Variety of strainers and fileter

8/14/2019 Variety of strainers and fileter

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As the marketplace becomes increasingly competitive, more emphasis has been placed on reducing plant downtimeand maintenance. In steam and condensate systems, damage to plant is frequently caused by pipeline debris suchas scale, rust, jointing compound, weld metal and other solids, which may find their way into the pipeline system.Strainers are devices which arrest these solids in flowing liquids or gases, and protect equipment from their harmfuleffects, thus reducing downtime and maintenance. A strainer should be fitted upstream of every steam trap,flowmeter and control valve.

Strainers can be classified into two main types according to their body configuration; namely the !type and thebasket type. "ypical e#amples of these types of strainers can be seen in $igure %&.'.%.

Fig.12.4.1 Typical strainers

Y-type Strainers$or steam, a !type strainer is the usual standard and is almost universally used. Its body has a compact cylindricalshape that is very strong and can handle high pressures. It is literally a pressure vessel, and it is not uncommon for

!type strainers to be able to handle pressures of up to '(( bar g. "he use of strainers at these pressures is howevercomplicated by the high temperatures associated with steam at this pressure; and subsequently e#otic materials suchas chrome molybdenum steel have to be used.

Although there are e#ceptions, si)e for si)e, !type strainers have a lower dirt holding capacity than basket strainers,which means that they require more frequent cleaning. *n steam systems, this is generally not a problem, e#ceptwhere high levels of rust are present, or immediately after commissioning when large amounts of debris can beintroduced. *n applications where significant amounts of debris are e#pected, a blowdown valve can usually be fittedin the strainer cap, which enables the strainer to use the pressure of the steam to be cleaned, and without having toshut down the plant.

!type strainers in hori)ontal steam or gas lines should be installed so that the pocket is in the hori)ontal plane+$igure %&.'.&+a. "his stops water collecting in the pocket, helping to prevent water droplets being carried over,which can cause erosion and affect heat transfer processes.

*n liquid systems however, the pocket should point vertically downwards +$igure %&.'.&+b, this ensures that theremoved debris is not drawn back into the upstream pipework during low flow conditions.

Although it is advisable to install strainers in hori)ontal lines, this is not always possible, and they can be installed invertical pipelines if the flow is downwards, in which case the debris is naturally directed into the pocket +$igure%&.'.&+c. Installation is not possible with upward flow, as the strainer would have to be installed with the opening ofthe pocket pointing downwards and the debris would fall back down the pipe.


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Fig. 12.4.2Correct orientation of strainers

Straight and angle type strainersIn addition to !type strainers, several different body configurations are used in steam systems, namely straight andangle type strainers. "hese are shown in $igure %&.'.-. "hese types of strainer function in a similar way to the !typestrainer and have similar performance. "hey are used when the geometry of the steam pipework does not suit a !type strainer being used.

Fig. 12.4.3Straight type and angle type strainers

Basket type strainer units"he basket type or pot type strainer is characterised by a vertically orientated chamber, typically larger than that of a!type strainer. Si)e for si)e, the pressure drop across a basket strainer is less than that across the !type as it has agreater free straining area, which makes the basket type strainer the preferred type for liquid applications. As the dirt


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holding capacity is also greater than in !type strainers, the basket type strainer is also used on larger diametersteam pipelines.

asket type strainers can only be installed in hori)ontal pipelines, and for larger, heavier basket strainers, the base ofthe strainer needs to be supported.

/hen basket type strainers are used on steam systems, a significant amount of condensate may be formed.

0onsequently, strainers designed for use in steam systems usually have a drain plug, which can be fitted with asteam trap to remove the condensate.

asket type strainers are commonly found in a duple# arrangement. A second strainer is placed in parallel with theprimary strainer, and flow can be diverted through either of the two strainers. "his facilitates cleaning of the strainerunit whilst the fluid system is still operating, reducing the downtime for maintenance.

Fig. 12.4.4 duple!"asket strainer

Filters/hilst strainers remove all visible particles in the steam, it is sometimes necessary to remove smaller particles, fore#ample, in the following applications1

/hen there is direct injection of steam into a process, which may cause contamination of the product.

