TANK DEFINITIONS

Helpful Information for Selecting Thickener / Clarifier Tanks

Jack Sederstrom

Tank and Installation Group

WesTech Engineering, Inc.

www.westech-inc.com Rev 2015-10-14

Image: A 70’ ø WesTech Solids Contact Clarifier™ anchor channel tank under construction.

ANCHOR CHANNEL TANK

An anchor channel tank is an at-grade tank with a concrete bottom and steel walls. A rolled steel channel anchors the steel shell to the concrete. The rolled channel is tied into the concrete bottom and embedded into the concrete at the time of pouring. The steel shell is welded to the rolled channel. This is one of the less expensive types of tank because the tank bottom is also the tank foundation. Foundation loads from the shell are distributed around the anchor channel in a generally uniform manner. The design of the foundation and anchorage of the steel channel are the responsibility of the foundation supplier.

Image: A 125’ WesTech thickener steel bottom tank is being installed.

STEEL BOTTOM TANK

A steel bottom tank is an on-grade tank with a steel bottom resting directly on a foundation made of concrete or other media. The shell wall is attached directly to the tank bottom. This type of tank is generally used when the contents are not suitable for containment in concrete. A steel bottom tank is more expensive than an anchor channel tank. The foundation loads are generally distributed on the tank bottom and foundation in a uniform manner. The design of the foundation pad or ring wall base is the responsibility of the foundation supplier.

Image: A bolted false bottom thickener tank is shown before startup.

FALSE BOTTOM TANK

A false bottom tank is an on-grade tank with a steel or concrete bottom resting directly on grade. This type of tank is generally used when earthworks excavation is undesirable and the underflow is not biologically active. This tank is less expensive than an elevated tank because it rests directly on grade. Underflow solids are allowed to build up a “false bottom” of solids to a point that the rakes begin to push solids into the cone. Two underflow pipes are required to connect to the cone and each pipe must have back flushing lines to help prevent plugging. The foundation loads are generally distributed on the tank bottom and foundation in a uniform manner. The design of the foundation pad or ring wall base is the responsibility of the foundation supplier.

Image: An 80’ ø WesTech elevated tank under construction.

ELEVATED TANK

An elevated tank is a steel bottom tank which is set above grade with space between the tank bottom and foundation. The tank is supported by legs or a skirt. The shell wall of the tank is attached to the tank floor. This type of tank is generally used when pumps are required to be under the tank, elevation requirements dictate an elevated tank, when the underflow or sludge is very thick and prone to plugging, or when the tank contents are such that the operator must be able to physically observe and verify tank integrity throughout the plant lifetime. An elevated tank is usually the most expensive type of tank. The loads on the foundation are typically high point loads at each leg. The leg loads are typically tied to the foundation through anchor bolts. Anchor bolt design is the responsibility of the foundation supplier.

Image: Plate steel sections for a WesTech knock down tank.

KNOCK DOWN

A knock down tank is a welded design tank that is fabricated in multiple pieces. The tank must be field erected by a tank crew using standard tank builder’s tools and techniques. It is estimated that 95% of sedimentation tanks are knock down tanks. The interpretation knock down is different from one tank supplier to another depending on the method of transport, site conditions, and manufacturer standards.

Image: A 23’ ø WesTech clarifier – modular in 4 major sections. Lower tank halves shown.

MODULAR TANK

A modular tank is shipped in large sections. Logistics make this design challenging. The right fabricator and engineer must be used since the tank must fit within the constraints on every road traveled. Modular tanks have a longer lead time in the shop since they are built in large, semi-finished sections, but are more cost effective in field erection when experienced tanks crews are used. It is most cost effective to have the same company who supplies the modular tank also function as the erector of the modular tank.

Image: A 24’ ø WesTech clarifier in a bolted tank on grade.

