STRAINERS FOR PIPELINES


 In this article, we explain what are strainers for pipelines: definition, function, types (Y and basket), and, materials.


 

WHAT ARE STRAINERS?

Strainers are mechanical devices that feature a cleanable screen (or mesh) used to remove and retain foreign particles from flowing liquids, such as oil, gas, slurries, steam, and solids along pipelines and protect equipment such as pumps, compressors and other mechanical devices that would be affected by such solids. If a particle is visible to the naked eye, a strainer should be chosen to remove it from the liquid stream.

If the device retains particles finer than 45 microns, it is rather defined as a filter instead of a strainer. In any case, the difference between filters and strainers is only semantic; A filter could be considered a coarse filter.

 

Strainer for pipeline

 

 

MESH SIZES

The relationship between the required mesh size and the dimension of the particles to be filtered is given in the table below.

Strainer Mesh and Opening Size

Mesh size

Particle diameter, um

4

5,205

8

2,487

10

1,923

14

1,307

18

1,000

20

840

25

710

30

590

35

500

40

420

45

350

50

297

60

250

70

210

80

177

100

149

120

125

140

105

170

88

200

74

230

62

270

53

325

44

400

37

550

25

800

15

1250

10

 

MESH CONVERSION TABLE (INCHES, MICRONS AND MM)

STRAINER MESH
USA
in INCH in MICRONS in MM
3 0.265 6730 6.73
4 0.187 4760 4.76
5 0.157 4000 4
6 0.132 3360 3.36
7 0.111 2830 2.83
8 0.0937 2380 2.38
10 0.0787 2000 2
12 0.0661 1680 1.68
14 0.0555 1410 1.41
16 0.0469 1190 1.19
18 0.0394 1000 1
20 0.0331 841 0.841
25 0.028 707 0.707
30 0.0232 595 0.595
35 0.0197 500 0.5
40 0.0165 400 0.4
45 0.0138 354 0.354
50 0.0117 297 0.297
60 0.0098 250 0.25
70 0.0083 210 0.21
80 0.007 177 0.177
100 0.0059 149 0.149
120 0.0049 125 0.125
140 0.0041 105 0.105
170 0.0035 88 0.088
200 0.0029 74 0.074
230 0.0024 63 0.063
270 0.0021 53 0.053
325 0.0017 44 0.044
400 0.0015 37 0.037

 


 

STRAINERS TYPES

There are two main types of strainers for pipelines:

Y STRAINER

Y strainers, illustrated below, have a compact in design and are used where there are space constraints. They can be installed either horizontally or vertically with the filtering element positioned towards the ground. Frequent cleaning is often required, as particles may deposit and limit the filtering ability of the device after some time of use.

This type of strainer is a good choice for high-pressure applications and for gases where pressures are higher and the amount of dirt present is low. Indeed, Y strainers have a lower capacity to store dirt than basket strainers of similar size. Very often, a valved pipe will be put in the removable end of the strainer so that the accumulated debris can be easily blown out while keeping the line in service.

 

Y strainers

BASKET STRAINER (T-TYPE)

Basket strainers, illustrated below, feature an upright and perforated basket able to trap the particles flowing through the pipeline. Thanks to its large size, this type of strainer can store large quantities of dirt and debris and generates a lower pressure drop (per unit of the captured foreign particle) than y-strainers of the same size. Therefore, they are used for pipelines that require a high flow capacity.

Self-cleaning t-strainers are also available on the market, and they require lower maintenance work. T strainers shall be installed upright, and the top of the strainer can be removed for frequent cleaning. In any case, to service a basket strainer, the cover can be removed so technicians get immediate access to the filtering element (cage) if it needs replacement (due to cumulated debris).

 

Basket type strainer

Basket strainers are manufactured also in the “duplex type” variant, which consists of two parallel basket strainers that facilitate the inline maintenance of this filtration device (the line is bypassed when one strainer is being cleaned).

Duplex basket strainer

Duplex basket strainer

 

 


MATERIALS FOR STRAINERS

BODY MATERIALS

The most common material used for Y strainers and Basket strainers bodies are cast iron, bronze, carbon steel, stainless steel, and plastic.

y strainer body

CAST IRON

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.

BRONZE

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

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 case of applications with fire risk.

