A General Guide to In-line Strainers

1.0 Purpose and Scope

Strainers are devices used in piping systems to mechanically remove and retain solid foreign particles like dirt, weld spatter, or pipe scale from a fluid stream. They are essential for protecting downstream equipment and instruments.

This guide provides general information for selecting and specifying common strainers. It does not cover specialized filtration units like de-misters or candle filters.

2.0 Strainer vs. Filter: What's the Difference?

Functionally, strainers and filters do the same job. The primary difference is the size of the particle they remove. A common rule of thumb is:

• Strainer: Removes particles visible to the naked eye (coarser than ~200 mesh).
• Filter: Removes particles that are *not* visible (finer than ~200 mesh).

In essence, a strainer is a coarse filter.

3.0 Types of Strainers

Strainers are broadly divided into two main categories: Temporary and Permanent.

• Temporary Strainers
  • Flat Disc
  • Conical
  • T-Type (or Bath Tub)
• Permanent Strainers
  • Y-Type
  • Basket Type (Simplex and Duplex)
  • Cartridge Type
  • Self-Cleaning (Automatic)

3.1 Temporary Strainers

These are installed for short-term use, typically during the commissioning and initial startup of a new or reconditioned piping system. Their goal is to catch coarse debris (rags, welding rods, rust, etc.) that may have been left behind during construction. They are usually removed, cleaned, and stored after the system is confirmed to be clean.

Flat Disc Strainer

A simple, flat perforated plate installed between two flanges. It has a low flow area and is generally used for very coarse straining (e.g., up to 2 mesh).

Conical Strainer ("Witch's Hat")

A cone-shaped perforated plate, often lined with wire mesh. The conical shape provides a much larger flow area than a flat disc. A 20-mesh cone is a common standard for temporary startup strainers.

T-Type Strainer ("Bath Tub")

A trough-shaped strainer element inserted into a "Tee" fitting in the pipeline. These are an alternative to conical strainers, especially where space is limited, as they don't require a special spool piece to be removed.

3.2 Permanent Strainers

These are designed to remain in the pipeline for the life of the plant, protecting equipment during normal operation.

Y-Type Strainer

These have a compact, Y-shaped body containing a cylindrical strainer element. They are strong and can handle high pressures.

• Installation: Can be installed in horizontal or vertical pipelines. In both cases, the Y-leg (with the element) must be on the "down" side to properly collect debris.
• Cleaning: Can often be cleaned in place by opening a drain valve on the Y-leg ("blow-down").
• Limitations: Small dirt-holding capacity. Not suitable for applications requiring frequent cleaning.
• Application: Best for relatively clean liquids or gases (e.g., steam, air). Typically used for line sizes 40 mm and smaller.

Basket Type Strainer (Simplex)

A housing containing a large, removable "basket" element. The top of the housing is unbolted to lift the basket out for cleaning.

• Advantages: Large dirt-holding capacity and a low pressure drop, especially in sizes above 100 mm.
• Limitations: Can only be installed in horizontal pipelines. The system must be shut down to clean the basket.
• Application: The standard choice for liquid lines, especially in sizes 300 mm and larger, or in services with a high dirt load (e.g., cooling water intake from an open basin).

Duplex (or Dual) Strainer

This setup consists of two basket strainers (or sometimes Y-types) connected by 3-way or 4-way valves. A differential pressure (ΔP) indicator monitors the pressure drop across the in-service strainer.

• Operation: When the ΔP gets too high, the flow is diverted to the clean, standby strainer. The clogged basket can then be removed and cleaned without interrupting the process.
• Application: Essential for any service that cannot be shut down for cleaning, such as fuel oil supply, boiler feed, or continuous process feeds.

Cartridge Filter

These units use a disposable filter element (cartridge) made of paper, yarn, or other media. They are technically "filters" as they often remove very fine particles. The cartridge is replaced with a new one when clogged.

Self-Cleaning Strainer

These are automated strainers that use a built-in mechanism (like a scraper) to clean the element. The cleaning cycle can be initiated automatically (based on high ΔP) or semi-automatically (by an operator). They are used when:

• The strainer is in an inaccessible location.
• Process shutdown for cleaning is not an option.
• Plugging is frequent and unpredictable.

4.0 Strainer Element Selection Criteria

The four key parameters for specifying a strainer are Mesh Number, Open Area Ratio, Pressure Drop, and Material of Construction.

4.1 Mesh Number

"Mesh" refers to the number of openings per linear inch of the screen. A higher mesh number means a smaller opening and finer filtration. The mesh should be the coarsest possible size that still protects the downstream equipment.

4.2 Open Area Ratio

This is the ratio of the total open area of the strainer element to the internal cross-sectional area of the pipe. A larger ratio means a lower pressure drop and a higher dirt-holding capacity.

4.3 Allowable Pressure Drop (ΔP)

The maximum allowable pressure drop is typically specified for a 50% clogged condition to signal when the strainer needs to be cleaned.

• Pump Suction: Specified with a low ΔP to protect the pump NPSH.
  • Clean ΔP: ~0.02 kg/cm²
  • Clogged (Alarm) ΔP: 0.05 kg/cm²
• Duplex (e.g., Fuel Oil): Can be slightly higher.
  • Clogged (Alarm) ΔP: 0.1 kg/cm²
• Pump Discharge: Can be much higher, as NPSH is not a concern.
  • Clogged (Alarm) ΔP: 0.15 kg/cm² or higher, depending on the process.

4.4 Material of Construction (MOC)

A simple rule for MOC selection:

• Strainer Body: The material should match the piping specification (e.g., Carbon Steel, Stainless Steel).
• Strainer Element (Mesh/Basket): The material should be equivalent to, or more corrosion-resistant than, the body (e.g., Stainless Steel element in a Carbon Steel body).

5.0 Installation & Layout Considerations

• Clearance: Always provide sufficient clear space around the strainer to allow for the removal of the element (e.g., dropping the basket from a Basket Strainer, pulling the element from a Y-Type).
• Lifting: For large basket strainers (e.g., > 500 mm), a davit or monorail should be available to lift the heavy top cover.
• Drains: Ensure adequate clearance below drain valves for a bucket or hose.
• End Connections: The pressure rating and connection type (flanged, welded) must match the piping specification.