Mechanical Process and Cost Analysis Between Peripherally and Center Driven Circular Collector Mechanisms
Nils Young, PE, Director of Engineering
In wastewater treatment processes the mechanisms used to rake solids in circular collectors (grit collectors, gravity thickeners and clarifiers) can be classified into two broad categories. The two types are those that are center driven and those that are peripherally driven.
In Europe peripherally driven mechanisms are the most commonly used mechanisms for both Primary and Final Settlement. In the United States, on the other hand, Primary and Final Sludge settlement is usually carried out in tanks with centrally driven rakes or scrapers. In both the US and Europe Center driven thickeners are normally used for sludge thickening.
There are several reasons for this clearly demarcated difference in design approach. It is most likely that the biggest factor is tradition. If this is the case it is probable that some equipment is being misapplied on technical and economic grounds. Considering the widespread use of circular collector equipment it seems unusual that there is so little discussion upon the benefits of the two approaches.
This article examines the mechanical differences between the two different designs and the implications that they have on maintenance and capital costs. The differences in process performance and the resultant impact in process economics are then discussed.
Outside of the water industry this sedimentation equipment is generally center driven and few differences in approach exist between North America and Europe.
It is important to note that clarifier design standards and safety factors used in the United States are higher than those used in Europe. The differences in engineering designs stem from European low cost concerns vs. American consulting engineering specifications. The cost of peripheral driven mechanism may appear to be less expensive than a center driven mechanism, however comparisons are generally not made on equal grounds. A fair amount of extra steel and larger gearboxes are used in the design of center driven mechanisms as opposed to peripheral driven mechanisms and if the peripheral driven clarifiers were to adhere to the same structural standards, there would not be a cost difference.
Another cost generally not considered when comparing the two systems is the extra amount of labor and materials for the tank and erection. These will be discussed later.
Center Driven Scraper Mechanisms: Center driven mechanisms have two different configurations; bridge mounted and pier mounted. The two different designs are described below.
Bridge Mounted Center Drives: The bridge mounted configuration has a bridge spanning the full diameter of the tank, at the center of which is mounted a drive. The output of the drive is a shaft, which is connected to the main rake drive shaft or torque tube. As tanks increase in diameter the torque generated, as a reaction to driving the rakes through the settled solids in the sludge blanket increases proportionally to the square of the tank diameter. This means that the strength of the bridge and rakes need to increase rapidly as the tank diameter increases. The weight of the rakes that need supporting from the drive and bridge will also increase with an increase in tank diameter. This in turn means an extra increase in the strength of the bridge. At a certain diameter it becomes more economical to construct a tank with a Pier Mounted Center drive.
Pier Mounted Center Drives: The pier mounted center drive requires the construction of a pier or post in the center of the thickener tank. A drive is positioned on this post. The drive has an output drum on the outside onto which is suspended a rake cage. The rake cage rotates and supports the rake arms.
Peripheral Driven Scraper Mechanisms: There are two categories of peripherally driven scraper mechanisms, namely half-bridge and full bridge scrapers. Peripherally driven units typically use small commercial gearboxes with elastomeric traction wheels. The tank wall needs to be wide, and the rim must be parallel, flat and in true circular form to ensure that the drive can operate correctly. It is also necessary to use a slip ring contactor to get power from the static tripod in the center of the tank out to the drives. All electrical power must be fed underground and come up through the center column.
Half Bridge Scraper – Peripherally Driven: Peripherally driven mechanisms require a static support in the center of the tank. A bearing located on the top of the static support allow the rotation of the complete bridge that spans from the center of the tank to the wall.
Suspended from the bridge are a set of rake arms. It is necessary to feed a half bridge scraper through a riding feed pipe coming underneath the tank. This is because, unlike on a center driven machine, a feed pipe going into the center of the tank to the feed well would collide with the rake arms suspended from the bridge.
Full Bridge Scraper – Peripherally Driven: Full bridge scrapers also require a static support or tripod at the center of the tank. Unlike a half bridge scraper, the bridge spans the full diameter of the tank passing over the center where it spins on the center bearing. Both ends of the full bridge scraper need to be driven and some form of load balancing is therefore necessary to ensure even wear across the two gearboxes.
Mechanical Design Differences and Discussion on Capital Cost and Maintenance Impact
As discussed above it is necessary to feed peripherally driven machines in the center of the tank through a rising feed pipe from underneath the tank. In treatment works that rely on gravity flow through the works this is the best way of conserving head. It does however require a level of excavation and concrete to bury the incoming feed pipe.
With a center drive it is easiest to convey the feed to the center of the tank through the side of the clarifier and supported under the bridge. This avoids any excavations to bury the feed pipe.
The motor may limit the torque available from a peripherally driven mechanism because the traction goes through a lower reduction than on a centrally driven mechanism
The peripherally driven rakes may be limited by the friction available between the traction wheel and the tank wall.
With a peripherally driven Half-bridge Scraper, it is not possible to obtain a raking action more frequently than once per revolution of the mechanism; however with a centrally driven mechanism it is normal to use two rake arms with the possibility of four arms if required. It is also possible to obtain a double sweep using a Full-bridge Scraper.
The less frequently scraped the tank floor the more likely is short-circuiting.
Because peripheral driven systems are designed for light duty applications, most rakes are designed to “swing up” and ride over the sludge when an overload of the clarifier occurs. The peripheral driven system simply will not perform under more demanding circumstances.
Concrete vs Steel Concrete and or steel tanks can be used for centrally or peripherally driven scraper mechanisms.
In the case of peripheral drives it is always necessary to ensure that the tank wall or specially installed rail are circular, flat and smooth within a tight tolerance.
The covering tanks with the objective of odor containment are becoming more widespread. This is inherently more difficult if a rotating bridge is being used.
Peripherally driven rake mechanisms carry a risk of personal injury as a result of something becoming trapped between the driving wheel and the tank wall. There is also a risk that somebody dis-embarking from a rotating bridge would fall and cause injury.
Neither of these two hazards arises with Center Driven mechanisms.
Peripherally driven rake mechanisms may include torque overload protection; however they are not generally as complete and accurate as most center driven mechanisms which include alarm and motor cut-off switches, a shear pin, and visual torque indication.
The Effect of Limited Torque
Torque will be a function of weight on the drive wheel and coefficient of friction = +/- 1. So if weight is 1,000 lbs., thrust could be 1,000 lbs. However, I would not depend on a Cf = 1.0., use a Cf = 0.5 max. Therefore, thrust = 500 lbs. max with 1,000 lb. load. Obviously, one could add weight over the drive wheel to increase thrust - like putting sandbags in your trunk when driving on snow or ice. Of course, ice, snow, or pond scum on the rim of the clarifier would also greatly reduce Cf.