Guidelines for The Preparation of Strainer Specification- Part 1
Topics covered1.
What is a strainer
2.
What is the requirement of a strainer in a system
3.
Identification of Strainers in various documents and preferred location of Strainer
4.
Various types of Strainers & their functions
5.
Definition of Strainer element and its Basis of Selection
6.
Material of construction of Strainer and Strainer element
7.
Pressure Rating and End Connection details
8.
Layout aspects to be considered during Installation of Strainers
9.
Sample Strainer Specification
1.
What is a strainer It is a device used in Piping Systems, its function being to arrest foreign particles like dirt, weld sputter, scale etc upstream of rotating equipment such as compressors, turbines, pumps, rotary instruments, steam traps etc.
2.
What is the requirement of a strainer in a system Rotating equipment such as centrifugal compressors, turbines, pumps etc and also rotary instruments such as PD meters, turbine meters operate on narrow clearance between static and rotating part. Also inline items like traps, spray nozzles have minute openings for the flow medium as required for the operation. In the above cases the efficiency and function of the equipment or device are largely hampered if they encounter foreign particles of unacceptable size. Hence in order to avoid such an occurrence a device called strainer is warranted in a system upstream of such critical items.
3.
Identification of Strainers in various documents and preferred location of Strainer Strainers are identified broadly in Basic Engineering Documents such as P&ID’s by Process Licensor. Strainers are also identified in Process Licensor’s Piping Specification. However strainers subsequently get added in a System by the discipline who establishes the warranty of strainer upstream of items handled by them.
Following are a few examples citing location of strainer and originating Department. Upstream of turbine meters/ PD metersInstrument
Location
Originating Department
As shown in the P&ID’S
Process Licensor / Process
Upstream of centrifugal, reciprocating pumps
Mechanical/Piping
Upstream of centrifugal compressors, turbines etc.
Mechanical/Piping
Upstream of fuel oil burner within furnace, boiler, incinerator etc
Mechanical/Piping
Upstream of spray nozzle
Process
Upstream of stream trap, safety shower/eyewash unit
Piping
Sr No
1
3
4
5
6
7
4.
Various types of Strainers & their functions They are broadly divided into two categories.
4.1 Temporary Strainer 4.2 Permanent Strainers Refer following Table and Attachment – I depicting most commonly used Strainer under the above mentioned types :
Types of strainers Attachment –I 4.1.1 Temporary Strainers These are designed for short periods of application for example fitted during start up period of a new system, or when restarting a system after shut-down and maintenance. These are also used in cases where the characteristics of the process fluid does not permit usage of a permanent strainer. Temporary strainers are selected based on the type either depicted in the P&ID by Process Licensor or at place such as upstream of pumps where such requirement is not indicated in the P&ID, in order to safeguard the equipment during start up or after a prolonged shutdown. They are intended to remove coarse debris particles present in the System. After a suitable period in use, they are removed and they can be cleaned and stored for future use. Strainers of this type are normally designed with standard flange faces so that they can be fitted at a suitable removed the gap can be compensated by either pulling up the flange (for small bore flexible piping) or by inserting a spacer flange and gasket. (Refer attachment – I). Specifying a particular type of temporary strainer depends also on the configuration of the Piping System. Temporary strainers are of the following types: a) Flat Disc Type These are normally perforated plates. b) Conical Basket Type These may be of perforated plate with supported wire mesh. They have higher dirt capacity, normally have a lower pressure drop and provide fine filtering. c) T-Type Strainer