2#ample1 In the food industry, and for the sterilisation of process equipment in the pharmaceutical industry.

/here dirty steam may cause rejection of a product or process batch due to staining or visible particle

retention.

2#ample1 Sterili)ers and paper3board machines.

/here minimal particle emission is required from steam humidifiers.

2#ample1 4umidifiers used in a 5clean5 environment.

$or the reduction of the steam water content, ensuring a dry, saturated supply.

In such 5clean steam5 applications, strainers are not suitable and filters must be used. A filter used in a steam systemtypically consists of a sintered stainless steel filter element. "he sintering process produces a fine porous structure inthe stainless steel, which removes any particles from fluid passing through it. $ilters capable of removing particles assmall as % 6m are available, conforming to the good practice needs of culinary steam.


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Fig. 12.4.# hori$ontal in-line filter

"he fine, porous nature of the filter element will create a larger pressure drop across the filter than that associatedwith the same si)e strainer; this must be given careful consideration when si)ing such filters. In addition, filters areeasily damaged by e#cessive flowrates, and the manufacturer5s specified limits should not be e#ceeded.

/hen the filter is used in steam or gas applications, a separator should be fitted upstream of the filter to remove anydroplets of condensate held in suspension. In addition to improving the quality of the steam, this will prolong the life ofthe filter. A !type strainer should also be fitted upstream of the filter to remove all larger particles which wouldotherwise rapidly block the filter, increase the amount of cleaning required and reduce the life of the filter element. yinstalling pressure gauges either side of the filter, the pressure drop across the filter can be measured, which can

then be used to identify when the filter requires cleaning. An alternative to this is to install a pressure switch on thedownstream side of the filter. /hen the downstream pressure decreases below a set level, an alarm light can beswitched on in a control room alerting an operator, who can then clean the filter.

"op

Strainer screens"here are two types of screens used in strainers1
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%erforated screens - "hese are formed by punching a large number of holes in a flat sheet of the required

material using a multiple punch. "he perforated sheet is then rolled into a tube and spot welded together.

"hese are relatively coarse screens and hole si)es typically range from (.7 mm to -.& mm. 0onsequently,perforated screens are only suitable for removing general pipe debris.

&esh screens - $ine wire is formed into a grid or mesh arrangement. "his is then commonly layered over a

perforated screen, which acts as a support cage for the mesh.

y using a mesh screen, it is possible to produce much smaller hole si)es than with perforated screens.4ole si)es as small as (.(8 mm are achievable. Subsequently, they are used to remove smaller particleswhich would otherwise pass through a perforated screen. 9esh screens are usually specified in terms of5mesh5; which represents the number of openings per linear inch of screen, measured from the centre line ofthe wire. $igure %&.'.: shows a - mesh screen.

Fig. 12.4.' (!a)ple of a 3 )esh screen

"he corresponding hole si)e in the mesh screen is determined from knowledge of the wire diameter and the meshsi)e; it is usually specified by the manufacturer. "he ma#imum particle si)e that will be allowed to pass through thescreen can be determined using geometry. If, for e#ample, a &(( mesh screen is specified and the manufacturer5sspecifications stated that the hole si)e is (.(8: mm, then the ma#imum particle si)e that will pass through the screencan be found using ythagoras5 theorem1

(*uation 12.4.1


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Fig. 12.4.+ ,eter)ining the )a!i)u) particle si$e

"he problem with this dimension is that the screens are two!dimensional and the particle must reach the hole in acertain orientation. "herefore, if a long thin particle reached the strainer 5face on5, it may be allowed to pass throughthe screen. 4owever, if it hit the hole 5side on5 it would be stopped. If this is likely to be a problem, a finer mesh shouldbe used.

"he screening area is the area available for removing debris. A larger screening area means that the frequency ofblowdown for cleaning the screen is considerably reduced.

"he free area is the proportion of the total area of the holes to the total screening area, usually e#pressed as apercentage. "his directly affects the flow capacity of the strainer. "he greater the free area +and the coarser thescreen, the higher the flow capacity and ultimately the lower the pressure drop across the strainer. As most strainerscreens have very large straining and free areas, the pressure drop across the strainer is very low when used onsteam or gas systems +see 2#ample %&.'.%.4owever, in pumped water or viscous fluid systems, the pressure drop can be significant. Strainers should have flowcapacities quoted in terms of a capacity inde# or '( strainer with a


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"his equates to a pressure drop of just over (.@C.

