Strainers are a type of pipe fitting where liquid passes through for removal of solids. Its purpose is to protect critical equipment such as pumps, heat exchangers, valves, regulators, flow meters, steam traps and other piping items.
Strainers can also be used to ensure clean raw materials, prevent contamination of products or even removal of solid finished products from the process.
In this article we discuss the following topics:
Types of Strainers
Y-type strainers are cost effective permanent strainers that can be installed along a pipe without any support. They can be installed horizontally or vertically. They are commonly used in pressurised lines for steam, liquids or gas.
Basket Type strainers have an advantage over Y-type as it has a higher dirt capacity size for size. Also, the pressure drop is lower. Basket type are used in all industries, mostly for liquid applications.
Duplex type strainers are basically 2 basket strainers installed in parallel, with a valve in between them to direct the flow between each strainer. Its purpose is that each filter element can be removed from the basket strainers without interrupting the process.
Common applications are where the process does allow for any downtime to clean the filter.
An automatic strainer is designed to clean the filter element automatically without having to even remove the filter element. A motor is mounted onto the strainer to continuously remove the suspended solids. They can be used where heavy debris is expected, such as river or seawater intake for fire protection or cooling purposes.
A conical strainer is designed to be clamped between two pipe flanges. It is fast and cheap to manufacture. However, the disadvantage is that the section of pipe must be dismantled in order to maintain the strainer.
For this reason, they are usually used for commissioning or temporary purposes, and sometimes known as ‘temporary strainers’.
Strainers Filter Element
The filter element is made up of fine wire formed into a grid or mesh and combined with a perforated screen for better durability.
Selection & Sizing of Strainers
Pressure drop is the difference in pressure between 2 points. Resistance to flow, such as pipe elbows, reducers, valves, strainers etc. causes pressure drop. The process medium itself may also contribute to pressure drop, such as a high viscosity medium or high flow.
Pressure drop is an important consideration when sizing of the pump.
Over time, the filter element will get clogged with debris and this will lead to higher pressure drop. The filter element has to be cleaned periodically.
The open area will affect the frequency of maintenance of the strainer. A strainer with a higher open area, such as a basket strainer, will need to be cleaned less frequently than a y-strainer.
Usually, the open area is compared to the pipe size. For example, the user may demand an open area that is 3X of the pipe size.
When specifying the strainer, the first thing that comes to mind is the type of strainer required. Both the pressure drop and open area are important considerations. Next, the amount of solids expected must be taken into account.
For applications where the process cannot be stopped to remove the filter element, consider using a duplex or automatic strainer.
Material of Construction
Cast Iron – Cheap and fairly resistatant to corrosion.
Cast Steel – Resistant to corrosion and able to handle high pressures
Stainless Steel – Very resistant to corrosion. Used in hygienic applications or extremely clean raw materials / products is a must.
Other materials such as Alloy 20, GRP/FRP, Bronze, Monel, Hastelloy and Super Duplex Stainless.are available.