Why a centrifuge sometimes discharges irregularly

With industrial centrifuges and decanters, a stable liquid discharge is essential for a reliable separation process. Yet process problems occur where the liquid discharge is uneven. The pipe starts to surge, the flow rate varies and pressure differences or irregular discharge flows arise.

This phenomenon is often called surging. One possible cause is air entrainment in the gravity discharge line of the centrifuge. Especially with free-falling liquid, the combination of pipe diameter, flow velocity and height differences can lead to unwanted air entrainment and unstable flow.

What causes surging in a centrifuge discharge?

In a centrifuge or decanter, the separated liquid, also called centrate, is discharged through a pipe. When this discharge is driven by gravity, the pipe must have sufficient capacity to carry the liquid away steadily.

An important consideration is the ratio between the liquid velocity and the characteristic velocity at which gravitational effects manifest in the pipe. The Froude number can be used for this:

Fr = v / √(g × D)

Where:

• Fr = Froude number.
• v = average liquid velocity (m/s).
• g = gravitational acceleration (m/s²).
• D = characteristic pipe diameter (m).

The Froude number helps assess flow behaviour, but on its own it is not a universal criterion for air entrainment or self-venting. Pipe geometry, back pressure and venting options also play an important role.

Why an undersized pipe causes problems

When a relatively large liquid flow is discharged through a small pipe, the liquid can flow at high velocity. In a partially filled pipe, this can draw air in from the centrifuge housing. This can have several consequences:

• Irregular liquid flow.
• Air bubbles and liquid slugs.
• Fluctuating back pressure.
• Temporary siphoning.
• Fluctuations in discharge flow.

If the air cannot escape properly, the discharge can alternately fill and vent. This causes unstable flow.

What solutions are possible?

The right solution depends on the existing pipe configuration and the properties of the liquid. There are several technical options.

1. Increase the pipe diameter

At the same flow rate, a larger pipe diameter lowers the average liquid velocity. This can contribute to more stable flow and reduces the chance of unwanted air entrainment. If the horizontal pipe is increased from 3 to 5 inches, for example, the average velocity drops considerably.

It is important to assess the complete pipe route. A larger horizontal pipe does not automatically solve the problem if a long, smaller vertical section remains further downstream.

2. Provide proper venting

Another solution is to improve the venting options. When air accumulates at the highest point of a pipe route, a suitable vent connection can help prevent unstable flow.

The venting must be compatible with process safety. If the centrate contains solvents or hazardous components, the vent must not simply discharge into the work area. A closed connection to a controlled ventilation or exhaust system may then be necessary.

3. Avoid air pockets and unfavourable pipe geometry

The geometry of the pipe also matters. A continuous downward slope without unnecessary height differences or local accumulations helps keep the flow under control. Pay attention to the following:

• Avoid unnecessary bends and local restrictions.
• Prevent air pockets at high points.
• Ensure sufficient pipe diameter along the entire route.
• Assess the venting of the centrifuge housing.
• Check the back pressure on the discharge side.

When is a centripetal pump a better solution?

When stable gravity discharge is difficult to achieve with the available pipe configuration, a centripetal pump can be an interesting alternative. Some decanters use a centripetal pump system for this, also called a paring disc.

This system uses the rotation of the centrifuge to discharge the liquid in a controlled way. The advantage is that the liquid does not depend solely on free fall through a long gravity line. This avoids a large part of the problems associated with unstable gravity discharge.

Applicability depends on the type of centrifuge, the liquid properties, the required capacity and the process configuration.

Do not forget back pressure and the centrifuge housing

When solving surging, attention sometimes goes only to the discharge pipe. Yet other factors can contribute to the problem. An insufficiently vented centrifuge housing can lead to pressure fluctuations, for example. Fluctuating back pressure on the discharge side can also disturb the liquid discharge.

In a decanter, this can affect the pond depth and the separation performance. It is therefore important to assess the complete discharge system, not just the pipe diameter.

A stable process starts with the right system integration

At Hecht Technology we work with our technology partners on process questions in the chemical and process industries. Through our network, including Riera Nadeu for centrifuge technology, we can help with the integration of separation equipment, liquid discharge and further process steps.

A well-designed installation takes the entire process chain into account: from separation and discharge to further processing and closed product transfer.

Conclusion

Fluctuations in the liquid discharge of a centrifuge can be caused by air entrainment, an unfavourable pipe diameter or insufficient venting. Increasing the pipe diameter, improving the pipe geometry and optimising venting are possible solutions. When gravity discharge causes structural problems, a centripetal pump can be an alternative.