"he pressure drop across a strainer may be determined either from the


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edge of the cleaning knife, and it is deposited into a solid, vertical groove formed in the outside surface of the strainerelement by special packing pieces. As there is no flow through this part of the element there is no force holding theaccumulated dirt against the element, and it falls into the sump at the bottom of the strainer.

Fig.12.4.0 The )etallic disc positi/e edge type strainer

Te)porary strainers"emporary strainers are designed for protection of equipment and instrumentation during start!up periods. "hestrainer is usually installed between a set of flanges for an initial period after a new plant has been installed.Installation of a spool piece equal or more than the length of the strainer is recommended for ease of installation orremoval.

"here are three basic configurations of temporary strainers, namely the conical type, the basket type and the platetype. Standard construction is of perforated screen or single ply heavy wire mesh. /ire mesh liners can be addedinside or outside of the strainer for finer straining capabilities. If a wire mesh is used, care must be taken to ensurethat the direction of flow is against the wire mesh with the perforated metal as a back!up.

Fig. 12.4. Te)porary conea and "asket " type strainers

!Strainers are devices for mechanically removing solids from flowing liquids or gases by means of a

perforated or wire mesh straining element. "hey are used in pipelines to protect equipment such as

pumps, meters, control valves, steam traps and regulators.

Although there are occasional e#ceptions, the use of !strainers generally follows several rules. $irst,


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they are ordinarily employed where the amount of material to be removed is small. Si)e for si)e, their dirt

holding capacity is less than a basket strainer. >e#t, !strainers are usually installed when frequent clean!

out is not needed. "here are !strainers in service on steam lines, for e#ample, that are not cleaned more

than once a year. Denerally, !strainers are used with gases such as steam or air. asket strainers are

used with liquids.

$or handling steam, a !Strainer is the standard and is almost universally used. Its compact, cylindrical

shape is very strong and can handle high pressures. It is, literally, a pressure vessel. !strainers which

handle pressures up to :,((( psi are not uncommon. *f course, in these cases, safety is very important

and !strainers, if properly designed, can be used at these pressures without fear of failure. /hen high

pressure steam is being handled, another complicating factor arises ! temperature. /ith steam pressures

of %@(( psi or higher, standard carbon steel is sometimes not suitable because the steam temperature

may be %((( degrees $ or even higher.

An official definition adopted by the $luid 0ontrols Institute is 1A closed vessel with cleanable screen

element designed to remove and retain foreign particles down to (.((% inch diameter from various flowing

fluids.>ote the termforeign particles. Strainers do not necessarily remove only dirt. "hey take out

material which is not wanted in the fluid and this can sometime be a valuable product which may besaved.

/hat is the difference between a strainer and a filter Actually there isnt any since a strainer is, in

reality, a coarse filter. "he question is then one of semantics. Denerally it is assumed that if the particle to

be removed is not visible to the naked eye, the unit is filtering, and if the particle is visible, the unit is

straining. "he average human eye can detect a specific particle between @( and 8( micros. 9ost people

cannot see anything smaller than -&@ mesh, or '' microns. Since &(( mesh is equivalent to 8' microns,

a general rule would be that if the screening device is coarser than &(( mesh, it is a strainer and if it is

finer than &(( mesh it is a filter.

*ne of the best uses for a strainer is in conduction with a filter. y installing a strainer directly ahead of a

filter, the large heavy pieces which would quickly clog the filter are removed. "he filter is then free to do its

major job of fine particle removal and does not have to be cleaned so often.

The selection of steam traps for specific applications is made in two steps :

A.Choice of typeB.Choice of size

Before discussing these steps it is necessary to make a general comment from the economic point of view.Giving as granted that the condensate must be discharged , it is highly important not to loose live steam in thisprocess. ssuming that today!s cost of steam is appro"imately #.#$ %.&. ' to a kilogram ( and this is veryconservative ) it follows that a trap , sized for $## *g + hour losing #- of its steam , in a refinery onstream $hour + day , costs in one year ( $## " #. " /01 " #.#$ ) %.&. ' 0 .2f this refinery has ### traps incorrectly sized and therefore in such conditions , the loss will be 0### %.&. 'per year 34ne can easily calculate what happens if the trap has failed in the open position instead of 5ust losing somesteam. The choice of the type and size to a steam trap is a matter of great importance .


