How a U-section distribution pipe works on a flap filter floor — its structure, the two air cushions, the parts it is supplied with, the backwash intensities it is sized for, and what to check when a filter is converted.
← Back to Flap Filter Bread PipeA flap filter is a gravity filter that uses flap valves instead of a conventional valve gallery. Backwash is run with the cell outlet closed, so the wash water is not discharged in the usual way and the bed is not carried out with it. That places the whole distribution duty on the filter floor: every part of it has to collect filtrate in one direction and deliver air and water in the other.
The bread pipe — U-shaped filter pipe, or multifunction filter pipe — is the element that carries that duty. It is a profiled pipe, arched in section, laid in rows across the filter floor and connected to vertical distribution risers. During filtration water passes down through the media and enters the pipes through their openings on the way to the clear-water channel. During backwash the flow reverses and air, then air together with water, then water alone is spread back up through the same openings and through the media bed.
One element therefore performs three duties: water distribution, air distribution and filtrate collection. A filter floor built this way is simple — a level concrete base, rows of pipes on support blocks, and the riser connections. There is no plate to level and no nozzle to set unit by unit, which is why the arrangement is used both on new cells and on filters being upgraded.
The section is U-shaped: an arched crown sitting on a flat base flange, the loaf-like profile that gives the pipe its name. The flow openings are set in a staggered — plum-blossom — pattern along the body rather than in a single line, so the pipe distributes and collects along its whole length instead of at a few points. The flat base flange is the seating face on the support blocks and the reference for lining the rows up to a common level.
A flap filter distribution system is made up of the following parts, fitted in this order:
The pipes are set into the graded gravel support layer: gravel is placed between the pipes to the bed grading, which holds them against movement and passes the air and water leaving the pipe openings up into the media as an even flow. This intermediate layer is what turns a row of openings into a uniform flow across the bed, and it is the reason the arrangement is not sensitive to how the bed is loaded or to the accuracy of a large number of individual fixings.
Air distribution through pipework fails in a characteristic way: the air arrives in slugs and the bed is scoured unevenly, strongly in one place and hardly at all in another. The pipe system addresses this with two air cushions — an upper cushion in the top of the collection and distribution channel, and a lower cushion at the crown of the transverse pipes. The upper cushion absorbs the pressure fluctuation coming from the blower; the lower one spreads the flow along each pipe.
With both cushions in place, air and water reach the whole cell rather than pulsing down the nearest available path, and the bed is cleaned evenly from the inlet end to the far end. This is the feature that distinguishes the arrangement from a plain large-resistance pipe distribution system.
The wash sequence is run in three steps, and the pipes are sized for the intensities that the filter design sets. The values below are published flap filter design values; they are given here for context and to show what the pipe is sized for — they are not a statement of pipe performance.
| Step | Air intensity | Water intensity |
|---|---|---|
| 1 — Air only | 15–18 L/(s·m²) | — |
| 2 — Air and water together | 15–18 L/(s·m²) | 4–8 L/(s·m²) |
| 3 — Water only | — | 14–16 L/(s·m²) |
In step 1 the compressed air leaves the pipe openings and lifts the bed so that the grains rub against one another and shed the solids held on their surface. In step 2 air and water are injected together, which disturbs the bed further and carries the loosened material clear. In step 3 the air is shut off and water alone is distributed evenly, flushing the remaining solids out of the bed. The sequence is normally run twice, followed by a settling pause of some tens of seconds while the media settles back under its own weight before the cell returns to service.
| Parameter | Value |
|---|---|
| Material | HDPE (polyethylene) · stainless steel 304 |
| Section | U-shaped — arched crown on a flat base flange |
| Openings | Staggered (plum-blossom) pattern along the pipe body |
| Length | Made to order to the filter drawing; transverse runs are published at 2–4 m |
| Section size | On request — matched to the filter drawing; published supplier sections vary, so the drawing governs |
| Service temperature | −20 to 80 °C (published range) |
| Load capacity | 1 t/m² (published) |
| Friction coefficient | Below 0.05 (published) |
| Hygiene | HDPE is non-toxic and odourless and meets hygiene testing requirements (published) |
| Corrosion behaviour | Acid- and alkali-resistant, low adsorption, non-clogging (published) |
| Media beds | Single-layer, dual-layer or multi-layer, with a graded gravel support layer between the pipes |
| Fixing system | Support blocks, riser clamps, upper and lower riser locating plates, 304 expansion bolts |
| MOQ / price | On request |
Both materials are preformed, so the site work is short: the pipes are placed on their support blocks, brought to a common level and clamped to the risers. The demand on the civil works is limited to a level floor and the riser positions, which is why the same system suits a new cell and a conversion.
Bread pipes are used for water distribution, air distribution and filtrate collection in:
The pipe rows are laid across the cell and connected to the risers that run down to the distribution channel, so the number of pipes and their spacing follow the cell geometry rather than a fixed catalogue size. Where a filter is being converted, the rows are usually added to the existing floor and the riser positions are set to suit the new layout.
Compiled from public supplier and industry references for flap filter distribution systems, together with the product photographs on this site. Section size, slot opening, length and material are made to order and are confirmed against the filter drawing and the backwash rates before shipment. Values marked as published are ranges reported in those references. Price is quoted on request.