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In this article, you will learn more about the pond depth concerning a decanter centrifuge. We address the following typical questions related to pond depth.
The decanter centrifuge bowl rotation causes the fluid to form a concentric layer or ‘pond’ on the bowl shell. The depth or thickness of this fluid layer is referred to as pond depth. The pond depth can be anywhere from 5 mm up to 50 mm for a conventional decanter centrifuge.
Weir plates (also known as Ring Dams) located on the decanter bowl’s liquid discharge end are radially adjustable to move the liquid discharge opening position. The image below shows a weir plate installed on a decanter centrifuge bowl.
A weir plate with a shorter radius from the bowl rotation axis causes more liquid to be retained in the bowl leading to a deeper pond.
Conversely, a weir plate with a longer radius causes less liquid to stay in the bowl, causing a shallower pond.
The pond depth directly affects the performance of the decanter centrifuge. In the following sections, we delve into pond depth's effect on the separated phases.
As shown in the illustration below, a shallow pond depth reduces the volume of fluid retained in the bowl while also exposing a longer ‘beach’ at the solids discharge side.
Reduced liquid volume in the bowl implies lesser retention time in the bowl under the high centrifugal force. This reduced fluid and solids residence time within the bowl, subject to the high centrifugal force, limits the solid's separation. As a result, the separated fluid tends to have higher solids content.
On the other hand, the shallow pond depth exposes a long beach at the solids discharge end. The separated solids are pushed up this exposed beach towards the solid's discharge ports.
The prolonged exposure of the solids to the centrifugal force allows more liquid to separate from the solids leading to dryer solids.
Therefore a shallow pond depth leads to less clarity of the separated liquid phase and drier solids.
The decanter centrifuge bowl cross-section with a deeper pond depth is shown in the diagram below. As seen, a deeper pond depth retains more volume of the process fluid with solids in the bowl. It also reduces the ‘beach’s exposed section at the solids discharge end.
The larger volume of process fluid in the bowl translates to longer residence time. This extended time allows for more separation of the solids from the fluid under the high centrifugal force. The better separation leads to clearer separated fluid.
The deeper pond depth also reduces the beach’s exposed portion or drying zone in the decanter bowl. This reduction in the beach length does not allow the solids to separate from the liquid, effectively leading to wetter solids.
In other words, a deeper pond depth tends to produce clearer fluid and wetter solids.
The following graph shows the typical effect of pond depth versus decanter performance parameters.
The following table shows the amount of liquid & solids volume for a specific decanter centrifuge. In this case, we are referring to the Alfa Laval NX-314 model. The table refers to various positions of the eccentric port weir plate, as shown below.
|Ring Position||Pond Radius (mm)||Bowl Volume (Gal)||Beach Length (mm)|
The pond depth formed within a rotating decanter significantly affects the decanter's performance. Adjusting this depth allows the operator to balance the separated liquid's clarity versus the separated solids' dryness.
In other words, decanter centrifuge pond depth is the tool to control the trade-off between centrate clarity and cake dryness.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge