6.3.7 Sewage Treatment Chain (Rectangular Sludge Collector)
Large conveyance: Sewage treatment equipment
One of main uses of large pitch conveyor chain is in water treatment facilities. In a large sewage treatment facility, sewage goes through several tanks in which solid wastes are eliminated by deposition and flotation.
In the silt tank, sand and dirt are removed using vacuum or V-buckets. In the settling tank, sludge in the water, or on its surface, is scraped to the exit with "flights" (boards) installed between two strands of chains at intervals of 3 m (Figure 6.26). Sewage Treatment Chain (ACS Chain) is used in this process (Figure 6.27). Accumulated dirt is removed with pumps.
Cast iron chains were once used in sewage treatment facilities. In such a corrosive environment, chain deterioration could not be avoided. As the volume of chain material decreased due to corrosion, wear was accelerated. To offset the loss of material due to corrosion, cast iron chains became quite heavy, yet the tension required in water-treatment applications did not justify the use of a chain with such high tensile strength.
In the mid-1960s, ACS stainless steel chains were developed in Japan especially for water treatment facilities. The stainless steel construction assured excellent corrosion resistance, so there was no need for extra-heavy cast iron chains. Because of their superior functions, ACS chains have gained wide acceptance.
This chain is also used to convey corrosive objects in general scraper conveyors.
Figure 6.26 Sewage Treatment Chain with Flights Installed
Figure 6.27 Sewage Treatment Chain
Construction and Features
ACS Chain has large-diameter bushings. It does not have rollers.
Plates, pins, and bushings are made of 403 stainless steel. The T-head cotter key is made of 304 stainless steel, which ensures high corrosion resistance.
SF-4 attachments are used for installing flights, and extended pins or LA-1 attachments for installing buckets in the dredger. Both of these attachments are placed on the outer plates. LA-1 attachments are made of heat-treated carbon steel.
Figure 6.28 compares the strength of cast iron chain and stainless steel chain in a long-term test.
Figure 6.28 comparison of Cast Iron and Stainless Steel Sewage Treatment Chain
Use special sprockets. Refer to the manufacturer's catalog.
When cast iron chain was used, cast iron sprockets were also required. Due to corrosion, the area of the sprocket tooth engaging with the chain would lose its original form and round off (Figure 6.29). Upon engagement with the chain, additional stresses would appear that would accelerate wear on the chain and the sprocket even further. As a result, the working life of cast iron chains and sprockets was short.
In an ideal situation, stainless steel sprockets are used with stainless steel chain to ensure the optimum performance. Cast iron sprockets will wear in a similar fashion even if stainless steel chain is used, resulting in increased wear on the chain bushings and shortened chain life. It is a basic point that you must use stainless steel chain and sprockets together. However, stainless steel sprockets are expensive. Chain manufacturers have designed the insert-tooth sprocket to reduce the cost. Only the part of the tooth that engages the chain is stainless steel; the sprocket body is carbon steel (Figure 6.30).
Figure 6.29 Rounding of Cast Iron Sprocket
Figure 6.30 Insert-Tooth Sprocket
Selection and Handling
- The chain speed of a scraper application is slow, 0.3 to 0.6 m/min., and 3 m/min. in the bucket application. The chain tension is the highest during the test period, before water is poured into the tank. Before water is poured into a 40-m tank, one chain is exposed to tension of 10 kN.
- 403 stainless steel chain has sufficient corrosion resistance for most sewage facilities. If there is a high concentration of chlorine (as found in sea water, for example), if there are high levels of sulfur from hot springs, or if the tanks are contaminated, 304 stainless steel should be used, at least for side plates.
Chains for Special Applications
Chains used in water treatment applications are operated at low speeds and not subjected to any heavy shock loads. It is not necessary in this application to consider chains with tensile strength greater than 19 tonf.
For that reason the following chains were developed:
- ACR 810 Chain is a small chain made of 403 stainless steel. It has a tensile strength of 10 tonf. This was the first chain to be used in scraper applications to be equipped with rollers. The rollers reduce wear on the sprocket and the chain. Insert-tooth sprockets have been developed for this chain as well. (See Figure 6.30.)
- Engineered plastic chain (ACP Chain, Figure 6.31), developed in the United States, is a light-weight chain with high corrosion resistance. It does not have rollers (similar to cast iron chain). Due to its light weight (one-half to one-fourth the weight of stainless steel chain), installation is relatively simple.
One of the problems with this chain is that engineered plastic expands and contracts as the water temperature changes. Therefore, it is difficult to keep the chain under constant tension. Tensile strength (25 to 40 kN) is much lower than either cast iron or stainless steel chain.
Figure 6.31 Engineered Plastic Chain (ACP Chain)
The material, strength, and dimensions of engineered plastic chains differ from one manufacturer to another. Compare these points when you select the chain.
For engineered plastic chains, there are plastic kits, which include sprockets, flights, and shoes (see Figure 6.32). Use them together. Never use cast iron sprockets with engineered plastic chain.
Figure 6.32 Sprocket for Engineered Plastic Chain