Where fitting of above strainers is difficult or impartial on a particularly configured Piping System, T-Type Strainer can be used which can be inserted to match a blind on one side T-junction fitting in the pipeline and can be also easily removed. These strainers are of a through shape, referred to as bath tub strainers. 4.1.2 Permanent Strainers These are of the following types: a) Y-Type Strainers These are of Y-Type configuration with a cylindrical strainer element housed in Y-Leg of Strainer. They can be used both in horizontal and vertical (down flow) lines. The element is retained by a plug end which can be opened for cleaning the element. In order to facilitate quick clean out without removing the element a drain connection with a valve is provided as part of “Y” Leg. These are generally used in pipelines upto 150NB to 200NB in size. The Y-pattern offers lesser resistance to the flow and hence lowers the pressure drop. The limitation of this type of strainer is its limited filtration area, hence it is used on a clean fluid where the necessity of cleaning the strainer is infrequent. b) Basket Type Strainers On larger pipelines basket type strainers are preferred. These may be of inline or off line type. The principal advantages of basket strainers is that they can provide greater dirt holding capacity and can have easier access for removal of the strainer for cleaning. They, do however, normally have a higher pressure drop than simple Y-Type Strainers. These are generally used with liquids wherein regular or frequent cleaning is required. Basket strainers may be further divided into the following types : i. Single Sieve Wherein only one basket doers the filtration, suitable where the dirt load is not very high and high pressure drop is permissible. ii. Double Sieve Wherein two baskets work in parallel, suitable for high flow rates and if high pressure drop is not permissible. This is achieved by providing a larger filtration area in comparison to single sieve design. iii. Multi-Sieve Basket This design has a number of sieves arranged in telescopic design so that large filtration area can be provided in a small housing. For highly viscous application the strainers can be steam jacketed so that the hot flow viscosity fluid is easily filtered and a lower pressure drop is registered. For removal of ferrous particles the basket strainer may have as special requirement magnetic inserts so that the ferrous particles are attracted to the suspend magnetic inserts. c) Duplex or Dual Type Strainers These are desired when a continuous operation is required and a shut-down is not possible. The duplex design facilitates cleaning of one element while the other is in operation.
This is generally preferred for highly viscous fluids such as fuel oil where chances of impurities are fairly higher as compared to other fluids. The basket type strainer covered above can be converted into duplex type by : i. Interconnecting two strainers covered by 2 numbers of 3-way valves for compact design or ii. By interconnecting two strainers by 4 nos. 2-way gate valves. d) Catridge Type Filters Are produced in a wide variety of configurations and material of construction. Filter media includes yarns, felts, papers, resin, bonded fibres and woven wire cloth. The filter element (actual catridge) of a catridge filter is normally designed as a disposable component to be replaced with a new one when clogged. e) Self Clearing Strainers These may be used as an alternative to duplex types where continuous supply is critical in a process system. Cleaning may be by using a brush or a scraper to comb out solid particles accumulated around catridge stack which will get deposited in the sump. Motorised self cleaning strainers can be used when : a) Constant pressure drop is desired b) Strainer is inaccessible for manual maintenance c) The fluid is too viscous d) The plant is costly and a shut-down is out of the question. These are being handled by Mechanical Department.
5.
Definition of Strainer element and its Basis of Selection The strainer element is the heart of all types of strainers. Its selection in terms of material of construction, area, mesh size etc is the key to prove its efficiency. Refer table below showing assumption for selection of strainer element.
Sr.No.
Type of Strainer
Type of Element
Mesh/Filtration Size Range
Filtration Area
Flat Disc
Perforated Plates
Coarse 1/8”
Less than pipe cross section area
Conical
Conical
20 to 200 mesh
Refer DPG Standard More than pipe cross section area available
1
2
“T” Type
Half cone shaped
20 to 200 mesh
“
“Y” Type
Cylindrical
20 to 200 mesh
“
Basket Type
Cylindrical
Upto 2 micron
“
Duplex
Cylindrical
Upto 2 micron
“
Catridge
Catridge
Filtration Upto 0.2 micron
“
Self Cleaning
Plate type with cleaning blade
Filtration Upto 25 micron
3
4
5
6
7
8
6. Material of construction of Strainer and Strainer element The material of construction of the strainer and strainer element must be keeping in view compatibility with the process fluid being handled. As a general rule MOC of strainer body is specified in line with relevant piping material spec code and the element material is specified at least equivalent or superior to the body material. The following Table is prepared as a general guideline for specifying material, based on what is offered as Standard product of the manufacturer:
Sr.N o.