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A. CHOICE OF TYPE

The main criteria for the selection of the type are ( they cannot be listed in order of importance since it variesfrom application to application ) :

6 7esistance to freezing

6 2nstallation versatility

6 ir venting6 7esistance to water hammer6 Cold condition ( if water logging is not allowed the trap must be the open type )6 Type of discharge ( with temperature regulating control valves , the modulating type is preferable )6 8eat e"change efficiency ( traps discharging sub cooled condensate do not allow an efficient heate"change )6 &ensitive to back pressure6 7eaction to load changes6 9ressure variations ( types reuiring changes of orifices for different pressure are unfit for wide variations )6 ;imension and weight

hen a trap discharges at the atmosphere the downstream pressure is zero ( we always refer to relative and notabsolute pressure ) and the differential pressure is the same of the line. >hen there is a condensate return

system , there is always some pressure inside it due to friction and line lifting. The best way to know the value ofdownstream pressure ( also called backpressure ) is to install a pressure gouge 5ust after the trap. 2f this is notpractical one should calculate the amount of backpressure by formulas of pressure drop in water ducts adding

appro". #. bar for each meter of rise .

2 CONDENSATE LOAD

This is the second parameter to be introduced into the capacity tables.


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pressure and with the condensate load multiplied by the safety factor. minimum safety factor .$ + .1 must bealways taken into consideration. 8igher safety factors $ + are reuired for certain applications .

INSTALLATION

&pecific suggestions for a correct installation depend on the application and on the type of select trap. The

following are some general comments :

6 The trap should always be installed below the drain point6 Try to avoid condensate lifting. 2f this is necessary install a check valve 5ust after the trap6 lways install = A = type strainer upstream, unless the trap has a built in = A = type strainer6 @echanical and thermodynamic traps should be installed as close as possible to the drain point6 Thermostatic traps should be installed at ? $ mt from drain point. ;o not insulate this cooling leg6 2t is advisable to install a check valve up stream of an inverted bucket trap to prevent water seal loss6 sight glass fitted down stream the trap allows a continuous check of the trap operation6 lways install isolating valves upstream and downstream for maintenance purposes.

STEAM TRAP SELECTION AND APPLICATIONS

D = Thermody!m"#

T = B!$!#ed %re&&'re (hermo&(!("#B = B"me(!$$"# (hermo&(!("#I = I)er(ed *'#+e(, = B!$$ -$o!( "(h (hermo&(!("# !"r )e(

APPLICATIONS TRAP CHOICE

STEAM MAINS D / T

TRACIN, LINES D / B

THAN0S

S(or!e (!+&

B T

O"$ (!+&

A&%h!$( (!+&

Dye )!(&

E)!%or!(or&

B$eder&

S'#("o he!(er&

HEATHER BATTERIES

"( he!(er&

, / I

Dry" room&

,reeho'&e #o"$&

F" #o"$&

S'!r dryer&

PANS

4!#+e(ed %!&

, / I

T"$(" +e(($e&

Bre +e(($e&

C!dy +e(($e&Chee&e +e(($e&

S'*mered #o"$&

HEAT E5CHAN,ERS

6!(er he!(er&

, / IF'e$ o"$ %rehe!(er&

P$!(" (!+&

DRYIN, CILINDERS P!%er dryer& , / I

P'$% dryer&

Ro(!ry dryer&


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C!$eder&

PRESSES

P$yood %re&&e&

D / T

Mo$d" %re&&e&

T"re mo$d %re&&e&

7'$#!8" %re&&e&

M"$+ dryer&

O7ENS

Dre&&" &(er"$"8er&

, / TPre&&'re #oo+er&

A'(o#$!)e&

Dr'm dryer&

IRONIN, MACHINES I / D

TRBINES D / I

MARINE APPLICATIONS D / B

Variety of strainers and fileter

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