Strainers and filters: simplex, duplex, Y, basket

Y-strainers take their name from the Y-shaped body, with a cylindrical strainer basket mounted at an angle to the main flow path. They are compact, lightweight and inexpensive, making them the default choice for protecting individual items of equipment: a pump suction, a control valve, a flow meter or a pressure regulator.

The strainer element is removed by unscrewing a plug at the end of the strainer pocket. Cleaning requires taking the pipeline section out of service, which makes Y-strainers unsuitable for continuous processes where any shutdown is unacceptable. For these applications, a duplex arrangement is required.

Y-strainers are available in screwed and flanged ends from DN15 upwards. Most are rated to PN40 or Class 300, and the body material follows the pipeline specification: bronze for water and low-pressure service, ductile iron for larger water lines, and carbon or stainless steel for process and high-pressure applications.

Basket strainers house a cylindrical or conical basket perpendicular to the flow, rather than at an angle. The larger basket volume gives substantially greater dirt-holding capacity between cleanings, making basket strainers the preferred choice for larger pipelines, slurry-prone services, and anywhere the Y-strainer would need cleaning too frequently.

Simplex basket strainers have a single basket and require a pipeline isolation to clean, the same as a Y-strainer. They are specified where high dirt-holding capacity is needed but continuous service is not a requirement: water treatment intake lines, cooling water circuits and filter protection duties.

Access is through a bolted or quick-release cover on top of the body. Large basket strainers (DN200 and above) may be fitted with a differential pressure gauge or transmitter across the strainer to indicate when the basket is approaching its loading limit and cleaning is required.

A duplex strainer contains two parallel basket chambers with a changeover valve or diverter that routes flow through either basket independently. When one basket needs cleaning, flow is diverted to the second basket, the first basket is isolated, and it is cleaned without any interruption to the pipeline. This makes duplex strainers essential for continuous processes, critical pump protection and any service where a strainer cleaning shutdown is not acceptable.

The changeover mechanism is typically a ball or plug valve integrated into the body. Some designs require manual changeover; others are fitted with automatic differential-pressure-triggered diversion for unattended operation.

Duplex strainers are larger and more expensive than simplex equivalents, but the cost is easily justified where the alternative is a planned shutdown each time the strainer needs cleaning. They are standard in offshore processing, chemical plants, power generation cooling water systems and LNG applications.

The strainer mesh is specified by aperture size, measured in millimetres or mesh count (holes per inch). Coarser meshes (3 to 6 mm aperture) are used for protecting pumps and general pipeline duty where fine filtration is not required. Finer meshes (0.5 to 1 mm) are used for protecting control valves and flow meters. Very fine strainers (below 0.3 mm) bridge into filter territory and carry a significant pressure drop penalty.

Strainer sizing is based on the pipeline bore and the acceptable pressure drop at maximum flow rate. An undersized strainer element will have an excessive clean pressure drop and will load rapidly. The general rule is to select a strainer body one or two pipe sizes larger than the pipeline, or to size the free-flow area of the element at a minimum of three to five times the pipe cross-sectional area.

Always consider the clean pressure drop alongside the loaded drop. A strainer that is just acceptable when clean may impose an unacceptable pressure loss when partially loaded. Specify the maximum acceptable differential pressure at the design flow and verify the strainer meets it when the basket is at its working load limit.

Y-strainers should be installed with the strainer pocket pointing downward or horizontally, never upward, so collected debris falls away from the screen rather than back into the flow when the pocket is opened. Basket strainers are mounted with the basket accessible from above.

In horizontal pipework, both types should be installed with the drain plug or basket cover accessible for cleaning without special equipment. Allow sufficient clearance above a basket strainer to withdraw the basket vertically; check this against your installation drawings before ordering.

Install a differential pressure indicator across any strainer that will be in continuous service. A bypass with isolation valves on either side of the strainer allows in-situ cleaning without a full pipeline shutdown for simplex designs, at the cost of the bypass path remaining unprotected during cleaning.