential Pressure (ΔP) Calculation
- Duplex vs Simplex — When to Choose Which
- Industry Applications
- Engineering Selection Checklist
- Frequently Asked Questions
1. Working Principle — Step-by-Step
The duplex filter consists of two identical basket strainer chambers (Chamber A and Chamber B) connected by a 3-way diverter valve (also called a changeover valve or transfer valve). Here is the complete operating sequence:
Normal Operation (Chamber A Active)
- Fluid enters through the common inlet port and is directed to Chamber A via the diverter valve
- Inside Chamber A, fluid passes from outside to inside through the perforated/mesh basket screen
- Contaminants larger than the screen mesh aperture are trapped on the outer surface of the basket
- Clean (filtered) fluid exits through the common outlet port to the downstream pipeline
- Chamber B remains pressurized on standby with a clean basket installed
Changeover Sequence (Switching to Chamber B)
- Differential pressure gauge across Chamber A reads above the set point (typically ΔP > 0.5–1.0 bar) — indicating the basket is clogged
- Operator rotates the diverter valve handle 180° — this simultaneously closes the flow path to Chamber A and opens the flow path to Chamber B
- Flow is now filtered through Chamber B's clean basket — zero flow interruption, zero pressure spike
- Chamber A is depressurized via the drain valve at its base
- The cover is removed, the clogged basket is lifted out, cleaned (or replaced), and reinstalled
- Chamber A cover is re-bolted, and the chamber is repressurized — ready for the next changeover
2. Design Standards & Codes
| Parameter | Standard / Code |
|---|---|
| Design / Manufacturing | ASME B16.34 / BS 1873 |
| Wall Thickness | ASME B16.34 (minimum wall) |
| End Connections (Flanged) | ASME B16.5 (150#, 300#, 600#, 900#) |
| End Connections (Butt-Weld) | ASME B16.25 |
| End Connections (Screwed) | BSP-F / NPT |
| Face-to-Face Dimensions | ASME B16.10 (where applicable) |
| Testing & Inspection | API 598 / BS EN 12266-1 |
| Flange Drilling (DIN/EN) | DIN EN 1092-1 (PN10, PN16, PN25, PN40) |
| Flange Drilling (BS) | BS 10 (Table D, E, F, H, J) |
| Screen Standard | ASTM E11 (woven wire), ASME PTC 19.3 (perforated) |
| Material Certificates | EN 10204 Type 3.1 MTC |
3. Bill of Materials (BOM)
The following table shows standard materials of construction. Special materials (Monel, Alloy 20, Hastelloy, Inconel, Duplex SS) available on request.
| Part No. | Part Name | Carbon Steel | SS 304 | SS 316 |
|---|---|---|---|---|
| 01 | Body (Housing) | ASTM A216 WCB | A351 CF8 | A351 CF8M |
| 02 | Cover (Lid) | ASTM A216 WCB | A351 CF8 | A351 CF8M |
| 03 | Screen (Basket) | SS 304 or SS 316 Perforated/Mesh (as per requirement) | ||
| 04 | Cover Bolts & Nuts | ASTM A193 Gr. B7 / ASTM A194 Gr. 2H (or SS equivalent) | ||
| 05 | Cover Gasket | SS 304/316 Spiral Wound with Graphoil Filler / PTFE | ||
| 06 | Drain Plug | MS / SS | SS 304 | SS 316 |
| 07 | Diverter Valve Body | A216 WCB | A351 CF8 | A351 CF8M |
| 08 | Diverter Valve Plug | AISI 410 / SS 304 / SS 316 with Stellite facing (optional) | ||
| 09 | Diverter Handle | MS / SS — with locking mechanism | ||
| 10 | ΔP Gauge Connections | ½" NPT tappings on inlet & outlet | ||
| 11 | Vent/Air Release | ½" NPT plug on each cover | ||
