Differences Between Simplex and Duplex Strainers

Comparison of Simplex and Duplex Basket Strainers

Basket strainers are closed pressure vessels with cleanable screen elements that remove particulate from process liquids to protect downstream valves, pumps, and equipment. They are designed for horizontal installation and quick access to the basket. The two most common styles are simplex (single basket) and duplex (dual basket).

Simplex Strainers

In a simplex design the basket can only be removed when the process is shut down. They are used for batch applications or processes that can tolerate brief interruptions for cleaning. Shut-down time is usually short thanks to quick-opening covers.

 

After isolating the strainer, a vent valve relieves chamber pressure and a drain valve or plug empties the chamber. The dirty basket is removed, cleaned (or replaced with a spare), and reinstalled so the process can resume. Keeping a spare clean basket on hand minimizes downtime.

 

Simplex strainers are available in both cast and custom fabricated designs to match pipeline pressure class and material.

Duplex Style Strainers

Duplex strainers are designed for continuous-duty applications that cannot tolerate any interruption in flow. Essentially they are two simplex strainers connected by a three-way valve (or set of valves) that diverts flow from the dirty chamber to the clean chamber.

 

Cast designs usually use a single three-way valve operated by one lever. As one side closes the other opens, keeping flow continuous.

 

Custom fabricated designs often use four butterfly valves. These may be linked to a single operator or operated individually. The correct sequence is important:

1. Open the inlet valve to the clean chamber
2. Open the outlet valve from the clean chamber
3. Close the outlet valve from the dirty chamber
4. Close the inlet valve to the dirty chamber

5. Once the dirty chamber is isolated, basket change-out is the same as with a simplex strainer.

 

Cast designs are preferred whenever they meet the pressure, temperature, and material requirements because they cost less and have shorter lead times than custom fabricated units.

 

Sizing Guidelines

Four key steps when sizing a strainer:

1. Confirm pipeline velocity and clean differential pressure fall within the strainer’s standard design range.
2. Ensure material compatibility and that the strainer meets design pressure and temperature.
3. Evaluate the quantity, type, and nature of the debris to be removed.
4. Select the proper perforation or mesh. An overly fine screen raises clean differential pressure and shortens cleaning intervals.

 

Open Area Ratio (OAR) and Cv

Basket strainers typically have an open area ratio of 6:1 and sometimes as high as 8:1 (net free area of the screen relative to pipe area). As the basket loads, the effective open area decreases, flow drops, and differential pressure rises.

 

Most elements start as perforated sheet; finer retentions add a layer of wire mesh welded to the interior. Finer mesh reduces open area because more structural material is required.

 

Baskets are generally designed to withstand up to 20 PSI differential. A properly sized strainer should start at ≤ 2 PSI clean differential and be cleaned when differential rises by about 5 PSI. Beyond that point pressure rises rapidly and risk of basket damage increases.

 

Summary

Choose a simplex strainer when the process can be stopped briefly for cleaning. Choose a duplex strainer when continuous, uninterrupted flow is required.