- Published on Thursday, 20 June 2013 09:41
- Written by Leo Minervini
Plant operators across the world are tasked with maximizing efficiency and output from existing facilities, which can be a major challenge when there are many valves and actuators. Plant management strategy to accomplish this task includes developing a better understanding of how valve and control assets perform in the field and to adopt preventative maintenance to minimize plant downtime. Valve positioner technology today has evolved to help plant managers with this major challenge.
VALVE POSITIONER TECHNOLOGY
The valve positioner market has grown significantly in recent years to about $1 billion globally. The technology grew from the need to maintain a steady control state for process valves such as globe, high-performance butterfly, ball and plug valves used to throttle or regulate a process within a plant. As plant network technology and infrastructure developed, so, too have innovations in valve positioners.
Three technology options are available to plant engineers looking to integrate a valve positioner solution:
Mechanical: A positioner is connected to the valve stem so that the position can be compared with that dictated by the controller via a force-balanced arm.
Electro-pneumatic: These positioners are based on the same force-balance principle as mechanical positioners, but the technology operates on a current-to-pressure transducer signal, typically 4-20mA, which then relays the valve position to the actuator. These systems are mounted directly and axially to the actuator stem and are suited to quarter-turn rotary actuator applications.
Smart/digital positioners: With built-in microprocessors, this positioner option allows greater, more precise control over plant assets and can also offer diagnostic data, which provides input for predictive maintenance schedules.
Over the last 10 years, developments in smart and digital valve positioners have outpaced the performance capabilities of mechanical and electro-pneumatic systems. Advancements in digital communications protocols have allowed smart controller technology to evolve, integrating more sophisticated functionality. Such innovations have led to digital valve positioners that operate as mini-steady-state controllers, enabling field level automatic controls.
For example, if an operator wants to send a signal to a valve to throttle by X degrees of rotation to a specific position for purposes of achieving a desired set point, the actuator applies only enough torque to move the valve Y degrees of rotation. The smart positioner can send a larger torque until the required set point has been achieved. Microprocessor-based technology gives the opportunity to make these necessary adjustments in the field. Unlike conventional mechanical and electro-pneumatic positioners, digital controllers develop accurate valve position feedback without the need for linkages, levers, and rotary or linear seals. Position sensing is performed totally by non-contacting means, which enables the use of advanced control strategies.
By accurately measuring and recording valve stem position, input signal and actuator pressure, smart positioners can provide information for control valve signature generation. These capabilities can be further enhanced with wireless technology to provide immediate status reports to engineers in the field, removing the need to access valves and actuators in difficult-to-reach or hazardous locations. This allows a plant to introduce predictive maintenance methodologies that can lead to better performance, improved safety and reduced inefficiencies through unnecessary downtime and subsequent costs.
Smart valve positioner technology introduces additional capabilities to plant operators over conventional valve controllers. For example, a smart positioner can use non-contact feedback to control the valve position, enabling the positioner to be mounted remotely from the valve. This offers benefits to plants where the control valve is located either in a high-vibration or corrosive environment.
In addition to rotary valve applications, the position-sensing performance and microprocessor technology of the smart valve positioner mean a single device can be used for rotary and linear valve control and non-valve applications.
DEVELOPMENTS IN PROTOCOLS
The HART communications protocol was an early implementation of a digital industrial automation protocol built on the common 4-20mA wiring infrastructure installed throughout the world. This allowed the protocol to offer a transition solution to many plant operators who were comfortable with the 4-20mA analog signal, yet wanted to implement a “smart” protocol.
As plant operators became more familiar with industrial automation protocols, the digital communication protocols emerged, such as FOUNDATION Fieldbus (FF) and PROFIBUS PA (Process Automation), which offered greater functionality and enhanced diagnostics.
FF is a powerful networking solution that reduces the cost and time required to install and wire industrial automation devices. With the capability to interconnect both complex and simple devices from multiple vendors on the same network, FF supports distributed control allowing for the configuration of devices for local control in the field, in the host or for both sources.
Developed from the PROFIBUS DP (Decentralized Peripherals) variation, PROFIBUS PA monitors measuring equipment via a process control system in process automation applications. PA uses the same data format as the much faster DP protocol, enabling easy and seamless integration between devices on both networks. As with FF, remote configuration, asset management and predictive maintenance scheduling are all possible with the PROFIBUS system.
In addition to offering greater functionality and enhanced diagnostics, FF and PA protocols have a distinct advantage when considering power availability compared to HART, allowing positioners to run processors faster, increasing communication performance.
RESPONDING TO CHANGING APPLICATIONS
Many oil & gas, petrochemical and process engineering plants are now operating with wide ranges of temperatures, pressures and flow rates. Harsh environments require valve and control products to function in severe service applications, requiring tougher and more reliable components with the ability to withstand extreme temperature or pressure fluctuations and maintain chemical and corrosion resistance.
Plant operators now look to control and monitoring solutions that can cross multiple applications. For valve positioner technology, developments in construction materials have needed to keep pace with these changing application requirements.
The valve position and control market is largely driven by changes in customer requirements, which then impact system design, enclosure construction or performance capability. In hazardous zones, for example, stainless steel has replaced aluminum in the construction of valve positioner enclosures—an innovation driven by customer requirements in the oil and gas industry.
VALVE POSITIONER SPECIFICATION
A key consideration for plant operators looking to specify and engineer a valve positioner solution is system integration with existing plant network infrastructure. Host neutrality, which allows integration with a digital control system and local plant infrastructure, ensures engineers can achieve quick calibration and integration without the need for major system investment.
Valve positioner technology that integrates innovative digital communications provides plant operators with greater visibility and control over plant assets. Smart valve positioners offer enhanced capability and practical benefits to plant operators in relation to improved plant performance and greater operational efficiencies. Many of the products available today provide features such as advanced auto-calibration and single design for linear and rotary applications, as well as spring return and double-acting actuators to reduce plant inventory requirements.
Remote mount capability and different materials of construction—such as engineered resin, aluminium and stainless steel—as well as the ability to operate in low- and high-flow capacities enables smart valve positioner technology to meet the needs of demanding plant applications. The enhanced performance available today, when combined with valuable diagnostic data and robust communications protocols, ensure plant operators can benefit from the latest innovations in plant control technology.
Leo Minervini is general manager, Westlock Controls Global (www.westlockcontrols.com).