4. Screen Mesh & Micron Rating Specifications
The filter screen (basket) is the critical component. It is available in two configurations:
A. Perforated Plate Screens (Coarse Filtration)
| Hole Diameter (mm) | Approx. Micron | Open Area (%) | Typical Application |
|---|---|---|---|
| 0.5 | 500μm | 23% | Fine chemical filtration |
| 0.8 | 800μm | 30% | Instrument protection |
| 1.0 | 1000μm | 33% | Pump protection (clean fluids) |
| 1.5 | 1500μm | 40% | General pipeline protection |
| 2.0 | 2000μm | 44% | Cooling water systems |
| 3.0 | 3000μm | 51% | Coarse debris removal |
| 4.0 | 4000μm | 55% | Raw water intake |
| 6.0 | 6000μm | 58% | Large debris (leaves, stones) |
B. Woven Wire Mesh Screens (Fine Filtration)
| Mesh Count | Wire Dia. (mm) | Aperture (μm) | Open Area (%) | Typical Application |
|---|---|---|---|---|
| 20 mesh | 0.45 | 841 | 46% | General pipeline |
| 30 mesh | 0.35 | 595 | 43% | Pump suction |
| 40 mesh | 0.28 | 400 | 40% | Standard protection |
| 60 mesh | 0.19 | 250 | 36% | Valve/actuator protection |
| 80 mesh | 0.14 | 177 | 31% | Pump seal protection |
| 100 mesh | 0.11 | 149 | 30% | Instrument protection |
| 150 mesh | 0.066 | 105 | 37% | Fine chemical/pharma |
| 200 mesh | 0.053 | 74 | 33% | Ultra-fine filtration |
| 325 mesh | 0.036 | 44 | 30% | Catalyst protection |
5. Dimension Tables by Pressure Class
The following tables provide key dimensions for duplex strainer housings based on ASME B16.5 flange connections. All dimensions in mm.
Class 150 Duplex Strainer Dimensions
| Size (Inch) | DN (mm) | Port (mm) | Flange OD (mm) | PCD (mm) | No. of Bolts | Bolt Hole (mm) | Approx. Length (mm) |
|---|---|---|---|---|---|---|---|
| 2" | 50 | 50.8 | 152.4 | 120.6 | 4 | 19.1 | 450 |
| 3" | 80 | 76.2 | 190.5 | 152.4 | 4 | 19.1 | 550 |
| 4" | 100 | 101.6 | 229.0 | 190.5 | 8 | 19.1 | 650 |
| 6" | 150 | 152.4 | 279.0 | 241.0 | 8 | 22.2 | 850 |
| 8" | 200 | 203.2 | 343.0 | 298.4 | 8 | 22.2 | 1050 |
| 10" | 250 | 254.0 | 406.0 | 362.0 | 12 | 25.4 | 1250 |
| 12" | 300 | 304.8 | 483.0 | 431.8 | 12 | 25.4 | 1450 |
| 14" | 350 | 336.6 | 533.0 | 476.2 | 12 | 28.6 | 1650 |
| 16" | 400 | 387.4 | 597.0 | 539.8 | 16 | 28.6 | 1850 |
| 18" | 450 | 438.2 | 635.0 | 577.8 | 16 | 31.8 | 2050 |
| 20" | 500 | 489.0 | 693.0 | 635.0 | 20 | 31.8 | 2250 |
| 24" | 600 | 590.6 | 813.0 | 749.3 | 20 | 34.9 | 2650 |
Class 300 Duplex Strainer Dimensions
| Size (Inch) | DN (mm) | Flange OD (mm) | PCD (mm) | No. of Bolts | Bolt Hole (mm) |
|---|---|---|---|---|---|
| 2" | 50 | 165.0 | 127.0 | 8 | 19.1 |
| 3" | 80 | 210.0 | 168.1 | 8 | 22.2 |
| 4" | 100 | 254.0 | 200.2 | 8 | 22.2 |
| 6" | 150 | 318.0 | 269.7 | 12 | 22.2 |
| 8" | 200 | 381.0 | 330.2 | 12 | 25.4 |
| 10" | 250 | 444.0 | 387.4 | 16 | 28.6 |
| 12" | 300 | 521.0 | 450.8 | 16 | 31.8 |
6. Pressure Testing per API 598
| Class | Body (Hydro) kg/cm² | Body (Hydro) PSI | Seat Test kg/cm² | Seat Test PSI | Air Test kg/cm² |
|---|---|---|---|---|---|
| 150# | 30 | 427 | 22 | 313 | 7 |
| 300# | 78 | 1,109 | 57 | 811 | 7 |
| 600# | 156 | 2,219 | 115 | 1,636 | 7 |
| 800# (Std) | 210 | 2,987 | 154 | 2,190 | 7 |
| 900# | 234 | 3,328 | 172 | 2,446 | 7 |
PN-rated equivalents: PN-10: Body 15, Seat 11 kg/cm². PN-16: Body 24, Seat 17.6 kg/cm². PN-25: Body 37.5, Seat 27.5 kg/cm². PN-40: Body 60, Seat 44 kg/cm².
