Jul-2024

Foam in the field: how proper level instrumentation can alleviate issues

The objective of this paper is to review challenges presented by foam and how to realise operational improvements through proper level instrumentation. Many liquid tanks in the chemical, refining, food & beverage, life sciences and other process industries can, at times, have foam present. The dynamic nature of foam means there is no ‘one-size-fits-all’ measurement solution.

Ametek

Article Summary

In addition to knowing what type of measurement is required, it is also critical to consider the characteristics of the foam itself. There are many reasons why foam can be present in a tank, including the introduction of air/gas into the product being measured or the operation of agitators/mixing blades. Regardless of the source, care must be taken when choosing a level measurement technology to prevent unnecessary errors or loss of measurement.

The objective of this paper is to review challenges presented by foam and how to realise operational improvements through proper level instrumentation.

Foam challenges
Foam’s dynamic mixture of gas and liquid can present difficulties for traditional level technologies that are commonly employed for liquid level measurement.

In general, there are three potential level measurements that would involve a liquid tank with foam:
Most of the time a continuous measurement of the liquid is desirable, as this is the valuable product being processed. Although a level switch may suffice when the goal is to simply monitor the presence or absence of the foam layer at a specific point, a transmitter can provide additional insight into vessel contents to improve the overall process. Many tanks have both a continuous level transmitter in operation along with a point level switch.

Determining a technology’s suitability on a foaming application is dependent on the goal relative to the measurement: detect foam (point or continuous), ignore foam (liquid only measurement) or measure both foam and liquid.

Most of the time a continuous measurement of the liquid is desirable, as this is the valuable product being processed. Although a level switch may suffice when the goal is to simply monitor the presence or absence of the foam layer at a specific point, a transmitter can provide additional insight into vessel contents to improve the overall process. Many tanks have both a continuous level transmitter in operation along with a point level switch.

Most of the time a continuous measurement of the liquid is desirable, as this is the valuable product being processed. Although a level switch may suffice when the goal is to simply monitor the presence or absence of the foam layer at a specific point, a transmitter can provide additional insight into vessel contents to improve the overall process. Many tanks have both a continuous level transmitter in operation along with a point level switch.

Determining a technology’s suitability on a foaming application is dependent on the goal relative to the measurement: detect foam (point or continuous), ignore foam (liquid only measurement) or measure both foam and liquid.

Additionally, the foam thickness and properties of the foam, such as density, bubble size and dielectric constant are key considerations that can impact the level measurement.

Realising operational improvements
If there are existing issues in a process attributed to foam, consider reviewing the level technology (point or continuous) currently in use.
The correct technology will reduce maintenance and environmental impact costs while enhancing process throughput.

Overfills
Often when choosing a level measurement technology, the liquid surface takes precedence, with foam measurement only given secondary attention. This leads to foam-overs. Repeated foam-over conditions, if left unchecked, can obstruct the passage to overflow vessels and flood the containment basin/system. Expensive cleanup, environmental impact and lost product are the result of less than adequate level controls on foam.

The need for foam detection will vary by facility and individual tank. It is important to choose a level technology that is sensitive enough to detect the top of the foam layer and, possibly, provide a level of redundancy for foam detection. Some situations may only require a liquid level transmitter. However, if foam-over is a concern, a high-level switch or transmitter that can detect foam should be considered. In a plant that is continually receiving fines or encountering productivity issues due to foaming problems, a level switch will easily pay for itself. Unfortunately, some of the most well-known liquid level switch technologies may not be suitable for detecting an upper layer of foam.

Maintenance
Aside from potential fines and clean-up due to a foam-over, monitoring foam levels may also reduce maintenance costs associated with starving pumps. Pumping foam instead of liquid can damage pumps, resulting in significant and avoidable production downtime and replacement parts costs. A liquid level transmitter will indicate low or high levels in the tank, but a switch could also be used to indicate liquid versus foam.

Productivity
Oftentimes facilities will sacrifice tank capacity by reducing the maximum allowable level to accommodate dynamic foam conditions. Deploying the right technology for proper detection and monitoring can allow additional headspace for dynamic foam conditions. Proper monitoring of the top foam layer provides access to and utilisation of a tank’s entire capacity. No longer do you have to add additional tank capacity as a buffer to optimise production throughput. With large tanks that are being severely underfilled, this can result in considerable productivity gains.
In addition to better utilisation of tank capacity, reliable measurement of foam location, thickness and level can reduce the costs associated with anti-foaming agents and chemical additives. A redundant and diverse monitoring and detection solution can also be implemented on the most severe foaming applications.

Using radar-based technologies on foam
The scenarios previously described highlight the importance of choosing the appropriate level instrumentation. One of the fastest growing and most dependable technologies to both cut through foam and potentially measure the foam level is guided wave radar (GWR). Thanks to the strong signal down the waveguide (probe), GWR can be utilised across many applications and tank shapes.

The signal-to-noise ratio (SNR) is important for any technology, but in particular GWR. The higher SNR allows the instrument to detect lower dielectric liquids or foams and helps reduce the dead zone at the top of the probe. In the case of GWR, this means actual, direct level measurement, from the bottom of the probe up to the process connection, as opposed to inferred measurement via firmware techniques. This enables increased utilisation of tank capacity and thus higher productivity.