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
Other piping items
Strainers can also be used to ensure clean raw materials. It prevents the 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. Both horizontal or vertical installation are possible. They are commonly used in pressurised lines for steam, liquids or gas.
The advantage of basket type versus Y-type is its higher dirt capacity, size for size. Also, the pressure drop is lower. In most liquid applications, basket type is preferred.
Duplex Type
Duplex type strainers are basically 2 basket strainers installed in parallel. A valve is put between them to direct the flow between each strainer. This is so that we can remove the filter element from the basket strainers without interrupting the process.
Common applications are where the process does allow for any downtime to clean the filter.
Automatic Strainer
An automatic strainer can clean the filter element automatically without having to remove the filter element. A motor is mounted onto the strainer to continuously remove the suspended solids. We recommending using them 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 we must dismantle the section of pipe in order to clean and maintain the strainer.
For this reason, they are usually used for commissioning or temporary purposes, and sometimes known as ‘temporary strainers’.
Strainer Filter Element
The filter element is made up of fine wire formed into a grid or mesh. Usually, it is combined with a perforated screen for better durability. Different mesh sizes are possible. The smaller the mesh size, the smaller the particles it can filter. However, there is a trade off. Debris will build up more quickly for smaller mesh sizes and more maintenance is required.
Selection & Sizing of Strainers
Pressure Drop
Pressure drop is the difference in pressure between 2 points. Resistance to flow, such as pipe elbows, reducers, valves, etc. causes pressure drop. A high viscosity medium or high flow may also contribute to pressure drop.
Pressure drop is an important consideration when sizing the pump.
Open Area of strainers
Over time, the filter element will get clogged with debris. This will lead to higher pressure drop. The filter element has to be cleaned periodically to unclog the filter and prevent the filter from breakage.
The open area will affect how frequently the strainer must undergo maintenance. A strainer with a higher open area, such as a basket strainer, will need less frequent cleaning 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.
Types of strainers
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, we must consider the amount of solids expected.
For applications where the process cannot stop, 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.