Actuator Key to Plant Optimization

Using electric actuators on critical applications improves control and reliability.
By Jay Tannan | November 15, 2010
A quality feed coproduct is vital to the success of an ethanol plant, therefore attention paid to critical sections in the distillers grains drying process is of utmost importance. As in any combustion process, the main objective is to maximize efficiency and in so doing, maximize product quality.

There are many factors to be considered when improving the efficiency of a dryer—drying time, air temperature and surface temperature—all impacting quality. Some studies have linked the variations in dried distillers grains with solubles (DDGS) moisture content to its digestibility in animals. Moisture content also affects the DDGS shelf life. Better dryer temperature control leads to less variability, more consistent product and better energy efficiency.

To achieve better dryer performance, air and fuel flows must be controlled precisely and consistently throughout the process. Traditionally, pneumatic actuators have been used to control the various throttling dampers and valves that regulate air and fuel flow, but pneumatic actuators don't have the consistent positioning capabilities required to optimize process control. Many ethanol plants are following the lead of the electric utility industry and installing specialty electric actuators. Choosing the right type of electric actuator capable of continuous modulation control can pay big dividends in terms of improved process control, product quality and efficiency.

Means for Good Actuation
What makes up a superior quality actuator and how can its performance influence the efficiency of a process? Since actuators come in a variety of different designs and sizes, one has to focus on specific application requirements. Actuators required for optimal process control must be capable of continuous modulation and provide consistent, precise positioning of the damper or valve over time and in varying conditions. An actuator needs to provide this control regardless of the number of starts and stops or changes in load and environmental conditions. Furthermore, the response of the actuator should not be affected by stiction (sticking caused by friction) that leads to a stick and slip response which adds dead time and causes overshoot and cycling.

With the inconsistent nature of damper and valve loads, and the simple fact that industrial equipment is not frictionless, pneumatic actuators will typically experience stiction. These actuators use compressible air for balancing dynamic and static loads which tends to trigger irregular control and positioning.

Electric actuators do not typically have issues with stiction. Most designs, however, incorporate induction motors that present another limitation. Every time an induction motor starts there is a high in-rush current that generates a lot of heat. This type of electric actuator design requires built-in thermal protection that turns the motor off if the temperature rises too high. The problem with this motor design is that the actuator's ability to start and stop, or modulate, is limited by motor temperature rise rather than by what is required to optimally control the process. Therefore, in order to maximize control performance, care must be taken to avoid selecting electric actuators that utilize induction motors with duty-cycle limitations, and select an actuator rated for continuous modulating duty.

An electric actuator that can precisely track the closed-loop demand is best suited for modulating applications. The actuator should respond quickly to demand changes without any dead-time, lag or overshoot. This not only requires an actuator capable of continuous duty but also the ability to make small, consistent position adjustments. Many typical electric actuators are incapable of making changes less than 0.5 percent of travel, and pneumatic actuators can be far worse over time combined with the potential effect of changing loads and conditions. Care should be taken to select actuators capable position changes in the range of 0.10-0.15 percent of full actuator travel consistently and under all conditions. This allows the control system to maintain the process demands, avoiding inconsistencies caused by poor positioning of the final control element.

Having good control makes a huge difference in the operating efficiency of a process. Good control is a function of how closely the dampers and valves can follow the demand and depend upon actuators that are capable of tracking the demand. In a combustion process typical of a DDG dryer, optimal control results in lower fuel consumption and more stable product quality. In addition, optimized control leads to less equipment fatigue caused by both mechanical and thermal cycling. Therefore, maintenance costs are reduced and the actuators, dampers and valves will last longer, provide better control and reduce unplanned shutdowns.

If chosen correctly, an electric actuator will respond instantaneously to demand changes without any overshoot or stiction. It will also provide a 100 percent continuous duty motor that never overheats and is capable of unlimited starts and stops. Currently, electric actuators are being used in ethanol plants on many critical damper and valve applications: bleed-off, combustion, recirculation air damper and other fan dampers. These are primarily used for flow and pressure control in the dryers. In some cases, flue gas recirculation dampers are also being controlled by electric actuators to reduce nitrous oxide emissions by recirculating flue gas to the combustion chamber.

Given that many of the processes in an ethanol plant require precise control, the desire to have a continuous duty actuator that will not coast or over-shoot the demand is extremely important. By regulating flow rates and pressures through various dampers and valves, electric actuators can improve efficiencies on many applications. Besides the dryers, the molecular sieve process and various tanks represent other opportunities for actuator upgrades. These applications involve the controlling of gases and liquids through a multitude of valves, which, if controlled effectively, could increase overall efficiency and reduce operating costs.

With a volatile global economy, one has to look in every direction for potential upgrades and improving automation is one key area. Maximizing returns in an increasingly competitive environment is key to plant success. One of the most critical, but often misunderstood parts of optimizing process control performance and efficiency is selecting the correct actuator. Installing reliable electric actuators that are designed for continuous modulating service, while providing precision positioning consistently over time and changing conditions, is an absolute necessity. In the end, improving a plant's performance can be as simple as investing in better actuator performance for the DDGS drying section of an ethanol plant.

Author: Jay Tannan
Sales Application Engineer
Harold Beck and Sons Inc.
(215) 968-4600, [email protected]