Control of fluid flows and temperatures in food & beverage plants presents a number of challenges, most of which can be addressed with correct selection and application of control valves.
Blake Coleman, Key Account Manager for Emerson’s flow controls products says temperature control is a very important variable in the food and beverage industry. His article, Selecting Valves for Food & Beverage Applications in the September 2018 issue of Flow Control magazine, points out that if temperature is poorly controlled, the best case result would be lost efficiency, with the worst case a completely unusable batch of product.
Due to such high stakes, producers have a tendency to err on the side of caution. While producing product in this manner ensures consistency, it also limits the maximum potential of a process. But when there is a high confidence in maintaining critical temperatures close to setpoints, the ability to run closer to safety thresholds can result in increased production, along with reduced water and steam usage.
Reactor Temperature Control
One estimate says that 30% of batches are not done right the first time. In many cases, Coleman says, this is because of poor temperature control.
The root causes of many temperature control problems are nonlinearity in the jacket loop from selecting inappropriate control valves, improper tuning, and excessive dead zones in a split range strategy. Split-range control can be used to maintain a reactor temperature where both heating and cooling are needed. When the temperature is below the desired setpoint, the cooling valve closes and the heating valve will open. When the temperature is above the setpoint, the heating valve closes and the cooling valve will open.
Processes requiring extended rangeability may use two or more control valves. One valve provides control within the lower operating range, and the second valve provides control within the upper operating range.
Tuning Problems Solved
Sometimes, problems are caused by improper tuning of the valve and control loop. For example, at one plant, temperatures of eight batch reactors were oscillating. With the reactor setpoint initially at 30°C, slow oscillation caused the jacket to continuously consume significant quantities of steam and chilled water in an alternating sequence. Later, after the exothermic reaction inside the reactor was completed, the jacket controller output began swinging almost full scale up and down. Average energy consumption was much greater than theoretically required to maintain the reactor temperature.
Plant personnel hadn’t been trained in modern loop-tuning methods such as Lambda tuning, which gives non-oscillatory response at the speed required. Tests required for systematic tuning also revealed nonlinearities in the split range logic and control valves. After applying corrections to three reactors, energy savings on steam alone paid for the project in less than three months.
Not matching controller settings to valve characteristics causes several problems. For example, the response in another batch reactor was too slow, taking more than two hours to reach a new setpoint. Due to the slow response, operators preferred to make frequent manual adjustments to the jacket setpoint until the correct reactor temperature was achieved. This interfered with the operators’ primary duties, such as sampling for quality control.
While many temperature control problems in food and beverage reactors and processes are because of poor tuning, selecting the correct control valve is very important because no amount of tuning will suffice if the control valve is not operating properly.
Importance of Valve Selection
Control valve selection is important for ensuring high levels of uptime and quality. Control valves associated with temperature control must be able to quickly get to setpoint, maintain a high level of accuracy during the batch cycle without oscillation, and quickly respond to end of batch setpoint.
Having high rangeability is preferable but must be balanced with response. Does one degree of temperature change impact quality? If so, then a control valve solution with highly accurate response should be considered. A butterfly valve can provide high rangeability, but has a much narrower control range compared to a segmented ball or globe valve.
In batch operations, time between batches has a major impact on plant profitability, so fast charging times are important, which may require a high-volume valve, with corresponding lower response. A butterfly or full port ball valve may be used if fast fill time is all that is required. If improved throttling control is needed, a segmented ball valve or globe valve is a better option.
An oversized valve will provide sluggish performance and result in a high rate of reflux, with correspondingly high energy costs to meet purity targets. Conversely, an undersized valve may not be able to properly respond to disturbances, causing large oscillations and negatively impacting product quality.
Once the right valve is in use, diagnostics can be used to monitor performance.
Valve diagnostics can be used to ensure valve performance has not significantly changed over time. By identifying issues early, a maintenance plan can be put into place, with the right parts and spares in stock. This eliminates unnecessary work and troubleshooting. Diagnostics can be run online in continuous operation, or offline between batches.
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