Regulations
Screening Mesh
The screening mesh, which must not have apertures greater than 0.65 mm (Regulation 16.2), is available in a variety of materials and as it is with the unit’s casing, it requires to be made from WRAS approved materials.
Generally, for the smaller sizes, say 15 mm to 54 mm dia., the McAlpine range of fittings constructed from plastic materials, will be satisfactory for the majority of cistern and tank installations and are available at low cost. It could be said, however, the 54 mm size is not as well thought through as it is for the smaller sizes in the range. This size being a concoction of two bellmouth fittings with the screen sandwiched between.
This ‘inline’ screen is out of step with the other ‘right angle’ designed models and is easily incorrectly horizontally installed and no means of inspecting the screen is available without dismantling the unit from the pipeline. A further criticism is the fact the screening area is much reduced proportionately when compared with the other sizes in the range.
In the smaller sizes the simple action of lifting the dust cover lid enables rapid, visual inspection of the screen and as necessary, its removal for cleaning.
Given Overflows require to be sized at 2 x the inlet diameter, it is common on larger tanks that 80, 100, 150 or 200 mm dia. sizes are required.
When the need arises these larger Overflow units can be fabricated from PVC pipe incorporating a st. st. 0.65 mm screen mesh.
Generally available, this mesh provides in the order of 40% free area for flow passage. Therefore the total screen area requires being 2.5 x larger than the Overflow nominal bore area. An 80 mm unit requires a minimum mesh area of 12570 sq. mm. i.e. A flat, circular screen would require to be 130 mm in diameter to provide the area necessary with no safety margin. (250 mm dia. for a 150 mm unit).
It is standard practice for any screen to incorporate 100% additional area for operational safety reasons and these units should be no different. Though I believe the McAlpine Fittings falls short in this regard.
Tank Specification
Therefore, to restrict the size envelope of these larger units, the practical approach is for the screen to take the form of a cylinder, located centrally within a casing.