7. Pressure-Temperature Ratings (WCB Material per ASME B16.34)
Maximum allowable working pressure (kg/cm²) decreases with temperature:
| Class | −29 to 38°C | 100°C | 200°C | 300°C | 350°C | 425°C |
|---|---|---|---|---|---|---|
| 150# | 20.0 | 17.6 | 14.0 | 10.4 | 8.5 | 5.6 |
| 300# | 52.1 | 47.5 | 44.6 | 40.6 | 38.3 | 23.2 |
| 600# | 104.1 | 95.1 | 89.3 | 81.1 | 76.6 | 58.6 |
| 900# | 138.8 | 126.8 | 119.1 | 108.3 | 102.1 | 78.2 |
8. Differential Pressure (ΔP) Calculation & Monitoring
Monitoring the pressure drop across the duplex filter is essential for determining when to switch chambers. The ΔP increases as the basket collects contaminants.
ΔP Operating Thresholds
| Condition | ΔP (bar) | ΔP (PSI) | Action |
|---|---|---|---|
| Clean Basket | 0.02 – 0.10 | 0.3 – 1.5 | Normal operation |
| Warning Level | 0.3 – 0.5 | 4.5 – 7.5 | Plan changeover, alert operator |
| Changeover Required | 0.5 – 1.0 | 7.5 – 15 | Switch to standby chamber immediately |
| Emergency — Bypass Risk | > 1.5 | > 22 | Basket may collapse. Switch immediately. |
Approximate ΔP Formula (Clean Basket)
ΔP = K × (ρ × V²) / 2
Where: ΔP = pressure drop (Pa), K = resistance coefficient (typically 2.0–5.0 for basket strainers), ρ = fluid density (kg/m³), V = velocity through the screen open area (m/s). For quick estimation: design for V < 1.5 m/s through the net open area of the screen for liquids, < 15 m/s for gases.
9. Duplex vs Simplex — When to Choose Which
| Feature | Duplex Filter | Simplex Filter |
|---|---|---|
| Downtime for Cleaning | Zero | Pipeline shutdown required |
| Continuous Flow | Yes — 100% | No — intermittent stoppage |
| Cost | 1.8× – 2.5× simplex cost | Lower |
| Footprint | Larger (2 chambers + valve) | Compact |
| Weight | Heavier | Lighter |
| Best For | Continuous processes, offshore, power plants | Batch processes, clean fluids, low-criticality |
| Cleaning Frequency | High contaminant loading OK | Low contaminant loading preferred |
| Operator Requirement | Changeover during operation | Shutdown, depressurize, clean, restart |
10. Industry Applications
| Industry | Application | Typical Mesh | Body Material |
|---|---|---|---|
| Oil & Gas | Crude oil, produced water, gas pipeline | 40–80 mesh | WCB / LCB |
| Petrochemical | Feedstock filtration, catalyst protection | 100–200 mesh | CF8M (316) |
| Power Generation | Cooling water, lube oil, fuel oil | 40–100 mesh | WCB / CF8 |
| Marine / Offshore | Seawater intake, ballast water | 20–60 mesh | CF8M / NAB / Monel |
| Water Treatment | Raw water, recycled water, WFI | 80–200 mesh | CF8M (316L) |
| Food & Beverage | Juice, milk, edible oil, syrup | 100–200 mesh | CF8M (316L, 3-A cert.) |
| Pharmaceutical | Process water, WFI, API filtration | 200–325 mesh | CF8M (316L, Ra < 0.8μm) |
| Chemical Processing | Acid, alkali, solvent filtration | 40–150 mesh | CF8M / Alloy 20 / Hast. |
| Hydraulic Systems | Hydraulic oil cleanliness (NAS/ISO) | 100–200 mesh | WCB / CF8 |
| Pulp & Paper | White water, black liquor | 20–60 mesh | CF8M / Duplex SS |
11. Engineering Selection Checklist
Use this checklist when specifying a duplex filter for your project:
| # | Parameter | Your Requirement |
|---|---|---|
| 1 | Pipeline Size (NPS) | 2" to 24" (DN50 to DN600) |
| 2 | Pressure Class | 150#, 300#, 600#, 900# / PN10–PN40 |
| 3 | Design Temperature | Min ___°C to Max ___°C |