CHROME MOLY ALLOY

For high pressures and temperatures over 1000 °F, chrome-moly steel is usually specified for bodies.

STAINLESS STEEL

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).

 

BASKET AND SCREEN MATERIALS

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.

strainer mesh element

The basket of a strainer (dirt filtering unit)

The size of the openings through a screen is referred to as mesh, and the size of the openings through a basket is referred to as perforations. The term mesh describes a screen that uses a woven wire cloth manufactured from the material chosen for the intended service.

The most common material for all applications is stainless steel. Mesh screens are generally available in standard sizes (from 20 to 200 mesh). The mesh size does not indicate the particle size retention since the size of the mesh opening is determined by the diameter and number of wires/inch. For example, an 80 mesh means 80 vertical and 80 horizontal strands of wire per inch. For critical applications, a screen should be selected on particle retention capability (not mesh size). Standard perforated screens are manufactured (from a light or heavy gauge sheet metal) available in standard sizes. The baskets should be assembled by either welding or brazing. The strongest are of welded construction. Solder is much weaker and easy to break. If a basket strainer is required for fine straining, it is common practice to add a wire mesh liner inside the perforated bucket since the screen alone is too weak to provide the necessary mechanical strength. The material to be used depends both on the intended water service and the body that the strainer is installed into.

The most usual of basket materials are brass and stainless steel. Cast iron strainer bodies usually use baskets of brass and stainless steel, depending on the service. Bronze bodies generally require Monel metal baskets because of the severe service. Stainless steel baskets are used with stainless steel bodies. The baskets are made, in general, from sheet metal with a wide variety of diameter perforations.

A Y strainer is generally furnished with 1⁄16-in perforations in sizes up to 4 in, 1⁄8-in perforations in larger sizes for liquid service, and 3⁄64-in perforations for steam service. If finer straining is required, a wire mesh screen fitted inside a basket is used. In this case, generally accepted practice limits the perforations in the basket to 50 percent of the wall area in order not to lose strength. This combination provides an excellent ratio of maximum flow rate with appropriate reinforcement. The mesh and basket should be an integral unit, with the mesh fastened to the basket both at the top and the bottom to prevent any debris from bypassing the unit. In general, the open area perforation ratio should be about 4:1 to avoid excess pressure drop through the unit. A smaller ratio will require frequent cleaning. Additional strainer basket area can be obtained by using a pleated basket. If finer filtration is required, a mesh liner can be added inside the basket. If the size particle to be removed is known, the perforations should be slightly smaller. Fluid streams may contain iron or steel particles small enough to pass through the finer screens. If this is a problem, a strong magnet capable of lifting several times its own weight should be suspended in the basket. The magnet must be installed so that water passes over it. This magnet should be encased in an inert material to prevent corrosion. The baskets can’t withstand the same pressure as the body. A particular phenomenon called runaway buildup is possible, in which the dirt builds up and plugs the mesh or perforations, thereby reducing the free area. The pressure in the sieve increases slowly at first but then increases more rapidly with time. The velocity and pressure of the water inside the basket intensify with speed, which causes the resulting flow to stop or to be reduced to a trickle. This full-line pressure can burst the basket.

STRAINER COVERS

The cover is the part of the device that should be opened to clean the basket. The most common type is bolted, the bolts must be loosened and removed to bring access to the basket. This type of cover is the strongest and should be used for high-pressure applications.

Another type of cover is the clamping yoke, in which threaded, tee-shaped handles are used to connect the cover to the body. Often, the cover is attached to the body with a hinge mechanism, making it easy to remove. This type of cover is more expensive than the standard one.

Another type of strainer is the automatic type (which does not require manual cleaning). A rotating, circular screen is used as the basket. The water inlet goes into the basket. A rotating backwash inlet inside the basket uses the differential pressure between the atmosphere and line pressure to produce a localized reverse flow across only a portion of the basket, thereby allowing continuous cleaning. This type of strainer is appropriate for large consumers of water such as raw water inlets from rivers and lakes used for cooling and process. Automatic pipe strainers are available to 60-in (150-mm) size.

Need to learn more? You can consult the technical information about strainers provided by Spirax Sarco – one of the most well-known manufacturers of pipeline strainers.