6.3.8 Sewage Treatment Chain (Bar Screen)
Large conveyance: Sewage treatment equipment
Application Example
At the water gate of sewage treatment plants, there are gratings—called bar screens—arranged lengthwise to catch floating objects. In addition to water treatment plants, bar screens may be installed at the mouths of rivers. In some bar screen setups, chains are set on guide rails and used as wide gratings or screens. (See Figures 6.33 and 6.34.)
Eventually gratings fill up with contaminants, and they have to be cleaned. A comb-shaped rake installed between two strands of chain is used for this purpose.
Figure 6.33 Bar Screen Chain Set-up
Figure 6.34 Bar Screen Chain
Construction and Features
- Bar Screen Chain is constructed like roller chain. There are three specifications in this kind of chain. Select the appropriate type based on the corrosiveness of the operating environment (Table 6.5).
- Available attachments—Y and A-2 (Type I)—are made of heat-treated steel.
- S-rollers and F-rollers may be used with this chain. The difference between the two include the following points: S-roller: Adapted to a rake with wheels. The rake rotates and sweeps out the waste. (This is sometimes called a rotating-rake design.) F-roller: Adapted to the rake without wheels. The rake is fixed on the chain. The F-rollers support the weight of the rake. In an F-roller set-up, the flange may alternate sides every one or two rollers. This arrangement prevents derailment. Because the inside width (W) of the chain is larger with the F-roller, the chain is called PJW specification to distinguish it from others (Figure 6.35). Each chain attachment is exposed to high load, because there are only two or three rakes installed on the chain. Y attachments are used in the rotating-rake design (Figure 6.36), and A-2 (Type I) attachments are used in the fixed rake type (Figure 6.37). Both attachments have additional features that increase their strength: the end of the connecting pin is threaded and equipped with a nut to prevent falling off.
| Side Plate | Pin, Bushing, Roller | |
|---|---|---|
| Standard Series
PJ Series SJ Series |
Heat-Treated Steel
Heat-Treated 400 Series Stainless Steel 304 Stainless Steel |
Heat-Treated 400 Series Stainless Steel
Heat-Treated 400 Series Stainless Steel 304 Stainless Steel |
Figure 6.35 PJW-Specification Bar Screen Chain
Figure 6.36 Y Attachment
Figure 6.37 A-2 (Type I) Attachment
- F-rollers are exposed to extremely high load from the rakes attached to the chain. To extend the working life, bushings usually have a larger bearing area, which reduces bearing pressure.
- Common chain sizes have tensile strength within the range of 68 to 490 kN.
Sprockets
Although these chains are based on RF conveyor chain, they require special sprockets because of the pitch (152.4mm = 6 inches). For S-roller type, insert-tooth sprockets are available.
Selection and Handling
- To select this chain, verify the chain tension required, and make sure to confirm the roller load and attachment strength (twisting moment and bending moment).
- Allow for a safety factor to withstand peak loads during jam-ups.
- Chain rollers might be exposed to high loads when following the curves of the guide rail. Make sure you take this load into consideration when selecting the chain. Minimize tension from the take-up (Figure 6.38).
- Of course, you should avoid contaminating the water with oil, but when the test run of the equipment is performed without water, the chain's moving parts should be lubricated.
Figure 6.38 High Roller Load Due to Curved Rail
Technical Trends
Bar Screen Chain is required to perform with low noise levels, because it is now commonly used near populated areas.