Type of Strainer
Body Material
Element Material
Flat Disc
–
Carbon Steel/Bronze, Stainless Steel
Conical
–
Sheet material of SS, CS or Brass with SS Wire mesh
1
2
Remarks
“T” Type
–
Carbon Steel/ Stainless Steel
“Y” Type
CS-A105 or A105 Galvanized
Stainless Steel
Size 15 to 40 NB Forged Construction
SS-A182 Gr F304,316,321 Bronze – ASTM B62
SS-304,316,321
For potable Water and instrument air service
“Y” Type or Basket Type
CS-A216 Gr WCB SS-A351 Cr CF 8 Cast Iron-A126 CLB IS 210 Gr 20 IS 2002 Gr FG 200
Stainless Steel
Size 50 to 100 NB Cast Construction
Basket Type
Pipe Material A53 Gr B,A106 Gr B Plate Material A515 Gr 60, IS 2062 Is 2992 Gr 2A
Stainless Steel
200 NB and above Generally for Utility Services Welded Construction
Duplex Type
Carbon Steel Seamless Pipe Viz., 106 Cr B
Stainless Steel
Cartridge Type
Same as Basket type Body Material
Honey Comb Wound With cotton, fibers glass, polypropylene, nylon yarn on GI/SS 304/SS 316
3
4
5
6
7
8
For cast iron strainers, bolts, studs and nuts shall conform to ASTM A307 Gr B or equivalent where as for steel trainers, bolts and studs shall conform to ASTM A193 Gr. B7 or equivalent and nuts to ASTM A194 Gr. 2H or equivalent.
7. Pressure Rating and End Connection details The pressure rating of the strainer shall be identical to that of other piping components used in a system such as valves etc. related to the specific fluid covered under the relevant piping Spec Code. Similarly end connection details shall be akin to whatever applicable to inline piping components like valves as covered in the relevant Spec Code.
Sr.No.
Type of Strainer
Pressure Rating
End Connection Details
Upto 250 PSI, 435°F 1
Y-Type
Upto 150 PSI, 400°F
Material /size range Gun metal body
Screwed Ends
Size 15 to 40 NB
Flanged Ends
Size 50 NB & above Cast iron body
Flanged Ends
Size 50 NB & above
Screwed Ends
Size below 50 NB
Class 800
2
Basket Type
Forged CS Body Socket Weld Ends
Size 15 NB to 40 NB
600PSI, 850°F
Flanged Ends
Cast Carbon SteelBody size50 NB & above
1400 PSI, 1004°F
Socket Weld Ends
Alloy Steel Body Size 15 NB to 25 NB
150 Class & above
Flanged Ends
CS Body Size 50 NB & above
8. Layout aspects to be considered during Installation of Strainers 8.1. Y-Type Strainer to be properly installed as per flow direction. Sufficient clear space to be provided in all strainers to facilitate easy removal of mesh element for cleaning purpose. 8.2. In large basket type strainers cover to be provided with a davit arrangement for ease in opening. Alternatively, monorail beam to be envisaged in the layout. 8.3. Be low drains of basket strainers sufficient clearance to be provided for drainage. 8.4. For temporary cone type strainers break off flanges (spool piece) to be considered for ease in removal. 8.5. Temporary conical strainers are to be placed with the cone pointing opposite to the direction of flow. 9. Pressure Drop across a Strainer / Mesh Number The mesh is effectively the number of opening per (25.4 mm) linear inch measured from the center of one wire to another one inch from it. Mesh numbers range from 2 to 400. Hence the mesh number is an indication of the operative opening size, higher the mesh number smaller the opening and higher the pressure drop. Liquids which are more viscous when used with finer meshes create increased pressure drop. Perforated plate strainers provide coarse filtration to above 150 microns. Finer filtering wire mesh elements are normally used, in conjunction with perforated plates for reinforcement. Wire mesh strainers can provide filtration down to about 40 microns. Attachment – II gives an idea of aperture sizes based on various mesh numbers. Numerically the service life of a strainer (between cleaning times) can be expressed as the ratio of open area of the strainer to the pipe cross section. Open area is determined by choice of perforations or mesh. Basket type strainers shall have a minimum free flow area of 1.5 times the cross sectional area and all Y-type strainers would have a minimum free flow area of twice the bore cross sectional area. Thus specifying mesh number would depend on : a) The smallest particle size to be filtered
b) The allowable pressure drop Generally the allowable pressure drop across a clean filter is specified as 0.01 kg/cm 2 (g) and across la clogged filter as 0.5 kg/cm2 (g). The viscosity of a fluid at working temperature is also to be specified since higher the viscocity higher the pressure drop. The data for Mesh No. and Pressure Drop if not given in the Process Licensor’s Documentation is to be confirmed with Process Department. Vendors are to provide pressure drop versus flow rate curves for all the strainers for clean condition as well as 50% clogged condition.
