Dolphin Centrifuge

Decanter Centrifuge Pond Depth | Effects on Cake Dryness & Liquid Clarity

Table of Contents

30 Second Summary Takeaway

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.

  • What is pond depth?
  • How do you adjust it?
  • How does it affect decanter centrifuge performance?
  • How does the weir plate regulate pond depth?
  • What does an adjustable weir plate work?

Decanter Centrifuge 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.

Adjusting the Depth

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 one such weir plate as installed on a decanter centrifuge bowl.

Pond depth adjustment weir plate

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.

Effects of Pond Depth on Decanter Performance

The pond depth directly affects the performance of the decanter centrifuge. In the following sections, we delve into the effect pond depth has on the phases separated.

Shallow Pond (Longer Weir Plate Radius)

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.

Decanter Centrifuge - Shallow Pond Depth
Decanter with Shallow Pond Depth

Reduced liquid volume in the bowl implies lesser retention time in the bowl under the high centrifugal force. This reduced residence time of the fluid and solids 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.

Deeper Pond (Shorter Weir Plate Radius)

The decanter centrifuge bowl cross-section with 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 exposed section of the ‘beach’ at the solids discharge end.

Decanter Centrifuge - Deep Pond Depth
Decanter with Deep Pond Depth

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.


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Correlation Graph Between Pond Depth and Decanter Performance

The following graph shows the typical effect of pond depth versus decanter performance parameters.

Pond Depth versus Decanter Performance Graph
Pond Depth versus Decanter Performance Graph

Bowl Liquid Volume as a Function of Weir Plate Radius

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.

Alfa Laval NX-314-B31 Pond Depth  & Bowl Volume

Ring PositionPond Radius (mm)Bowl Volume (Gal)Beach Length (mm)
11542.3348
21502.8322
31403.7267
41344.7217
51324.9205
61384.2239
71463.2295
81522.5341
Decanter Pond Depth Weir Plate Setting
Decanter Pond Depth Weir Plate Setting

Summary

The pond depth formed within a rotating decanter has significant effects on the decanter performance. The adjustment of this depth allows the operator to balance the clarity of the separated liquid versus the dryness of the separated solids.

In other words, decanter centrifuge pond depth is the tool to control the trade-off between the centrate clarity and cake dryness.

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