SPECIAL TOPIC : Actuators & Automation
• Delayed Response to Process Control : Deadband introduces a delay in the valve ’ s response to alternations in process conditions . In dynamic environments where quick adjustments are essential , such delays can result in process inefficiencies , waste , heightened safety risks , and operational instability .
• Increased Hysteresis : A direct consequence of deadband is the escalation of hysteresis within valve control systems . Characterized by a variation in actuation points based on the signal ’ s direction of change , hysteresis introduces control inaccuracies , complicating the task of achieving precise process management .
• Reduced Energy Efficiency : The inefficiencies introduced by deadband can lead to processes being either over-operated or under-operated as attempts are made to counteract the imprecision , thereby inflating operational costs and energy usage .
• Increased Risk of Oscillations : Deadband present in the system increases the likelihood of oscillations and system instability . Without proper adjustment of control strategies to accommodate deadband effects , the system may experience undesired oscillations , undermining process control stability and reliability .
Sources of Deadband
Deadband can compromise the precision and efficiency of valve control systems . Understanding and addressing these sources are crucial for enhancing system accuracy and responsiveness .
Figure 3 : Performance analysis of the KVA38 in a natural gas production process control setting .
• Mechanical Backlash : Typically present in both pneumatic and electric motorized valves , mechanical backlash – looseness within actuator drivetrains – introduces a delay characteristic of deadband . Initial actuator movements fail to directly translate into valve adjustments , creating a zone where control inputs do not produce expected outcomes .
• Friction : A universal challenge , friction occurs within the valve mechanism , notably between the valve stem and packing in addition to friction from the several moving components typical of pneumatic or electric motor actuators . This friction requires additional force to both initiate movement and maintain it , contributing to deadband by delaying actuation until a higher input threshold is reached .
• Positioner Issues : Problems with the valve positioner , crucial for adjusting the actuator ’ s operation based on the control signal , can significantly magnify deadband . This issue is particularly pertinent for pneumatic actuators , where the precision of the positioner is essential for converting air pressure changes into accurate valve movements .
• Duty Cycle Limitations : Typical electric motors cannot run continuously ( for example , less than 100 % duty cycle ) to maintain safe operating temperatures to prevent motor failure . A deadband may be intentionally introduced to prevent motorized actuators from continuous motion in hunting for the process setpoint .
Shape Memory Alloy Actuators : Advancing Valve Control Precision and Response
SMA actuators ( Figure 2 ) represent an advancement in industrial valve control technology . SMA ’ s possess the extraordinary ability to return to their original shape when energized , a property that SMA actuators harness to offer smooth control of valve position with fine precision and accuracy .
How SMA Actuators Work
Shape memory alloys ( SMA ) belong to a distinct category of metal alloys capable of “ remembering ” a predefined shape . This two-way shape memory effect enables precise and consistent shape manipulation . The core functionality of SMA bundled wire actuators , a type of SMA actuator , is derived from the contraction of SMA wires upon being energized , biased against a mechanical spring . This dynamic converts electrical energy into mechanical movement , facilitating precise and smooth valve adjustments in response to control signals . The application of electrical current to these wires triggers the shape memory effect , allowing the wires to shorten by overcoming the spring force . This contraction , when linked to a valve stem , facilitates the precise regulation of the valve ’ s position . This ability for precise control allows for extremely fine responses that minimize deadband that arises in control valve applications ( Figure 4 ).
Figure 4 : Example of the Kinitics Valve Actuator demonstrating a deadband of less than 0.3 %.
Minimizing Deadband with Shape Memory Alloy Actuators
SMA actuators introduce a suite of advantages directly addressing the issue of deadband :
• 100 % Duty Cycle : SMA actuators excel in continuous throttling operation without the need for pauses or downtime , crucial for processes requiring uninterrupted control . This constant operability ensures that there is no lag between control input and actuator response , effectively eliminating deadband caused by operational pauses .
• Micron-level Precision : From the ability of precise control over the SMA ’ s length adjustments , these actuators can achieve incredibly fine positional precision . This ability directly combats deadband by ensuring precise valve movements and positioning that track the command signal .
• Mechanically Efficient and Reliable : SMA actuators on the market ( Figure 2 ) feature a single moving part in the drivetrain that moves in-line with the valve stem , thus minimizing friction losses due to internal components ( e . g ., gears , bearings , etc . - typical of motorized or pneumatic equipment ). Also , the absence of high-speed rotational components reduces wear , minimizes maintenance schedules , and prolongs actuator life .
Conclusion
SMA actuators represent a solution in combating challenges due to deadband in control valve systems . SMA technology provides the ability for continuous operation at 100 % duty cycle with a mechanically efficient drivetrain . When applied to process control , SMA actuators provide precision and responsiveness that mitigate deadband ’ s negative impact on system performance . SMA actuators are being used in critical industrial applications including within the natural gas sector ( Figure 3 ). Their use provides a reliable solution for process control in a sector that aims to electrify operations including the adoption of electrically powered control valve actuators .
ABOUT THE AUTHOR
Adam Nguyen , M . Eng is an Automation Solutions Specialist at Kinitics Automation . He is dedicated to pushing the boundaries of technology in robotics , A . I ., computer vision , and industrial automation . His commitment to innovation fuels his work , with a goal for a more advanced and sustainable world through cutting-edge solutions .
www . valve-world-americas . net • April 2024 | Valve World Americas 15