Strainers are mechanical devices to retain (and remove) foreign particles from flowing liquids and protect equipment such as pumps and compressors. There are several types of strainers, including the Y, basket and duplex types. A critical element is the strainer’s screen, specifically its mesh size (perforation density).
There are two basic designs for pipe strainers, i.e. “y-strainers” and “basket strainers”.
A Y-strainer has a y-shaped compact design and is used to capture foreign particles flowing through the pipes of a pipeline.
Y strainers can be installed either horizontally or vertically, with the filtering element positioned towards the ground, and generate a pressure drop in the pipeline.
This type of strainer for pipeline needs to be cleaned often, to make sure that the filtering ability of the device is not jeopardized by the accumulation of debris. The pipeline can be kept in service, generally, when y-strainers need a cleanup.
Y-strainers suit high-pressure gas pipelines with a low concentration of foreign particles and debris. Indeed, Y strainers have a lower capacity to store dirt than basket strainers of similar size.
A basket strainer features an upright and perforated basket that traps the foreign particles carried by the fluid conveyed by the pipeline.
A basket strainer can store bigger quantities of dirt and debris than y-strainers of the same size, due to the larger size of the filtering element, and is the preferred choice when pressure drops in the pipeline are not acceptable.
T strainers shall be installed upright, and the top of the strainer can be removed for frequent cleaning (self-cleaning t-strainers are also available on the market).
A special type of T-strainer is the so-called “duplex basket strainer”: this device, consisting of two parallel basket strainers, facilitates the maintenance of the filtration element keeping the line in service (the line is bypassed when one of the two strainers is cleaned).
Duplex basket strainer
(Source: Piping Design Channel)
A critical element to consider is the y-strainer mesh size. The mesh represents the number of the holes per inch. of the filtration element of the strainer (called, screen).
The screen mesh size can be designated in mm, in inches, in microns or by a mesh number.
The factors to consider to select the proper mesh size are:
If the mesh is too small, some unwanted debris may not be filtered and affect the downstream equipment. If the mesh is too small, extra loads of debris may accumulate in the strainer cage, causing pressure drops in the piping system and higher than needed maintenance costs.
The most common materials used for Y strainers and Basket strainers bodies are cast iron, bronze, carbon steel, stainless steel, and plastic. Essentially, Y-strainers have bodies made of the same cast and forged materials of other commercial valves.
Because of its low initial cost, cast iron is the most popular strainer body. It is used in systems where the pressure and temperature of the water are not high and the system is not subject to high thermal or mechanical shock.
Cast iron is mostly used for larger size potable water lines and many non-potable water systems in addition to a variety of other product and process uses.
A bronze body is preferred for brackish, saline, and seawater service. It is often used for potable water services. Its cost doubles that of cast iron.
Carbon steel bodies are used with high temperature and pressure applications and when a good resistance to thermal and mechanical stress is required. Carbon steel is used also in the case of applications with fire risk.
For high pressures and temperatures over 1000 °F, chrome-moly steel is usually specified for bodies.
Stainless steel is the preferred body, basket, and screen material for the pharmaceutical, food processing, and chemical industries because of its resistance to corrosion and contamination and ease of cleaning. Stainless steel costs about four times as much as cast iron.
The typical material grades for strainers’ bodies are cast steel (bore size above 2 inches) and forged steel (bore size below 2 inches).
The actual collection and retaining of the debris, dirt, and other particles in all pipe strainers is done by the basket or screen that is placed inside the body of the strainer.
The basket of a strainer (dirt filtering unit)
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