Components and accessories for sedimentation units

Tanks / Basins materials of construction

steel - industrial 30m/100 ft

Concrete - municipal and larger industrial

wood

Compacted earth - very large units with impermeable liners as economical option

Plastic sheeting

Soil

Cement

Tanks / Basins basis of selection of materials of construction

Environmental cost

Availability

Topography

Water table

Ground conditions

Climate

Operating temperature

Chemical-corrosion resistance

Drive support structures’ three basic drive mechanisms

bridge supported mechanism

center column-supported mechanism

Traction-drive thickener containing a centercolumn-supported mechanism with the driving arm attached to a motorized carriage at the tank periphery

Bridge supported thickeners

Common: up to 30m in diameter, Max diameter - 45 m

ability to transfer loads to the tank periphery

ability to give a denser and more consistent underflow concentration with the single drawoff point

fewer structural members subject to mud accumulation

access to the drive from both ends of the bridge

lower cost for units smaller than 30 m in diameter

Center column supported thickeners

20m diameter

mechanism is supported by a stationary steel or concrete center column

raking arms are attached to a driving cage which rotates around the center column

Traction thickeners

most adaptable to tanks >60 m (200 ft) in diameter

Maintenance generally is less difficult than with other types of thickeners, which is an advantage in remote locations.

The drive may be supported on the concrete wall (the wall would be a structural member) or supported outside the wall on the ground (a standard tank wall could be used).

(1) no practical lifting device can be used (2) operation may be difficult in frosty climates (3) the driving torque effort must be transmitted from the tank periphery to the center, where the heaviest raking conditions occur

Traction thickeners (rakeless ultrahigh-rate thickeners, elevated tanks up to 20 m diameter

pro:no drive high throughput rate small footprint

con:height of elevated tank

Drive Assembles - key component of a sedimentation unit

usually have steel / iron main spur gears mounted on bearings, alloy-steel pinions, or a planetary gear.

includes a torque-measuring system with torque indicated on the mechanism and often transmitted to a remote indicator.

If the torque becomes excessive, it can automatically activate such safeguards against structural damage as sounding an alarm, raising the rakes, and stopping the drive.

(1) the force to move the rakes through the thickened pulp and to move settled solids to the point of discharge

(2) the support for the mechanism which permits it to rotate

(3) adequate reserve capacity to withstand upsets and temporary overloads

(4) a reliable control which protects the mechanism from damage when a major overload occurs

RAKE-LIFTING MECHANISMS should be provided

provided when abnormal thickener operation is probable

▪ Abnormal thickener operation or excessive torque may result from:

insufficient underflow pumping

surges in the solids feed rate

excessive amounts of large particles

sloughing of solids accumulated between the rakes and the bottom of the tank or on structural members of the rake mechanism

miscellaneous obstructions falling into the thickener

RAKE-LIFTING MECHANISMS

The lifting mechanism may be set to raise the rakes automatically when a specific torque level (e.g., 40% of design) is encountered, continuing to lift until the torque returns to normal or until the maximum lift height is reached.

Generally, corrective action must be taken to eliminate the cause of the upset.

Once the torque returns to normal, the rake mechanism is lowered slowly to “plow” gradually through the excess accumulated solids until these are removed from the tank

Motorized rake-lifting devices typically are designed to allow for a vertical lift of the rake mechanism of up to 90 cm (3 ft).

The cable arm design uses cables attached to a truss above or near the liquid surface to move the rake arms, which are hinged to the drive structure, allowing the rakes to raise when excessive torque is encountered.

RAKE MECHANISM

assists in moving the settled solids to the point of discharge

aids in thickening the pulp by disrupting bridged floccules, permitting trapped fluid to escape and allowing the floccules to become more consolidated

designed for specific applications, usually having two long rake arms with an option for two short rake arms for bridgesupported and center-column-supported units

Traction units usually have one long arm, two short arms, and one intermediate arm

The conventional design typically is used in bridge-supported units, while the dual-slope design is used for units of larger diameter.

Rake blades can have attached spikes or serrated bottoms to cut into solids that have a tendency to compact.

Lifting devices typically are used with these applications.

Rake-speed requirements depend on the type of solids entering the thickener.

Peripheral speed ranges used: ▪ for slow-settling solids, 3-8 m/min (10-25 ft/min)

for fast-settling solids, 8-12 m/min (25-40 ft/min) ▪ for coarse solids or crystalline materials, 12-30 m/min (40-100 ft/min)

Feedwell

designed to allow the feed to enter the thickener with minimum turbulence and uniform distribution while dissipating most of its kinetic energy

Feed slurry enters the feedwell, which is usually located in the center of the thickener, through a pipe or launder suspended from the bridge.

To avoid excess velocity, an open launder normally has a slope no greater than 1-2%.

Pulp should enter the launder at a velocity that prevents sanding at the inlet.

With nonsanding pulps, the feed may also enter upward through the center column from a pipeline installed beneath the tank.

The standard feedwell for a thickener is designed for a maximum vertical outlet velocity of about 1.5 m/min (5 ft/min).

High turbidity caused by short-circuiting the feed to the overflow can be reduced by increasing the depth of the feedwell.