| 4 | Fluid / Media | ___ (specify density, viscosity) |
| 5 | Flow Rate | ___ m³/hr or GPM |
| 6 | Body Material | WCB / CF8 / CF8M / Special |
| 7 | Screen Type | Perforated (___mm) / Mesh (___mesh) |
| 8 | Screen Material | SS 304 / SS 316 / Monel / Special |
| 9 | End Connection | Flanged / Butt-Weld / Screwed |
| 10 | Max Allowable ΔP | ___ bar (clean basket) |
| 11 | ΔP Instrumentation | Gauge / Transmitter / Switch |
| 12 | Certifications | EN 10204 3.1 / PED / NACE / 3-A |
12. Frequently Asked Questions
What is a duplex filter?
A dual-chamber filtration device with a diverter valve. One chamber filters while the other is on standby or being cleaned. Ensures 100% continuous flow — zero downtime for basket maintenance. Bodies in WCB, CF8, CF8M per ASME B16.34. Tested per API 598.
Duplex filter vs simplex filter — what is the difference?
Simplex = one chamber = pipeline shutdown for cleaning. Duplex = two chambers + diverter valve = zero downtime. Choose duplex when continuous flow is mandatory, contaminant loading is high, or the process cannot tolerate any interruption.
What mesh size should I choose?
40 mesh (400μm) for general pipeline protection. 80 mesh (177μm) for pump protection. 100 mesh (149μm) for instruments. 200 mesh (74μm) for fine chemical/pharma. Match mesh to the smallest particle that could damage downstream equipment.
How do I know when to switch chambers?
Install a differential pressure gauge or transmitter across the filter. When ΔP exceeds 0.5–1.0 bar (depending on your design), rotate the diverter valve to switch to the standby chamber. Some installations use automated ΔP switches that trigger alarms or auto-switchover actuators.
What pressure classes are available?
ASME: Class 150 (20 bar), 300 (52 bar), 600 (104 bar), 900 (138 bar). DIN: PN10, PN16, PN25, PN40. BS: Table D, E, F, H. Test pressures per API 598 — e.g., Class 150 body test = 30 kg/cm², seat test = 22 kg/cm².
Can duplex filters be automated?
Yes. The diverter valve can be fitted with a pneumatic or electric actuator. Combined with a ΔP transmitter and PLC, the system can auto-switch chambers when the basket becomes clogged — fully automatic, no operator intervention required. This is common in offshore platforms and unmanned installations.
Conclusion
The duplex filter is an indispensable piece of pipeline equipment wherever continuous, uninterrupted flow is non-negotiable. From crude oil pipelines to pharmaceutical clean rooms, from offshore seawater intakes to food processing lines — the dual-chamber design with diverter valve ensures zero downtime for filtration maintenance.
When specifying a duplex filter, the critical engineering parameters are: pressure class (per ASME B16.34 P-T ratings), body material (matched to fluid corrosivity and temperature), screen mesh/micron rating (matched to the smallest damaging particle size), and ΔP monitoring instrumentation (to trigger timely changeover). Get these four right, and your duplex filter will deliver years of reliable, maintenance-friendly service.
Need Duplex Strainers & Filters?
Supreme Valves India manufactures duplex strainers in Class 150 to 900. WCB, SS 304, SS 316, and exotic alloys. Sizes 2" to 24". EN 10204 3.1 MTCs. Third-party inspection available.
Also available: Y-Type Strainers, Basket Strainers, Tee-Type Strainers, Bucket Strainers