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A paring disc pump within the disc centrifuge bowl converts the rotational momentum of the fluid into pressure. This pressure causes the liquid to discharge out of the bowl.
Any restriction in the discharge passage of the liquids exerts pressure on the bowl. This externally created pressure is known as centrifuge backpressure.
In this article, we discuss the benefits of centrifuge backpressure, methods to create backpressure, and the disadvantages of back pressure.
Centrifuge backpressure enables higher separation efficiency and, therefore, better results. The backpressure also has some operational benefits to centrifugation.
By applying backpressure, the discharge liquid in the outlet chamber accumulates. This accumulation of fluid causes a fluid layer within the paring disc housing area. The adequate depth of this fluid layer ensures immersion of the paring disc in the centrate.
Therefore, the paring disc pump can perform its function of pumping out the separated fluid.
Without the backpressure, an air gap might develop between the paring disc and fluid layer. This gap leads to the inconsistent flow of the separated fluid.
Given the dynamic flow within the centrifuge bowl, the oil-water interface formed within the bowl is not always stable.
The back pressure on the liquid transfers to the oil side of the oil-water interface. This pressure has a stabilizing effect on the interface, which leads to better separation efficiency.
The externally exerted backpressure transfers from the paring disc into the light liquid phase inside the bowl. This pressure helps the light phase liquid push the heavy phase (water) column radially outward.
The outward movement of the oil-water interface expands the light liquid column radially. The result is an increase in the diameter of the light fluid column. A larger diameter increases the centrifugal force acting on the liquid, leading to better separation.
As explained in our Disc Centrifuge Breakover article, a liquid breakover occurs when the light fluid passes through the heavy fluid outlet.
Under normal conditions, the control system monitors the backpressure in the light fluid discharge line. In case of a breakover, this pressure drops to zero as there is no fluid flowing in the clean outlet. This pressure drop allows for the detection of the breakover condition.
Disc centrifuges are flow-through type separation devices. The separated clean fluids discharge through different outlets from the centrifuge. A paring disc pump creates pressure, which causes the fluids to flow out.
A flow control device, such as a valve, restricts the liquid’s flow and builds pressure in the line. This pressure is known as back pressure.
Throttling of the valve in the liquid outlet discharge pipe causes the backpressure to increase. Conversely, opening the valve reduces the back pressure.
We have encountered several centrifuge installations where the liquid discharge pipe connects to a far-away tank with elevation. The resistance in the pipeline and head pressure from the height causes inadvertent backpressure on the centrifuge.
In some cases, pipeline restrictions such as solids buildup can restrict the flow, which leads to back pressure. This source of pressure is not apparent and unexplained.
It is also important to note that increasing the centrifuge flow during operation (without adjusting the backpressure valve) will increase the backpressure. Conversely, reducing the flow without throttling the valve reduces back pressure.
Therefore, any flow adjustment of the centrifuge should accompany backpressure valve adjustment.
Backpressure generated unintentionally has a direct effect on the centrifuge performance and functionality of the system. We explain some of these adverse effects below.
Backpressure exerted on the centrifuge bowl causes the light phase liquid to push the heavy phase liquid. This phenomenon causes the oil-water interface to move radially outwards.
However, too much backpressure can push the interface beyond the top disc. This interface movement allows the light phase to discharge through the heavy phase outlet, and we have a breakover condition.
Any restriction in the discharge line of the centrifuge is a resistance to flow through the line. This resistance causes the flow to reduce.
Therefore, any backpressure, intentional or unintentional, causes the flow rate to decrease.
Consider a setup where the centrifuge discharge pipe connects to the bottom of the clean liquid tank. As the liquid accumulates in the tank, the level in the tank rises.
The rising liquid level increases head pressure from the tank liquid, increasing the centrifuge discharge line’s backpressure.
In this case, we have a situation where the backpressure on the centrifuge varies (increasing) as the centrifuge operates. Varying backpressure affects the position of the oil-water interface within the centrifuge bowl.
If this pressure exceeds the breakover threshold, the centrifuge will experience a breakover.
If the operator reduces the mechanically applied pressure to compensate for this increasing backpressure, the centrifuge will not operate at a consistent efficiency level. This changing efficiency leads to bad separation results.
Based on the above discussion, centrifuge backpressure is an essential factor that affects centrifuge performance. Monitoring and controlling this pressure is crucial for the reliable operation of the centrifuge. The following are two ways to check and control backpressure in centrifuges.
The easiest and most efficient way to monitor backpressure is by installing a pressure gauge upstream of the valve.
The indicator will display the pressure in the discharge line. Manual opening of the control valve reduces the back pressure.
Installing a pressure sensor on the outlet line automatically monitors the back pressure. The centrifuge controller has a preset pressure threshold.
When the line’s pressure exceeds this level, the controller activates corrective action, such as triggering an alarm or adjusting a servo valve. The opening or closing of the valve regulates the backpressure.
In summary, a centrifuge operator should be aware of centrifuge backpressure, its importance in centrifuge performance, and ways to adjust it. The article above highlights most of the issues and fixes related to centrifuge backpressure.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge