STRAINERS FOR PIPELINES

DEFINITION

The answer to the questions, What is a strainer? or How do you define a strainer? is the following: pipes strainers are closed mechanical devices (belonging to the valves family) that contain a cleanable screen or mesh used to remove and retain foreign particles larger than 45 microns (325 mesh) from flowing liquids, such as oil, gas, slurries, steam and solids along pipelines. 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 generally considered a filter. The relationship between mesh and opening size is given in Table 1. The difference between filters and strainers is only semantic; A filter could be considered a coarse filter.  All types of strainers are available with a large variety of jointing methods for insertion into a pipeline (threaded, socket weld, butt weld, flanged). Manufacturers can construct units for special conditions. 

Pipeline strainer

 

TABLE 1. Relationship Between Mesh and Opening Size

Mesh size

Particle diameter, um

Pipeline Strainers

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

Filter

325

44

400

37

550

25

800

15

1250

10

 

CONVERSION US MESH TO INCHES/MICRONS AND MM

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

 

 

TYPES OF STRAINERS

There are two main types of pipeline strainers:

Y-STRAINER

The Y strainer , illustrated below, has compact in design and is considered for use where space constraints exist. Frequent cleaning is often required. Its a good choice for high pressure applications and for gases where pressures are higher and amount of dirt present is low. Y strainer has less capacity to store dirt than similar sizes of basket strainers. It is installed in a pipeline with its strainer element in the down position, and can be positioned either horizontally or vertically. 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 Strainer

BASKET STRAINER (T-TYPE)

The Basket strainers, illustrated below as well, gets its name from the upright, perforated basket used to trap particles. It is installed upright in the strainer body, and the top of the strainer must be removed for cleaning. Because of its large size, it can store large quantities of dirt and so has a lower pressure loss than a similar sized of Y strainer. Basket strainers are available in self-cleaning models. 
Basket Strainers find wide application in pipelines that require a high flow capacity. To swrvice 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). These types of strainers are manufactured also in the variant of "duplex type" which consists of two parallel basket strainers that facilitate the maintenance of the strainers in conditions when the flow of the line cannot be stopped.

Basket Strainer (T-strainer)

Duplex basket strainer

Duplex basket strainer

MATERIALS FOR STRAINERS

BODY

The most common material used for Y strainers and Basket strainers bodies are cast iron, bronze, carbon steel, stainless steel, and plastic. 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 where high temperature and pressure conditions are encountered and where resistance to thermal and mechanical shocks is required. Carbon steel components are the materials of choice where fire hazards exist. 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. Plastic strainer bodies are accessible in all of the materials used for pipelines. Baskets and screens are available in all metal materials.

It has to be added, that strainers body may be manufactured either in cast or forged steel.

 

BASKET AND SCREEN

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 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, a 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 

A cover is supplied to remove and 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 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 other one. Another type of strainer is the automatic type (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.

 

 

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