9. Sample Strainer Specification Before commencing on preparation of strainer specification the first step involves identifying the types of strainers and quantifying the number of strainers from the P&ID’S. Alternatively Strainers can be included in Piping MTO for quantification purpose. The following gives the user an idea for the input sources for specifying a Strainer.
Tag no
To suit Special Material Index & P&ID
Type
As per P&ID or to suit the purpose
Size mm NB Line Strainer
As per P&ID Line Size to be specified By vendor
Quantity (N.)
As per P&ID or Material Status Report.
End Connection
To suit Type & Size and Pipe Spec
Flange Facing and Finish
To suit Pipe Spec
Strainer Element Size (mm2)
To be specified by Vendor
Mesh Number
To be obtained from Process Dept.
Piping Spec
As per Line Designation / P&ID
Line Number / Location
As per P&ID
P&ID No.
As per P&ID
Manufacturer
Same as Vendor (To be incorporated at the time of Order Specification after receipt of LOI/Purchase order from clients).
Manufacturer Drawing No.
Same as above
Dimension face to face And center line to Bottom strainer
As per Piping Layout
Body Material
To suit Type & Size and Piping Spec.
Mesh Material
To suit Piping Specification (equivalent or superior to body material)
Flanges
As per Piping Spec
Gasket
As per Piping Spec
Bolting
As per Piping Spec
Plug
As per Piping Spec
Service Fluid
As per Piping Spec Index.
Design Flow Rate
To be obtained from Process Dept. Alternatively it can be obtained from the Process Data Sheet of the equipment for which the Strainer is warranted.
Design Pressure
As per Piping Spec Index.
Design Temperature
As per piping Spec Index.
Density at Operating Temp (Kg/m3)
To be obtained from Process Dept.
Viscosity at Operating Temp.
To be obtained from Process Dept.
Maximum Allowable Pressure Drop across clean filter
Generally specified as 0.01 kg/cm2(g). However the same to be confirmed with Process Department
Hydrostatic TestTest Pressure
1.5 times the Design Pressure
For basket strainers on pump suction it is advisable to calculate the design pressure based on operating conditions to establish hydro test pressure at shop or site in order to avoid a robust strainer construction which may be unacceptable to layout. Remarks: The following special requirements are to be observed by Vendor. 1 Vendors are to provide pressure drop versus flow rate curves for all the strainers for clean conditions as well as 50% clogged condition. 2 Supporting of Large Strainers: All basket strainers 200 NB and above are to be provided with leg supports and above 500 NB, 3 legs are required. Depending on layout constraints and foundation requirement the limitation on the maximum distance between bottom face of leg support and center line of nozzle is to be specified as also the distance between center line of nozzle and drain plug. 3 Drains & Vents All basket strainers are to be provided with drain plugs and vent cocks. If Valves are to be used and are not in vendors scope then vendor to be informed of the appropriate end connections required. 4 Tag No to be clearly identified.
