As plant network technology and infrastructures developed, innovations in valve positioner technology have likewise grown to meet these demands.
By Leo Minervini
Chemical and petrochemical plant owners and operators throughout the world continuously seek to optimise the efficiency and production capacity of their facilities. Developing a better understanding of how their valves and controls assets perform in the field and adopting preventive maintenance techniques to minimise plant downtime and equipment failure are key strategies that support the goals of plant performance and production optimisations. However, doing so represents a significant challenge for many plant engineering and maintenance personnel, particularly within facilities that have a high number of installed valves and actuators. In many plants, the number of valves and actuators can run in the thousands.
Valve positioner technologies and solutions
The global valve positioner market has grown significantly in recent years and is valued at approximately $1 billion. As plant network technology and infrastructures have developed, innovations in valve positioner technology have likewise grown to meet these demands. Valve positioning technology has grown from the need to maintain a steady control state of process valves such as globe valves, high-performance butterfly, and ball and plug valve types used to throttle or regulate a process within a plant.
Three valve positioner technology options are available to plant engineers who want to integrate a valve positioner solution:
- Mechanical – the 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 – based on the same force-balance principle as mechanical positioners, this 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; and,
- Smart/digital positioners – with built-in microprocessors, this option enables greater and more precise control over plant assets and can also offer diagnostic data, which thus provides input for predictive maintenance schedules.
Over the last 10 to 15 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 enabled smart controller technology to evolve and integrate more sophisticated functionality. Such innovations have led to digital valve positioners operating as mini steady state controllers, thereby enabling field level automatic control. For example, if an operator sends a signal to a valve to throttle by X degrees of rotation to a specific position to achieve the desired set point, and the actuator only applies 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.
The 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, enabling the use of advanced control strategies.
By accurately measuring and recording valve stem position, the input signal and actuator pressure, smart positioners can provide information for a 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 enables a plant to introduce predictive maintenance methodologies which can lead to better performance, improved safety, and reduced inefficiencies by avoiding unnecessary downtime and subsequent costs.
Smart valve positioner technology introduces additional capabilities to plant operators in comparison to conventional valve controllers. One such product is the ICoT series of positioner from Pentair Valves & Controls’ Westlock Controls brand, the ICot products use non-contact feedback to control the valve position, which enables it to be mounted remotely from the valve. This offers benefits to plants in which the control valve is in either a high vibration or a corrosive environment. In addition to rotary valve applications, the position sensing performance and microprocessor technology of the smart valve positioner enables a single device to be used for rotary and linear valve control and non-valve applications.
Developments in digital communications protocols
The HART™ (Highway Addressable Remote Transducer) 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 significant installed base allowed the protocol to offer a transition solution to many plant operators who were comfortable with the 4-20mA analogue signal, yet wanted to implement a ‘smart’ protocol.
As plant operators became more familiar with industrial automation protocols, this led to the emergence of digital communication protocols, such as FOUNDATION™ Fieldbus (FF) and PROFIBUS™ PA (Process Automation) that 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 of interconnecting 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 both.
Developed from the PROFIBUS™ DP (Decentralised 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.
Anticipating and addressing evolving application needs
Many oil and gas, chemical, petrochemical and process engineering plants operate with wide ranges of temperatures, pressures, and flow rates. Harsh environments require valves and control products to function in severe service applications; i.e., tougher and more reliable components with the ability to withstand extreme temperature or pressure fluctuations and maintain chemical and corrosion resistance are essential.
Plant operators need and expect control and monitoring solutions that can cross multiple applications effectively, safely and reliably. With regard to valve positioner technology, construction materials have evolved to keep pace with these changing application requirements. The valve position and control market is driven predominantly by changes in customer requirements that then correspondingly impact system design, enclosure construction, and/or performance capability. In hazardous zones, for example, stainless steel has replaced aluminium in the construction of valve positioner enclosures - an innovation driven by customer requirements in the oil and gas industry that has transferred to and been adopted by virtually every industry sector.
Valve positioner specifications
System integration with existing plant network infrastructure is a key consideration for plant operators looking to specify and engineer a valve positioner solution. Host neutrality, which allows integration with a digital control system (DCS) and local plant infrastructure, ensures that engineers can achieve calibration and integration without the need for major system investment in an expedited manner.
Valve positioner technology that integrates innovative digital communications provides plant operators with greater visibility and control over their plant assets. Smart valve positioners offer enhanced capability and practical benefits to plant operators in relation to improved plant performance and greater operational efficiencies. Products such as the ICoT series provide advanced auto-calibration features, a single design for linear and rotary applications and also 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. This enhanced performance, when combined with valuable diagnostic data and robust communications protocols, ensures that plant operators can benefit from the latest innovations in plant automation and control technology and solutions.
Read More: Role of valve positioner in achieving greater efficiencies
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