Specifications Part 1 - Water Supply, Sewerage and Tube Well Works

5.7.1 Bimetal strainersFor small driven tube wells generally of 3 cm to 5 cm dia, the strainers aregenerally of bimetal-sometimes called jacketed strainers. It consist of agalvanized iron pipe with about 300 rectangular slots of 1 cm x 1 ½ cm in astandard length of pipe of 1.8 m, having an area of opening of about 17 %covered with a brass wire-mesh or 24 or 32 mesh which again is enveloped bya perforated brass metal sheet of 26 gauge, having about 2 to 3 holes of 3 mmdia per cm² or an area of opening of about 18.5 %. The effective area ofopening resulting is roughly 5 to 7 %. These are available up to 150 mm dia.Gravel shrouding is not essential for this type of strainer.5.7.2 Monometal strainersThe monometal type is of a single metal with diameters in the range 30 mm to300 mm usually fabricated from brass sheets 2 or 3 mm thick. These have V-shaped slots of varying sizes to permit a proper selection of strainer to suit thesand size in the aquifer. Slot 2.5 mm wide and 30 mm long are usual.5.7.2.1 A thin gravel shroud (50 to 75 mm) is also provided in some cases. Sometimes the brass monometal strainer is strengthened with an inner G.I. slottedpipe for greater rigidity and longer service.5.7.3 Slotted pipe strainers5.7.3.1 Galvanized iron or brass pipes having bigger slots about 3 mm in width and750 mm length are provided in conjunction with pea gravel shroud, 100 mm to250 mm thick.5.7.3.2 The slots are V-shaped with the smaller opening on the outside. The gravelshroud makes it possible to use strainers with large sized slots and abstract alarger sized slots and abstract a larger yield than is otherwise possible. Theslots are preferably to be kept horizontal with un-slotted strips left betweensuccessive rows or columns of slot.The advantage with this type of strainer over the other is that there is lessdamage by galvanic action or chock age due to incrustation.5.7.3.3 Strainers of different makes are marketed claiming specific advantage for each.One such is a slotted mild steel pipe or, coated with special anti-corrosiveplastic paint and provided with an enveloping graded sand shroud boundedwith heat resistant, water repellant Plastic.5.7.3.4 Strainers made of special alloys such as stainless steel (type 304 and 316),monelmatel red brass etc., are also used where indicated and if available.5.7.3.5 High density polythene or PVC and metal combined strainers are gainingpopularity in view of their non-choking, non-corroding and non-incrustingproperties which give long and uninterrupted service.5.7.3.6 Lowering of well assembly:Urban Administration & Development Department Page 213