BOLTED TANK (ON GRADE)

Bolted on grade tanks are commonplace as storage tanks. Special considerations must be made in order to use them for clarifiers or thickeners. Launders, weirs, baffles, scum supports, bridge supports, and any other structural load on the tank shell must be fully taken into account on this type of tank. On grade bolted tanks come factory painted with full paint or with a more expensive “glass” fired lining. There are 3 major manufacturing plants in the United States where the majority of all on-grade tanks are made. Most companies claiming to be “manufacturers” of this type of tank are really a distributor who also erects the tanks. Very few “manufacturer” erectors of on grade bolted tanks will know how to properly install a clarifier or thickener into a bolted tank.

Fully bolted shells Fully bolted floors

Full trial fit in the shop All components are match marked

Images: Sample WesTech bolted elevated tank pictures from a 48’ ø bolted thickener.

BOLTED TANK (ELEVATED)

An elevated bolted tank uses only field bolting for assembly. Elevated bolted tanks are usually completely shop painted and require even more time to fabricate than a modular welded tank. Bolted elevated tanks must be fabricated by shops who understand this type of tank fabrication and design. These tanks must be fully trial fitted at the shop before being painted. These tanks require significant design changes in shell layout, floor layout, fabrication, packing for shipment, and lead time for design. Bolted tanks can save time in field erection and require less skilled labor than a welded tank. However, if a bolted tank is not correctly assembled the risk of tank failure and/or leaking is much greater than the risk of a welded tank. The tank must be field erected by a crew using bolted tank builder’s tools and techniques. Elevated bolted tanks are suspended bottom or scalloped bottom designs.

Image: A 12’ ø WesTech clarifier – shop built with clarifier fully installed before shipment.

ELEVATED SUSPENDED BOTTOM TANK

An elevated suspended bottom tank is an elevated tank with a conical steel bottom on legs. Because the floor is a true cone, biologically active slurry can be used in this tank as long as a flexible squeegee is used on the rakes to clean the tank floor. Over time, the paint may wear if a squeegee is used. The suspended floor does not use radial beams nor scallops. The tank floor is a structural component because the floor has no radial beams. The greater the floor slope on a suspended bottom tank, the stronger the floor. Normally a thickener or clarifier in an elevated suspended tank will have a 3:12 slope to keep the floor steel thinner. Steeper slopes are occasionally used for process or structural reasons. Shallower floor slopes will lead to thickener floor materials. A compression ring on the outside of the tank is usually required to keep the floor from sagging under the water pressure and collapsing the tank. This type of tank is generally used when the tank is small and the side shell is fairly shallow or when a very steep cone bottom is acceptable. Generally, most tanks smaller than 20’ in diameter with a shell depth of less than 18’ deep are economical candidates for a suspended bottom design. Outside of this range, a radial beam supported tank is generally more economical.

Image: A 45m ø WesTech paste thickener tank in Russia – deep scallops on the floor.

ELEVATED SCALLOP BOTTOM TANK

An elevated scallop bottom tank is an elevated tank with a steel bottom constructed on radial beams on legs. Because the floor is a scallop, biologically active slurry should not be used in this tank because it will collect in the scallops and become septic. Elevated scallop tanks are used extensively in mining and minerals applications. Scallops allow the tank floor to be thinner because the steel is placed into tension under the water loads on the floor. Scallop bottom tanks generally start to be used on any tank 20’ diameter and larger. The largest elevated scalloped tank designed by WesTech was a 295’ diameter (it was several million pounds for one tank and the client did not move forward with the fabrication). The largest known elevated scallop tank in use as of 2015 is 213’ diameter and is not a WesTech tank. WesTech has fabricated up to 148’ diameter scallop tanks.

Image: 46’ ø WesTech phosphate thickener tank foundation layout – true cone floor.

Elevated True Cone / Elevated Segmented True Cone Radial Beam Tank

This is an elevated tank on radial beams that is structurally too large to be a suspended bottom tank. This is the most rare and most expensive type of elevated tank. The tank floor is generally 3x to 4x+ thicker than a scallop floor and the tank structure is 2x to 3x+ as much as a scallop tank in order to keep the tank floor from flexing. It is used when the tank must be brick lined (acid brick, carbon brick, other brick) or when the tank will contain biologically active sludge which must be removed by squeegees. WesTech has supplied several of these in our history, but they were very special applications.

©WesTech Engineering, Inc.