When overflow clarity is important or the solids specific gravity is close to the liquid specific gravity, deep feedwells of large diameter are used, and measures are taken to reduce the velocity of the entering feed slurry

Shallow feedwells - may be used when overflow clarity is not important, the overflow rate is low, and/or solids density is appreciably greater than that of water.

Some special feedwell designs used to dissipate entrance velocity and create quiescent settling conditions split the feed stream and allow it to enter the feedwell tangentially on opposite sides.

The two streams shear or collide with one another to dissipate kinetic energy.

When flocculants are used, often it will be found that the optimum solids concentration for flocculation is considerably less than the normal concentration, and significant savings in reagent cost will be made possible by dilution of the feed prior to flocculation.

achieved by recycling overflow or more efficiently by feedwell modifications, including openings in the feedwell rim

allow supernatant to enter the feedwell, and flocculant can be added at this point or injected below the surface of the pulp in the feedwell

Another effective means of achieving this dilution prior to flocculant addition

This approach utilizes the energy available in the incoming feed stream to achieve the dilution by momentum transfer and requires no additional energy expenditure to dilute this slurry by as much as three to four times.

Overflow Arrangements

Clarified effluent typically is removed in a peripheral launder located inside or outside the tank.

The effluent enters the launder by overflowing a V-notch or level flat weir, or through submerged orifices in the bottom of the launder.

Uneven overflow rates caused by wind blowing across the liquid surface in large thickeners can be better controlled when submerged orifices or V-notch weirs are used.

Radial launders are used when uniform upward liquid flow is desired in order to improve clarifier detention efficiency.

This arrangement provides an additional benefit in reducing the effect of wind, which can seriously impair clarity in applications that employ basins of large diameter.

The hydraulic capacity of a launder must be sufficient to prevent flooding, which can cause short-circuiting of the feed and deterioration of overflow clarity.

Standards are occasionally imposed on weir overflow rates for clarifiers used in municipal applications; typical rates are 3.5 to 15 m3/(h⋅m) [7000 to 30,000 gal/(day⋅ft)], and they are highly dependent on clarifier side-water depth.

Industrial clarifiers may have higher overflow rates, depending on the application and the desired overflow clarity.

(in overflow arrangements) Launders can be arranged in a variety of configurations to achieve the desired overflow rate.

Alternatives to improve clarity:

• annular launder inside the tank (the liquid overflows both sides)

• radial launders connected to the peripheral launder (providing the very long weir that may be needed when abnormally high overflow rates are encountered and overflow clarity is important)

• Stamford baffles, which are located below the launder to direct flow currents back toward the center of the clarifier

In many thickener applications, on the other hand, complete peripheral launders are not required, and no difference in either overflow clarity or underflow concentration will result through the use of launders extending over only a fraction (e.g., one-fifth) of the perimeter.

For design purposes, a weir-loading rate in the range of 7.5 to 30.0 m3/(h⋅m) [10 to 40 gpm/ft] can be used, the higher values being employed with well-flocculated, rapidly settling slurries.

The overflow launder required may occupy only a single section of the perimeter rather than consisting of multiple, shorter segments spaced uniformly around the tank.

UNDERFLOW ARRANGEMENTS

Concentrated solids are removed from the thickener by use of centrifugal or positive displacement pumps or, particularly with large-volume flows, by gravity discharge through a flow control valve or orifice suitable for slurry applications.

Due to the risk to the thickener operation of a plugged underflow pipe, it is recommended that duplicate underflow pipes and pumps be installed in all thickening applications.

Provision to recycle underflow slurry back to the feedwell is also useful, particularly if solids are to be stored in the thickener.

Three basic underflow arrangements:

• (1) the underflow pump adjacent to the thickener sidewall with buried piping from the discharge cone

• (2) the underflow pump under the thickeners or adjacent to the sidewall with the piping from the discharge cone in a tunnel

• (3) the underflow pump located in the center of the thickener on the bridge, or using piping up through the center column

PUMP ADJACENT TO THICKENER WITH BURIED PIPING

This arrangement of buried piping from the discharge cone is the least expensive system but the most susceptible to plugging.

It is used only when the solids do not compact to an unpumpable slurry and can be easily backflushed if plugging occurs.

Typically, two or more underflow pipes are installed from the discharge cone to the underflow pump so that solids removal can continue if one of the lines plugs.

Valves should be installed to permit flushing with water and compressed air in both directions to remove blockages.

UNDERFLOW PUMP IN A TUNNEL

A tunnel may be constructed under the thickener to provide access to the discharge cone when underflow slurries are difficult to pump and have characteristics that cause plugging.

The underflow pump may be installed underneath the thickener or at the perimeter.

Occasionally thickeners are installed on legs or piers, making tunneling for access to the center unnecessary.

A tunnel or an elevated thickener is more expensive than the other underflow arrangements, but there are certain operational and maintenance advantages.

The hazards of working in a tunnel (flooding and interrupted ventilation, for example) and related safety regulations must be considered.

CENTER-COLUMN PUMPING

may be used instead of a tunnel.

Bridgemounted pump with a suction line through a wet or dry center column

• The pump selection may be limiting, requiring special attention to priming, net positive suction head, and the maximum density that the pump can handle.

Underflow pump located in a room under the thickener mechanism and connected to openings in the column

• Access is through the drive gear at the top of the column