Tank bottom: flat, conical or ellipsoidal? Impact on draining and cleaning

Two tanks made from the same material and offering the same capacity can behave very differently during draining. The difference is often determined by the bottom: flat, conical or ellipsoidal. Its geometry affects the direction of flow, the amount of product left after the pump stops, deposit accumulation, inspection access and the operation of cleaning devices.

There is no single best shape for every application. A water tank, fermenter, process liquid buffer and dense slurry storage tank have different priorities. When selecting a used process tank, it is therefore important to ask not only about litres and steel grade, but also about the actual geometry of the lowest point, outlet location and bottom support.

What really determines effective draining?

The name of the bottom type alone does not indicate how much medium will remain inside. The slope towards the outlet, nozzle diameter and location, the height of its lower edge, product viscosity, solid particles and the way the entire tank is levelled all matter. A bottom described as flat may have a small process slope, while a cone with a side outlet above its tip can still retain a dead volume.

Before selecting the geometry, describe the full process cycle:

• whether the medium will drain by gravity or be removed by a pump,
• how its viscosity changes with temperature and standing time,
• whether sedimentation, crystallisation, foam or sticky deposits occur,
• how much residue is acceptable before cleaning or changing a batch,
• whether the interior will be cleaned manually, by circulation or by a spray device,
• whether the structure is intended for documented pressure or vacuum conditions.

In a hygienic process, "almost empty" does not necessarily mean "ready for the next batch". Residue in a recess, beside a weld or behind a protruding nozzle may require separate rinsing. In hygienic design, tank geometry strongly affects cleaning device performance, while cleanability and drainability are basic criteria. These are process requirements, not guarantees created by the name of the bottom shape.

Flat bottom: low height and critical foundation conditions

A flat bottom uses space without the additional height required beneath a cone. In large storage tanks it may rest on a prepared foundation, transferring the load over a wide area. This design can suit water and low-viscosity liquids, particularly where simple installation and a low overall height are important.

The main challenge is directing liquid towards the outlet. Even a small foundation irregularity, local deflection or a nozzle installed too high can leave a pool of product. A flat bottom also provides a large surface for deposits. If the tank is opened periodically, good access can assist manual residue removal, but entry into the tank requires a separate safety procedure and cannot be treated as ordinary cleaning work.

In a used tank, inspection must cover more than the centre of the bottom. The shell connection, welds around nozzles, support contact areas and signs of long-term product retention are important. Continued use should not be decided by surface appearance alone. A competent person must select the inspection scope for the material, service history and intended use.

Conical bottom: gravity flow depends on angle and outlet design

A cone directs the medium towards one point, which makes it a natural choice for processes that require frequent draining, sediment removal or dosing of powders and suspensions. A central valve at the cone tip can reduce dead volume. For valuable products, lower residue also increases yield, but the actual result depends on the valve design and its connection to the bottom.

A steeper cone usually promotes flow, but there is no universal angle suitable for every liquid. A yield-stress product, dense paste or suspension can remain even on a clearly inclined surface. An agitator, vibration, heating or flushing may be required, and each solution must be checked against the process and structural strength. A cone also increases tank height and needs supports that provide space for the valve, pump and service access.

During cleaning, the area around the outlet needs particular attention. A valve mounted below the bottom assists draining, but its seat, seal and transition can retain product. Installation at the lowest point and a design without dead spaces support complete draining. In practice, the whole outlet assembly must be assessed, not only the slope of the plate.

Ellipsoidal bottom: structural strength and smooth transitions

An ellipsoidal bottom is a formed head with a curved profile. It is found in process tanks and vessels where shell geometry affects the transfer of pressure or vacuum loads. However, the presence of this head does not prove that every such tank is suitable for pressure operation. Permitted parameters must follow from the design, material, thickness, workmanship, documentation and current technical condition.

A smooth transition can reduce sharp corners and support surface coverage during cleaning, while a bottom nozzle at the lowest point promotes drainage. At the same time, the profile is shallower than a steep cone, so heavy deposits will not always move to the outlet by themselves. The effect also changes if the head curves upwards or the outlet is on the side. The term "ellipsoidal" is not a substitute for a drawing with dimensions and nozzle locations.

When comparing the three options, consider not only draining but also building height, transport, agitator position, access beneath the tank, support loads and modification cost. For a tank selected from the Agrotex range, paid modernization may include improving the outlet or nozzle arrangement. The service is agreed before the purchase is finalized, and its scope is matched to the process and the construction of the specific tank. Selecting a tank whose existing geometry is already close to the process requirements is often the most economical approach.

Summary

A flat bottom limits overall height and may simplify support, a conical bottom directs the medium towards the outlet, and an ellipsoidal bottom combines a curved profile with structural characteristics important for process vessels. None of these shapes alone guarantees complete draining or easy cleaning. Check the slope, lowest point, valve, viscosity, deposits, cleaning method and tank documentation. To compare available designs for a specific process, describe the medium and required draining method through the Agrotex contact form.